Species response curves along environmental gradients. A case study from SE Norwegian swamp forests

Abstract. Vegetation science has relied on untested paradigms relating to the shape of species response curves along environmental gradients. To advance in this field, we used the HOF approach to model response curves for 112 plant species along six environmental gradients and three ecoclines (as represented by DCA ordination axes) in SE Norwegian swamp forests. Response curve properties were summarized in three binary response variables: (1) model unimodal or monotonous (determinate) vs. indeterminate; (2) for determinate models, unimodal vs. monotonous and (3) for unimodal models, skewed vs. symmetric. We used logistic regression to test the influence, singly and jointly, of seven predictor variables on each of three response variables. Predictor variables included gradient type (environmental or ecocline) and length (compositional turnover); species category (vascular plant, moss, Sphagnum or hepatic), species frequency and richness, tolerance (the fraction of the gradient along which the species occurs) and position of species along each gradient. The probability for fitting a determinate model increased as the main occurrence of species approached gradient extremes and with increasing species tolerance and frequency and gradient length. Appearance of unimodal models was favoured by low species tolerance and disfavoured by closeness of species to gradient extremes. Appearance of skewed models was weakly related to predictors but was slightly favoured by species optima near gradient extremes. Contrary to the results of previous studies, species category, gradient type and variation in species richness along gradients did not contribute independently to model prediction. The overall best predictors of response curve shape were position along the gradient (relative to extremes) and tolerance; the latter also expressing gradient length in units of compositional turnover. This helps predicting species responses to gradients from gradient specific species properties. The low proportion of skewed response curves and the large variation of species response curves along all gradients indicate that skewed response curves is a smaller problem for the performance of ordination methods than often claimed. We find no evidence that DCA ordination increases the unimodality, or symmetry, of species response curves more than expected from the higher compositional turnover along ordination axes. Thus ordination axes may be appropriate proxies for ecoclines, applicable for use in species response modelling.     

Patterns of density and structure of natural populations of Taxus baccata in the Hyrcanian forests of Iran

Iran's Hyrcanian forests cover a relatively narrow strip in the northeastern part of the country, and are among the most important and valuable ecosystems inscribed in United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage List. European yew Taxus baccata L. is a Tertiary relict in the region and a long-lived dioecious tree with high ecological and economic importance in the Hyrcanian forests. To study the structure and analysing the survivorship of yew stands, we selected two forest reserves (Gazou and Afratakhteh) with European yew. In the two study areas, we established 165 0.1 ha circular sample plots (75 of the sample plots were in Gazou and 90 sample plots were in Afratakhteh) and measured three characteristics of each tree. The structure of the stands was quantified by means of 1) size distributions by diameter at breast height (DBH), 2) height classes, 3) and stand basal area. Comparison of the diameter distribution by DBH classes in two the forest reserves showed that there was no statistical difference between the two populations. The highest number of yew trees in the Afratakhteh and Gazou populations were in the 12–16 and 18–24 m height classes, respectively. In Gazou, abundance was greatest at 1000 m a.s.l. on mesic exposures and intermediate slopes (40–50%), whereas in Afratakhteh it was found at 1600–1620 m a.s.l. on east-facing areas with 55–65% slopes. Static life tables indicated that the first two age classes have particularily high mortality. Based on these results, we conclude that forest managers should support the regeneration of stands and increase survival rates by applying treatments such as thinning and selective removal of shrubs and saplings of other tree species.     

