Interaction between tree species populations and windthrow dynamics in natural beech-dominated forest, Czech Republic

Forest Ecology and Management 280 (2012) 9–19

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Interaction between tree species populations and windthrow dynamics in natural beech-dominated forest, Czech Republic Barbora Šebková a,⇑, Pavel Šamonil a, Martin Valtera a,b, Dušan Adam a, David Janík a a b

The Silva Tarouca Research Institute for Landscape and Ornamental Gardening, Department of Forest Ecology, Lidicka 25/27, 657 20 Brno, Czech Republic Mendel University in Brno, Faculty of Forestry and Wood Technology, Zemedelska 1, 613 00 Brno, Czech Republic

a r t i c l e

i n f o

Article history: Received 3 January 2012 Received in revised form 23 May 2012 Accepted 24 May 2012 Available online 7 July 2012 Keywords: Pit-mound dating Tree-uprooting disturbance Tree regeneration Natural forest Fagus sylvatica Picea abies

a b s t r a c t Interactions between pit-mound dynamics and tree species populations were studied in a natural mountain (fir)-spruce-beech forest. Pit-mounds are special habitats with unique erosion-sedimentation and microclimatic conditions, which continually influence the trees growing there. Our assumption was that these factors would impact on the competitive potential of the trees and that the interaction between the pit-mound dynamics and the tree layer would not be static, but would change depending on the ages of both the trees and the pit-mounds. Over an area of 74.2 ha that was repeatedly studied in 1975, 1997, and 2008 in terms of the tree layer structure (about 23,000 trunks), pit-mound evaluation was performed on a regular network of 354 circular plots with a 23 m diameter (1733 pit-mounds in total). Dendrochronological cores were drilled in 1986 samples in order to establish an age structure of the tree layer. Using tree-census, dendrochronological, and mathematical methods, direct or indirect dating of the pit-mounds was performed. The actual occurrence of trees on the pit-mounds was compared with a null model corresponding to random occurrence for various age categories of the trees/pit-mounds. The number of trees decreased smoothly with age in the respective classes. The dominant species was Fagus sylvatica, which like Picea abies and Abies alba reached an age of >450 years. A multi-peak pitmound age structure suggested the occurrence of stronger episodic disturbance events in the past. Mounds covered 8.5% and pits 3.7% of the studied area (121 pieces/ha) and the average pit-mound size was 9.92 m2. F. sylvatica and other marginally represented trees (A. alba, Sorbus aucuparia, Acer pseudoplatanus, Ulmus glabra) significantly preferred mounds over other microsites (v2 = 147.37, p < 0.001; resp. v2 = 14.73, p = 0.005). The preference to mounds by marginally represented tree species decreased with the age of the trees, whereas the affinity of F. sylvatica increased with age. Also, older individuals of P. abies were more frequently found on mounds compared to the null model, although the overall presence of P. abies on mounds was significantly deficient (v2 = 11.21, p = 0.024). These results suggest that the competitive potential of the trees on mounds decreases with age only in trees of initial succession stages. Mounds older than 101 years were most favourable to natural regeneration. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Disturbances represent important impulses in natural forest dynamics. Understanding their impacts on forest development is crucial to understanding the basic processes that take place within forests. Ecosystems can be influenced by disturbances of varying type, frequency, duration, spatial extent, intensity or strength (Bengtsson, 2002; Holling et al., 1994) and disturbance regimes differ greatly between the various forest biomes of the world (Pickett and White, 1985). Small-scale but frequent disturbances including ⇑ Corresponding author. Tel.: +420 541 126 285; fax: +420 541 246 001. E-mail addresses: [email protected] (B. Šebková), pavel.samonil@ vukoz.cz (P. Šamonil), [email protected] (M. Valtera), [email protected] (D. Adam), [email protected] (D. Janík). 0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.05.030

tree-uprooting events predominate in the dynamics of the natural European forests dominated by Fagus sylvatica (Splechtna et al., 2005). Whereas the maximum longevity of pit-mounds ranges from 5–10 years in the tropical forests of Panama (Putz, 1983), to 2420 years in the northern hardwoods of Michigan (Schaetzl and Follmer, 1990), in Central Europe the maximum established longevity of pit-mounds is 220–680 years (Skvorcova et al., 1983; Šamonil et al., 2009). The surface ratio of the pit-mounds is not insignificant either; in the natural forests of Central Europe it ranges between 8–15% (Simon et al., 2011; Skvorcova et al., 1983; Šamonil et al., 2009); globally pit-mounds may cover up to 90% of the surface area (south Siberia, Karpachevskiy et al., 1980). The intensity of the tree uprooting disturbances poses a significant question: what is the impact of this phenomenon on the further development of the tree layer?

