Effects of gap size and associated changes in light and soil moisture on the understorey vegetation of a Hungarian beech forest

In European beech forests windstorms often create canopy gaps and change the level of incident light, soil moisture and nutrient availability on the forest floor. Understanding the interrelations between gap size and environmental change, and the effects these have on regeneration processes is a prerequisite for developing techniques of nature-based forestry. The aims of this study were to investigate the effects of gap size on the resulting spatial distributions of key abiotic environmental variables (light and soil moisture) in gaps, and to study how light and soil moisture affect the abundance and distribution of herb layer species. To do this we used eight artificially created gaps – three large (diameter: 35–40 m) and five small (diameter: 10–15 m) – in a mesotrophic submontane beech forest. Data on species’ importance and substrate types were collected in systematically distributed 1 m×1 m quadrats before gap creation and on four occasions during the next two growing seasons. Hemispherical photographs were taken and analysed to estimate relative light intensity. Soil moisture was measured by frequency domain and capacitance probes. It was found that gap size had a profound effect on the environmental variables measured. While relative light intensity values in small gaps did not reach those in large gaps, soil moisture levels did reach similar maximum values in gap centres regardless of gap size. Richness, composition and total cover of herbaceous vegetation were different in small versus large gaps. Much of this difference was attributed to the presence of specific relative light intensities and also to the increased amount of available soil moisture in gaps. Species were differently affected by the combined effects of light and soil moisture, as well as by differences in available substrates. All this resulted in species-specific distribution patterns within gaps.     

The gap theory in forest dynamics

Since the late 1970s, ecologists interested in forest dynamics have focused their attention to the responses of individuals, populations and communities to “gaps” which are openings created in the forest canopy. This review intended to introduce some collective knowledge on major subjects of the gap theory in forest dynamics, in relation to gap-disturbance regimes, tree regeneration behaviour and community structure.     

Factors influencing sapling composition in canopy gaps of a temperate deciduous forest

To detect the factors that affect sapling species composition in gaps, we investigated 55 gaps in an old-growth temperate deciduous forest in Ogawa Forest Reserve, central Japan. Gap size, gap age, gap maker species, topographic location, adult tree composition around gaps, and saplings of tree species growing in the gaps were censused. For gaps 5 m², mean gap size was 70 m² and the maximum was 330 m². Estimated ages of gaps had a tendency to be concentrated in particular periods relating to strong wind records in the past. The sapling composition in gaps was highly and significantly correlated to that under closed canopy, indicating the importance of advance regeneration in this forest. However, some species showed significant occurrence biases in gaps or under closed canopy, suggesting differences in shade tolerance. The result of MANOVA showed that gap size and topography were important factors in determining the sapling composition in gaps. Species of gap makers affected the sapling composition indirectly by influencing gap size. The existence of parent trees around gaps had effects on sapling densities of several species. Gap age did not have clear influences on sapling composition. Variations in gap size and topography were considered as important factors that contribute to maintenance of species diversity in this forest.     

Vegetation dynamics: patterns in time and space

This paper introduces the collection of contributions in this special volume on temporal and spatial patterns of vegetation dynamics. First, it is pointed out that the dynamics of any piece of vegetation, large or small, is always dependent on the degree of isolation of that piece towards its environment. Then ten types of island situation are treated ranging from very much to very little isolated: remote species-rich oceanic islands, remote species-poor islands, young big islands near a continent, small off-shore islands, emerging islands, isolated hills, landscape islands, isolated patches of vegetation, and gaps in stands of vegetation.Also, eight forms of vegetation dynamics are treated, ranging from short-term to long-term changes and involving larger and larger units: individuals, patches, communities, landscapes and vegetation regions. The forms of dynamics are fluctuation, gap dynamics, patch dynamics, cyclic succession, regeneration succession, secondary succession, primary succession, and secular succession. Each form of dynamics may occur under varying degrees of isolation.The general conclusion is that processes and patterns of vegetation dynamics cannot be generalized in any simple manner. The 20 papers collected in this volume, divergent as they are, express the complexity of vegetation dynamics.     

Canopy structure and vertical patterns of photosynthesis and related leaf traits in a deciduous forest

Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A _max per unit area (LMA, N/area and A _max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A _max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A _max/area among different canopy positions (r²=0.81 and r²=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.     