Freshwater species distributions along thermal gradients

The distribution of a species along a thermal gradient is commonly approximated by a unimodal response curve, with a characteristic single optimum near the temperature where a species is most likely to be found, and a decreasing probability of occurrence away from the optimum. We aimed at identifying thermal response curves (TRCs) of European freshwater species and evaluating the potential impact of climate warming across species, taxonomic groups, and latitude. We first applied generalized additive models using catchment‐scale global data on distribution ranges of 577 freshwater species native to Europe and four different temperature variables (the current annual mean air/water temperature and the maximum air/water temperature of the warmest month) to describe species TRCs. We then classified TRCs into one of eight curve types and identified spatial patterns in thermal responses. Finally, we integrated empirical TRCs and the projected geographic distribution of climate warming to evaluate the effect of rising temperatures on species’ distributions. For the different temperature variables, 390–463 of 577 species (67.6%–80.2%) were characterized by a unimodal TRC. The number of species with a unimodal TRC decreased from central toward northern and southern Europe. Warming tolerance (WT = maximum temperature of occurrence—preferred temperature) was higher at higher latitudes. Preferred temperature of many species is already exceeded. Rising temperatures will affect most Mediterranean species. We demonstrated that freshwater species’ occurrence probabilities are most frequently unimodal. The impact of the global climate warming on species distributions is species and latitude dependent. Among the studied taxonomic groups, rising temperatures will be most detrimental to fish. Our findings support the efforts of catchment‐based freshwater management and conservation in the face of global warming.     

Population ecology of yew (<Emphasis Type="Italic">Taxus baccata</Emphasis> L.) in the Central Apennines: spatial patterns and their relevance for conservation strategies

Understanding the ecological mechanisms that allow a species to transition from an occasional understory component to the dominant type in the forest canopy is essential for predicting future shifts in the distribution of species. We investigated this issue with regard to yew, also because mature yew trees have been reported to inhibit self-regeneration and seedling survival, prompting concerns for the long-term preservation of the species. Our objectives were (a) to quantify spatial patterns of yew (Taxus baccata L.) populations near the southern limit of the species’ ecological distribution, (b) to determine the relationships between yew presence and topographic gradients, and (c) to answer the question of how yew regeneration is affected by such patterns and relationships. We analyzed three extensive yew populations (90–165 ha, including 3–12 thousand established individuals) that mostly occupy the understory of beech forests located in protected areas of the central Apennines (Italy). Overall, the realized niche of yew (either as established trees, saplings, or seedlings) followed the expected bell-shaped curve of a species response to an environmental gradient. Yew was mainly found at 1,000–1,600 m elevation on mesic exposures (north and west) and intermediate slopes (30–60%). Geostatistical analysis revealed that yew occurred in patches, as shown by variogram ranges of 40–110 m for yew tree basal area and regeneration abundance. Yew regeneration over the landscape was directly related to basal area of yew trees. At local scales (~10 m), presence of established trees favored regeneration in relatively less developed stands, whereas high density of mature yews suppressed regeneration. Healthy yew populations in beech forests had a minimum size of 0.5–3 ha. As yew density increased within these patches, regeneration dropped, so that yew conservation cannot be limited to presently occurring populations, despite the longevity and potential for vegetative reproduction of the species. Disturbance from grazing and wildfire was also found to impact yew survival. Long-term existence of yew in the Italian Apennines depends on maintaining and expanding old-growth beech forests that incorporate yew patches, and have a minimum continuous cover equivalent to a relatively undisturbed regime (10–50 ha).     

Plant species richness in continental southern Siberia: effects of pH and climate in the context of the species pool hypothesis