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There are unique microclimatic conditions within pit-mound microsites untill their levelling, which differ significantly from the currently non-disturbed, ± flattened areas. Compared to pits, mounds are generally drier, warmer and show greater temperature fluctuations and higher insolation intake (Beatty, 1984; Peterson and Cambell, 1993). Pits accumulate run-off water and under a thick snow layer soil-formation processes may also be active during winter (Schaetzl, 1990). Due to the different microclimatic and sedimentation-erosion conditions, the development of organic horizons on the mound is delayed by many years compared to the pit, where excessive amounts of organic matter accumulate (Šamonil et al., 2008a). The process of transformation of organic matter varies between microsites due to other communities of decomposers (Nachtergale et al., 2002; Ponge and Delhaye, 1995). The results of the different microsite soil-formation processes are different chemical properties of the soil horizons and their configuration (Schaetzl et al., 1989b; Šamonil et al., 2008b, 2010a). These differences have inspired studies focusing on the relationship between the pit-mound microsites and the natural regeneration of trees. It was found that microsites may differ significantly with regard to the diversity of species (Nakashizuka, 1989; Palmer et al., 2000; von Oheimb et al., 2007) and that some species prefer specific types of microsite (Kuuluvainen and Juntunen, 1998; Phillips et al., 2008). The interaction between tree species regeneration and the pit-mound dynamics differs according to the disturbance regime, tree species and forest structure (Schaetzl et al., 1989a). Despite much evidence of a greater participation in the natural regeneration on the mounds (Kuuluvainen and Juntunen, 1998; Phillips et al., 2008), trees more often preferred the pits in dry and inhospitable areas, as they provide protection and better access to water (Cook, 1971). It is therefore difficult to transfer the relationships described in a single forest type to another forest type. The issue of the relationships between pit-mounds and tree species populations deserves more detailed research in the beech-dominated natural forests of Europe. However, information on the occurrence of tree regeneration on the pit-mound microsites provides only a limited picture of the actual importance of pit-mounds in the dynamics of the tree layer. This information is insufficient to assess how the trees growing on pit-mounds survive in a long-term perspective. Up to now, the importance of pit-mounds for forest dynamics was determined by extrapolating results acquired for the youngest category of trees and implicitly presuming that the same relationship holds for older trees as well. The legitimacy of this presumption has not been validated yet and there have been numerous records of processes that dispute it. In addition to wider environmental conditions, the growth of trees is markedly dependent on the properties of the microsite throughout their lives. Specific microclimatic and erosion-sedimentation conditions of the pit or mound can influence the competitive potential of trees, or may affect their phenology. Soils on mounds are generally loose and tree roots are therefore easily exposed. This process is so common in some forests that it can be used for indirect dating of the pit-mounds (Zeide, 1981). The loose soil further weakens the stability of trees that have exposed roots due to the displacement of their natural centre of gravity (Mayer, 1989); the trees must invest a lot of energy into coping with the changing conditions. It can be anticipated that the competition potential of such trees will be reduced and that they will be more prone to damage, e.g., windfall, or damage by insect and fungal pathogens as a result of root exposure. There is not enough information on the relationship between pit-mounds and trees of various ages, although there has been some scientific work in this area. Most of the earlier work recorded fast development of pioneer species on very young pit-mounds (Carlton and Bazzaz, 1998; Palmer et al., 2000) and then later the number of species declined with a gradual decrease in the

dynamics of this process (Beatty and Sholes, 1988; Peterson and Cambell, 1993). Due to problems with dating pit-mounds no detailed information has yet been obtained regarding their entire age spectrum. This could lead to an incorrect assessment of the interactions between the plants and the pit-mounds. A deeper study of this interaction can contribute to a better understanding of the influence of historical and geographical contingency in forest dynamics (Phillips, 2004, 2006). We focused on dealing with the relationship between tree species populations and pit-mound dynamics in the beech-dominated natural forests of Central Europe that have not yet been sufficiently studied regarding this aspect. We also focused on a holistic approach to this process, as far as possible throughout the entire life of the trees and the longevity of the pit-mounds. Hypothetically, we presume that the image we gain for the youngest developmental phase of the trees (or pit-mounds) will not be static in time, but that it will evolve upon the changing acceptability of the pitmounds for the trees, as well as upon the competition potential of the trees. Our study therefore focuses on the following questions: (i) Is there a significant relationship between trees and pitmound microsites in beech-dominated forests? (ii) How is the initial relationship between the trees’ microsites modified in the case of older trees? (iii) How does the relationship of a pit-mound and the natural regeneration of trees change relative to its age? 2. Materials and methods 2.1. Study area This study took place in the Zofinsky Prales National Nature Reserve (hereafter Zofin) (Fig. 1). The reserve was established in 1838 as the 4th oldest forest reserve in Europe (Welzholz and Johann, 2007). Zofin (total area of 102.7 ha) is located within the Novohradske Hory mountains on the border between the Czech Republic and Austria (Fig. 1). The site is located along an elevation gradient of 735–830 m a.s.l., mainly on the NW-facing slopes. The average annual temperature is 4.3 °C and the average annual rainfall is 917 mm (www.chmu.cz). The bedrock is mostly homogeneous and composed of fine, to medium-grainy porphyritic and biotite granites. The dominant soils (classified according to Michéli et al., 2006) at terrestrial sites are: Entic Podzols and Haplic and Dystric Cambisols. At water-affected sites there are Histic and Haplic Gleysols, Endogleyic Stangonosols, or Fibric, Hemic, and Sapric Histosols (Šamonil et al., 2011). Plant communities can be classified into Galio-odorati Fagetum association (most frequent), Mercuriali perennis-Fagetum, Calamagrostio villosae-Fagetum and Luzulo-Fagetum. Areas around the frequently occurring springs differ significantly in composition from the surrounding terrestrial sites. These communities can be classified as Caricion remotae alliance (further division of the associations is usually impossible), or as EquisetoPiceetum association (Boublík et al., 2009). The representation of tree species within the reserve in 2008 was: Fagus sylvatica L. 70%, Picea abies L. Karst. 28%, Abies alba Mill. 1% and other tree species (Sorbus aucuparia L., Acer pseudoplatanus L., Acer platanoides L., Ulmus glabra Huds.) 1% (www.pralesy.cz). 2.2. Data collection The basis for the surveys was the establishment of a regular network of 354 points with a 44.25 m spacing that covered the entire core, well-preserved zone of the reserve (74.2 ha) in 2008. The points in the network were geodetically surveyed to an accuracy

