Regeneration gap and microsite niche partitioning in a high alpine forest: Are Norway spruce seedlings more drought-tolerant than beech seedlings?

Forest Ecology and Management 455 (2020) 117688

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Regeneration gap and microsite niche partitioning in a high alpine forest: Are Norway spruce seedlings more drought-tolerant than beech seedlings?

T

⁎

Jurij Diacia, , Jurij Rozmanb, Andrej Rozmana a b

Department of Forestry and Renewable Forest Resources, Biotechnical Faculty, University of Ljubljana, Vecna Pot 83, 1000 Ljubljana, Slovenia Slovenia Forest Service, Regional Unit Kranj, Cesta Staneta Žagarja 27b, 4000 Kranj, Slovenia

A R T I C LE I N FO

A B S T R A C T

Keywords: Gap partitioning Microsite partitioning Topsoil moisture Micro-relief Ground vegetation Overbrowsing

Mixed mountain forests composed of Picea abies, Abies alba and Fagus sylvatica represent one of the most important alpine ecosystems with respect to economics, environmental effects and social functions. However, many stands are characterised by even-aged forest structure and altered tree species composition, making them more prone to disturbances and less effective in protection against natural hazards. Changes to the forest microclimate and nutrient cycling delay natural regeneration and favour the successional development of ground vegetation. In this study we hypothesized that the creation of gaps of appropriate size and shape would facilitate natural regeneration and forest conversion. In 2003, we selected 15 gaps, ranging in size from 0.01 to 0.62 ha and more than 50 years in age, and three areas below closed canopy on a south facing P. abies dominated mixed mountain forest (1380–1480 m a.s.l.). Within the gaps we established 542 systematically distributed research plots (1.5 × 1.5 m) and analysed regeneration attributes according to height classes, ground vegetation coverage, light climate, topsoil moisture, microsite relief and soil features. Measurements were repeated after five vegetation seasons and indicated minimal changes in vegetation structure and the slow development of regeneration overall. Vegetation ordination and generalized linear mixed models showed a positive association of P. abies seedling abundance with diffuse light, thickness of the organic horizons, presence of CWD and moss coverage and a negative association with ground vegetation cover, soil water content, Landolt indication value for nutrients and direct light. Fagus sylvatica seedlings were more abundant closer to seed trees, on less acidic soils with higher moisture content and on concave microrelief. The results indicate that P. abies and F. sylvatica seedlings may successfully establish under slightly open forest canopies, but in a few years the former require amounts of diffuse light comparable with that in the medium sized gaps in this study (0.15 ha). Due to the negative association between direct light and regeneration, gaps should be elliptical with the long axis oriented east-west. Recruitment of all species was significantly retarded by overbrowsing.

1. Introduction Mixed mountain Norway spruce-silver fir-beech forests cover extensive areas in European mountains (Brus et al., 2012). Their range extends from northeast Spain to western Ukraine and from the Sudeten Mountains in the north to almost the southern tip of Italy (Bohn and Katenina, 1996). Norway spruce (Picea abies (L.) H. Karst., hereafter spruce) is an economically important tree species in Europe and has been strongly favoured within this forest site type through planting, thinning, clear-cut management, overbrowsing and grazing. Therefore, many stands on sites of mixed mountain forests are characterised by even-aged forest structure and dominant spruce. This has resulted in problems with natural regeneration (Ott et al., 1997) and high susceptibility to natural disturbances (Schütz et al., 2006; Seidl et al., ⁎

2011). The predominant silvicultural goal is therefore to convert stands towards mixed uneven-aged stands (Hanewinkel and Pretzsch, 2000; Spiecker, 2003). In younger stands conversion may be carried out by structural or group thinning and in mature stands through natural regeneration (Bachofen and Zingg, 2001; Schütz, 2001). Improvement of the species mixture can most efficiently be implemented during the stand regeneration phase (Schütz, 2004). However, natural regeneration of altered mixed mountain stands is delayed due to changes in nutrient cycling and forest climate, which is reflected in retarded humus decomposition and the development of competing ground vegetation (Fanta, 1997; Diaci, 2002). Additional difficulties include a lack of seed trees of the original species, seed predation (Diaci, 1997; Rozman et al., 2015) and a deficiency of decaying wood, which represents an important seedbed for spruce and

Corresponding author at: Department of Forestry, Biotechnical Faculty, University of Ljubljana, Vecna Pot 83, 1001 Ljubljana, Slovenia. E-mail address: [email protected] (J. Diaci).

https://doi.org/10.1016/j.foreco.2019.117688 Received 8 July 2019; Received in revised form 2 October 2019; Accepted 12 October 2019 0378-1127/ © 2019 Elsevier B.V. All rights reserved.