Seed bank and vegetation composition of forest stands of varying age in central Belgium: consequences for regeneration of ancient forest vegetation

Vegetation composition differs significantly between ancientand recent forest, due to slow colonization capacity of typical forest speciesand the higher abundance of early successional species in recent forest.However, little is known about differences in persistent seed bank compositionbetween ancient and recent forest and about the interaction between seed bankand vegetation in relation with forest age. We surveyed the seed bank and theunderstorey vegetationcomposition in transects from ancient to recent forest. Seed bank and fieldlayer vegetation characteristics and similarity between seed bank andvegetationwere analysed in relation to recent forest age and distance to the ancientforest. A total of 39 species and 14,911 seedlings germinated, whichcorresponds with a seed density of 12,426 seeds/m².Total seed density is significantly higher in the youngest recent forest parcel(55 years). Also the seed bank composition in the youngest forest parceldifferssignificantly from the other parcels. After a longer period of reforestation,the seed bank approaches that of the ancient forest, suggesting seed bankdepletion, although the seed bank is permanently replenished to some extent byseed bank forming species from local disturbances. Seed bank composition doesnot change significantly with distance to the ancient forest. Similaritybetween seed bank and vegetation composition, nomatter the forest age, is very low, but decreases with increasing forest age.The most frequent species in the vegetation are absent in the seed bank andvice versa. The contribution of forest species is highin the vegetation and they almost not occur in the seed bank, while species offorest edges and clearings, and species of disturbed environments are morefrequent in the seed bank. The seed bank is mainly composed of earlysuccessional species of former forest stages or species which temporary occurinsmall-scale disturbances. The seed bank may enhance the negative effects ofearlysuccessional, mainly competitive species to the forest species richness in therecent forest. In this respect, forest management should minimise forestdisturbances, to prevent germination of competitive species form buriedseeds.     

Simulating forest succession along ecological gradients in southern Central Europe

Forest stand development was simulated using a forest succession model of the JABOWA/FORET type. The environmental conditions are representative for a wide spectrum of Swiss forest sites ranging from 220 m to 1 700 m a.s.l. Each model run covers a period of 1 200 yr and is based on the averaged successional characteristics of 50 forest plots with an individual size of 1/12 ha. These small forest plots serve as basic units to simulate establishment, growth, and death of individual trees of 29 species. Existing light in the forest stand, climatic conditions, soil properties, and other environmental factors control the growth of each individual tree. Compared with previous simulation studies, some major modifications were made, including the incorporation of the indicator values of Ellenberg (1979) to describe the ecophysiological behaviour of the species considered. As a test, the simulated species composition through time was compared with the actual vegetation and the potentially natural species composition on the corresponding site types. The extensive comparison revealed that approximately 80% of the simulations match the expected species configurations. Thus, it was concluded that the model is valid for the purpose of evaluating impacts of natural and human disturbances on forest communities.     

Gap formation and species diversity in Japanese beech forests: a test of the intermediate disturbance hypothesis on a geographic scale

To evaluate whether the intermediate-disturbance hypothesis applies on regional scales, the relationship between the species diversity and gap formation regime of beech forests was examined. The mean gap size and the variation of gap sizes showed no correlation with species diversity. The mean windstorm interval varied widely, but geographical trends, such as latitudinal gradient, were not observed. However, locations that sustained an intermediate frequency of disturbance had the highest species diversity. Although a latitudinal gradient of disturbance was not apparent, the intermediate-disturbance hypothesis was partly supported on a geographic scale. The most predictable model for species diversity was a multiple regression model composed of two factors, the windstorm interval and the cumulative temperature of the growing season. The fact that the temperature was of greater importance than the disturbance interval indicates that the most important factor in predicting forest species diversity is the amount of available energy on a geographic scale.     

Spatial scale and sampling resolution affect measures of gap disturbance in a lowland tropical forest: implications for understanding forest regeneration and carbon storage

Treefall gaps play an important role in tropical forest dynamics and in determining above-ground biomass (AGB). However, our understanding of gap disturbance regimes is largely based either on surveys of forest plots that are small relative to spatial variation in gap disturbance, or on satellite imagery, which cannot accurately detect small gaps. We used high-resolution light detection and ranging data from a 1500 ha forest in Panama to: (i) determine how gap disturbance parameters are influenced by study area size, and the criteria used to define gaps; and (ii) to evaluate how accurately previous ground-based canopy height sampling can determine the size and location of gaps. We found that plot-scale disturbance parameters frequently differed significantly from those measured at the landscape-level, and that canopy height thresholds used to define gaps strongly influenced the gap-size distribution, an important metric influencing AGB. Furthermore, simulated ground surveys of canopy height frequently misrepresented the true location of gaps, which may affect conclusions about how relatively small canopy gaps affect successional processes and contribute to the maintenance of diversity. Across site comparisons need to consider how gap definition, scale and spatial resolution affect characterizations of gap disturbance, and its inferred importance for carbon storage and community composition.     