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Fig. 1. Location of the study area Zofinsky Prales National Nature Reserve (Zofin) in the Czech Republic. The survey was carried out in a regular network of circular plots (diameter 23 m) with a spacing between centres of 44.25 m on terrestrial sites (total area of 12.02 ha). Image includes preview of stem position map with position of standing (circles) and lying trees (lines) and pit-mounds (triangles – pit-mounds without lying stem; pentagons – pit-mounds with lying stem).

of ca. 0.05 m and were used as the basis for the follow-up tree-census, soil, disturbance, and dendrochronological surveys. 2.2.1. Tree-census survey Tree-census surveys were carried out in the core zone of the Zofin in 1975, 1997 and 2008 (74.2 ha; Pru˚ša, 1985; www.pralesy.cz) and serve as the basic data set describing the tree layer structure. All trees with a diameter at breast height (DBH) of P 10 cm were surveyed in terms of their position, species, DBH and health condition (e.g., standing living tree, stump, lying dead tree). In addition, fallen trunks were also studied in terms of their degree of decomposition (hard, half-rotten, and rotten). The output from each treecensus survey was a map of the stems with a position of all the individuals and their current data (Král et al., 2010a, 2010b). The comparison of these maps over time enables the study of the dynamics of each individual. 2.2.2. Soil survey A soil survey was performed between 2008 and 2011 on the entire area of 74.2 ha (detailed description in Šamonil et al., 2011). The occurrence of terrestrial and (semi-)hydromorphic soils was investigated and determined through using a regular network of 1780 shallow soil profiles and through the use of a soil corer. Additionally, the presence of specific plant species (e.g., Carex remota, Cardamine amara, Sphagnum spp.) as well as the characteristics of root plates (Norman et al., 1995) was used as an accessory indicator. Histosols, Gleysols, Stagnosols and Fluvisols belong to (semi-)hydromorphic soils and these sites were considered as water-affected. Terrestrial sites were represented by Cambisols and Podzols (Michéli et al., 2006). Both site types were usually strictly delimited (transition zone of ca. 1.0 m), and were delin-

eated as polygons overlying the map of the current tree layer (scale of 1:250). Results by Šamonil et al. (2011) were used in this study to reject the water-affected sites, because we wanted to deal with terrestrial sites only. In both site types the pit-mounds differ in their shape and course of levelling over time, as well as their importance for the tree regeneration. Retaining both terrestrial and water-affected sites in the analysis would be a source of substantial interference. Hence, all the calculations and surveys mentioned in this text include data from terrestrial sites only.

2.2.3. Study of the pit-mounds The characteristics of the present pit-mounds (defined as the microtopographical form caused by a single uprooted tree, with or without an uprooted trunk; see Šamonil et al., 2010b) were studied in the years 2009 and 2010 on 354 circular plots of 23.0 m in diameter staked out around the centres of a regular network (see above). The total area of the circles was 14.70 ha (Fig. 1). Only pit-mounds on terrestrial sites (12.02 ha) whose height or depth reached 20 cm (in the case of pit-mounds without a fallen trunk), or when the fallen trunk had a DBH P 10 cm were included in the analyses. The positions of the pit-mounds were recorded using Field-Map technology (http://field-mapping.com). The following properties were recorded for each pit-mound: the length and width (to calculate the area of the pit-mound, an approximation to an ellipse was used), the height of the mound and the depth of the pit, estimation of the pit and mound area proportion (as a sub-multiple), thickness of forest floor horizons (L – litter, F – fermented, H – humification horizons, see Klinka et al., 1997) on the mound (mm) and the thickness of the upper mineral A-horizon on mound (mm). If fallen trunks were present, the following

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characteristics were noted: tree species, DBH, the visually evaluated degree of trunk decomposition (categories: hard, half-rotten, rotten), occurrence (classified as ‘‘absent/present = 0/1’’) of branches up to the second order of branching, branchlets from the third order of branching, leaves, bark, roots and bryophytes on the trunk. In addition, the degree of decomposition was empirically measured by penetrometer as a penetration depth (see Šamonil et al., 2009). The presence of trees growing on mounds and in pits, their species and DBH were also recorded. 2.2.4. Age structure of the forest Between 2008 and 2011 the entire area of 74.2 ha was subject to a dendrochronological survey focusing on the study of the disturbance history of Zofin. For each of the 354 circular plots within the regular network we performed the standard dendrochronological cores at the height of 1.3 m, one core per tree. The cores were visually assessed in the field (absence of rot, knags, and acceptable distance from the pith) and then studied in detail in the laboratory. Discarded samples were replaced by new ones until six cores were taken on each of the plots from presently non-suppressed trees closest to the centre of the plot. From the total number of 23,111 trunks we took 3020 samples. The data of the dendrochronological research served as a basis for determining the relationship between the age of the tree and its DBH, as well as the age structure of the forest. 2.2.5. Direct dating of the pit-mounds Direct dating of pit-mound events proceeded according to the modified methodology by Šamonil et al., 2009 as a combination of dendrochronological and tree-census surveys. A total 237 pitmounds suitable for dating were selected on the basis of the spatial pattern of the trees and pit-mounds (places with high frequency of pit-mounds or lack of trees suitable for coring were eliminated). Within an approximate circle of 9 m from the dated pit-mounds, cores were taken from trees growing directly on the dated pitmound, or in the vicinity at the height of 1.3 m. The representative set of 43 individuals of F. sylvatica and 26 individuals of P. abies were also cored in the trunk base to determine the mean age on reaching a height of 1.3 m. A total of 537 cores were analysed. 2.3. Data processing 2.3.1. Direct dating of the pit-mounds The extracted cores were processed at a laboratory of the Department of Forest Ecology, VUKOZ (www.pralesy.cz). The cores were dried and smoothed with fine sandpaper. Damaged cores, cores without the sub-bark growth rings, or cores with excessive deviation of the coring from the pith (accepted deviation 6 3.0 cm) were rejected from further study. Because we also sampled trees with DBH > 150 cm, the accepted accuracy of pith reaching had to be decreased (e.g., Splechtna et al., 2005). A total number of 1986 cores was accepted for the follow-up analysis. The widths of the growth rings were measured using the PAST 4 programme (SCIEM, 2007) with 0.01 mm accuracy. Ring width series were cross-dated by identifying marker rings (Bayliss, 2004; Yamaguchi, 1991) and visual inspection of the series. Cross-dating was statistically validated using the PAST 4 programme and by COFECHA (Holmes 1983). However, the series intercorrelation (range 0.151–0.359) and the average mean sensitivity (range 0.231– 0.323) were relatively poor. High growth plasticity corresponds well with other natural beech-dominated forest in Central Europe (e.g., Splechtna et al., 2005; Šamonil et al. 2009). Pit-mounds were considered successfully dated by cross-validation of information from: (i) the development of the tree layer since 1975 based upon tree-census data, (ii) the minimum age of