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silver fir (Abies alba Mill., hereafter fir) (Szewczyk and Szwagrzyk, 1996; Hunziker and Brang, 2005). In recent decades overbrowsing has been a growing problem in these forests (Ammer, 1996; Motta, 1996; Nagel et al., 2015). Factors that inhibit regeneration often act in synergy and may completely halt gradual conversion; however, with appropriate silvicultural measures, many difficulties can be reduced (Ott et al., 1997). This requires a good knowledge of the associations between the successional development of the regeneration and the spatial and temporal dynamics of gaps. While the latter has received considerable research attention (Mosandl and Kateb, 1988; Van Couwenberghe et al., 2010), there have been far fewer studies addressing the variability of microsites within gaps (but see Kern et al., 2017; Downey et al., 2018). These are similarly important for the success of silvicultural measures, as they help us to understand 1) the spatial distribution of ecological factors that are related to the variability of solar radiation within gaps and factors that are less dependent on gap features, for example soil and micro-relief; 2) the response of tree species to this variability; and 3) how to adjust gap geometry and successive expansion to this variability. The coexistence of fir, spruce and beech (Fagus sylvatica L., hereafter beech) is based on a preference for different microsites as well as complementing the exploitation of resources both from the point of view of the resource structure (e.g. beech exploits deeper soil horizons than spruce and requires a species specific set of nutrients) and that of time (e.g. spruce may assimilate when beech is leafless) (Nakashizuka, 2001). These mechanisms are affected by altered tree species composition and also contemporary environmental changes. From this point of view, we were particularly interested in shedding light on the differences between the tree species regarding microrelief, soil structure and drought tolerance. The overall goal of the study was to characterise ecological factors and regeneration within and among gaps of different sizes and derive silvicultural prescriptions for facilitating natural regeneration leading towards gradual conversion.

Fig. 1. Projection of medium canopy gap 15 with plot arrangement.

carried out occasionally, but this did not significantly change the gap climate. Only the large gap was partly created by sanitary felling and regeneration harvest. Small gaps (n = 7) were smaller than < 0.03 ha, medium gaps (n = 8) covered areas between 0.03 and 0.20 ha and the large gap was about 0.50 ha. In 2003, we delineated within these 18 research units square plots of 1.5 × 1.5 m on a systematic grid of 5 × 5 m. In total, we marked with iron sticks and georeferenced 542 plots (Fig. 1). Microrelief (flat, concave, convex), slope inclination and orientation, soil depth and the depth of the organic horizons (H – humus horizon, F – fermentation horizon, Ah – topsoil horizon) were measured. Humus structure was assessed with Möller’s (1981) humus form index (IHF). This index is a quotient between the thickness of the Ah horizon (enriched with humus) and the sum of the fermentation horizon (partly decomposed vegetation debris), humus horizon (largely decomposed vegetation debris) and Ah horizon. Biologically active humus forms (i.e. mull) have an index value of 1. Regeneration density and coverage according to species were estimated corresponding to four height classes (one-year old seedlings, seedlings with height < 20 cm, seedlings with height ≥20–50 cm and seedlings taller than 50 cm). Due to the low densities of the regeneration, only two classes were used in the statistical models: one-year-old seedlings and older seedlings (hereafter seedlings). Seedlings were categorized as browsed if the leading shoot was damaged. Plant cover per species was estimated visually from above on each plot and was recorded to the nearest 10% from 10 to 100% and to the nearest 1% from 1% to 10%. Moss coverage was estimated with the same scale but only as a total coverage. The same scale was also used for estimation of woody debris coverage, which was also classified into type (stump, branches, log) and decay stages (Diaci et al., 2012). Light climate 1.3 m above each plot was assessed with a digital fish-eye method (WinScanopy) described in (Rozenbergar et al., 2011). On 16.06.2008 (moisture1), 11.10.2008 (moisture2) and 12.07.2010 (moisture3), the water content of the topsoil (0–10 cm) was measured on all plots using time domain reflectometry (IMKO, 2006). Each time measurement was taken after a period of dry weather. In terms of rainfall and air temperature, the year 2008 did not