Aim Many high‐latitude floras contain more calcicole than calcifuge vascular plant species. The species pool hypothesis explains this pattern through an historical abundance of high‐pH soils in the Pleistocene and an associated opportunity for the evolutionary accumulation of calcicoles. To obtain insights into the history of calcicole/calcifuge patterns, we studied species richness–pH–climate relationships across a climatic gradient, which included cool and dry landscapes resembling the Pleistocene environments of northern Eurasia. Location Western Sayan Mountains, southern Siberia. Methods Vegetation and environmental variables were sampled at steppe, forest and tundra sites varying in climate and soil pH, which ranged from 3.7 to 8.6. Species richness was related to pH and other variables using linear models and regression trees. Results Species richness is higher in areas with warmer winters and at medium altitudes that are warmer than the mountains and wetter than the lowlands. In treeless vegetation, the species richness–pH relationship is unimodal. In tundra vegetation, which occurs on low‐pH soils, richness increases with pH, but it decreases in steppes, which have high‐pH soils. In forests, where soils are more acidic than in the open landscape, the species richness–pH relationship is monotonic positive. Most species occur on soils with a pH of 6–7. Main conclusions Soil pH in continental southern Siberia is strongly negatively correlated with precipitation, and species richness is determined by the opposite effects of these two variables. Species richness increases with pH until the soil is very dry. In dry soils, pH is high but species richness decreases due to drought stress. Thus, the species richness–pH relationship is unimodal in treeless vegetation. Trees do not grow on the driest soils, which results in a positive species richness–pH relationship in forests. If modern species richness resulted mainly from the species pool effects, it would suggest that historically common habitats had moderate precipitation and slightly acidic to neutral soils.     

Mollusc community patterns and species response curves along a mineral richness gradient: a case study in fens

Aim The goals of this study were to: (1) compare water conductivity and pH as proxy measures of mineral richness in relation to mollusc assemblages in fens, (2) examine the patterns of mollusc species richness along the gradient of mineral richness based on these factors, (3) model species–response curves and analyse calcicole–calcifuge behaviour of molluscs, and (4) compare the results with those from other studies concerning non‐marine mollusc ecology. Location Altogether, 135 treeless spring fen sites were sampled within the area of the Western Carpathians (east Czech Republic, north‐west Slovakia and south Poland; overall extent of study area was 12,000 km²). Methods Mollusc communities were recorded quantitatively from a homogeneous area of 16 m². Water conductivity and pH were measured in the field. The patterns of local species diversity along selected gradients, and species–response curves, were modelled using generalized linear models (GLM) and generalized additive models (GAM), both using the Poisson distribution. Results When the most acid sites (practically free of molluscs) were excluded, conductivity expressed the sites’ mineral richness and base saturation within the entire gradient, in contrast to pH. In the base‐rich sites, pH did not correlate with mineral richness. A unimodal response of local species diversity to mineral richness (expressed as conductivity) was found. In the extremely mineral‐rich, tufa‐forming sites (conductivity > 600 μS cm^?1) a decrease in species diversity was encountered. Response curves of the most common species showed clear differentiation of their niches. Significant models of either unimodal or monotonic form were fitted for 18 of the 30 species analysed. Species showed five types of calcicole–calcifuge behaviour: (1) a decreasing monotonic response curve and a preference for the really acid sites; (2) a skewed unimodal response curve with the optimum shifted towards the slightly acid sites; (3) a symmetrical unimodal model response curve with the optimum in the base‐rich sites, with no or slight tufa precipitation; (4) a skewed unimodal response curve but with the optimum shifted to the more mineral‐rich sites; and (5) an increasingly monotonic response curve, the optimum in the extremely base‐rich sites with strong tufa precipitation. Main conclusions Conductivity is the only reliable proxy measure of mineral richness across the entire gradient, within the confines of this study. This information is of great ecological significance in studies of fen mollusc communities. Species richness does not increase with increasing mineral richness along the entire gradient: only a few species are able to dwell in the extremely base‐rich sites. The five types of calcicole–calcifuge behaviour seen in species living in fens have a wider application: data published so far suggest they are also applicable to mollusc communities in other habitats.     

