Martin Diekmann

Species indicator values as an important tool in applied plant ecology – a review

Abstract Species indicator values, especially those defined by Ellenberg, have been used widely in applied plant ecology, forestry and agriculture. In spite of being criticised by many, there is a growing interest in using them also outside central Europe in order to analyse trends of change in the vegetation and their underlying environmental variables. Various aspects of indicator values are reviewed: their main features, the most important applications (notably the use of weighted site averages), problems and pitfalls of indicator analyses, the relationship between weighted site averages and measurements as well as their extension to other regions. Some aspects are illustrated using data sets of forest plots from S Sweden. Provided that the limitations of indicator values are recognised, these have a high reliability and can complement or, in some cases, replace measurements to determine the values of environmental variables and to monitor their change. Zeigerwerte, besonders die von Ellenberg vorgeschlagenen Zahlen, haben in der Pflanzenokologie, Forst- und Landwirtschaft eine breite Verwendung gefunden. Obwohl viele Wissenschaftler Zeigerwerten kritisch gegenuberstehen, werden sie in zunehmendem Mase auch auserhalb Mitteleuropas benutzt, um Veranderungen der Vegetation und der sie bedingenden Umweltfaktoren zu analysieren. Verschiedene Aspekte von Zeigerwerten werden erortert: ihre wichtigsten Merkmale und Anwendungen (insbesondere die Verwendung mittlerer Zeigerwerte), Probleme und Fehlerquellen, die Beziehung zwischen mittleren Zeigerwerten und Messwerten sowie die Uberprufung und Kalibrierung in Regionen auserhalb Mitteleuropas. Einige dieser Aspekte werden anhand von Datensatzen von Waldern Sud-Schwedens illustriert. Zeigerwerte haben, wenn ihre Begrenzungen berucksichtigt werden, eine hohe Zuverlassigkeit und konnen im Hinblick auf das Monitoring von Umweltveranderungen Messungen erganzen oder in manchen Fallen sogar ersetzen.

Nitrogen deposition threatens species richness of grasslands across Europe.

Evidence from an international survey in the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is reducing plant species richness in acid grasslands. Across the deposition gradient in this region (2-44 kg N ha(-1) yr(-1)) species richness showed a curvilinear response, with greatest reductions in species richness when deposition increased from low levels. This has important implications for conservation policies, suggesting that to protect the most sensitive grasslands resources should be focussed where deposition is currently low. Soil pH is also an important driver of species richness indicating that the acidifying effect of nitrogen deposition may be contributing to species richness reductions. The results of this survey suggest that the impacts of nitrogen deposition can be observed over a large geographical range.

Latitudinal gradients as natural laboratories to infer species' responses to temperature

Macroclimatic variation along latitudinal gradients provides an excellent natural laboratory to investigate the role of temperature and the potential impacts of climate warming on terrestrial organisms. Here, we review the use of latitudinal gradients for ecological climate change research, in comparison with altitudinal gradients and experimental warming, and illustrate their use and caveats with a meta-analysis of latitudinal intraspecific variation in important life-history traits of vascular plants. We first provide an overview of latitudinal patterns in temperature and other abiotic and biotic environmental variables in terrestrial ecosystems. We then assess the latitudinal intraspecific variation present in five key life-history traits [plant height, specific leaf area (SLA), foliar nitrogen:phosphorus (N:P) stoichiometry, seed mass and root:shoot (R:S) ratio] in natural populations or common garden experiments across a total of 98 plant species. Intraspecific leaf N:P ratio and seed mass significantly decreased with latitude in natural populations. Conversely, the plant height decreased and SLA increased significantly with latitude of population origin in common garden experiments. However, less than a third of the investigated latitudinal transect studies also formally disentangled the effects of temperature from other environmental drivers which potentially hampers the translation from latitudinal effects into a temperature signal. Synthesis. Latitudinal gradients provide a methodological set-up to overcome the drawbacks of other observational and experimental warming methods. Our synthesis indicates that many life-history traits of plants vary with latitude but the translation of latitudinal clines into responses to temperature is a crucial step. Therefore, especially adaptive differentiation of populations and confounding environmental factors other than temperature need to be considered. More generally, integrated approaches of observational studies along temperature gradients, experimental methods and common garden experiments increasingly emerge as the way forward to further our understanding of species and community responses to climate warming.

