LONG-TERM EFFECT OF WILDFIRES ON VASCULAR PLANT AND SOIL INVERTEBRATE DIVERSITY IN PRIMARY FIR-SPRUCE FORESTS OF THE URAL MOUNTAINS (NORTH EURASIA) Текст научной статьи по специальности «Биологические науки»

О RUSSIAN JOURNAL OF ECOSYSTEM ECOLOGY Vol. 7 (1), 2022

Reœived 12.10.2021 Revised 20.02.2022 Accepted 10.03.2022 Open Access

DOI 10.21685/2500-0578-2022-1-5

LONG-TERM EFFECT OF WILDFIRES ON VASCULAR PLANT AND SOIL INVERTEBRATE DIVERSITY IN PRIMARY FIR-SPRUCE FORESTS OF THE URAL MOUNTAINS (NORTH EURASIA)

T. Yu. Braslavskaya1, A. P. Geraskina2, A. A. Aleinikov3, R. Z. Sibgatullin4, N. V. Belyaeva5, N. L. Ukhova6, V. N. Korotkov7, D. S. Shilov8, D. L. Lugovaya9, O. V. Smirnova10

1'2'310 Center of Forest Ecology and Productivity of Russian Academy of Sciences, 84/32 Profsoyuznaya street, Moscow, 117899, Russia

4, s, 6,8 visim State Nature Biosphere Reserve, 23 Stepan Razin street, Kirovgrad, Sverdlovsk Region, 624140, Russia

7 Yu. A. Israel Institute of Global Climate and Ecology of Federal Service for Hydrometeorology and Environmental Monitoring, Russian Academy of Sciences, 20b Glebovskaya street, Moscow, 107258, Russia

9 WWF Russia, 19 Nikoloyamskaya street, Moscow, 109240, Russia

t.braslavskaya@gmail.com

Abstract. One of the essential tasks of sustainable forest management is to maintain native biodiversity. Primary forest research is one of the ways to understand what this biodiversity is. Matherials and methods. The primary, as confirmed by their land-use history and structural peculiarities, mesic dark-conifer forests remain in Visim and Pechora-Ilych nature biosphere reserves (boreal and sub-boreal zones respectively, the Ural Mountains, Russian Federation). We compared the primary forests and post fire 100-year small-leaved deciduous forests by diversity of vascular flora and soil invertebrate macrofauna. Results and discussion. The diversity of some functional groups of species (low boreal herbs, earthworms) in post fire forests is lower than in primary forests, the research shows. These species largely depend on deadwood and other tree-related microhabitats common in the primary forests but not so in the 100-year post fire forests. Repeated fires at intervals of several decades, as is the case with the use of prescribed fires in forest management, will be expected to reduce the biodiversity quality of these specialist species. Additionally, we revealed that post fire forest flora is more synanthropic in the woodland of a small area (Visim reserve) than in the intact forest landscape (Pechora-Ilych reserve). It demonstrates that, within extensive woodlands, native forests are more resilient to sporadic stand-replacing disturbances than small woodlands. Conclusion. Strict conservation of intact forest landscapes is necessary as they serve as large buffer areas around the remaining primary forests to maintain native biodiversity.

Keywords: boreal and sub-boreal forests, functional groups of species, land-use history, plant and soil-invertebrate species richness, post-fire recovery, tree populations

Funding. This work has been supported by the Russian Foundation for Basic Research (project 19-04-00609) and by the Ministry of Science and Higher Education of Russian Federation (project 121121600118-8).

For citation: Braslavskaya T. Yu., Geraskina A. P., Aleinikov A. A., Sibgatullin R. Z., Belyaeva N. V., Ukhova N. L., Korotkov V. N., Shilov D. S., Lugovaya D. L., Smirnova O. V. Long-term effect of wildfires on vascular plant and soil invertebrate diversity in primary fir-spruce forests of the Ural Mountains (North Eurasia). Russian Journal of Ecosystem Ecology. 2022,7(1). Available from: https://doi.org/10.21685/2500-0578-2022-1-5

УДК 630.228 + .182 + .62 (470.13 + .54)

ДОЛГОВРЕМЕННОЕ ВЛИЯНИЕ ПОЖАРОВ НА РАЗНООБРАЗИЕ СОСУДИСТЫХ РАСТЕНИЙ И ПОЧВЕННЫХ БЕСПОЗВОНОЧНЫХ В МАЛОНАРУШЕННЫХ ПИХТОВО-ЕЛОВЫХ ЛЕСАХ УРАЛА (СЕВЕРНАЯ ЕВРАЗИЯ)

Т. Ю. Браславская1, А. П. Гераськина2, А. А. Алейников3,

Р. З. Сибгатуллин4, Н. В. Беляева5, Н. Л. Ухова6, В. Н. Коротков7,

Д. С. Шилов8, Д. Л. Луговая9, О. В. Смирнова10

© Braslavskaya T. Yu., Ge raskina A. P., Al eíníkov A. A., Síbgatull¡n R. Z., Belya eva N. V., Ukhova N. L.f Korotkov V. N., Shilov D. S., Page 1 from 2 6

Lugovaya D. L., Smirnova O. V. 2022 Данная статья доступна по условиям всемирной лицензии Creative Commons Attribution 4.0

International License (http://creat¡vecornmovs.org/l¡censes/ba/4.c)/), которая дает разрешение на неограниченное использование,

копирование на любые носители при условии указания авторства, источника и ссылки на лицензию Creative Commons, а также

изменений, если таковые имеют место.

12 310 Центр по проблемам экологии и продуктивности лесов РАН, Россия, 117899, Москва, ул. Профсоюзная, 84/32

4 5 6 8 Висимский государственный природный биосферный заповедник, Россия, 624140, Свердловская область, Кировград, ул. Степана Разина, 23

7 Институт глобального климата и экологии имени академика Ю. А. Израэля Федеральной службы по гидрометеорологии и мониторингу окружающей среды РАН, Россия, 107258, Москва, ул. Глебовская, 20б

