Thomas Dirnböck

A resampling approach for evaluating effects of pasture abandonment on subalpine plant species diversity

Abstract The decline of species-rich semi-natural calcareous grasslands is a major conservation problem throughout Europe. Maintenance of traditional animal husbandry is often recommended as an important management strategy. However, results that underpin such management recommendations were derived predominantly from lowland studies and may not be easily applicable to high mountain areas. In this study we analyse the importance of traditional low-intensity summer farming (cattle grazing) for vascular plant species diversity of a subalpine region in the northern calcareous Alps in Austria by resampling from an existing dataset on its vegetation. Results indicate a significant long term decline of plant species diversity following abandonment at the landscape scale. In contrast, within-community effects of pasture abandonment on plant species diversity are equivocal and strongly depend on the plant community. We suppose these differences to be due to diet preferences of cattle as well as to the differential importance of competition for structuring the respective communities. From our results we infer that the main mechanism by which pasture abandonment affects vascular plant species diversity, at least during the first ca. 100 yr documented here, are not local-scale competitive exclusion processes within persisting communities. Instead, post-abandonment successional community displacements that cause a landscape scale homogenization of the vegetation cover seem to be primarily responsible for a decline of species diversity. We conclude, that successful management of vascular plant species diversity in subalpine regions of the Northeastern Calcareous Alps will depend on the maintenance of large scale pasture systems with a spatially variable disturbance regime. Nomenclature: Adler et al. (1994). Abbreviations: DEM = Digital Elevation Model; GIS = Geographical Information System; WET = Topographic Wetness Index; EROS = Topographic Erosion Index; GLM = Generalized Linear Model; S = Species Richness; H = Shannon Index of diversity; E = Evenness; CC = Number of plant communities; dC = Berger-Parker Index of community diversity.

Assessing the Long-Term Dynamics of Endemic Plants at Summit Habitats

Evidence from high summits in the Alps, that mountain plants have migrated upwards (Gottfried et al. 1994;Grabherr et al. 1994, 2001; Pauli et al. 1996), prompted the initiation of a Global Observation Network (GLORIA, see http://www.gloria.ac.at) to study climate change induced effects on alpine biodiversity (Grabherr et al. 2000; Pauli et al. 2001). Mountain tops or summits form comparable environmental units, where habitats of every exposure (N, E, S, and W) are present within a small area and are little affected by shading from neighbouring land features. Mountain summits often have a high habitat diversity. They are of particular interest for detecting any upward migration of species. The summits are prominent landmarks that can be readily relocated for re-investigations and the highest summit points can be characterised by an average climate at any given altitude.

Habitat distribution models, spatial autocorrelation, functional traits and dispersal capacity of alpine plant species

Abstract We evaluate the potential influence of disturbance on the predictability of alpine plant species distribution from equilibrium-based habitat distribution models. Firstly, abundance data of 71 plant species were correlated with a comprehensive set of environmental variables using ordinal regression models. Subsequently, the residual spatial autocorrelation (at distances of 40 to 320 m) in these models was explored. The additional amount of variance explained by spatial structuring was compared with a set of functional traits assumed to confer advantages in disturbed or undisturbed habitats. We found significant residual spatial autocorrelation in the habitat models of most of the species that were analysed. The amount of this autocorrelation was positively correlated with the dispersal capacity of the species, levelling off with increasing spatial scale. Both trends indicate that dispersal and colonization processes, whose frequency is enhanced by disturbance, influence the distribution of many alpine plant species. Since habitat distribution models commonly ignore such spatial processes they miss an important driver of local- to landscape-scale plant distribution. Nomenclature: Adler et al. (1994). Abbreviation: DEM = Digital elevation model.

Vegetation distribution in relation to topographically driven processes in southwestern Australia

Abstract This study assesses the utility of modelling approaches to predict vegetation distribution in agricultural landscapes of southwestern Australia. Climate surfaces, hydrologic and erosion process models are used to link vegetation to environmental variables. Generalized additive models (GAM) are derived for presence/absence data of mapped vegetation types. Vegetation distribution shows significant responses to rainfall and subsequent water redistribution due to the relief; however, these variables are insufficient to effectively explain vegetation patterns at the local scale. Accordingly, prediction accuracy remains low (k‐values below 0.5). The striking unpredictability of the local distribution of the vegetation in the Wheatbelt is discussed with regard to the performance of topographically driven processes in subdued landscapes and with regard to geological, historical and biological factors determining the southwestern Australian plant species distribution. Nomenclature: Chapman et al. (in prep.). Abbreviations: AVHGT = average altitudinal height of the upslope area; AVSLP = average slope of the upslope area; CURVPL = plan curvature; CUPL500 = plan curvature in a 1.5 km window; CURVPR = profile curvature; DEM = Digital Elevation Model; DIRIDGE = distance from ridges; DROUGHT = drought index; EROS = sediment transport index; GAM = Generalized Additive Model; HABOVE = height above streamlines; RAIN = annual rainfall; SLOPE = maximum slope of the surface; STRP = stream power index; WET = wetness index.