Plant species richness–productivity relationships in a low-productive boreal region

Local species richness–productivity (SR–P) relationship is usually reported as unimodal if long productivity gradients are sampled. However, it tends to be monotonically increasing in low-productive environments due to the decreasing part of the SR–P curve being truncated. Previous work indicated that this can hold true for forest herb layers, because of an upper bound on productivity caused mainly by canopy shading. Here, we ask whether the same pattern exists in a region with an upper bound on productivity caused by a harsh climate. We sampled herbaceous vegetation of boreal forests and grasslands in a low-productive region of central Yakutia (NE Siberia) with dry and winter-cool continental climate. We collected data on species composition, herb-layer productivity (aboveground herbaceous biomass), soil chemistry and light availability. We applied regression models to discriminate between monotonically increasing, decreasing and unimodal responses of herb-layer species richness to measured variables and analysed trends in the species-pool size and beta diversity along the productivity gradient. Our expectation of the monotonically increasing SR–P relationship was confirmed for neither forest herb layers nor grasslands. In the forest herb layers, no relationship was detected. In grasslands, the relationship was unimodal with species richness decline starting at much lower productivity levels than in more productive temperate grasslands. Potential causes for this decline are either limitation of local species richness by the species pool, which contains few species adapted to more productive habitats, or competitive exclusion, which can become an important control of species richness under lower levels of productivity than is the case in temperate grasslands.     

Trait-specific response of ground beetles (Coleoptera: Carabidae) to forest fragmentation in the temperate region in Korea

The objective of this study was to examine the trait-specific response of ground beetles in terms of abundance, species richness, and composition for habitat fragmentation in temperate forests. In addition, we examined how different ecological groups and species of ground beetles responded to environmental variables. During middle May to early November in 2013, ground beetles were sampled using pitfall traps in 27 sites (18 patches and 9 continuous forests) in central Korea. A total of 51 species were identified from 17,845 ground beetles in the 27 study sites. Continuous forests had generally higher abundance and species richness of ground beetles than forest patches. Canonical correspondence analysis for ground beetles was significant, and the proportion of variance explained by environmental variables, such as patch size, elevation, organic matter (C), and soil moisture and pH, was 43.61%. In addition, species composition of ground beetles in continuous forests was grouped distinctively away from those in medium and small-sized patches. In conclusion, both small (1.1–9.6 ha) and medium patches (12.8–51.2 ha) were failed to preserve similar ground beetle biodiversity compared to continuous forests. However, our study revealed that medium-sized forest patches clearly had higher conservation value for forest specialist ground beetles than small-sized forest patches irrespective of forest type.     

Little owl (Athene noctua) around human settlements and agricultural lands: Conservation and management enlightenments

Ecological studies have been focused on large, rare or endangered species but species which are adapted to traditional agroecosystems or rural environments received less attention. For example, Little owl is a typical inhabitant of agricultural lands and its population had greatly reduced due to the mechanized agriculture. In this paper, we built the first country level habitat suitability map for the little owl and identified the most influential environmental predictor of its distribution in Iran. We used species presence data (177 distribution points) and eight uncorrelated environmental variables to model the species suitable habitats using the maximum entropy approach (Maxent). Results showed that suitable habitats for this species are located in the northwest, parts of north east and southwest of Iran. Distance to forests, distance to human settlements and distance to agricultural lands are the most important determinants of the little owl distribution in Iran. Human migrations from rural areas to the cities and availability of many vacant human settlements surrounded by traditional and semi-modern agricultural landscapes produce suitable habitats for little owl in Iran.     

Growth phenological variations in the narrow-leaved ash (Fraxinus angustifolia) over the Mediterranean region: A simulation study

Tree species inhabiting riparian forests under Mediterranean climate have evolved to face summer water shortage but may fail to cope with future increases in drought severity. Thus, understanding tree growth phenological variations in response to environmental conditions is necessary to assess the impact of seasonal drought in riparian forests. In this study, we investigated the response of stem radial growth to climate in the narrow-leaved ash (Fraxinus angustifolia) over its distribution in southern Europe. We simulated intra- and inter-annual growth patterns using the Vaganov-Shashkin (VS) model considering five sites subjected to summer drought but showing different climate conditions. The growth pattern in this species varied from unimodal in cool-wet sites to facultative bimodal in warm-dry sites. Bimodal patterns were characterized by two growth peaks coinciding with favorable climate conditions in spring and autumn. The spring growth peak occurs earlier (May) in warm-dry sites than in wet-cool sites (June–July). The variation in the season growth length and growth timing suggests different strategies adopted by this species to cope with summer drought. The VS model revealed different growth patterns across which would be relevant in predicting the response of this and other riparian tree species to climate warming and aridification. Differences in the length of the growing season, timings of growth peaks and the shift from unimodal to bimodal growth patterns should be considered when assessing growth adjustments to future climate scenarios.     