Effects of environment, habitat configuration and forest continuity on the distribution of forest plant species

Abstract In present day European landscapes many forest plant species are restricted to isolated remnants of a formerly more or less continuous forest cover. The two major objectives of this study were (1) to determine the relative importance of habitat quality (mainly in terms of soil parameters), habitat configuration (patch size and isolation) and habitat continuity for the distribution of herbaceous forest plant species in a highly fragmented landscape and (2) to examine if groups of species with different habitat requirements are affected differently. Deciduous forest patches in northwestern Germany were surveyed for the presence of a large set of forest species. For each patch, habitat quality, configuration and continuity were determined. Data were analysed by Redundancy Analysis with variation partitioning for effects on total species composition and multivariate logistic regression for effects on individual species, for two different data sets (base-rich and base-poor forest patches). Overall, we found strong effects of habitat quality (particularly of soil pH, water content and topographic heterogeneity in the base-rich forest patches; and of calcium content and disturbance in the base-poor patches), but only relatively weak effects of habitat configuration and habitat continuity. However, a number of species were positively affected by patch area and negatively affected by patch isolation. Furthermore, the relative importance of habitat configuration tended to be higher for species predominantly growing in closed forests compared to species occurring both in the forest and in the open landscape. Abbreviations: GIS = Geographic information system; RDA = Redundancy analysis. Nomenclature: Wisskirchen & Haeupler (1998).

Acidification and eutrophication of deciduous forests in northwestern Germany demonstrated by indicator species analysis

A new method of indicator species analysis is suggested that can be used to demonstrate changes in the species composition of forests resulting from atmospheric deposition of acidifying substances and nitrogen. The method is not dependent on an exact re-localization of sites. 2162 published releves of Fagus sylvatica and Quercus robur- Carpinus betulus forests from two study areas in Germany (S Niedersachsen and Nordrhein-Westfalen) were compiled and divided into releves from before 1975 and more recent releves. Weighted averages of Ellenberg indicator values for light (L), moisture (M), reaction (R), and nitrogen (N) were calculated and for each of the four data sets - mesic and dry sites in two areas - N was regressed on R to give a predicted value (Npred) for any given R. The differences between observed values Nobs and Npred were calculated separately for old and new releves (Ndev-old and Ndev-new). The same analysis was applied to model data including 16 different scenarios of acidification and/or eutrophication. The model runs resulted in six different types of regression plots for Ndev and R. All models had in common that the regression lines for Ndev-new lay, completely or largely, above the zero line of no change and above the lines for Ndev-old. Also, in the four data sets, Ndev-new was generally larger than Ndev-old, and particularly so in Nordrhein-Westfalen. The values of Ldev-old and Ldev-new indicate that the differences in Ndev are unlikely to be caused by altered forest management, but must result from atmospheric pollution.

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Changes in species density along the soil pH gradient — evidence from German plant communities

The relationship between small-scale species richness and soil pH in plant communities was reviewed using 40 (mainly phytosociological) studies primarily from the northern parts of Germany. Four formations were distinguished (forest, grassland, mire and sand). To examine the above relationship, two approaches were applied: (a) regression analysis using linear and quadratic models, and (b) calculation of the mean pH of the five plots with the highest species density. Despite methodological problems (lack of single plot data in some studies, varying plot sizes, different ways of measuring pH) some general patterns could be identified. In forests, the regressions in most cases indicated a linear or curved increase in species density with increasing pH, whereas maximum species density was, except in one case, found at an intermediate pH of around 5. In contrast, in grasslands and mires, most studies showed hump-backed relationships between species density and soil acidity, and, accordingly, maximum species density was found at moderately high pH between 5 and 6. For sand vegetation, the few studies available revealed an increase in species density with increasing pH. The findings imply that the predominance of calciphilous speciesvs. acidophilous species in the Central European flora does not always translate into positive relationships between plant species density and pH on a local plot scale. The hump-backed species density-pH pattern may be explained by confounding effects of other environmental factors and of productivity, but also by the large overlap of species tolerances in the middle of the acidity gradient.