2. Materials and methods 2.1. Stand and site characteristics The study was carried out in a high mountain mixed spruce-firbeech forest or Homogyno sylvestris-Fagetum according to the BraunBlanquet approach. The study area is located in the Karavanke Mountains, which are part of the Eastern Alps. The two stands are located on a smooth to undulating slope at 1380–1480 m a.s.l. with an inclination of 14–26° and a mostly south-eastern orientation. Both stands were approximately 160 years old, and about 30 m in height. The mean annual temperature in the period 1960–1990 was about 5.9 °C (meteorological station Zgornje Jezersko, 894 m a.s.l.) and precipitation was 1709 mm with a peak in summer (meteorological station Jelendol, 760 m a.s.l.). The predominant parent materials are limestone and dolomite on which shallow rendzinas with some pockets of deeper brown soil developed. The growing stock of the stands ranged from 330 to 550 m3 ha−1, which was comprised of 91% spruce, followed by 8% beech and 1% silver fir. Although there are alpine pastures nearby, the forest is also depicted as forest on the oldest maps from late 18th century (Rajšp et al., 1997). However, it was used for forest grazing and probably also for charcoal burning. 2.2. Sampling design and recordings The research unit consisted of two sub-compartments, totalling 25 ha in size. In 2003, we selected 15 non-overlapping gaps in a recently non-managed part of the forest. Within the same research unit three areas with relatively closed canopies were also selected, hereafter referred to as stands. The gaps were created by regular regeneration harvests several decades ago and they were relatively stable in time. After the creation of gaps, sanitary fellings of individual trees were 2

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3. Results

significantly deviate from the long-term average; therefore, the topsoil moisture values can be considered as a proxy for the lowest topsoil moisture content in average years. In 2010, only 621 mm of rainfall or 71% of the average of the last 50 years fell in the period from January to July. At the same time the average July air temperature at the Krvavec meteorological station was 2.3 °C above the long-term average. Measurements of topsoil moisture in 2010 thus indicate topsoil moisture for extremely hot and dry summers. All beech seed trees in the vicinity of the research units were georeferenced and the shortest distance to each plot calculated. Other admixed tree species were rarely represented as seed trees or regeneration. The species of the dominant seedling – the one most likely to outgrow all other seedlings on the plot and having a minimal height of 5 cm – was additionally recorded along with its height, increment and vitality. The measurements were repeated in 2008.

3.1. Variability of ecological factors within and between gaps In 2003, the median diffuse light under closed stands amounted to 11.7%. This was followed by small gaps with 12.8%, medium gaps with 22.9% and the large gap with about 75.3% (Fig. 2a). Between 2003 and 2008 diffuse light slightly increased due to sanitary felling. Somewhat lower values were estimated for relative direct light (Fig. 2b). In spite of the irregular shape of the gaps, the spatial distribution of diffuse light was relatively symmetrical within all gaps, which indicates homogeneity of the canopy layer surrounding the gaps (for example, Fig. 3a). On the other hand, the distribution of relative diffuse light indicated NS asymmetry (Fig. 3b). The variability of both light components increased with gap size. Spearman rank correlation between measured light values and LIV for light indicated a good match (rs = 0.50***). Topsoil moisture values during the summer of 2008 were higher when compared to the dry year of 2010, amounting to 6.7%, 7.2%, 7.1% and 5.6% within the stand and small, medium and large gaps, respectively (Fig. 2cd). Microsites within gaps had higher topsoil moisture when compared with those at gap edges under the canopy. In all measurements the lowest values were recorded in the large gap. The association between the LIV and measured topsoil moisture was weaker when compared to light. The strongest association was exhibited by moisture2 (rs = 0.23***). The spatial distribution of topsoil moisture indicated highest values within gap centres and a decline towards gap edges, which was steeper for the less dry year of 2008 (Fig. 3cd). In the extremely dry year of 2010, the central part of the gap retained about 10% of topsoil moisture, while the rest of the gaps had significantly less than 10% of topsoil moisture. Within individual inventories there were large differences between plots, for example between 3.7% and 88.7% for moisture2.