Can we trust gradients extracted by Detrended Correspondence Analysis?

Abstract. In this study, data from dry temperate grassland vegetation in Denmark are used to compare the predictive power of gradients obtained by ordination. One of the problems of ordination methods based on weighted averaging, namely the assumption of symmetric, unimodal response curves, is investigated by smoothing species responses to simple and complex gradients respectively. It was found that species response types to a pH gradient are diverse and often deviate from a unimodal, symmetrical shape. Bimodal responses were also found, but they disappeared when more influential gradients were considered. Many species showed truncated responses with optima near gradient ends. In order to assess the impact of unrealistic response assumptions and reported problems with instability on the performance of DCA, the predictive potentials of measured variables and coenoclines extracted by DCA were compared. Despite field data violating the assumption of unimodal response, DCA was found to extract gradients predicting species abundance better than the environmental variables available. Shortcomings and advantages of indirect and direct methods in plant ecology are discussed.     

Distribution of twelve moist forest canopy tree species in Liberia and Côte d'Ivoire: response curves to a climatic gradient

Abstract. The occurrence and abundance of 12 canopy tree species from the moist tropical forests of West Africa have been studied in relation to a climatic gradient. We focused on environmental factors related to water availability: annual amount of rainfall, the length of the dry season, and cumulative water deficit. Species occurrence and abundance data are used for 39 forest sites in Liberia and southwest Côte d'Ivoire. Species responses are modelled using a set of five increasingly complex models, ranging from a no-trend model to a skewed bell-shaped response curve. The study species show different distribution patterns. Most of them suggest a close relationship to climatic conditions. Fitting of species occurrence data to each of the three climatic factors results in most cases in simple models. In only one out of 36 cases a bell-shaped response curve is needed to describe the data. Four of the 12 species show no response to the climatic factors when only occurrence is evaluated. When abundance data are used, in 33 of the 36 cases significant response models are found. In general these are much more complex than in the cases of species occurrence data: in 10 of the 36 cases a bell-shaped response model is found to describe the data best. This is in contrast with the widespread belief that species response curves are bell-shaped: within the forest zone in the area studied this is not generally the case. The importance of the three climatic factors for the distribution of the species is evaluated: for four species mean annual rainfall is the most important variable, for four species the length of the dry period, and for one species cumulative water deficit. Consequently, the assumption that mean annual rainfall is the most important factor determining tree species distribution in West African forests is not correct. Species response models to climatic factors show where species have their geographical optima. Implications for forest management are briefly discussed.     

穆棱东北红豆杉年轮-气候关系及其濒危机制

东北红豆杉是国家I级重点保护野生濒危植物,其径向生长与气候的关系对于探讨其种群濒危机制具有极其重要的意义。利用穆棱自然保护区内天然东北红豆杉树木年轮样本,建立了长300多年的树轮年表(1700—2005年)。分析径向生长特征发现东北红豆杉总体上呈波动式下降的生长方式,在1770年和1930年左右存在两个生长释放区,该地区调查的样本最大年龄达到370a左右。年表与气候变量之间相关分析表明,冬春季节最低气温、平均气温是树木生长的主要限制因子,其他气候因子也有一定的影响,但不起主要作用。1942年以后东北红豆杉年轮指数与Wilson重建的北半球气温变化吻合度降低,温度对东北红豆杉的径向生长限制性降低,这表明气候变暖可能有利于东北红豆杉的径向生长,但不足以说明气候变暖与东北红豆杉衰退的关系,其他气候因子的年轮气候响应分析也不能确定气候变化在东北红豆杉衰退中的主导作用。人为破坏可能是东北红豆杉衰退的主要因素,但气候变化在其中起到多大的贡献还有待深入研究。     