Factors influencing vegetation gradients across ancient‐recent woodland borderlines in southern Sweden

. We studied gradients in field layer vegetation across ecotone-type borderlines between 12 ancient woodlands and adjacent secondary deciduous woodlands on former arable land. The aim of the study was to determine how distance from the borderline influences species distributions as compared with soil factors and degree of canopy closure. Correspondence Analysis showed that distance from the borderline is closely related to the first ordination axis at all study sites. Canonical Correspondence Analysis with variation partitioning revealed that distance from the borderline was the single most important factor in explaining vegetation variation. In general, the results suggest the following order of decreasing importance: Distance from the borderline < Soil reaction < Soil nitrogen < Soil moisture < Canopy cover. However, the sum of soil variables, as estimated by weighted averages of Ellenberg indicator values for moisture, reaction and nitrogen, accounted for as much as ca. 50–70% of the total variation explained by environmental variables. Important gradients in field layer vegetation are due to a decrease in typical woodland species and an increase in other species with increasing distance from the ancient woodland. The results suggest dispersal limitation of woodland species as an important determinant of secondary forest succession. However, the importance of distance to species distributions decreases with increasing stand age as most woodland species gradually colonize the recent woodlands. After 70 yr, ca. 50 % of the woodland species present at a site showed complete colonization within 50 m from the ancient woodland border.

Nitrogen mineralisation in deciduous forest soils in south Sweden in gradients of soil acidity and deposition

Abstract Net nitrogen mineralisation was studied in 600 deciduous forests in four geographically separate regions of southern Sweden. The total nitrogen deposition in the forests varied between 7 and 17 kg ha −1 year −1 . The most frequent canopy-dominant tree species was Quercus robur , others being Acer platanoides, Carpinus betulus, Fraxinus excelsior, Tilia cordata and Ulmus glabra . Soil pH (0.2 M KCl), varied between 3.1 and 7.7 and the C/N ratio between 8 and 50 in soil sampled 5 cm below the litter layer. The potential net mineralisation rate, measured in a 15-week laboratory incubation experiment, could be explained to 20% by soil pH and to 4.5. The most striking finding was that rates of net nitrogen mineralisation and nitrification were by far the highest in the southernmost region, especially in the most acid soils, rates about twice as high as in the other regions. Sites that differed in the dominant tree species did not differ in net nitrogen mineralisation. Nitrogen deposition explained 30% of the variation in mineralisation rate. It is suggested that the high nitrogen deposition in recent decades has favoured ammonification through increasing the nitrogen pool in the soil and altering the quality of the organic matter, the microbial community adapting to the acidified soils and the increased nitrification.

Shifts in ecological behaviour of herbaceous forest species along a transect from northern central to North Europe

We investigated the ecological behaviour (the response to environmental factors in the field, synonymous to the term realized niche) of four closely related species pairs (Melica nutans, M. uniflora; Primula veris, P. elatior; Veronica chamaedrys, V. montana; Viola riviniana, V. reichenbachiana) across a transect from northern Central to North Europe. The second-mentioned species of each pair is confined in its geographical distribution to the southern parts of the studied transect. Sample plot data of deciduous forests were compiled from (1) Germany, S Niedersachsen, (2) Germany, northern Schleswig-Holstein, (3) Denmark, and (4) Boreo-nemoral Sweden. We compared the ecological optima and amplitudes of the response curves of species along the gradients for moisture, pH and nitrogen by means of phytosociological data, detrended correspondence analysis (DCA) and Ellenberg indicator values. pH measurements from Sweden were significantly correlated with the corresponding DCA sample plot scores and the plot averages of the Ellenberg values for reaction (pH).In accordance with our main hypothesis, the wide range species appeared to have broader ecological amplitudes on the northern margins of their distributional ranges, especially in Boreo-nemoral Sweden, than in the southern parts of the study area. Our findings are in contrast to theories claiming a reduced niche breadth of range-margin populations of species compared to range-centre populations. The shifts in ecological behaviour were particularly obvious with respect to soil acidity. We believe that these shifts are caused by changes in the competitive relationships between the species: in the north, the total pool of species in deciduous forests is comparatively small. The low species richness is likely to lead to reduced competition and to an expansion of the ecological amplitude, known as competitive release.