2.3. Data analysis Seedling density in relation to ecological factors was modelled with a series of generalised mixed-effects models (GLMMs) with logistic regression, since the models with negative binomial distribution did not converge. The models were built with the drop1 functions from the “lme4” package (Bates et al., 2015). The best models were selected based on the anova likelihood ratio test of fits (Bolker et al., 2009). The associations between ecological factors and regeneration features (height, height increment, herb coverage) were studied with linear mixed models (LMMs). The models were built with the ‘‘nlme’’ package (Pinheiro and Bates, 2000). In both modelling approaches research units (i.e. gaps) were considered as random factors. For model selection we followed a top-down approach (Zuur et al., 2009) and started with a fixed model structure containing all ecologically meaningful fixed factors and their interaction terms (for the models and factors considered, see Tables 2 and 3). For model diagnostics we carefully examined confidence intervals of parameters and analysed sets of graphical summaries proposed by Robinson and Hamann (2011). Principle coordinate analysis (PCoA) was used to ordinate plots based on the 2008 vegetation survey. The distance matrix was calculated by applying the Bray-Curtis distance measure to the 4-th root of the plant species coverage data. Based on inspection of the eigenvalues, we selected only the first and third principal coordinate for presentation. Ecological variables (light and topsoil moisture measurements, distance to beech seed trees, etc.), mean Landolt indicator values (LIVs) for the assessment of site conditions (temperature regime, continentality, light quantity, topsoil moisture, soil reaction, nutrient content in the soil, humus content and soil aeration) and regeneration density according to species groups were fitted onto an ordination using the vegan package (Oksanen et al., 2011). Mean LIVs were estimated as a weighted average of the Landolt indicator values (Landolt et al., 2010) of all present species, and their abundances were used as weights (Lepš and Šmilauer, 2003). Data were analysed in R Version 3.3.1 (R Core Team, 2016). For visualisation of the spatial distribution of light and topsoil moisture within individual gaps, we applied Surfer 8 software with objective spatial interpolation called general kriging (Nielson and Wendroth, 2003).

3.2. Natural regeneration In 2003, the overall regeneration density excluding one-year-old seedlings was 10.324 per ha (Table 1). Still, 60% of the seedlings were shorter than 10 cm. Over five years the density moderately increased by about 1115 seedlings older than one year, and there was a shift towards taller seedlings (Fig. 4). The number of one-year-old seedlings in 2003 and 2008 was 8487 and 23920, respectively, and indicated a fair seeding potential. This was because 2007 was a beech seed year. As a result, species mixture within one-year-old seedlings changed significantly in favour of beech (from 18% to 52%), while the share of spruce decreased from 74% to 46% in 2003 and 2008, respectively. Although the density and composition of one-year-old seedlings changed between inventories, the distribution according to gap sizes remained the same. The highest overall one-year-old seedling density was found in small and medium gaps, followed by the stand, while the large gap had extremely low numbers in both years (Fig. 4a). The species composition among seedlings older than one year remained relatively stable through the years. In 2003, spruce predominated with 59%, followed by beech with 36%, rowan (Sorbus aucuparia L.) with 2%, and fir and other tree species with 1%. By 2008, the share of beech increased by 7 percentage points at the expense of

Table 1 Average seedling density per hectare according to gap size, developmental class and tree species in 2003. Numbers in brackets denote standard error. Spruce

Stand Small gaps Medium gaps Large gap

Beech

Other trees

One-year-old

Seedlings

One-year-old

Seedlings

One-year-old

Seedlings

3259(864) 7336(1593) 7696(1086) 222(126)

519(214) 2035(577) 7971(927) 6963(1867)

3037(874) 1339(408) 1599(293) 0

3630(861) 3213(691) 4523(686) 148(104)

370(160) 750(199) 734(117) 74(74)

370(192) 482(171) 420(75) 741(262)

3

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Table 2 Results of the GLMM analysis for the density of woody seedlings according to species groups predicted by factors (n = 1084). The research units (stand, gap) were considered as a random factor. Labels in brackets at factor names denote the factor level for comparison, while numbers in brackets within the results denote the standard error (SE). Dependent variables Spruce

Intercept Diffuse light Herb coverage Moss coverage Distance to beech seed trees Topsoil moisture2 Topsoil moisture3 Soil depth Rockiness Coverage woody debris Slope LIV nutrients LIV aeration LIV humus LIV soil reaction Cat.: trunk (vs stump) Cat.: no (vs stump) Cat.: branches (vs stump) Direct light: medium (vs low) Direct light: high (vs low) Relief: concave (vs flat) Relief: convex (vs flat) Position: gap (vs canopy) Year: 2008 (vs 2003) Random effects: Std. dev. intercept/residual

Beech

One-year-old seedlings

Older seedlings

−3.22(2.81) 0.59(0.27)* −0.67(0.16)*** ns ns ns ns ns ns ns ns −3.06(0.51)*** 1.65(0.41)*** 1.62(0.36)*** ns −0.50(0.32) −0.34(0.21) −1.04(0.24)*** −0.41(0.19)* −0.75(0.27)** ns ns ns 0.60(0.15)*** 0.84/0.92

Rowan

Herbs

One-year-old seedlings

Older seedlings

All seedlings

−10.12(2.82) 2.27(0.32)*** −0.38(0.17)* 0.83(31)** ns ns −0.89(0.24)*** ns ns ns −0.04(0.01)*** −2.21(0.48)*** 2.16(0.41)*** 1.70(0.37)*** ns −0.56(0.32) −0.88(0.20)*** −1.31(0.24)*** −0.25(0.20) −0.80(0.30)** ns ns ns −0.33(0.15)*