9 WWF России, Россия, 109240, Москва, ул. Николоямская, 19

t.braslavskaya@gmail.com

Аннотация. Одна из важных задач устойчивого управления лесами - сохранение биологического разнообразия, в том числе аборигенных и эндемичных видов. Оценки потенциала такого разнообразия возможны только при изучении сохранившихся малонарушенных лесов. Материалы и методы. Исследования проведены в государственных природных биосферных заповедниках Висимском и Печоро-Илычском (соответственно - южная и средняя тайга, Уральские горы, РФ), где сохранились мезофитные темнохвойные леса, история и структура которых свидетельствуют, что они относятся к малонарушенным. Мы сравнили показатели разнообразия флоры сосудистых растений и макрофауны почвенных беспозвоночных в этих малонарушенных темнохвойных лесах и в березняках, сформировавшихся в тех же условиях после пожаров 100-летней давности. Результаты и обсуждение. В послепожарных березняках разнообразие в некоторых функциональных группах видов (бореальное мелкотравье, дождевые черви) ниже, чем в малонарушенных темнохвойных лесах. Жизнедеятельность этих видов часто связана с присутствием в сообществах крупных древесных остатков и других микроместообитаний, формируемых в результате полноценной жизнедеятельности лесообра-зующих видов деревьев и поэтому обычных в малонарушенных лесах, но не успевающих появиться в течение первых 100 лет после пожара. Основываясь на наблюдаемых результатах 100-летнего послепожарного восстановления лесов, можно ожидать, что под влиянием пожаров, повторяющихся каждые несколько десятилетий (в том числе при использовании профилактических выжиганий в лесном хозяйстве), уровень и качество биоразнообразия лесов будет снижаться вследствие исчезновения таких специализированных видов. Наше исследование также выявило, что в лесном массиве небольшой площади (Висимский заповедник) це-нофлора послепожарного березняка включает больше синантропных видов, чем в аналогичном сообществе в составе крупного малонарушенного лесного массива (Печоро-Илычский заповедник). Это свидетельствует, что в сообществах, сохраняющих эталонное биоразнообразие лесов, видовой состав меньше трансформируется при эпизодических катастрофических нарушениях их древостоев, если такие сообщества расположены в крупных лесных массивах. Заключение. Для успешного поддержания биоразнообразия в сохранившихся малонарушенных лесных сообществах необходимо обеспечивать сохранение вокруг них крупных не фрагментированных лесных массивов, которые могут способствовать расселению сохраняемых видов и в то же время затруднят проникновение синантропных.

Ключевые слова: бореальные и гемибореальные леса, функциональные группы видов, история природопользования, видовое богатство растений и почвенно-беспозвоночных, послепожарное восстановление, популяции деревьев

Финансирование. Работа выполнена при поддержке РФФИ (проект 19-04-00609) и Министерства науки и высшего образования РФ (проект 121121600118-8).

Для цитирования: Браславская Т. Ю., Гераськина А. П., Алейников А. А., Сибгатуллин Р. З., Беляева Н. В., Ухова Н. Л., Коротков В. Н., Шилов Д. С., Луговая Д. Л., Смирнова О. В. Долговременное влияние пожаров на разнообразие сосудистых растений и почвенных беспозвоночных в малонарушенных пихтово-еловых лесах Урала (северная Евразия) // Russian Journal of Ecosystem Ecology. 2022. Vol. 7 (1). https://doi.org/ 10.21685/2500-0578-2022-1-5

Introduction

Maintaining native biodiversity is one of the crucial tasks of sustainable forest management. Certain principles of community and population ecology help to achieve it. First, communities are combinations of species populations [1] therefore generations should replace each other in the correct order in all populations [2] to maintain biodiversity. Second, some keystone species strongly influence the lives of others and general community processes so that total biodiversity level depends on self-maintaining popu-

lations of keystone species [3, 4]. Third, community structure and composition often change due to events influencing population mortality [5-7] but community self-maintaining is possible only when all species' populations can reproduce new generations [2].

In forests, tree species play a keystone role as community modifiers [3] and accumulate and regulate most of the community resources. They form a community framework and various tree-related microhabitats used by many other species [8-14]. Forest management should ensure trees grow to maximum size (at least sometimes) and are re-

tained when dead to provide the availability of tree-related microhabitats [15].

Canopy gap mosaics, trees of wide size and age range, large number of deadwood and tree-related microhabitats are well developed in sub-boreal and boreal forests where populations of various species (autotrophs and heterotrophs) thrive [10-12, 16-20]. Such structure features result of tree population dynamics affected by fine-scale disturbances over the lifespan of several tree generations [10, 21-24]. Such forests without traces of anthropogenic impact are often considered primary [25]. However, in certain old-growth forests weak anthropogenic disturbances, e.g., selective logging could form a similar structure [18, 23].

Forest dynamics have often to be regulated by large-scale stand-replacing disturbances related to various factors. Those may be extreme weather, ge-omorphological or hydrological events, fires or steep increase in consumer abundance. Human economic activity has been a cause of such disturbances in forests for a long time too. The regularity and intensity of stand-replacing disturbances vary depending on the influencing factors and general environmental conditions [5, 26-28]. Large open plots or other resources appearing after stand-replacing disturbances allow some species including commercial trees to develop rapidly and reach their maximum abundance [11, 29-31]. However, the studied stand-replacing disturbance impact on general biodiversity is not favourable in many cases [32].

Simulation of some stand-replacing disturbances considered natural and providing successful reforestation are the current trends in forest management [27]. Within boreal (taiga) and sub-boreal biomes in the Eurasia northwest, wildfires are regarded as common natural disturbances that should be simulated by forestry actions to provide recovery of typical forests [33]. Thus, fires every 40-60 years in xeric Scots pine forests and fires every 60-100 years in mesic spruce or secondary deciduous forests are considered natural [27, 34]. However, such periodicity detected in historical time is disputably natural [35, 36]. In forest management, use of fire may have both planned results and undesirable effects [37, 38].

We compared the diversity of vascular flora and soil invertebrate macrofauna in mesic primary fir-spruce forests (PrFS) which remained in the Ural mountain forests and tundra ecoregion (North Eurasia, Russian Federation - Fig. 1,A, B) and in post fire 100-year birch forests (PfB) that replaced those dark-conifer communities. In the studied cases, the span of post fire succession was the same as the suggested natural periodicity of fires in mesic spruce forests [27, 34]. However, the observed post fire forests still haven't restored their previous stand composition (as expected for the forests adapted to such a span between natural

fires). Thus, our research hypothesis H1 was that the composition of flora and fauna has not recovered in post fire forests when compared to primary forests, and the difference may be found in vascular plants and soil invertebrate macrofauna.

Fig. 1. The study area in (A) Eurasia and (B) within the Ural montane forests and tundra ecoregion. Legend: I - Visim NBR, II - Pechora-Ilych NBR; 1 - Intact Forest

Landscapes [39, updated in 2018]; 2-7 - Terrestrial Ecoregions [40]: 2 - Scandinavian and Russian taiga, 3 - West Siberian taiga, 4 - Sarmatic mixed forest,

5 - Western Siberian hemiboreal forests, 6 - East European forest steppe, 7 - Ural montane

forests and tundra; 8-9 - geographic provinces in the Urals [41]: 8 - the North Urals, 9 - the Middle Urals; 10 - areas of state nature reserves

Material and Methods

Study area

The study area comprises the Middle and the North Urals [41], which are two provinces within the Ural montane forests and tundra ecoregion (see Fig. 1,B). In these parts of the Ural Mountains, most landscapes are not suitable for agriculture because of harsh environmental conditions. Therefore, people did not develop anthropogenic infrastructure there for a long time. Many forested areas have remained hard-to-reach and primary up

to now. Only the nomadic Finno-Ugric people named Mansi inhabited these provinces of the Ural ecoregion before the 17th century. Mansi hunted, fished, gathered wild plants; they also knew blacksmith handcraft and used it to make hunting weapons and heathen religious amulets [42-45]. Russian people established their first settlements (along trade routes used to carry furs from Siberia) at the end of the 16th century in the south of the Middle Urals [46]. They built river vessels there and used it for cargo trans-shipment. Local agricultural clearings appeared near them too. Primitive industrial mining of iron and non-ferrous metals started in the developed southern districts of the Middle Urals in the first half of the 18th century. Then a few metallurgical manufactories were built there [47, 48]. Consequently, logging, charcoal burning, and road building began to develop [49, 50]. Appeared then, a particular kind of land-use was gathering Siberian pine seeds (edible and high-calorie): commonly, to get the cones, people cut down entire reproductive trees [51, 52].