the pit-mounds using dendrochronological data, (iii) the growth reaction of the surrounding tree to the disturbance event. (i) Repeated whole-area surveying of the tree layer from 1975 (Pru˚ša, 1985; www.pralesy.cz) provided exact information on whether the individual was uprooted in 2007 (impact of the Kyrill storm on 18 January 2007), or in the periods between 2007–1997 and 1997–1975, or before 1975. (ii) The minimum age of the pit-mound was determined as the age of the tree newly grown on the pit-mound plus the mean age on reaching the height of 1.3 m. On average, F. sylvatica reached the height of 1.3 at the age of 8.16 years (standard deviation, SD = 7.60 years) and P. abies at the age of 10.81 years (SD = 7.09 years). Young trees growing close to the pit-mound were assessed in terms of the character of the initial growth – the so-called gap origin. If its initial growth exceeded the threshold of gap-origin it was considered to be regenerated in the gap after the tree uprooting. Its age, plus the age on reaching a height of 1.3 m also indicated the minimum age of the pitmound. The threshold of gap origin was calculated according to Lorimer and Frelich (1989). Mean early growth rate was evaluated on 5 rings, 3 cm away from the pith, in 57 juvenile suppressed F. sylvatica, 44 juvenile suppressed P. abies, 64 juvenile F. sylvatica in gaps, 47 juvenile P. abies in gaps, 108 mature F. sylvatica and 254 mature individuals of P. abies. The threshold of gap origin was 1.45 mm for F. sylvatica and 1.58 mm for P. abies. Due to the absence of appropriate trees within the locality, the remaining broadleaved trees were evaluated according to F. sylvatica; A. alba according to P. abies. (iii) Release in radial growth of trees growing near the uprooted tree was sought after as a reaction to a disturbance event. The value of the release was determined using an identification method by comparing the percentage change of the average increment in two consecutive 10-year periods (Nowacki and Abrams, 1997) with the maximum growth potential represented by the boundary line (Black and Abrams, 2003). The boundary lines for individual tree species were used according to Splechtna et al., 2005 and its use in the Czech Republic (the Carpathians) was verified by Šamonil et al., 2009. The value of release for F. sylvatica and other deciduous trees was defined as 12% of the boundary line value (empirically derived by Šamonil et al., 2009) and 20% for P. abies and A. alba (Splechtna et al., 2005). Direct dating was carried out for 178 pit-mounds (52 cases upon the dendrochronological techniques including 4 cases with only minimal age, 126 young pit-mounds upon the condition of the trees according to the stem position maps). 2.3.2. Indirect dating of the pit-mounds The age of the remaining 1289 pit-mounds was deduced from the regression model calculated in the R programme (R Development Core Team, 2006). A linear regression calculation was based on a set of 178 directly dated pit-mounds. The initial set of variables (Table 1) suitable for next modelling the pit-mound age was selected based on research of the indirect dating of pitmounds using ecological characteristics (Šamonil et al., 2009, 2010b). A final regression model was created using backward selection upon the p-value until only statistically significant variables remained. We applied the model of the pit-mound age (y) in a form: y = 183.0066 + 0.0826l 21.3542ln(h) 22.5948ln (d) + 49.7501p + 1.6533Ah. Criteria set by Akaike, Bayes’ information, Mallow’s Cp served as a measure of the model accuracy and complexity. The resulting distribution of pit-mound age was

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Table 1 Initial set of variables used in the process of backward regression for indirect dating of pit-mound age (for detailed description of variables and their relationship to pit-mound age see Šamonil et al., 2009, 2010b). Variable