Patterns of tree species richness in Jalisco,Mexico: relation to topography,climate and forest structure

The objective of this study was to identify the major environmental variables and components of forest structure associated with variability in tree species richness on a network of 806 permanent plots in the State of Jalisco, Mexico. Tree data recorded on the sample plots were used to characterize tree species richness by forest type and climatic conditions (temperature and precipitation) in the State. Species composition and other diversity indices were also calculated. Explanatory variables identified in a Poisson regression identified forest cover type, elevation, tree basal area, canopy closure, and winter precipitation as being important to changes in tree species richness. An “extreme quantile curve estimation” approach was then used to approximate the boundary that represented the maximum potential species richness response to the various levels of important variables. Maximum tree species richness decreased with increasing elevation. The relationships between maximum species richness and tree basal area, canopy closure, and winter precipitation followed a hump-back unimodal model, with intermediate values supporting the largest species richness. We believe that results of the current study will contribute to further development of a conservation plan for tree species in the State of Jalisco, Mexico.     

Climatic Control on Forests and Tree Species Distribution in the Forest Region of Northeast China

North-east （NE） China covers considerable climatic gradients and all major forests types of NE Asia. in the present study, 10 major forest types across the forest region of NE China were sampled to Investigate forest distribution in relation to climate. Canonical correspondence analysis （CCA） revealed that growing season precipitation and energy availability were primary climatic factors for the overall forest pattern of NE China, accounting for 66% of the explanatory power of CCA. Conversely, annual precipitation and winter coldness had minor effects. Generalized additive models revealed that tree species responded to climatic gradients differently and showed three types of response curve： （i） monotonous decline; （ii） monotonous Increase; and （iii） a unimodai pattern. Furthermore, tree species showed remarkable differences in limiting climatic factors for their distribution. The power of climate in explaining species distribution declined significantly with decreasing species dominance, suggesting that the distribution of dominant species was primarily controlled by climate, whereas that of subordinate species was more affected by competition from other species.     

Environmental controls of diatom species in northeast Pacific sediments

Non-linear, unimodal techniques are suitable for estimation of environmental properties in the northeast (NE) Pacific based on fossil diatom taxa (species and/or species groups) found in modern (core-top) sediments. Canonical Correspondence Analysis (CCA), a constrained ordination technique, discerned best-fit relationships among two multivariate datasets (the floral and the environmental) and thus yielded insight into the environmental variables that best explain the species variance within diatom populations. Based on these insights, we developed predictive functions for annual Primary Productivity (PP) and seasonal range of sea Sea-Surface Temperature using unimodal models and cross validation techniques. Estimates of annual PP (r²_jack = 0.92; RMSEP = 91.94 gC/m²/y) explained the highest percentage of variance in the core-top diatom record (22.1%).     

The richness–heterogeneity relationship differs between heterogeneity measures within and among habitats

The positive monotonic relationship between habitat heterogeneity and species richness is a cornerstone of ecology. Recently, it was suggested that this relationship should be unimodal rather than monotonic due to a tradeoff between environmental heterogeneity and population sizes, which increases local species extinctions at high heterogeneity levels. Here, we studied the richness–heterogeneity relationship for an avian community using two different environmental variables, foliage‐height diversity and cover type diversity. We analyzed the richness–heterogeneity within different habitat types (grasslands, savannas, or woodlands) and at the landscape scale. We found strong evidence that both positive and unimodal relationships exist at the landscape scale. Within habitats we found positive relationships between richness and heterogeneity in grasslands and woodlands, and unimodal relationships in savannas. We suggest that the length of the environmental heterogeneity gradient (which is affected by both spatial scale and the environmental variable being analyzed) affects the type of the richness–heterogeneity relationship. We conclude that the type of the relationship between species richness and environmental heterogeneity is non‐ubiquitous, and varies both within and among habitats and environmental variables.     