There were no permanently inhabited human localities until the end of the 18th century north of the N 60° latitude [45]. The first trading settlements were established along rivers and land trade routes from the Pechora basin to the Kama basin. Clearings were small and used mainly as hayfields near these villages. There were no mining and logging within the North Urals until the early 20th century due to lack of roads and population [53]. Now the most intact areas of the North Urals are included in the World Natural Heritage UNESCO list [54] and mapped as intact forest landscapes [39] (see Fig. 1,A). In some forest communities of the North Urals palaeoecological studies revealed no fire marks dating back the last few millennia [55].

Our study deals with montane forests in two Nature Biosphere Reserves (NBRs) situated on the western macro-slope of the Ural Mountains (see Fig. 1,B). These NBRs have been under the longest continuous protection and research in the Ural ecoregion. Their land-use history thoroughly studied really confirms that the forests chosen for our study have remained primary.

The Visim NBR (Sverdlovsk Region) is situated in the sub-boreal zone in the Middle Urals (see Fig. 1,B, I), mainly in the basin of the upper Sulyom River, a tributary of the Chusovaya River (Volga-Kama basin). The climate of this district is moderate continental, average annual temperature is +1.3°C, and annual precipitation is about 620 mm. Snow makes up about 150 mm and covers soil for 150-165 days [56]. The land-use history of the contemporary Visim NBR is troublesome. In the early 18th century several small mine-and-

metallurgical manufactories appeared in this district. By the middle of the 19th century, logging and fires that accompanied the industry converted a half of woody vegetation from dark-conifer forests to small-leaved deciduous ones. The formerly intact dark-conifer woodland was divided into western and eastern segments. Hay meadows also became common [49]. In the 20th century, alternate clear-strip fellings were practiced in the eastern segment. The first state nature reserve was established in 1946 on the area of 563 km2, but it was abolished in 1951. The reserve established in 1971 comprised only a small part (93 km2) of the former one. This new Visim reserve was expanded later, its contemporary area is 335 km2. The last felling was carried out in the reserve in 1995. Visim reserve got the biosphere status for conserving the reference forests of the Middle Urals in 2001. Woodlands cover 87 % of the area within the Vi-sim NBR now, but most forests are middle-aged. They are mainly small-leaved deciduous forests formed by two birch species (Betula pendula Ehrh., B. pubescens Roth) and aspen (Populus tremula L.). Mixed stands of spruce (Picea obo-vata Ledeb.) and fir (Abies sibirica Ledeb.) are also present. The last primary uneven-aged dark-conifer forests have remained as several plots of 2-5 km2 area (total is about 15 km2) located mainly in the southeast part of the reserve.

The Pechora-Ilych NBR (Komi Republic) is situated in the boreal zone in the North Urals (see Fig. 1,B, II) and comprises the basin of the upper Pechora River. The climate is moderate continental with an average annual temperature of -0.4 °C and annual precipitation of about 800 mm. Snow makes up about 200 mm of the precipitation and covers soil for 180-190 days [57]. The reserve of a total area of 7210 km2 was established in 1930. Foothill and mountain landscapes cover most of its territory. There were no permanent settlements in these landscapes until the second half of the 19th century. Several small villages (not more than 2-3 households) appeared later, but agricultural clearings occupied less than 0.5 % of the total area, and fires disturbed less than 11 %. In the early 20th century selective fellings were carried out only in Scots pine forests at the southwest extreme of foothill-mountain area [53, 58, 59]. Now woodland covers 88 % of the Pechora-Ilych NBR and the rest are alpine tundra and open bogs [60]. Most montane and foothill forests are uneven-aged and dark-conifer, therefore mainly formed by spruce (Picea obovata Ledeb.) and fir (Abies sibirica Ledeb.) with an admixture of Siberian pine (Pinus sibirica Du Tour) and pubescent birch (Betulapubescens Roth). The Pecho-

ra-Ilych NBR was assigned a status of a biosphere reserve in 1985. It is a part of one of the largest European intact landscapes widely known as UNESCO World Heritage "Virgin forests of Komi Republic" since 1995 [54].

Study objects

Primary fir-spruce (PrFS) and post fire birch (PfB) forests are in montane taiga belt in both NBRs. All studied communities belong to the tall-herb-fern type. The field layer is dominated by plants of 1-1.5 m height in this type of forests. Those are various tall herbs (Aconitum septentrionale Koelle, Cacalia hastata L., Chamaenerion angustifolium (L.) Scop., Crepis sibirica L., Delphinium elatum L., Paeonia anomala L., Trollius europaeus L., Veratrum lobelianum Bernh., etc.) and tall ferns (Dryopteris species, Diplazium sibi-ricum (Turcz. ex G. Kunze) Kurata, Athyrium species) with an admixture of tall grasses (Cala-magrostis species, Cinna latifolia (Trev.) Griseb., Milium effusum L.). The ground layer is typically composed of green mosses (without lichens or sphagnous mosses) and is not continuous. In forests where species composition includes a large set of tall herbs, diverse shrubs, and low boreal and nemoral herbs, total species richness is much higher [61] than in widely distributed boreal and sub-boreal forests of green-moss and low-herb types [62-64]. Tall-herb-fern forests occur in plain and montane landscapes within sub-boreal and boreal zones in northeast European Russia [19, 65]. In the Ural ecoregion, Siberian pine (Pinus sibirica Du Tour) and fir (Abies sibirica Ledeb.) are common in tall-herb-fern dark-conifer forests. This peculiarity distinguishes the Ural forests out of similar forests of Eastern Europe.

In the Visim NBR, the study was carried out in PrFS (57.380° N, 59.765° E, 610 m a.s.l.) located on the gentle western slope of the Mt. Bolshoi Sootook, and in PfB (57.389° N, 59.743° E, 560 m a.s.l.) located on the gentle south-western slope of the Mt. Malyi Sootook. Both forests grow on montane-forest burozem [66], i.e., Cambisol [67] or In-ceptisol [68]. Soil moisture is moderate and flowing in these communities. The two mountains are situated at least 10 km away from all former and existing villages. This explains the absence of logging or other economic activity on their slopes before the 20th century. The PrFS is located within one of the last primary woodlands in this reserve (see above), in a place not disturbed by logging or fires [49, 50]. This primary forest is the most accessible on foot in the NBR. The studied PfB grows in comparable environmental conditions with the PrFS, and this forest was chosen for our

study because of an easy access too. Logging was carried out in 1920-193 0s, and ground fire occurred after this [50] in the PfB.