1.20(0.47)* ns ns ns −0.25(0.03)*** ns ns ns ns ns −0.04(0.02)* ns ns ns ns ns ns ns ns ns ns ns ns 2.09(0.23)***

−2.15(0.81) ns ns ns −0.06(0.01)*** 0.56(0.23)* ns ns ns ns ns ns ns ns ns ns ns ns ns ns 0.69(0.30)* 0.52(0.28) ns 0.11(0.18)

−3.77(0.39)*** ns ns ns 0.02(0.01)** ns ns ns ns ns ns ns ns ns ns ns ns ns −0.64 (0.24)** −0.62 (0.31)* ns ns 1.92 (0.30)*** 0.003 (0.20)

4.97(0.50)*** 0.62(0.04)*** nt −0.01(0.002)*** 0.003(0.001)* ns 0.29(0.04)*** −0.01(0.002)*** −0.02(0.003)*** −0.01(0.00)*** ns −0.22(0.09)* −0.65(0.06)*** −0.69(0.07)*** −0.27(0.07)*** ns ns ns ns ns ns ns ns −0.07(0.03)***

0.59/0.77

0.66/0.81

2.04/1.43

0.29/0.54

0.20/0.40

***

**

ns: not significant; nt: not tested. *** Significance code: p < 0.05. ** Significance code: p < 0.01. * Significance code: p < 0.001.

Table 3 Results of the LMM for dominant seedling height (h) and height increment (ih) predicted by factors (n = 1084). The research units (stand, gap) were considered as a random effect. Labels in brackets at factor names denote the factor level for comparison, while numbers in brackets within the results denote the standard error (SE). Dependent variables Spruce Height Intercept Seedling height Vitality: medium (vs excellent) Vitality: poor (vs excellent) Diffuse light Direct light: medium (vs low) Direct light: high (vs low) Moss coverage Topsoil moisture1 Topsoil moisture3 IHF Soil depth Slope LIV aeration Distance to beech seed trees Year: 2008 (vs 2003) Random effects: Standard dev. intercept/residual

Beech Height increment

Height

Height increment

−0.95(0.54) nt −0.33(0.09)*** −0.25(0.11)* 0.74(0.08)*** ns ns ns ns ns −0.69(0.20)*** 0.02(0.01)** ns 0.56(0.17)** ns 0.37(0.07)***

1.02(0.29) 0.67(0.08)*** −0.61(0.13)*** −0.91(0.15)*** ns ns ns −0.10(0.05)* ns ns ns ns 0.02(0.01)*** ns ns 0.03(0.11)

0.94(0.25) nt −0.25(0.06)*** −0.36(0.08)*** 0.44(0.07)*** ns ns ns ns 0.22(0.09)* ns 0.02(0.004)** ns ns 0.01(0.003)* 0.32(0.06)***

2.17(0.82)** ns −0.57(0.17)** −1.20(0.23)*** ns −0.24(0.17) −0.51(0.20)* ns 0.45(0.23)* ns ns ns −0.02(0.01)* ns ns 0.03(0.15)

0.09(0.57)

0.14/0.82

0.03(0.45)

3E-05/1.23

***

ns: not significant, nt: not tested. *** Significance code: p < 0.05. ** Significance code: p < 0.01. * Significance code: p < 0.001. 4

***

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Fig. 2. (a) Median relative diffuse light according to gap size and year. (b) Median relative direct light according to gap size and year. (c) Topsoil moisture in a dry period in July in a normal year (2008). (d) Topsoil moisture in a dry period in July in a hot and dry year (2010).

3.3. Herb vegetation and indication of ecological factors

spruce, while the share of other species remained relatively constant. Also the distribution of seedlings according to gap size did not change significantly between years. In both years the highest density of seedlings shorter than 20 cm was in the medium gap, followed by the small gap, closed stand and large gap (Fig. 4b). The density of seedlings taller than 20 cm significantly increased with gap size (Fig. 4c). Seedlings taller than 1 m were recorded only in medium and large gaps. Over both size classes, beech seedling density sharply declined with gap size, while the density of spruce increased. The same was true for other tree species: sycamore maple (Acer pseudoplatanus L.), ash (Fraxinus excelsior L.) and goat willow (Salix caprea L.). Rowan was less represented in the large gap. Seedlings were not evenly distributed within plots. In both years the same plots did not have any seedlings, and their share remained rather high at about 42%. On 43% of plots there was only one species present, while two or more were present on 15% of plots. In both years the stand and small gaps contained the highest percentage of plots without any tree regeneration (Fig. 4d).