Comparison of vascular plant diversity and species composition of coppice and high beech forest in the Banat region,Romania

Abandonment of coppicing is one of the main reasons of diversity decline in European temperate forests. To reverse this trend, coppicing has been reintroduced in several forests, especially in areas of high conservation value. However, empirical information on the effects of coppicing on plant diversity and composition is still scarce. By comparing vegetation data from active coppices and beech-dominated high forests in the Banat region in Romania, we tested the hypothesis that coppices have a higher plant diversity and different plant species composition than high forests. Data were collected in 60 randomly placed phytosociological relevés and were analysed using linear models and multivariate methods. As expected, we found differences in understorey plant composition. Herb species with various environmental demands–sciophytes-(hemi)-heliophytes and grassland species–were more frequent in coppices whereas high forests had more vernal species and sciophytes. Coppices also had slightly greater plant diversity but did not differ in herb species richness. Our results demonstrate that coppicing has a small positive effect on plant diversity but a large effect on species composition. Coppice restoration may be especially beneficial for populations of thermophilous and non-forest species.     

Drivers of plant species composition in alder-dominated forests with contrasting connectivity

The effects of local and regional environmental variables as well as spatial gradients on the plant species composition of two types of alder-dominated forests (riparian forests and alder carrs) with contrasting connectivity were studied across the Western Carpathians from Hungary through Slovakia to Poland. We used large vegetation (240 sampling plots) and environmental (24 variables) datasets, which were accompanied by spatial variables represented by principal coordinates of neighbour matrices. Canonical correspondence analysis (CCA) of the two datasets revealed 13 and 29 variables with significant effects on variation in species composition of alder carrs and riparian alder forests, which jointly explained 41.2% and 36.4% of the variability, respectively. Altitude was the most important factor explaining 7.7% of the variability in the species composition of alder carrs and 8.2% in riparian alder forests. Variation partitioning in CCA revealed that local variables were crucial drivers for species composition patterns in alder carrs, while spatial processes unrelated to the measured environmental variables shaped the vegetation structure of riparian forests.

     

The species richness–productivity relationship in the herb layer of European deciduous forests

Aim In contrast to non‐forest vegetation, the species richness–productivity (SR‐P) relationship in forests still remains insufficiently explored. Several studies have focused on the diversity of the tree layer, but the species richness of temperate deciduous forests is mainly determined by their species‐rich herb layer. The factors controlling herb‐layer productivity may differ from those affecting tree layers or open herbaceous vegetation, and thus the SR‐P relationship and its underlying processes may differ. However, the few relevant studies have reported controversial results. Here we explore the SR‐P relationship in the forest herb layer across different areas from oceanic to continental Europe, and put the effect of habitat productivity on species richness into context with other key factors, namely soil pH and light availability. Location North‐western Germany, Czech Republic, Slovakia and southern Urals (Russia). Methods We measured herb‐layer species richness and biomass, soil pH and tree‐layer cover in 156 vegetation plots of 100 m² in deciduous forests. We analysed the SR‐P relationship and the relative importance of environmental variables using regression models for particular areas and separate forest types. Results We found a consistent monotonic increase in the herb‐layer species richness with productivity across all study areas and all forest types. Soil pH and light availability also affected species richness, but their relative importance differed among areas. Main conclusions We suggest that the monotonically increasing SR‐P relationship in the forest herb layer results from the fact that herb‐layer productivity is limited by canopy shading; competition within the herb layer is therefore not strong enough to exclude many species. This differs fundamentally from open herbaceous vegetation, which is not subject to such productivity limits and consequently exhibits a unimodal SR‐P relationship. We present a conceptual model that might explain the differences in the SR‐P relationship between the forest herb layer and open herbaceous vegetation.