For the forests studied in the Visim NBR the stand and deadwood parameters are shown in Table 1. Both forests have developed sub-coenosis parcelle mosaics, showing that the communities consist of alternating gaps and closed-canopy patches with the tree layer cover not less than 50 % (Fig. 2). These parcelles are of 0.01-0.04 ha. We distinguish two variants of the closed-canopy parcelles: fir-spruce (dark-conifer species dominate in the tree layer) and mixed spruce-deciduous (pioneer deciduous species, Betula spp. or Populus tremula, dominate, admixture of dark conifers in the tree layer), according to A. Shirokov with coauthors [69]. We also distinguish two gap variants: with cover of tree understorey and/or shrub undergrowth varying from 20 to 50 % and with under-storey and/or the undergrowth cover not exceeding together 20 %.

All parcelles reflect different stages of woody vegetation recovery after treefall in the PrFS. There are multiple tree-related microrelief forms as deadwood of various decay stages (see Table 1), tree-uprooting mounds and pits of various age, tree-base hillocks, and flat ground sites in this primary forest [70]. In the PfB, gaps remaining after irregular reforestation of the former burnt place are most common but treefall gaps are few and small (formed by the fall of one or two trees). Most of the ground surface is flat in this secondary forest because deadwood and other tree-related microrelief forms are few (ibid.).

In the Pechora-Ilych NBR, the studied forests are in the foothill landscape, at least 30 km away from former and existing villages. There was no logging due to this. The PrFS (62.077° N, 58.954° E, 350 m a.s.l.) is located in the lower reaches of the Bolshaya Porozhnaya River (a right tributary of the upper Pechora River), on the gentle eastern slope of its valley. On this site fire marks were not revealed both on the tree trunks and in specially analysed soil samples [71]. Such unique evidence of primary forest status was the main criterion for choosing this site for our study. The PfB (62.038° N, 58.945° E, 330 m a.s.l.) is located on a gentle southern slope of the upper Pechora River valley. It is the only post fire forest growing in environmental conditions similar to the chosen PrFS. The secondary stand formed after the fire in the 1930s according to archive documents [72]. Both forests grow on montane forest podzolic burozem [64], i.e. Dystric Cambisol [67], exposed to seasonal flowing moderate moisture [73]. Main features of the Pechora-Ilych NBR forests are shown in Table 2. Fig. 3 illustrates the physiognomy of these forests.

RUSSIAN JOURNAL OF ECOSYSTEM ECOLOGY

Vol. 7 (l), 2022

Table 1

Stand and deadwood parameters of the tall-herb-fern forests in the Visim NBR

Tree species Living trees Fallen woody debris

Density, trees per ha Age, years H, m DBH, cm Sum of basal area, m2ha-1 Volume, m3ha-1 Volume, m3ha-1

Primary fir-spruce forest (sample plot - 0.5 ha)

Picea obovata 210 40-230 2.6-28.0 4.1-59.0 16.5 171.7 59.5 105.1a

Abies sibirica 428 40-205 2.6-24.0 4.0-42.0 10.6 83.1 46.4

Betula species'3 2 ndY 2.9-3.2 4.5-7.0 <0.01 <0.01 9.6

Total 640 - - - 27.1 254.8 220.6

Post-fire birch forest (sample plot - 0.08 ha)

Picea obovata 538 nd 5.0-22.5 6.0-32.0 13.4 102.3 2.1

Abies sibirica 160 nd 6.0-17.0 6.0-19.0 1.8 11.5 0.0

Betula species'3 463 100ö 6.0-25.0 6.0-44.0 16.3 152.1 7.8

Populus tremula 163 nd 20.0-25.0 16.0-37.0 7.1 80 1.7

Salix caprea 13 nd 15.0-16.0 18.0-20.0 0.4 2.9 0.2

Pinus sylvestris 13 nd 20.0-23.0 35.0-38.0 1.4 14.9 0.0

Total 1350 - - - 40.4 363.7 11.8

aPicea obovata+Abies sibirica,13Betula pubescens+B. pendula, Yno data, sIn 2000 the age was 80 years, according to the information from the forestry inventory.

Fig. 2. Physiognomy of the studied communities in the Visim NBR: (A) the primary fir-spruce forest (PrFS); (B) the post-fire birch forest (PfB). Photo by R. Sibgatullin

Table 2

Stand and deadwood parameters of the tall-herb-fern forests in the Pechora-Ilych NBR

Tree species Living trees Fallen woody debris

Density, trees per ha Age, years Н, m DBH, cm Sum of basal area, m2ha-1 Volume, m3ha-1 Volume, m3ha-1

Primary fir-spruce forest (sample plot - 1 ha)

Picea obovata 262 39-229 2.0-36.0 2-76 16.40 192.9 161.0 16.8a

Abies sibirica 337 29-165 2.0-28.0 3-47 8.03 66.7 47.3

Betula pubescens 18 65-125 3.2-20.0 3-43 0.29 2.3 2.3

Pinus sibirica 15 35-350 16.5-30.5 26-77 4.02 48.5 26.1

Total 632 - - - 28.74 310.4 253.1

Post-fire birch forest (sample plot - 0.12 ha)

Picea obovata 600 33-74 2.5-27.9 3-52 11.90 90.6 20.6

Abies sibirica 25 30-33 6.1-7.3 8-9 0.14 0.5 0.0

Betula pubescens 900 ndß 6.7-23.5 8-35 30.20 268.0 10.8

Total 1525 - - - 42.24 359.1 31.4

aPicea obovata+Abies sibirica, ßno data.

в

Fig. 3. Physiognomy of the studied communities in the Pechora-Ilych NBR: (A) the primary fir-spruce forest (PrFS); (B) the post-fire birch forest (PfB). Photo by A. Aleinikov

Trees are distributed irregularly in the PrFS: standalone or groupwise. The tree canopy is open enough due to 0.01-0.02 ha gaps. There is a sufficiently large stock of coarse woody debris (see Table 2), including logs of different decay stages. Other tree-related microrelief forms are: numerous tree-base hillocks and

rare tree-uprooting sites (mounds, pits) [74]. In the secondary PfB, trees are disposed quite regularly, so the tree canopy is mainly closed. The stock of coarse woody debris is negligible (see Table 2) and there are no other tree-related microrelief forms, so most of the ground surface is flat.

Data collection

The data collected during routine biota inventory was used in the analysis in both NBRs. The PrFS was inspected in 2019 in the Visim NBR; the collected data included vegetation relevés, soil invertebrate samples, and results of tree registration within the plot. Invertebrate samples were collected in the PfB in 2017, tree registration within the plot was conducted in 2021, and vegetation relevés were collected in both years. In the Pechora-Ilych NBR the PrFS was inspected in 2008 (vegetation relevés) and in 2011 (tree registration), and the PfB was inspected in 2013 (both vegetation and tree population studies). Since forest succession dynamics are slow processes, the compelled interval of several years between different parts of the study does not influence the dynamic state of the studied forests or its structure and composition.

General vegetation study

The vegetation relevés were carried out on 100 m2 plots. In the Visim NBR, the relevés were distributed in accordance with the sub-coenosis parcelle mosaics (see above). 10 relevés in the PrFS and 6 relevés in the PfB were described. In the Pecho-ra-Ilych NBR, the relevés were disposed along the line at a distance of 50 m within each studied forest. 6 relevés in the PrFS and 3 relevés in the PfB were described. The following vegetation layers were distinguished on the plots: A - large trees, B - tree understorey and shrub undergrowth, C - herbs and dwarf shrubs (the latter are almost absent in communities of the studied tall-herb-fern type), and D - mosses. We detected the total cover for each layer. We made a complete list of vascular plant species and registered their abundance by the Braun-Blanquet scale for layers A-C.