Abbreviation

Description

Intercept Width Length Ln height Ln depth Acuteness of mound Acuteness of pit Penetrometer

i w l ln(h) ln(d) am

Absolute co-ordinate (modelled age if all other quantities are zero) Pit-mound width (cm) Pit-mound length (cm) Natural logarithm of mound height, ln(h + 1) Natural logarithm of pit depth, ln(d + 1) am = ln((h2/Sm)+1), where Sm (area of mound) = pl0.5w0.50.0001decimal quotient of the mound from the pit-mound area

ap p

A-horizon Ordinal classification

Ah OK

ap = ln((h2/(Sp) + 1), where Sp (area of pit) = pl0.5w0.50.0001(1-decimal quotient of the mound from the pit-mound area) Ratio of average depth of the penetrometer incision and DBH of fallen trunk; where fallen trunk was absent the penetrometer incision = 1 Thickness of A horizon (mm) on mound Classification of trunk decomposition developed as a logical decomposition sequence (categories of trunk conditions: live standing, lying hard, lying half-rotten, lying rotten, pit-mound without lying trunks present) of particular trunks assessed during surveys in 1975, 1997, 2008; scale 1–32 (Šamonil et al., 2009)

further specified by auxiliary attributes (ordinal classification, the presence of leaves and branches on the fallen trunk, tree regeneration on the trunk). 2.3.3. Age structure of the forest The set of living trees on terrestrial sites within the circular plots detected during the tree-census survey in 2008 (total of 2677 individuals) was assessed approximately in terms of age structure. This was done using the relationship between DBH and age of the tree detected during the whole-area dendrochronological survey of 1912 individuals. Because of the absence of data for marginal tree species, models were only created for F. sylvatica and P. abies. Regression models were created using Chapman-Richards´, Korf´s and Michajl´s growth functions in the R programme (R Development Core Team, 2006). For tree age modelling, the function used was that which best corresponded, according to the residual sum of squares (RSS), to the relationship between age (x) and DBH (y), namely Chapman-Richards´ for F. sylvatica and other deciduous trees (RSS = 198286) with the prescription of y = 91.4882127(1-exp( 0.0055892x))^1.1870041; and for P. abies and A. alba Korf’s function (RSS = 232061) with the prescription of y = 100.6240exp(32.7632/((1–1.8912)x^(1.8912–1))). Because of the lower accuracy of models for higher DBH categories, we did not divide the higher age classes. 2.3.4. Assessment of the relationship between tree species populations and pit-mound dynamics To assess the importance of pit-mounds for tree regeneration and survival we compared the populations of tree species on pitmounds with non-disturbed sites. A chi-square test was used for evaluating the difference between the observed and expected values of the occurrence of trees at particular microsites in relation to the age distribution of trees or pit-mounds. We always tested those parts of trees or pit-mounds, which logically could enter the analysis (e.g., during the analysis of trees 50–100 years old, pit-mounds younger than 50 years old were disqualified). We do not assume that regeneration and mortality of trees on pit-mounds are linear in time. We tested the null hypothesis of the congruence of random and observed distribution of the tree species by age classes, between the pit-mound microsites and a non-disturbed area, i.e., that the distribution of the individual tree species on a specific microsite will – upon the validity of H0 – correspond to the proportion of all living trees by percentage distribution of the microsites on the studied area (= null random model). The issue regarding the favourability of pit-mounds for trees over time was assessed by using the distribution of the trees on the pit-mounds of respective age classes, analogically with the

inverse calculation of the importance of pit-mounds for tree survival. Null hypothesis was expressed as a congruence of observed and random distribution of trees on the mounds or in pits, where the expected distribution of trees was calculated as a share of all living trees in respective pit-mound age classes (e.g., for the 101– 200 year microsite age class, a population of trees younger than 200 years was assumed) by the area share of pits or mounds in individual age classes. The value of significance level was selected as a = 0.05 for all statistical calculations. 3. Results 3.1. Structure of the tree layer and the pit-mounds The number of trees in age classes on terrestrial sites within Zofin decreased smoothly with age (Fig. 2). According to the regression model the most frequently occurring trees were up to 100 years of age, but there were also a relatively high proportion of trees over 350 years old. Although this category could be slightly exaggerated by the regression model compared with reality, the dendrochronological survey confirmed the high maximum age of trees in the sample set: P. abies – 485 years, F. sylvatica – 476 years, A. alba – 455 years (including a mathematic derivation of some missing growth rings to the pith). The structure of the forest was dominated by F. sylvatica, it was only in the oldest age class that the share of P. abies and F. sylvatica was more balanced. Accessory tree species like S. aucuparia, A. pseudoplatanus, U. glabra were

Fig. 2. Age distribution of all living standing trees (per hectare) according to tree species on terrestrial site of Zofin (12.02 ha) derived from DBH structure.

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for both mound and pit. However, in the oldest age group of mounds the portion of the oldest trees (over 200 years) was high and the share of the oldest trees was almost equal to the share of trees aged up to 200 years. F. sylvatica in all age classes both on mound and in pit was represented P 90%. P. abies was most frequently present on mounds between 101–150 years old (7%); other tree species occurred mostly on mounds between 151–200 years old (6%). 3.2. Development of tree species populations on the pit-mounds

Fig. 3. Number of pit-mounds per hectare and distribution of pit-mound area per hectare according to the age of pit-mounds.

represented in the younger age categories, and A. alba was almost dominant in the oldest age classes. On terrestrial sites within the total area of 12.02 ha we recorded 1457 pit-mounds with an area of 1.47 ha (average of 121 pitmounds per hectare) and of this area, mounds covered 8.5% and pits covered 3.7%. Mean pit-mound size was 9.92 m2 (SD = 6.33 m2). In the age distribution of the pit-mounds (Fig. 3) the least represented were pit-mounds aged between 13– 100 years; the largest category was between 151–200 years (Fig. 3). The development of pit-mound abundance was similar to their development expressed through the area of the microsites (Fig. 3). Only in the category of the youngest pit-mounds did their area not correspond with their abundance. The average size of the pit-mound in this class was relatively small (6.90 m2). So far, the generally known process of the spreading of pit-mounds over time has not been demonstrated in this category and neither has the factor of longer survival of the larger pit-mounds. The largest mean size of the pit-mounds was 12.06 m2 in the 101–150 year age category. The process of gradual filling of the pit and the spreading of the mound was demonstrated in the change of the area ratio of both the microsites. Whereas in the youngest category (up to 12 years) the pit areas represented 92% of the sum of the mound areas, in the 151–200 year category it was just 31% and in the over 200 year old category the share of the pits was only marginal. The most frequently occupied pit-mounds were aged between 101–200 years old with trees that were 30–100 years old (Fig. 4). The occurrence of older trees on these pit-mounds was infrequent, which corresponded with the overall decrease in abundance of older trees in the forest structure (Fig. 2). These results were identical