There were no major differences in the composition of vascular plants, including woody species, between years. We recorded 144 species in both inventories. Most species were found in medium gaps (120 in 2008), followed by the large gap (95 in 2008), small gaps (69 in 2008) and the stand (46 in 2008) in both years. Overall vegetation coverage sharply increased with gap size, from 6.8% within the stand to 72.7% within the large gap in 2008. The share of organic soil uncovered by plants decreased with gap size. Plots in the large gap had a slightly larger part of the surface covered with mineral soil and rocks compared to the other gap sizes (5% vs < 1% respectively), which may indicate erosion. Coverage of woody debris was relatively stable across gap sizes and years. It ranged from 3% in the small gaps to 6% in the large gap. The ordination indicated regeneration niche partitioning between spruce and beech, with regeneration of the former clustered in the upper part of the biplot and regeneration of the latter on the lower part (Fig. 5). Early developmental stages of beech regeneration were positively associated with less acidic soils and negatively associated with diffuse and direct light and distance to seed trees. Taller beech regeneration was positively related to LIV for soil nutrients and soil 5

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Fig. 3. Above left: relative diffuse light in medium sized gap number 14. Above right: relative direct light in the same medium sized gap. Below left: topsoil moisture during a dry period in July of a normal year (2008). Below right: topsoil moisture during a dry period in July of a dry year (2010).

related to available light. This trend was evident also for taller beech seedlings but to a lesser extent. One-year-old rowan seedlings and seedlings shorter than 20 cm showed analogous relations with ecological factors to that of the early developmental stages of spruce.

moisture. On the other hand, early developmental stages of spruce regeneration were negatively associated with soil nutrient and moisture content and positively associated with soil acidity and content of soil organic matter. Spruce seedlings taller than 20 cm were more closely

Fig. 4. a–c) Seedling density in 2008 according to gap size, species and seedling stage. d) Share of plots with tree seedlings. 6

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Fig. 5. Ordination-biplot of a principle coordinate analysis (PCoA) for vascular plant cover in the herb layer (Bray-Curtis distance measure) in 2008. Grey crosses represent the location of plots on the 1st and 3rd axes of the PCO analysis, while the upper and lower semicircles are proportional to the square root of the number of spruce and beech seedlings, respectively. Grey crosses without semicircles denote plots with no tree regeneration. The arrows show the direction of the gradient, and the length of the arrows is proportional to the correlation between the variable and the ordination. Only factors with a p-value < 0.05 are plotted. Smaller angles between the arrow and axis represent a higher correlation between the two values. Small dashed arrows represent regeneration density per species and developmental stage, e.g. Spruce.oyo for one-year-old spruce seedlings, Spruce.20 for spruce seedlings < 20 cm and Spruce.21 for spruce seedlings ≥20 cm. Black arrows represent measured ecological factors, e.g. topsoil moisture (Soil.moisture1), diffuse and direct light (Dif.light, Dir.light), coverage of woody debris (DW.cov), minimum distance to beech seed tree (Beech.seed.tree) and humus index (IHF). Large dashed arrows are for Landolt phytoindication values for light (L), continentality (K), humus (H) and the soil related factors of aeration (A), reaction (R), nutrients (N) and moisture (M).

coverage. The LMM for the height increment of beech also included a positive association with topsoil moisture1 and low levels of direct light and a negative association with slope inclination. The height of both species significantly increased over the years, while the height increment did not. Browsing damage on seedlings was high. In 2003, about 33% of seedlings were damaged. The share of browsed seedlings decreased to 28% in 2008. Browsing damage differed between species and height classes. Seedlings of all species were most damaged within the height class > 50 cm. Spruce was the least damaged tree species, yet almost 40% of seedlings larger than 20 cm were damaged in 2008. This was significantly higher for beech seedlings. In 2008, the share of beech seedlings damaged by browsing amounted 62% and 83% in height classes 21–50 cm and above 50 cm, respectively.