The corresponding sets of relevés by vascular plant species richness, combined and separately by life forms (trees, shrubs, herbs) and by ecological coenotic groups [75] were compared for primary and post fire forests in each NBR. We also compared the relevé sets by total cover of each vegetation layer. The Mann-Whitney test (with p < 0.05) was used.

Study of tree populations' demographic structure

We counted all tree trunks over 1 m high within the plots (see Tables 1 and 2 for their size) and registered their species and ontogeny stages according to the concept of discretised plant ontogeny [1, 76-78]. The corresponding ontogeny stages are as follows for different tree sizes. Saplings up to 2.5 m high are at the late immature (im2) stage. The young trees with maximal annual height increment and

sharp-conic crowns but without seeds are at the virginile (v) stage. They may belong to the early virginile group (v1) or the late virginile one (v2, includes young trees with bark at the trunk base). Young reproductive (or generative, g1) trees also grow rapidly and have a conic crown. Mature reproductive (g2) trees fruit regularly and form seeds on upper and middle parts of their crowns but they grow slower than g1 trees (due to this, their crown tops are not very sharp, especially ones of deciduous trees). Old reproductive (g3) trees have apparent features of ageing: ceased height growth, obtuse crown top (both with conifer and deciduous trees), very small radial increment, and multiple dry frame branches. The concept states that each ontogeny stage duration broadly varies, depending on the environment and coenotic conditions, and does not strongly correspond to the plant real age. Immature and virginile stages of tree species last from several years to one or two decades, and each reproductive stage lasts from several decades to one century.

We compared the demographic spectra of the studied tree populations (between different tree species within same communities and same species in the two communities) using the chi-square test. We evaluated the dynamic state (normal, invading, regressive or fragmentary) of each population by the following criteria based on the observed demographic spectrum [1, 77]. A normal population includes individuals of several reproductive stages (g1 and/or g2 together with or without g3). Two kinds of normal populations are distinguished: complete (which include individuals of all ontogeny stages) and incomplete (which include almost all stage except one or two). An invading population does not include reproductive ontogeny stages. If there are no individuals of stages younger than g3, the population is regressive. If there are only one or two ontogeny stages, the population is fragmentary. The demographic completeness is an indicator of the population sustainability.

Study of soil invertebrate macrofauna

In the Visim NBR, we sampled invertebrates in each forest twice a year - in May and August. In both PrFS and PfB communities we excavated soil monoliths of 15^15 cm, 20 cm depth, including all sub-horizons of litter. Sample places were located along the line at a distance of 4 m within each forest in the PrFS. We sampled 12 soil monoliths and 5 deadwood fragments (pieces of heavily decomposed fir and spruce logs of 40-60 cm, 10-12 cm in diameter) in the designated locations in May. We sampled 12 soil monoliths and 4 deadwood fragments in August. In the PfB the distance between samples was 1 m (less than in the

PrFS because previous studies revealed all soil invertebrate taxa to be very unevenly spaced in secondary birch forests in this reserve). In each season we collected 40 soil monoliths whereas no dead-wood logs were met in the designated locations in this secondary forest. We disassembled soil monoliths and deadwood fragments (to the state of a friable substrate) after excavation and manually parsed them. All invertebrates larger than 2 mm in size except Formicidae and large microarthropods, such as Acari (Oribatidae) and Collembola, were extracted from each piece of substrate to quantify and identify their taxa. The revealed insects, molluscs, crustaceans, and millipedes were identified up to supraspecific taxa using field guides [79-81]. The identification of species and morpho-ecological groups of earthworms were obtained using special guides [82, 83] with one correction according to the current taxonomic state (Perelia dip-lotetratheca (Easton, 1983) was renamed to Rhiphaeodrilus diplotetratheca (Perel, 1967) - see http://taxo.drilobase.org/index.php?title=Rhiphaeo drilus_diplotetratheca). Prior to statistical analysis we recalculated all invertebrate numbers detected in soil monoliths and deadwood fragments to values per 1 m2 in proportion to the corresponding horizontal projection square (multiplied length and width) of each sample. Invertebrate biomass was also calculated per 1 m2, basing on the average weight of one individual in each taxonomic group (species in the case of earthworms). Material fixed in ethanol (96 % for earthworms, 70 % for other groups) was weighed before this calculation. The obtained datasets were compared by the Mann-Whitney test (with p < 0.05) for the two studied forests.

All statistical processing of vegetation and soil-fauna data was carried out using the PAST software [84].

Results

Structural and floristic vegetation diversity

In the Visim NBR, the PrFS and the PfB are similar in some parameters but different in others (Fig. 4). Tree and undergrowth covers vary similarly broadly due to gap mosaics in both communities (ibid. A, B). However, the cover of the herb and moss layers is generally lower in the PfB than in the PrFS (ibid. C, D). This may be due to the patchy pattern of ground vegetation recovery after past fire disturbances because the lower layers are still fragmented in some strongly burnt places. For example, the patchy recovery pattern in the Visim NBR was observed after the 1998 fire that disturbed another tall-herb-fern dark-conifer forest [85]. The higher cover of the moss layer (see Fig. 4,D) in the PrFS may also be caused by a large stock of heavily decomposed woody debris (see Table 1) and

tree-uprooting mounds and pits because those all are favourable microhabitats for moss settlement.

One can find a significant difference in their floristic diversity when comparing the PrFS and PfBs, analysed distinctly by functional groups of species, e.g. either by life forms or by ecological-coenotic groups. The species richness of shrubs or forest boreal species (all life forms together) is generally less in the PfB relevés than in the PrFS (see Fig. 4,G, I). As for the forest boreal species only three low boreal herbs often occur within the PfB secondary community, so the list of boreal species is quite strict at each relevé (ibid. I). Eight low boreal herbs, in contrary, are constant in relevés of the PrFS, so the total species richness of boreal species is commonly high in this forest. Such different states of low boreal herbs in the two communities may be due to fire that occurred in the PfB about 100 years ago. These species are hemicryptophytes and geophytes whose renewal buds localised not deep in the litter, have to be burnt strongly [85]. That is why low boreal herbs may be the group most damaged and the least recovered after fire. The recovery of some low boreal herbs may be limited for a long time by a lack of suitable microhabitats in the community at the early stages of post fire succession. For example, the low herbs Circaea alpina L. and Viola selkirkii Pursh ex Goldie commonly grow on heavily decomposed deadwood, and ferns Gymnocarpium dryopteris (L.) Newm., Phegopteris connectilis (Michx.) Watt, and Rhizomatopteris sudetica (A. Br. et Milde) A. Khokhr. grow in tree-uprooting pits where their spores germinate the best and game-tophytes develop the same on moist bare soil.