长白山自然保护区阔叶红松林林隙更新的研究

通过对林隙及非林隙林分组成树种数量特征的对比分析,研究了长白山自然保护区阔叶红松林中主要树种对林隙的更新反应特点,阐述了林隙在阔叶红松林结构与多样性维持中的作用．随着林隙与非林隙的交替变化,红松和阔叶树以及主林层和中下层树种的相对优势（或重要性）亦呈现出交替变化的规律．林隙提高了阔叶红松林的物种丰富度,增加了其多样性,为不同特性物种的共存提供了可能,从而保持了阔叶红松林的整体稳定性．     

Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance

Effects of annual additions of mineral N and P (100 kg ha^–1) on plant species composition and annual aboveground net primary production (ANPP) were investigated during the first three years following disturbance in a semi-arid ecosystem. Additions of N reduced richness of perennial plant species during years 2 and 3, while P reduced the number of perennial species only in year 3. From year 1 to year 2, annual and biennial species richness declined in all treatments while ANPP of annual species increased greatly. Added N increased ANPP of annual species while it decreased ANPP of most perennial species relative to the unfertilized control treatment. Community similarities were higher for the control and native vegetation than for other pairs of treatments using both species presence and plant production data. Nitrogen additions have retarded but not completely arrested secondary succession in this system.     

Population dynamics of coniferous and broad-leaved trees in a Japanese temperate mixed forest

The growth and survival of coniferous and broad-leaved trees were followed over a 5-yr period in a temperate old-growth mixed forest in Japan, and dynamic features of the forest were studied in relation to the life history of the dominants, the coniferous Abies homolepis and the broad-leaved Fagus crenata. During this period, the gap formation rate was 31m² ha^?1yr^?1, the mortality of trees > 2m high was 1.7%/yr, and the rate of loss in basal area 1.4%/yr. These values were much higher than the recruitment, 0.3%/yr, and the total growth of surviving and new trees, 0.6%/yr, owing to the inhibition of regeneration by understorey dwarf bamboo (Sasa borealis). A transition matrix model based on DBH size classes predicts that the basal area of the forest will decrease by 14% in 50 yr, but that the DBH distribution of trees > 10 cm diameter will change little. Equilibrium DBH distributions assuming recruitment being equal to mortality, were quite different between broad-leaved and coniferous trees, reflecting different survivorship curves of the two dominants. The composition and structure of the forest may change depending on the pattern and frequency of disturbances, or episodic events, notably the synchronous death of Sasa borealis.     

Resprouters vs reseeders in South African forest trees; a model based on forest canopy height

1. We expect tree species that regenerate primarily by sprouting to produce fewer seedlings than co-occurring species that regenerate mainly from seedlings, because of the trade-off between allocating resources either to ensuring vegetative reproduction (e.g. protective bark/latent buds) or to sexual reproduction (e.g. seeds).
2. Furthermore, resprouting species, because of their multi-stemmed nature, should be at a relative disadvantage, and therefore relatively infrequent, in tall forests. This is because a resprouting individual allocates resources to a number of basal branches/stems and buds rather than maximizing vertical extension of a single leader, as is the case in a seeder. Also, many tall stems arising from the same multi-stemmed base, as is the case in resprouters, will be relatively poorly supported in comparison to the single stem of a reseeder.
3. To test these two ideas we surveyed a number of plots in a range of South African forests and thicket communities. We noted the numbers of seedlings and resprouts for each species and determined a mean for each site.
4. Short forests and thickets were dominated by multi-stemmed species and there was an almost total absence of seedlings. In contrast, tall forests were dominated by single-stemmed reseeding species and were accompanied by seedlings.     

Wind disturbance in remnant forest stands along the prairie-forest ecotone,Minnesota, USA

Strong winds are an important disturbance agent in northern Minnesota forests. On June 19, 1994, strong winds (>160 km h^(-1)) associated with a tornado damaged forested areas within the Rydell National Wildlife Refuge, situated in Polk County Minnesota along the prairie-forest boundary. Field sampling was conducted immediately following the storm to quantify the type and extent of damage in four different community types, and to project future composition based on the nature of the storm damage and current understory characteristics, including the impact of overbrowsing by deer. Basal area in six sampled remnant forest stands was reduced by 33.5%, although the damage was heterogeneous; basal area in one stand was reduced by 68.1%. The overall effect of the storm was the removal of early- successional species (primarily Populus tremuloides) in larger size classes. Trees situated at stand edges were not more susceptible to snapping or uprooting than interior trees. Projections of future stand composition indicate that wind disturbance, unlike other agents of disturbance such as fire, may accelerate succession on the Refuge, such that early-successional stands will assume a later-successional character, while Acer-Tilia stands should maintain their late-successional character. Overbrowsing and preferential foraging by deer may significantly alter stand recovery patterns.     