The observed age distribution of F. sylvatica at both the pit and mound sites significantly differed from the expected (=random) distribution. On mounds, the occurrence exceeded the random distribution (Fig. 5, v2 = 147.37, p < 0.001), especially in the higher age classes and the biggest differences were in trees older than 200 years. On the other hand, the observed distribution of F. sylvatica did not produce the expected values in pits (v2 = 29.71, p < 0.001). The expected numbers were not achieved mainly by the youngest trees, among the older trees the differences disappeared, and the occurrence of F. sylvatica was close to a random distribution. Differences between age classes were demonstrated more markedly in the pits, whereas development on the mounds was less dynamic. The observed representation of P. abies on the mound was lower than expected in almost all of the age classes of the trees and these differences were statistically significant (Fig. 5, v2 = 11.21, p = 0.024). Also, the observed frequency of P. abies was significantly lower than random in pits (v2 = 9.95, p = 0.041). The significant result of the test was caused mainly by the differences in the youngest trees. With an increasing age of the trees, the occurrence of P. abies in the pits and on the mounds was near to random and this trend was similar to the case of F. sylvatica in the pit. The occurrence of other tree species on the mound was significantly higher, especially in the lower age classes, against the random distribution (Fig. 5, v2 = 14.73, p = 0.005). This related mainly to the species of S. aucuparia, A. pseudoplatanus, U. glabra. On undisturbed sites the actual distribution of F. sylvatica was lower than random, but the results were not statistically significant (v2 = 12.77, p = 0.120). The opposite result was achieved for P. abies (v2 = 2.00, p = 0.981). Also the occurrence of other tree species on undisturbed sites did not differ statistically from the random (v2 = 0.97, p = 0.998). 3.3. Relationship of the pit-mounds and trees in time A higher representation of all tree species (as well as individual representation of F. sylvatica) on the mounds against random

Fig. 4. Distribution of tree age classes on pit-mounds according to the age of the pit-mounds; numbers converted to hectare values. In each pit-mound age class the trees are included up to the age class of the pit-mounds, older tree age classes are therefore zero. Pits totally lacked trees exceeding 200 years of age.

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Fig. 5. Observed and expected (=random) frequency of trees growing on mounds and in pits according to tree species and tree age. Expected distribution was counted as proportion of tree populations in relation to area proportion of mounds (8.5%), and pits (3.7%). Value of abundance of trees on mounds in the graph is listed in conversion to 1 ha. The graph of distribution of expected and observed abundances of other tree species in pits is missing due to the absence of these species on this microsite. The value of observed abundance of F. sylvatica in pits in the age category older than 200 years and observed abundance of P. abies in the category 51–150 years and 201 years and over, was zero, i.e., there were no trees recorded. In other categories the numbers were non-zero (in some cases their representation was limited by the scale of the graph).

distribution was observed along the entire age gradient of the pitmounds (Fig. 6, v2 = 344.20, p < 0.001; resp. v2 = 438.89, p < 0.001); the highest positive deviations were among mounds aged between 101–150 years. The distribution of age when the pit-mound was colonised by a tree (with DBH P 10 cm) was unimodal, with its maximum at the age of 101–140 years (Fig. 7). The differences between main tree species from the point of view of pit-mound occupation were not too strong. In the pits, the highest actual absences of trees against the expected numbers were also in pit-mounds aged between 101–150 years old (Fig. 6, F. sylvatica: total v2 = 8.32, p = 0.040). The observed occurrence of P. abies on mounds of various ages, or in pits did not differ significantly from a random distribution (mound: v2 = 6.98, p = 0.072, pit: v2 = 4.48, p = 0.213). On the mounds aged between 101– 150 years old and older than 200 years, the actual occurrence of P. abies exceeded expectations of the random model, whereas in other age categories it was the opposite. Other tree species on mounds occurred much more frequently especially in the older age categories (v2 = 64.23, p < 0.001, especially A. alba). The highest differences were among pit-mounds aged between 151– 200 years old.

4. Discussion 4.1. Age structure of the pit-mounds While disturbance history has been widely discussed in connection with forest age pattern or pattern of tree releases in former studies (e.g. Lorimer and Frelich, 1989; Splechtna et al., 2005), age structure of the pit-mounds has been evaluated rarely (e.g. Šamonil et al., 2009). The high percentage of pit-mounds in the 101–200 year period and the low proportion of pit-mounds aged between 13–100 years old document that the process of tree uprooting was of episodic character. The described age structure of the pit-mounds probably relates to several disturbances that affected region during the 19th century; 11 destructive strong wind events and 1 intense snow breakage event were recorded in historical documents for areas within 50 km of Zofin (Brázdil et al., 2004; Kruml, 1960). This suggests that the Kyrill storm of 18th January 2007 was not a unique event in the forest development. In addition to the traditionally emphasised small-scale dynamics (Král et al., 2010a, 2010b), coarse-scale dynamics probably play an important role in the long-term perspective.