However, forest-edge nitrophilous species (all life forms together) richness is higher in the PfB in the compared communities demonstrating an opposite pattern (see Fig. 4,N). Deciduous forest with light and nutrient-rich leaf litter are abundant in this mosaic, there are good conditions for growth of these species. Moreover, their successful survival and fast settling often just before the tree canopy closes outside the burnt area may explain their high species richness in the post fire forest [85]. The species richness of meadow herbs is generally higher in the PfB than in the PrFS the comparison also shows (ibid. O). This is mainly because eight meadow species were registered in the single relevé of an opened gap. However, meadow herbs are absent even in gap parcelles in the PrFS, so the general difference between the analysed relevé sets (PrFS and PfB) is significant. Finally, the total species richness of vascular plants in the PfB is similar to the PrFS (see Fig. 4,E). This is due to an increase of species richness in forest-edge nitro-philous and meadow ecological-coenotic groups that compensated the post fire decrease in some other groups.

Fig. 4. Parameters of structure and vascular plant diversity in the primary fir - spruce (PrFS) and the post fire birch (PfB) forests in the Visim NBR. A-D - the cover of vegetation layers: A - trees, B - tree understorey and shrub

undergrowth, C - herbs, D - mosses. E-O - number of species: E - total for vascular plants, F - all trees, G - all shrubs, H - all herbs; I-O - in the considered functional species groups (all life forms together): I - forest boreal, J - forest-edge boreal, K - forest nemoral, L - forest-edge nemoral, M - forest nitrophilous, N - forest-edge nitrophilous, O - meadow. Symbols represent the vegetation relevés by variants of sub-coenosis parcelles: 1 - fir-spruce, 2 - spruce-deciduous, 3 - gap with abundant tree understorey and shrub undergrowth, 4 - opened gap with little tree understorey and shrub undergrowth. For the compared relevé samples, significant differences (Mann - Whitney test, p < 0.05) are indicated by arrows: \ - higher values, \ - lower values

In the Pechora-Ilych NBR, the analysis of vegetation relevés confirms that the tree layer is significantly more closed in the secondary PfB than in the PrFS (Fig. 5, A), but other differences were not revealed in the two communities structure (ibid. B-D). Some parameters of their floristic diversity differ in

turn. For instance, species richness in the PrFS relevés is higher than in the PfB both in trees (ibid. F) and in forest boreal species (ibid. I). The latter is due to irregularly spaced boreal species including fir and spruce, whose individuals partially survived the fire in the PfB. Additionally, a lack of

Vol. 7 (l), 2022

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tree-related microhabitats in PfB may cause a strict set of low boreal herbs in this forest because these niches are favourable for some of them. For example Linnaea borealis, absent in the PfB usually settles deadwood, whilst moist soil in tree-uprooting pits is the optimal substrate for low spore-forming vascular herbs (Lycopodium annotinum L., Gymno-carpium dryopteris (L.) Newm., Phegopteris con-

nectilis (Michx.) Watt). Species richness does not vary in the two communities in other ecological-coenotic groups. This is mainly due to similar composition of light-demanding species of the meadow group and in three forest-edge groups in the sparse old-growth PrFS and in the PfB developed from an open burnt place. The total species richness also does not vary in the two communities (ibid. E).

Fig. 5. Parameters of structure and vascular plant diversity in the primary fir - spruce (PrFS) and the post fire birch (PfB) forests in the Pechora - Ilych NBR. A-D - the cover of vegetation layers: A - trees, B - tree understorey and

shrub undergrowth, C - herbs, D - mosses. E-O - number of species: E - total for vascular plants, F - all trees, G - all shrubs, H - all herbs; I-O - in the considered functional species groups (all life forms together): I - forest boreal, J - forest-edge boreal, K - forest nemoral, L - forest-edge nemoral, M - forest nitrophilous, N - forest-edge nitrophilous, O - meadow. Symbols represent relevés from the (1) PrFS and (2) PfB. For the compared relevé samples, significant differences (Mann - Whitney test, p < 0.05) are indicated by arrows: f - higher values, [ - lower values

We did not register the mosses species composition in the studied PfBs in detail in both NBRs. So there is no way to compare moss species richness in the post fire and primary forests (although in the PrFS we identified and listed moss species). In general, moss diversity is known to be high in forests with high decaying wood stock and potential substrates for mosses are diverse [86, 87]. We can only remark that in sub-boreal (Visim NBR) primary forest with multiple tree-related microhabitats (including deadwood) the total moss cover is higher than in the post fire forest without such microhabitats, basing on our collected data (see Fig. 4,D). However, total moss cover does not differ in boreal (Pechora-Ilych NBR) primary and post fire forests (see Fig. 5,D).

The demographic structure of tree populations

In the Visim NBR, fir and spruce have formed normal and complete populations in the PrFS where density of their stand-forming g1-g3 trees is about the same (Fig. 6,A and C). However, the fir population (ibid. C) looks more sustainable than

the spruce one because of higher summarised density of v1 and v2 young trees. The observed significant demographic difference between spruce and fir populations can reflect a stage in regular (about 100-year period) natural alternation of their dominance in the Middle Urals dark-conifer forests [88]. Canopy gaps favour the growth of birch in the studied late-successional forest, but the gaps are not large enough to allow the light-demanding pioneer species develop up to maturity. Therefore, birch population registered in the PrFS (ibid. B) is invasive and small. Probably the maternal birch reproductive trees grow near the plot (that is indicated by presence of birch understorey), but they are not numerous too. Additionally, several Siberian pine seedlings less than 1 m high (early immature stage - iml) were found within the plot but we did not register it because of its small size. Siberian pine seedlings indicate reproductive tree(s) growing somewhere near the plot as in the birch population. Nevertheless, the total Siberian pine population looks too fragmentary, small and unsustainable in the PrFS.

Fig. 6. Demographic structure of tree species populations in the studied forests of the Visim NBR. The ontogeny spectra

by the studied communities: A-C - the primary fir-spruce (PrFS) forest, D-I - the post fire birch (PfB) forest. The ontogeny spectra by tree species: A, D - fir (Abies sibirica); B, E - birch species (Betula pendula + B. pubescens); C, F - spruce (Picea obovata); G - Scots pine (Pinus sylvestris); H - aspen (Populus tremula); I - goat willow (Salix caprea). Explanations of the ontogeny stages see in the text section "Study of demographic structure in tree populations"

Both birch (pioneer species) and spruce (late-successional species) populations are numerous and normal but incomplete in the PfB recovered about 100 years past fire. The fast-developing birch trees are mature and old reproductive (g2 and g3, respectively), they dominate the first post fire stand generation (see Fig. 6,E). Nevertheless, birch population tends to regress, as few v2 individuals and no younger understorey (im2 or v1) show. The spruce population includes more vir-ginile young trees (ibid. F) than the birch one, but they are not numerous to provide future popula-

tion sustainability. The fir population (ibid. D) still looks invading, and this indicates some factors impede fir seed rain or seedling survival. Several pioneer tree species such as aspen, Scots pine, and goat willow (ibid. G-I) have formed fragmentary, very small and regressive populations. The observed diversity of tree species is likely to decrease soon in the PfB because the extinction of several pioneer species is very probable. Moreover, there is no evidence that spruce and fir populations can stabilise community dynamics in foreseeable future.