3.4. Drivers of regeneration dynamics The density of one-year-old and older spruce seedlings was positively associated with diffuse light, coverage of stumps on the plot and LIV for humus and negatively associated with herb cover, coverage branches on the plot, LIV for nutrients and high levels of direct light (Table 2). The model for older seedling density additionally indicated a positive association with moss coverage and a negative association with slope inclination and topsoil moisture3. GLMM models for one-year-old and older beech seedling density shared a negative association with distance to seed trees. One-year-old beech seedling density was additionally negatively associated with slope, while older seedlings were positively related to topsoil moisture2 and concave relief. The rowan seedling sample was too small to be analysed by developmental stages. The model for all rowan seedlings indicated higher density with increasing distance to beech seed trees, position within the gap (vs extended gap) and low direct light levels. The LMM model for herb coverage, as a strong competitor to spruce seedlings, indicated a positive association with diffuse light, distance to beech seed trees and topsoil moisture3 and a negative association with soil depth, rockiness, coverage of woody debris, as well as LIV for nutrients, aeration, humus and soil reaction. We recorded dominant seedlings on about 57% of plots in both years. Species composition was relatively stable at 47% beech, 44% spruce, 5% rowan, 2% fir and 1% other tree species in 2008. The average height of dominant spruce and beech seedlings in 2008 was 24.6 and 29.3 cm, respectively. LMM models for seedling height for both species indicated a positive association with increasing vitality, diffuse light and soil depth (Table 3). The height of dominant spruce seedlings was additionally positively associated with LIV for aeration and negatively associated with the IHF index. The dominant beech seedling height model also included a positive association with distance to beech trees and topsoil moisture3. The height increment of both beech and spruce increased with increasing vitality scores. Additionally, the height increment of spruce was positively related to vitality, seedling height and slope and negatively related to moss

4. Discussion 4.1. Variability of ecological factors within and between gaps Median diffuse radiation increased from 12% below the stand to 75% within the large gap and did not differ much between the years, indicating minor changes in canopy cover. The distribution of solar radiation showed the expected symmetrical attenuation from centre towards gap edges for diffuse and N-S asymmetry for the direct component (Krečmer, 1966; Diaci, 2002) despite the irregularities of the gaps. Such gap forms are common as a part of the irregular shelterwood system, where lesser-quality trees are successively removed (Schütz, 2004). Unlike light, topsoil moisture indicated a slight decrease with increasing gap size. Still, soils at microsites covered by canopy were consistently drier when compared to microsites within gaps. Similar results were reported by Gray et al. (2002) and Vilhar et al. (2010, 2015). This may be attributed to interception and water consumption of canopy trees. Between-gap variability of topsoil moisture was lower compared to within-gap variability, which may be due to the differences in microrelief (Clinton, 2003). We found a good association between LIV for light and light measurements and a poor association 7

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between spruce and beech. While models for spruce seedling density suggested a negative relation to herb coverage and positive relation with moss coverage, the model for beech seedling density did not show any association. In the first few years, spruce seedlings are generally smaller and slower in growth compared to beech; therefore, regeneration studies often reported their great sensitivity to herb competition, while microsites overgrown with mosses are generally less problematic (Hunziker and Brang, 2005), but this is not generally valid (Diaci, 2000). On the other hand, beech seedling density, height and height increment indicated a positive association with topsoil moisture, while topsoil moisture was only included in the model for spruce seedling density, but with a negative sign. Spruce seedling density was positively related to less developed humus forms (high LIV H, low IHF and LIV N), which have better aeration properties (high LIV A) and lower waterholding capacity (Piussi, 1970). The association of spruce regeneration with the organic layer has been found in many other studies (Ott et al., 1997; Diaci et al., 2000). As regards humus formation and topsoil moisture, the results are concordant and suggest that spruce and beech occupy different micro-sites within the research area. The better performance of spruce seedlings in drier micro-sites was not expected. For example, Brang (1998) indicated that southern slopes in the Swiss inner Alps may have a smaller window of opportunity for spruce regeneration regarding gap size because of risk of drought. This difference compared to this study may be a result of the higher yearly sum of precipitation and dissimilarities in herb assemblages on limestone substrate in the Slovenian Alps. Regarding beech seedlings, the better performance of beech regeneration in more mesic microsites and susceptibility to drought were not anticipated. There may be several reasons for this; young trees may have different requirements for ecological factors than mature trees, and these requirements also vary with sites (Wardle, 1958; Landolt et al., 2010). In light of climate change, it seems rational to verify the results of this study on other stand and site combinations. Overall, the results showed adequate seedling potential. Also, changes in the tree species composition from 91% of spruce in the mature stand, 59% among seedlings and 44% among dominant seedlings indicate stand development towards a more natural tree species mixture. However, both the regeneration density and mixture were strongly impeded by overbrowsing.