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Fig. 6. Observed and expected frequency of trees growing on mounds and in pits according to tree species and pit-mound age. Expected distribution was counted as portion of tree populations in individual age categories of the pit-mounds. The abundance values of tree occurrence on the pit-mounds are listed in conversion to 1 ha. The graph of distribution of expected and observed abundances of other tree species in pits is missing due to the absence of these species on this microsite. The value of observed abundances of all tree species, F. sylvatica and P. abies in pits in the last age category of the pit-mound and the observed abundance of P. abies and other tree species in the youngest age category on the mound, were zero, i.e., there were no trees recorded. In other categories the numbers were non-zero (in some cases their representation was limited by the scale of the graph).

4.2. Development of tree species populations on the pit-mounds Our study showed the significant influence of the pit-mound dynamics on the development of the tree layer. On terrestrial sites 24.7% of all mounds were occupied by trees of P 30 years of age and there were only 2.0% occupied pits. This observation is in general agreement with the literature, which mostly refers to different forest types. Although many authors describe higher density of cover and greater diversity of tree species on the mounds compared with the pits (Kuuluvainen and Juntunen, 1998;

Nakashizuka, 1989; von Oheimb et al., 2007), there are also opposite results (Ilisson et al., 2007). Except for the generally higher preference of the mounds by the trees, we found out that the relationships to microsites were significantly species-specific. While F. sylvatica and A. alba preferred the mounds to other microsites, the relationship of P. abies to this microsite was rather negative. A similar relationship, but in the youngest developmental phase of the tree seedlings, was found in beech-dominated Alpine forests by Simon et al. (2011). Species-specific relationships to pit-mound microsites were also discovered in the hardwood forests of USA

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Fig. 7. Age distribution (per hectare) of occupation of pit-mound by tree with DBH P 10 cm.

(Peterson et al., 1990; Peterson and Cambell, 1993) and in boreal forests (Kuuluvainen and Juntunen, 1998). We observed a gradual areal dominance of the mounds over the pits as a result of the different dynamics of the microsites over time. Hence, higher sensitivity of some tree species to specific microsites could have a long-term impact on the species diversity of the plant community. The proportion of F. sylvatica has significantly increased in recent decades (from 62.3% in 1975 to 70.3% in 2008; Pru˚ša, 1985; Král et al., 2010a; www.pralesy.cz), which was also demonstrated by the high proportion of P. abies in the oldest age category. However, we assume that the affinity of F. sylvatica to mounds just plays a catalytic role in this process and that it is not a primary cause. For a better understanding of the coexistence of tree species and the prediction of future development, it is necessary to include more factors in the analysis. The results suggest that the historical and geographical contingency could be factors more important in influencing ecosystem development via sequence, timing and initial conditions of disturbance events, than some global laws. Phillips (2004, 2006) supposed that these local factors are irreducibly significant and lead the researcher to shift away from finding general rules that incorporate these local factors. In our case, the historic indirect human impact (game overstocking periods, etc.) could have significantly influenced the portion of F. sylvatica and A. alba in the past (Kozáková et al., 2011; Šamonil and Vrška, 2007, 2008) and in the last few decades there could be a similar influence by air pollution (Elling et al., 2009; Oulehle et al., 2010; Šebesta et al., 2011). The affinity of tree species to specific microsites was often explained by the size of seeds. In the tropical forests of Panama, Putz (1983) claimed that small seeds might not be able to germinate on a forest floor with thick litter and therefore regenerated in places with exposed mineral soil, or with just a thin forest floor layer. Nakashizuka (1989) described a higher frequency of plant species with small seeds on mounds and fallen trunks. Peterson and Cambell (1993) and Peterson et al. (1990) explained a higher percentage of Fagus grandifolia on undisturbed sites compared with mounds, by the large weight of the seeds and their higher predation by wildlife on the mounds. On the other hand, Beatty (1984) did not found any relationship between seed size and microsite and we found mounds as more favourable microsites for F. sylvatica (large seeds) and less favourable for P. abies (small seeds). We therefore do not consider the size of seeds to be the reason for the different occurrence of trees species, but we assume that the actual reasons are the specifically different microclimatic (temperature and humidity; Beatty, 1984; Peterson and Cambell, 1993), and the competitive (Kuuluvainen and Juntunen, 1998; Nakashizuka, 1989) and pathogenic (Simon et al., 2011) conditions of