The PrFS maintains populations of all tree species which are characteristic for the dark-conifer forests in the North Urals in the Pechora-Ilych NBR. The late-successional species, fir and spruce, have formed normal and complete populations in the PrFS (Fig. 7,A and C). im2 saplings of fir and spruce indicate that the populations are permanently replenishing, whereas the low density of im2 group can be due to rapid sapling transition to the virginile stage in favourable light conditions (under the sparse tree canopy). Numerous virginile young trees allow fir and spruce populations to be sustainable enough in the future. The late-successional Siberian pine has formed normal but incomplete population in the PrFS (ibid. D). This species is shade-tolerant and long-living, so its immature and virginile ontogeny stages also last long. Siberian pine seedlings survive most successfully if settling near the trunk

base of their maternal trees where the shade from tree crowns inhibits photophilous tall herbs. However, the seedlings in such microhabitats are also partially inhibited. They can survive until the maternal trees treefall that speed up their ontogeny development [74] in some cases. These biological peculiarities of Siberian pine explain the observed demographic structure of its population in the PrFS and allow considering the population not hopeless, despite strong demographic incompleteness and low understorey density. The pioneer species, pubescent birch (see Fig. 7,B), has formed a normal but incomplete and small population in this sparse forest. Birch saplings (im2 stage) were not registered within the plot. This can be due to competition between birch seedlings and tall herbs dominating the field layer. However, numerous tree-related microrelief forms, especially deadwood, are the microhabitats where birch may overcome the competition.

Fig. 7. Demographic structure of tree species populations in the studied forests of the Pechora-Ilych NBR. The ontogeny spectra by the studied communities: A-D - the primary fir-spruce (PrFS) forest, E-G - the post fire birch (PfB) forest. The ontogeny spectra by tree species: A, E - fir (Abies sibirica); B, F - pubescent birch (Betula pubescens); C, G - spruce (Picea obovata); D - Siberian pine (Pinus sibirica). Explanations of the ontogeny stages see in the section "Study of demographic structure in tree populations"

In the PfB, the normal birch population is numerous (see Fig. 7,F) but tends to age and regress because of the closed forest canopy and lack of tree-related microhabitats. The spruce population is normal and complete (ibid. G). Probably, the contemporary g2 and g3 spruce trees had survived the fire and could play an important role in population recovery, because they facilitated the appearance of spruce understorey in the forest. Currently, the spruce population has the best perspectives amongst all tree species and can stabilise the community in the future. In contrast, the fir population (ibid. E) had poorly recovered after the fire, and its perspectives are indefinite. Siberian pine is absent in the PfB. Therefore, the total diversity of tree species is not complete after a 100-year succession

and has no apparent prerequisites for future increase or maintenance in this secondary forest.

Diversity of soil invertebrate macrofauna

Soil invertebrate macrofauna includes a similar set of supra-specific taxa (families, orders) and trophic groups, e.g., saprophages (Lumbricidae, Julidae, Gastropoda, larvae of Nematocera and Brachycera), phytophages (Elateridae and larvae of Coleoptera), predators (Aranei, Lithobiidae, Geo-philidae, Hemiptera, Carabidae, Staphylinidae), and aphages (pupa of Lepidoptera) in both studied communities in the Visim NBR. The total invertebrate numbers are higher in the PrFS than in the PfB in spring and in summer: the annual average values are 560 ± 77 ind.m-2 and 388 ± 33 ind.m-2,

respectively (Fig. 8,A). The total average biomass is also higher in the PrFS (53.5 ± 3.5 gm-2) than in the PfB (33.3 ± 2.8 gm-2). Additionally, there are some quantitative differences in the taxonomic and trophic structure of the soil macrofauna in these forests. The saprophage numbers in the PrFS (209 ± 23 ind.m-2) are higher than in the PfB (127 ± 11 ind.m-2) (ibid. B). The saprophage bio-

mass demonstrates the same: it is higher in the PrFS (50.4 ± 2.5 gm-2) than in the PfB (31.5 ± 2.1 gm-2). The effect of the corresponding variation in the abundance and biomass of invertebrates was reported previously [89]. As for taxonomic groups, the most numerous in the PrFS are Aranei and Staphylinidae, whereas, in the PfB those are Ara-nei, Lumbricidae, Lithobiidae, and Carabidae.

Fig. 8. The soil invertebrate numbers (ind. per 1 m2, annual average values) in the primary fir-spruce (PrFS) and the post fire birch (PfB) forests in the Visim NBR. A - total, B - saprophages, C - earthworms

Earthworm numbers are higher in the PrFS than in the PfB, 105 ± 25 ind.m-2 and 74 ± 10 ind.m-2, respectively which is similar to other saprophages (see Fig. 8,С). The earthworm biomass is also higher in the PrFS (47.4 ± 2.4 gm-2) than in the PfB

(22.7 ± 1.8 gm-2). As for the earthworm taxonom-ic diversity, there are seven species (and one of them includes two subspecies) in the PrFS where we found all earthworm species and subspecies both in soil monoliths and in pieces of heavily de-

composed fir and spruce logs. Notably, sexually mature earthworms were more numerous in these logs than in the soil, the same were earthworm co-

coons and juvenile individuals in spring. The earthworm diversity is poorer (only four species) in the PfB than in the PrFS (Fig. 9,A).

Fig. 9. Taxonomic (A) and morpho-ecological (B) structure of earthworm fauna (the shares of the total population numbers) in the studied forests of the Visim NBR. Decoded abbreviations see in Fig. 8 and in the text section "Study objects"

In both studied forests, four morpho-ecological groups of species form the earthworm fauna, but the proportion between the groups is different (see Fig. 9,B). Epigeic species (Dendrobaena oc-taedra (Savigny, 1826), Dendrodrilus rubidus tenuis (Eisen, 1874), and D. rubidus subrubicundus (Eisen, 1874), i.e., two species, one of which includes two subspecies) make up 40 % of the total Lumbricidae numbers in the PrFS. The rest of the groups include only one species. Those are epi-endogeic Rhiphaeodrilus diplotetratheca (Perel, 1967) (Ural endemic, makes up 30 % of the total earthworm numbers), anecic Eisenia nordenskioldi nordenskioldi (Eisen, 1873) (the East-Asian subspecies, almost 20 % of the total numbers, and this share is higher than in the PfB), and endogeic Perelia tuberosa (Svetlov, 1924) (else Ural endemic, about 10 % of the total numbers that is comparable to the PfB). In the PrFs in contrast each group includes only one species: epi-endogeic Rh. diplotetratheca makes up 80 % of the total Lumbricidae numbers, epigeic D. octaedra (cosmopolite species) and endogeic P. tuberosa are not very numerous, and individuals of anecic E. n. nor-denskioldi are scarce.

Discussion

As expected, we found compositional and functional differences in vascular plant flora and soil invertebrate macrofauna in the studied PrFSs and PfBs. A notable peculiarity is that the species demonstrating the differences commonly inhabit such substrates (soil litter, or deadwood and other

tree-related microhabitats), which used to be significantly damaged by fires.