The response of forest floor vegetation and tree regeneration to deer exclusion and disturbance in a riparian deciduous forest,central Japan

The response of forest floor vegetation and regeneration of major treespecies to deer exclusion in a riparian forest were studied for 3 years withtheinteractive effects of natural disturbances. At the start of this study, deerdensity had quickly increased to a fairly high level (29–31 individualskm^–2) during the last decade and had severely reduced theamount of forest floor vegetation in the study area. Dwarf bamboos, which weredominant before, had almost diminished from the forest floor. During the studyperiod, aboveground biomass increased steadily but species diversity did notchange much in the exclosures. Outside the exclosures, the seedlings of alltreespecies were damaged greatly by deer browsing, especially the taller ones. Deerbrowsing had little effect on the emergence of tree seedlings, but deertrampling might have accelerated emergence indirectly by disturbing the soilsurface for some species. Differences in plant responses were also observed fordeer browsing and the presence of dwarf bamboo that strongly inhibits therecruitment of tree seedlings. The nine major species were classified intothreegroups according to the response of seedlings to deer browsing and disturbance.Group 1 consists of the species whose seedling survival is affected bybrowsing,but seldom by disturbances (Phellodendron amurense,Kalopanax pictus, Quercus crispulaandMalus toringo). Groups 2 and 3 consist of species adaptedto tree-fall disturbance (Betula spp.) and ripariandisturbance (Alnus hirsuta var.sibirica, Ulmus davidiana var.japonica, Populus maximowiczii andSalix sachalinensis), respectively, and seedling survivalof these two groups is principally affected by light conditions. The effect ofdeer browsing on seedling survival and growth was greater for Group 2 than forGroup 3.     

Gap regeneration of major tree species in different forest types of Japan

Gap regeneration of major tree species was examined, based on the pattern of gap phase replacement, in primary old-growth stands of warm-temperate, cool-temperate and subalpine forests, Japan. Using principal component analysis, the gap-regeneration behavior of major tree species could be divided into three guilds and that of Fagus crenata (monodominant species of cool-temperate forests). The criteria used for this division were total abundance of canopy trees and regenerations and relative abundance of regenerations to canopy trees. The gap-regeneration behavior of species in the first guild was that canopy trees regenerate in gaps from seedlings or saplings recruited before gap formation; they had higher total abundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of F. crenata was same as species in the first guild, but F. crenata had less abundant regenerations relative to its canopy trees. Species in the second guild had lower total abundance and less abundant regenerations to their canopy trees. The guild contained species whose canopy trees regenerate in gaps from seedlings or saplings recruited after gap formation or regenerate following largescale disturbance. The third guild consisted of species with lower total aboundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of some species in this guild was that trees regenerate in gaps from seedlings or saplings recruited before gap formation, and grow, mature, and die without reaching the canopy layer, while the gap-regeneration behavior of other species was same as that of species in the first guild or F. crenata. Major tree species of subalpine forests were not present in the third guild.     

Hurricane disturbance in a temperate deciduous forest: patch dynamics, tree mortality, and coarse woody detritus

Patch dynamics, tree injury and mortality, and coarse woody detritus were quantified to examine the ecological impacts of Hurricane Fran on an oak-hickory-pine forest near Chapel Hill, NC. Data from long-term vegetation plots (1990–1997) and aerial photographs (1998) indicated that this 1996 storm caused patchy disturbance of intermediate severity (10–50% tree mortality; Woods, J Ecol 92:464–476, 2004). The area in large disturbance patches (>0.1 ha) increased from <1% to approximately 4% of the forested landscape. Of the forty-two 0.1-ha plots that were studied, 23 were damaged by the storm and lost 1–66% of their original live basal area. Although the remaining 19 plots gained basal area (1–15% increase), across all 42 stands basal area decreased by 17% because of storm impacts. Overall mortality of trees >10 cm dbh was 18%. The basal area of standing dead trees after the storm was 0.9 m²/ha, which was not substantially different from the original value of 0.7 m²/ha. In contrast, the volume and mass of fallen dead trees after the storm (129 m³/ha; 55 Mg/ha) were 6.1 and 7.9 times greater than the original levels (21 m³/ha; 7 Mg/ha), respectively. Uprooting was the most frequent type of damage, and it increased with tree size. However, two other forms of injury, severe canopy breakage and toppling by other trees, decreased with increasing tree size. Two dominant oak species of intermediate shade-tolerance suffered the largest losses in basal area (30–41% lost). Before the storm they comprised almost half of the total basal area in a forest of 13% shade-tolerant, 69% intermediate, and 18% shade-intolerant trees. Recovery is expected to differ with respect to vegetation (e.g., species composition and diversity) and ecosystem properties (e.g., biomass, detritus mass, and carbon balance). Vegetation may not revert to its former composition; however, reversion of biomass, detritus mass, and carbon balance to pre-storm conditions is projected to occur within a few decades. For example, the net change in ecosystem carbon balance may initially be negative from losses to decomposition, but it is expected to be positive within a decade after the storm. Repeated intermediate-disturbance events of this nature would likely have cumulative effects, particularly on vegetation properties.