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the microsites after seed germination and during tree growth. Tree mortality at an early age is surely influenced by the different chemical properties of the forest floor (Šamonil et al., 2008a), as well as by differences between decomposer communities (Beatty and Sholes, 1988; Hautala et al., 2008). Simon et al., 2011 thus found a lower density of seeds of F. sylvatica on the mounds compared with pits, but the mortality rate was so high that it eventually led to a significant absence of regeneration in the pit microsite; Harrington and Bluhm (2001) also pointed out the difference between tree growth rates on different microsites. The soil thickness being another factor that affects tree growth, which is modelled by pit-mound dynamics (e.g., Gabet and Mudd, 2010; Meyer et al., 2007). However, because the mean soil thickness exceeds 70 cm in the locality (Šamonil et al., 2011) we do not assume this variable to be a determining factor for tree regeneration on pit-mounds. According to our current knowledge, none of the studies comprehensively resolved interactions between trees and pit-mound microsites along the tree and pit-mound age gradient. Previous work has focused on the influence of microsites on natural regeneration up to a maximum age of 20 years and has not dealt with further development (Palmer et al., 2000; Peterson and Cambell, 1993; Simon et al., 2011; von Oheimb et al., 2007). The results of our study show that knowledge of further development may be very important for understanding the interaction between trees and pit-mounds. The erosion-sedimentation pit-mound dynamics (Cremeans and Kalisz, 1988; Lyford and MacLean, 1966) have not a negative influence on the competitive potential of the tree species and thus these species are not less successful on pit-mound microsites. The increase in the affinity of trees for disturbed microsites over time probably relates to several factors: (i) microclimatic specifics of the pit-mounds that may positively influence the growth of trees (shift of phenological phases due to unique temperature conditions, high content of nutrients in the pit, etc.), (ii) causal positive relationship between gaps and the presence of pit-mounds (large pit-mounds often also represent large gaps with a lower initial competitive pressure from surrounding trees), (iii) blocking of pit-mound destruction by tree roots (pit-mounds occupied by trees may survive longer), (iv) in exceptional (rather hypothetical) cases, old pit-mounds could be overlaid with younger uprooted tree; it could bias the record of mortality of older trees on pit-mounds. Dissimilarity in occupying microsites by trees may also have feedback on pit-mound dynamics. We assume, that mounds with a changed structure and loose soil (e.g., Šamonil et al., 2010b; Vassenev and Targulian, 1995) with a higher probability of being colonised by trees (according to our results), could have a higher probability of further disturbance events. Conversely, this probability is very low in pits as they are almost always without natural regeneration. These differences influence the course of soil formation on disturbed and undisturbed soils and may lead to fragmentation of the disturbance regime into a varied pattern (Gabet and Mudd, 2010; Phillips and Marion, 2005; Šamonil et al., 2010b, 2011). The result of this process may be a divergent evolution of natural forest soils instead of a convergent evolution, which presumes the traditional paradigm of soil science related to the direction towards climax soils and steady-state model (see Phillips, 2002, 2010). Divergent evolution occurs together with a persistence or deepening of the initial soil heterogeneity instead of its gradual diminishment (Phillips, 2001). 4.3. Relationship of pit-mounds and trees over time In earlier studies, the highest diversity and cover of herb species and tree seedlings were recorded on young mounds up to approximately 12 years old (e.g., Palmer et al., 2000; von Oheimb et al., 2007). Later, the diversity of herb species decreased and the

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proportion of shrubs and trees on the mounds gradually increased (Palmer et al., 2000). Von Oheimb et al. (2007) found a large number of exclusive and infrequent vascular plant species, among them several disturbance specialists, on mounds in the beech-dominated forests of Germany. In Zofin as well, the highest species diversity of trees was found on the youngest pit-mounds, although for pitmounds up to the age of 12 years old, only 4.5% of mounds and 0.8% pits were actually occupied. Young pit-mounds were occupied mostly by tree species of early-successional stages (S. aucuparia, Sambucus nigra L., Sambucus racemosa L., Salix caprea L.), rarely accompanied by F. sylvatica; P. abies and A. alba were missing. The older pit-mounds, on the other hand, were occupied only by F. sylvatica, P. abies, and A. alba. The unimodal character of age distribution in the colonisation of the pit-mounds by trees was probably caused by the significant instability of young pit-mounds (Beatty and Stone, 1986; Peterson et al., 1990) and because the microclimatic conditions tend to be more extreme in young pitmounds. On the other hand, tree growth can be limited by accumulation of organic matter in the older pit-mounds (Šamonil et al., 2008b). 4.4. Development in managed forests The results of the study show that the factors influencing regeneration and dynamics of natural forests are more complex than we previously assumed. In managed forests, the described relationships are broken by human intervention even if their techniques are quite natural. Intentional blocking of pit-mound formation in managed forests leads to the forming of incomplete relationships and the spatial pattern and dynamics of such forests are far from natural. Pit-mound dynamics have a provably significant influence on the character of soils, spatial, and age structure of natural regeneration, and these relationships also have an historical character. The character of tree-soil interactions could be, depending on the forest type, recognised as an important criterion in the assessment of historical human impact and respective evaluation of the naturalness of the forest. This complex issue has not yet been sufficiently addressed and it deserves further attention in the future. Acknowledgments The authors would like to thank their colleagues from the ‘‘Blue Cat research team’’ for field data measurement. We thank Petra Dolezˇelová, Libor Hort, Filip Král, Kamil Král, Jan Novák, Pavel Unar, and Tomáš Vrška. The authors would also like to thank all the anonymous reviewers, as their comments and suggestions considerably improved the quality of the paper. The research was supported by the Czech Science Foundation (Project No. 526/09/P335 and No. P504/11/2135). References Bayliss, A., 2004. Dendrochronology: Guidelines on Producing and Interpreting Dendrochronological Dates. English Heritage. . Beatty, S., 1984. Influence of microtopography and canopy species on spatial patterns of forest understory plants. Ecology 65, 1406–1419. Beatty, S., Sholes, O., 1988. Leaf litter effect on plant species composition of deciduous forest treefall pits. Can. J. Forest Res. 16, 539–559. Beatty, S.W., Stone, E.L., 1986. The variety of soul microsites created by treefalls. Can. J. Forest Res. 16, 539–548. Bengtsson, J., 2002. Disturbance and resilience in soil animal communities. Eur. J. Soil Biol. 38, 119–125. Black, B.A., Abrams, M.D., 2003. Use boundary-line growth patterns as a basis for dendroecological release criteria. Ecol. Appl. 13, 1733–1749. Boublík, K., Lepší, M., Lepší, P., 2009. Vegetation of the Zˇofínsky´ Prales nature reserve (Novohradské Hory Mts., Czech Republic). Silva Gabreta 15, 121–142, in Czech.

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