Thus, the species richness of low boreal herbs is higher in the PrFSs than in the PfBs. The effect is similar in both NBRs (situated in different climatic zones). Low boreal herbs diversity and abundance decrease immediately after fire [85] previous observations (in the same forest type) show. Therefore, we suppose the observed difference to be a consequence of fire (probably additionally enhanced by the lack of treefall microrelief in the PfBs, which still are early-successional forests). Low boreal herbs are considered indicators of rich soils both in boreal and in sub-boreal forests [62, 64]. However, our study results incite us to rethink if the presence or absence of low boreal herbs reflect chemical content of soil or previous fire history in landscapes, especially in those transformed by human activity over a long time.

In the Visim NBR, we observed higher numbers of soil invertebrates, especially saprophages, in the PrFS than in the PfB. Furthermore, earthworms' total species and functional diversity (saprophages of the largest size) are higher in the PrFS, and epigeic earthworm species are more diverse and numerous too. In the Pechora-Ilych NBR we earlier also revealed a high number and diversity of earthworms in the studied PrFS [90]. However, we have no corresponding faunistic data from the PfB for comparison. The observed few numbers and restricted diversity of epigeic (litter-eating) earthworms are surprising effects because the leaf litter of deciduous trees is a more beneficial trophic re-

source than conifer needles which are difficult to decompose [91-93]. The revealed differences of the soil invertebrate macrofauna may be due to past logging and fire in the PfB because invertebrate inhabiting litter and other upper soil horizons (2-3 cm depth under the burnt zone) die off during fires [94]. Those who primarily die in fires are individuals of the little-mobile taxa, such as Acari, Collembola, Gastropoda, Insecta, and Lumbri-cidae, which inhabit the organic soil horizons [38, 95]. Moreover, fires negatively influence even an-ecic earthworms because most of their cocoons and juvenile individuals are usually located in the upper soil horizons [96]. Logging and fires are agents that destroy organic soil horizons partly or completely, and fires transform even lower mineral horizons [38, 97].

Region-specific tree species lost their positions in the PfBs (Siberian pine is absent, fir forms small and incomplete populations). Despite the decrease of low boreal herbs and region-specific trees in the PfBs, the total species richness of vascular plants is similar in the inspected PrFSs and PfBs. This is mainly due to the increased diversity of forest-edge herbs in the PfB communities. Pioneer trees and meadow herbs also contribute to the total species richness in the PfB located in non-extensive woodland transformed powerfully by human economic activity in the Visimsky NBR. The listed three functional groups of plant species (forest-edge herbs, meadow herbs, and pioneer trees) present apophytes, i.e., native plants whose distribution extends and abundance increases after the anthropogenic transformation of ecosystems. The higher diversity of such plant species in forest communities indicates degradation of native biodiversity. Such species mark synanthropization of flora and vegetation [98] and inhibition of species intolerant to anthropogenic disturbances (as Siberian pine and fir in the PfBs in our study). Furthermore, the current apophyte species richness is not sustainable: population demography provides evidence that some pioneer tree species will extinct from the secondary forest communities in future. A previous comparative study of several state nature reserves within the Middle and North Urals demonstrated a negative relation between flora synanthropization level and the corresponding reserve area [99]. Our study also shows that, in the vast and hard-to-reach intact forest landscape (Pechora-Ilych NBR), the PfB is less synanthropic than similar forest in smaller woodland (Visim NBR) where past land-use was more intensive. This demonstrates large woodlands provide resistance of primary forests to episodic and local stand-replacing disturbances.

We also observed that soil invertebrate fauna is a sufficient forest biodiversity component. In par-

ticular saprophages, especially epigeic earthworms, which use deadwood, as well as soil litter, are the most informative functional group of soil invertebrates for forest naturalness assessment. Previous publications on forest biodiversity [11, 12] have not always highlighted this issue. The general state of the soil invertebrate macrofauna is an essential factor influencing the biogeochemical cycles and other ecosystem functions of forests. If macrofauna abundance is low, especially if earthworms are few, then micro- and mesofauna species mainly decompose litter, and this type of decomposition is associated with soil fertility reduction and intensive CO2 emission from the soil [100]. Moreover, earthworms are very influent ecosystem engineers (a kind of keystone species) because they migrate in soil horizontally and vertically and form up to 50 % of pore space (biopores). The pores regulate the hydrological regime and gas diffusion inside and out of the soil [101, 102], turning atmospheric precipitation from the surface into ground runoff that protects soil from erosion [103]. By killing soil invertebrate macrofauna fires disturb forest ecosystem functions in a complex manner. The opinion that fires are necessary for forest functioning (e.g. [33]) appeared as the result of studying significantly and long transformed ecosystems, simplistic and devoid of the essential keystone species.

Late-successional tree species formed uneven-aged populations of complete demographic structure, multiple tree-related microhabitats, large stock of woody debris, sub-coenotic spatial mosaics and consequently high species diversity in the studied PrFSs. Such peculiarities correspond to the theoretical concept of late-successional forests formed by long-term autogenic succession under a fine-scale disturbance regime [104]. Missing fire marks (especially charcoals in soil) indicate absence of fires in these PrFSs. The studied land-use history of the Visim and the Pechora-Ilych NBRs proves these forests are primary according to the contemporary criteria (see review in [25]). Studying these forests significantly inputs the understanding of naturalness and boreal biomes biodiversity potential level [20, 61]. In addition to Jasinski and Angelstam [55] we found other evidence that the fire period in natural mesic dark-conifer forests can be much longer than 100 years. We may also conclude that, in primary dark-conifer forests, a 100-years fire periodicity would not be favourable for their native biodiversity maintenance. Therefore, a shorter fire interval is an influential factor that deflects biodiversity and ecosystem functions from its native state in mesic forests.

The hydro-construction activity of beavers can severely limit the fire spread as is now observed at

several sites [105]. When beavers were numerous in the past, common periodicity of fire emergence could be similar to the current regime. However, then each fire was localised and could not influence vast areas as nowadays. That might be the reason why some forests remained untouched by fires and why in others in contrary some pioneers (e.g. pyrophilous species) inhabited wild nature (within the boreal and sub-boreal zones of Eurasia). Therefore, forest management could make an effort to emulate a beaver-regulated hydrological regime restricting wildfires to minimise fire effect on biodiversity.

Conclusion

Primary forests can be especially valuable biodiversity refugia in any landscape condition we conclude. The current concern with its continuous loss [106-109] is justified. However, detailed studies in land-use history are necessary to detect whether an old-growth forest in landscapes influenced by humans long ago is truly primary (as re-

vealed for the PrFS in the Visim NBR). In such landscapes, small areas' forests are vulnerable to synanthropization when exposed to various (even originally natural) stand-replacing disturbances. Not only conservation of old-growth forests or even simulation of some natural forest-disturbance processes are entirely appropriate ways to maintain native biodiversity in landscapes changed by humans. In current conditions, forest management should apply scientifically sound rewilding actions, including reintroducing and maintaining keystone species important for forest ecosystem services. But most appropriate could be preserving the remaining primary forests with high general biodiversity in large-area intact forest landscapes [39], where forest synanthropization is minimal after stand-replacing disturbances and species demanding large habitats meet their well-being. Conservation is preferable for intact forest landscapes. It is crucial to prevent woodland fragmentation and emerging anthropogenic infrastructure in intact forest landscapes and restrict human accessibility that often induces fires.

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