Mapping alpine vegetation based on image analysis, topographic variables and Canonical Correspondence Analysis

The objective of the present study was to map dominant plant communities of an alpine area in the northeastern Alps (Austria), based on computer modelling. We employed gradient analysis by means of Canonical Correspondence Analysis (CCA) as a prediction tool and image segmentation as a filter for reducing the number of incorrect predictions. Topographical variables reflecting relief properties at different scales were used as surrogates for environmental conditions in combination with spectral band values from infrared orthophotographs. Coupling topographic correlation using CCA and image analysis proved practicable to map the distribution of alpine plant communities. Although plant communities often showed similar spectral response, they were mapped according to their specific topographical niches. Generally, topographic variables, indicative of environmental gradients controlling plant distribution, provided this information in most cases. The importance of spectral vs topographic variables varied among plant communities. Whereas the correlation between topography and plant species distribution was particularly significant for mapping alpine grasslands, spectral texture measures proved to be of major importance in discriminating between pioneer communities. Post-processing by image segmentation improved overall accuracy by 12%. A total of 17 plant communities and their mosaics were mapped, with an overall accuracy of 69.4% and a k value of 0.64. Inaccuracy resulted from insufficient resolution of the available digital elevation model and confounding effects of additional controls like land use history, which could not be accounted for by topographic descriptors.

The impacts of climate change and disturbance on spatio-temporal trajectories of biodiversity in a temperate forest landscape

Summary The ongoing changes to climate challenge the conservation of forest biodiversity. Yet, in thermally limited systems, such as temperate forests, not all species groups might be affected negatively. Furthermore, simultaneous changes in the disturbance regime have the potential to mitigate climate‐related impacts on forest species. Here, we (i) investigated the potential long‐term effect of climate change on biodiversity in a mountain forest landscape, (ii) assessed the effects of different disturbance frequencies, severities and sizes and (iii) identified biodiversity hotspots at the landscape scale to facilitate conservation management. We employed the model iLand to dynamically simulate the tree vegetation on 13 865 ha of the Kalkalpen National Park in Austria over 1000 years, and investigated 36 unique combinations of different disturbance and climate scenarios. We used simulated changes in tree cover and composition as well as projected temperature and precipitation to predict changes in the diversity of Araneae, Carabidae, ground vegetation, Hemiptera, Hymenoptera, Mollusca, saproxylic beetles, Symphyta and Syrphidae, using empirical response functions. Our findings revealed widely varying responses of biodiversity indicators to climate change. Five indicators showed overall negative effects, with Carabidae, saproxylic beetles and tree species diversity projected to decrease by more than 33%. Six indicators responded positively to climate change, with Hymenoptera, Mollusca and Syrphidae diversity projected to increase more than twofold. Disturbances were generally beneficial for the studied indicators of biodiversity. Our results indicated that increasing disturbance frequency and severity have a positive effect on biodiversity, while increasing disturbance size has a moderately negative effect. Spatial hotspots of biodiversity were currently found in low‐ to mid‐elevation areas of the mountainous study landscape, but shifted to higher‐elevation zones under changing climate conditions. Synthesis and applications. Our results highlight that intensifying disturbance regimes may alleviate some of the impacts of climate change on forest biodiversity. However, the projected shift in biodiversity hotspots is a challenge for static conservation areas. In this regard, overlapping hotspots under current and expected future conditions highlight priority areas for robust conservation management.