between LIV for moisture and topsoil moisture measurements. A possible reason for this is the fact that we measured the extreme values, while LIV indicates average topsoil moisture during the growth period (Landolt et al., 2010). The coverage of ground vegetation increased with gap size, while the highest diversity of vascular plants was recorded in medium gaps. The latter may be attributed to suitable conditions for both shade-tolerant and light-demanding species (Connell and Slatyer, 1977). 4.2. Drivers of tree regeneration dynamics Factors influencing regeneration can be divided into three groups. The first includes factors related to canopy gap size, shape, and position within the gap, all of which have an influence on gap microclimate, root density, and distance to seed trees (Gray et al., 2002). The second group includes factors not directly related to gap characteristics, such as soil and microrelief features, woody debris and browsing (Kern et al., 2017). The third consists of factors associated with both factor groups, for example topsoil moisture, humus form and herb coverage. Spruce and beech seedlings indicated gap microsite partitioning in regard to all three factor groups. Diffuse light was positively associated with spruce seedling density and spruce and dominant beech seedling height. The association of regeneration density and diffuse light suggested a stronger association between spruce and diffuse light when compared to beech, which is in line with previous research (Stancioiu and O'Hara, 2006). Also, an increasing share of spruce and a decreasing share of beech in species mixture with increasing gap size confirms this difference in light requirement. In contrast to diffuse light, direct light indicated, in all models where it was included, a negative relation with the characteristics of spruce, beech and rowan seedlings. Also, Brang (1998) and Streit et al. (2009) reported the negative impact of direct light on spruce regeneration on a southern slope and slightly higher altitudes in the Swiss Alps. In all models for beech regeneration, the distance to seed trees was included. Regarding the second group of factors, which are non-related to gap characteristics, spruce was more frequent in the vicinity of elevated areas near stumps, while piles of branches decreased its density. Tall beech seedlings were more frequent on concave microsites. Hunziker and Brang (2005) have also reported a negative association between spruce regeneration and concave microrelief. The association between seedling features and slope inclination was mostly negative, which has often been reported in studies from the Alps (Brang, 1998; Baier et al., 2007). The height of both species indicated a positive relation to the soil depth. Many researchers have confirmed the beneficial effect of tree residues on the natural regeneration of spruce and fir, which is more apparent on extreme sites (Moser, 1965; Szewczyk and Szwagrzyk, 1996; Hunziker and Brang, 2005). Woody debris was rare on this study site, covering about 5% of the area. This was due to occasional sanitary felling for prevention of bark beetle infestation. Still, spruce seedling density was positively associated with coverage of tree stumps and sharply increased with increasing decay rates. Browsing damage to seedlings in the 20–50 cm height class was 40% and 62% for spruce and beech, respectively, which is significantly higher than the maximum values that enable successful recruitment (Eiberle and Nigg, 1987). Fir and rowan were almost totally browsed. Such conditions are common throughout the Alps, although both species have important functions in mountain mixed forests (Ott et al., 1997). The few surviving rowan seedlings exhibited relations with ecological factors similar to that of spruce. The overbrowsing by ungulates was also indicated by the regeneration height structure. Namely, we expected faster height recruitment of beech below the closed stand and in small gaps due to its shade tolerance. This problem may have been compounded by the slow growth on the study site, which exposed regeneration to browsing for a longer time period. Also, within the third group of factors that are related to both of the previously discussed factor groups, there were many differences

5. Conclusions The results of this study indicated that spruce and beech seedlings may successfully establish under slightly open forest canopies (i.e. small to medium gaps) where seed rain is the strongest. However, in less than a decade, spruce requires diffuse light levels comparable to that in the medium sized gaps in this study (0.15 ha) or higher. Therefore, successive extension of gaps up to ca. 0.5 ha is needed. This study addressed only one large gap that was comparable to the rest of the sampling area in terms of forest site, yet caution should be exercised in generalizing from medium to large gaps. Due to the negative association between direct light and spruce regeneration and susceptibility of beech to drought, irregular elliptical gaps with their long axis oriented E-W may be more appropriate than circular or S-E oriented gaps. For the same reasons, the north direction should be avoided for the successive extension of gaps. For favouring both species, a combination of different cutting regimes would be appropriate. For example, scattered medium sized and larger gaps for regeneration of spruce and the gradual creation and extension of smaller gaps for regeneration of beech. This is in line with the free-style silviculture practised in Slovenia. However, effective gradual conversion of spruce monocultures is possible only with a significant decrease in game density, which would enable the recruitment of fir, rowan and other deciduous trees. In addition, all seed trees of species other than spruce should be preserved. While the adaptation of the cutting regime to gap partitioning seems feasible, the adaptation of the felling regime to microsite partitioning is harder to achieve in practice, although the composition of advance 8

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regeneration can provide advice. In addition, it also seems efficient to leave several damaged trees during sanitary fellings as a seed bed and to enrich organic matter and to preserve some overturned root plates of fallen trees.

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