Uniform climate sensitivity in tree-ring stable isotopes across species and sites in a mid-latitude temperate forest

Tree-ring stable isotopes, providing insight into drought-induced eco-physiological mechanisms, are frequently used to reconstruct past changes in growing season temperature and precipitation. Their climatic response is, however, still not fully understood, particularly for data originating from non-extreme, mid-latitude environments with differing ecological conditions. Here, we assess the response of δ(13)C, δ(18)O and tree-ring width (TRW) from a temperate mountain forest in the Austrian pre-Alps to climate and specific drought events. Variations in stem growth and isotopic composition of Norway spruce, common beech and European larch from dry, medium and moist sites are compared with records of sunshine, temperature, moisture, precipitation and cloud cover. Results indicate uniform year-to-year variations in δ(13)C and δ(18)O across sites and species, but distinct differences in TRW according to habitat and species. While the climate sensitivity of TRW is overall weak, the δ(13)C and δ(18)O chronologies contain significant signals with a maximum sensitivity to cloud cover changes (r = -0.72 for δ(18)O). The coherent inter-annual isotopic variations are accompanied by substantial differences in the isotopic signatures with offsets up to ∼3‰ for δ(13)C, indicating species-specific physiological strategies and varying water-use efficiencies. During severe summer drought, beech and larch benefit from access to deeper and moist soils, allowing them to keep their stomata open. This strategy is accompanied by an increased water loss through transpiration, but simultaneously enables enhanced photosynthesis. Our findings indicate the potential of tree-ring stable isotopes from temperate forests to reconstruct changes in cloud cover, and to improve knowledge on basic physiological mechanisms of tree species growing in different habitats to cope with soil moisture deficits.

Long‐term impacts of nitrogen and sulphur deposition on forest floor vegetation in the Northern limestone Alps, Austria

ABSTRACT Question: Are there effects of long-term deposition of airborne nitrogen and sulphur on the forest floor vegetation from permanent plots collected in 1993 compared to 2005. Location: Northern limestone Alps in Austria. Methods: Single species responses were analysed by correlating trends in cover-abundance values, as derived from marginal models, with Ellenberg indicator values. Changes in the species composition of plots were analysed by correlating changes in mean Ellenberg indicator values with the displacement of plots within a multidimensional scaling ordination. Results: Trends in single species abundance were positively correlated with indicator values of soil pH but were independent of nutrient availability. A general trend towards the homogenisation of vegetation, due to convergent time vectors of the relevés, became obvious. Oligotrophic sites previously situated at the distal ends of ordination axes shifted towards the centre since they were enriched by species preferring mesotrophic conditions. The bulk of plots with intermediate site conditions hardly showed any trends. A concomitant analysis demonstrated that temporal changes in species composition exceed the variation in cover abundance estimates among different field botanists. Conclusions: N deposition can lead to a homogenisation of forest floor vegetation. Larger limestone areas with diverse soil conditions, such as the Northern limestone Alps in Austria, as a whole are thus negatively affected by airborne N deposition. Nevertheless, the vegetation was at least as strongly affected by an increase of basiphilous species as a result of decreasing S deposition. Nomenclature: Adler et al. (1994).

Combining Biodiversity Resurveys across Regions to Advance Global Change Research

More and more ecologists have started to resurvey communities sampled in earlier decades to determine long-term shifts in community composition and infer the likely drivers of the ecological changes observed. However, to assess the relative importance of and interactions among multiple drivers, joint analyses of resurvey data from many regions spanning large environmental gradients are needed. In this article, we illustrate how combining resurvey data from multiple regions can increase the likelihood of driver orthogonality within the design and show that repeatedly surveying across multiple regions provides higher representativeness and comprehensiveness, allowing us to answer more completely a broader range of questions. We provide general guidelines to aid the implementation of multiregion resurvey databases. In so doing, we aim to encourage resurvey database development across other community types and biomes to advance global environmental change research.

GIS Assessment of Vegetation and Hydrological Change in a High Mountain Catchment of the Northern Limestone Alps

Abstract Large-scale vegetation mapping (1:10,000) was applied to obtain estimates of the hydrological properties and dynamics in catchment areas that supply water to the capital city of Austria (Vienna). Vegetation types as defined by standard relevé technique, such as alpine grassland, snow bed vegetation, and krummholz were related to habitat conditions. A GIS served as the focal exploration tool. The vegetation units show specific evapotranspiration rates, which were derived from literature on experimental research covering similar vegetation types in the Alps. Additionally, physical soil properties from field data were used to derive the specific soil water balance in relation to the mapped vegetation types. Finally, the hydrological balances for each landscape unit, as well as for the total catchment area, were presented by combining the estimates for evapotranspiration and soil water properties. The consequences of environmental change (forestry, pasturing, and climate warming) are a focus of attention for water management. Predicting general changes in vegetation patterns reveals contrasting scenarios about the consequences to be expected for the water supply of Vienna.