Jürgen Böhner

Land-Surface Parameters Specific to Topo-Climatology

Publisher Summary This chapter presents a brief overview of climate regionalization approaches. It introduces land-surface parameters relevant to assessing the short- and long-wave radiation flux of the surface. The chapter also deals with land-surface parameters, suitable to assessing the orographic effects on thermal conditions and cold air flow. The influences of the land-surface on near-ground thermodynamics, on wind velocities and the closely related precipitation distribution are discussed, emphasizing the passive effects of terrain in particular. The land-surface parameters, discussed in this chapter are either physically proved expressions or measures suitable to parameterize the multitude of orographically induced or affected atmospheric processes in the boundary layer. Land surface is one major control within the scope of atmosphere surface interactions, however, land use pattern, type and state of natural or managed vegetation cover likewise significantly affect the topo-climatic settings. Relevant surface properties, such as the meteorological roughness and vegetation/land cover, require consideration of the nature of underlying ground and its influence on the regional climate modeling applications.

Climatologies at high resolution for the earth’s land surface areas

High-resolution information on climatic conditions is essential to many applications in environmental and ecological sciences. Here we present the CHELSA (Climatologies at high resolution for the earth’s land surface areas) data of downscaled model output temperature and precipitation estimates of the ERA-Interim climatic reanalysis to a high resolution of 30 arc sec. The temperature algorithm is based on statistical downscaling of atmospheric temperatures. The precipitation algorithm incorporates orographic predictors including wind fields, valley exposition, and boundary layer height, with a subsequent bias correction. The resulting data consist of a monthly temperature and precipitation climatology for the years 1979–2013. We compare the data derived from the CHELSA algorithm with other standard gridded products and station data from the Global Historical Climate Network. We compare the performance of the new climatologies in species distribution modelling and show that we can increase the accuracy of species range predictions. We further show that CHELSA climatological data has a similar accuracy as other products for temperature, but that its predictions of precipitation patterns are better.

The impact of livestock grazing on plant diversity: an analysis across dryland ecosystems and scales in southern Africa

A general understanding of grazing effects on plant diversity in drylands is still missing, despite an extensive theoretical background. Cross-biome syntheses are hindered by the fact that the outcomes of disturbance studies are strongly affected by the choice of diversity measures, and the spatial and temporal scales of measurements. The aim of this study is to overcome these weaknesses by applying a wide range of diversity measures to a data set derived from identical sampling in three distinct ecosystems. We analyzed three fence-line contrasts (heavier vs. lighter grazing intensity), representing different degrees of aridity (from arid to semiarid) and precipitation regimes (summer rain vs. winter rain) in southern Africa. We tested the impact of grazing intensity on multiple aspects of plant diversity (species and functional group level, richness and evenness components, alpha and beta diversity, and composition) at two spatial scales, and for both 5-yr means and interannual variability. Heavier grazing reduced total plant cover and substantially altered the species and functional composition at all sites. However, a significant decrease in species alpha diversity was detected at only one of the three sites. By contrast, alpha diversity of plant functional groups responded consistently across ecosystems and scales, with a significant decrease at heavier grazing intensity. The cover-based measures of functional group diversity responded more sensitively and more consistently than functional group richness. Beta diversity of species and functional types increased under heavier grazing, showing that at larger scales, the heterogeneity of the community composition and the functional structure were increased. Heavier grazing mostly increased interannual variability of alpha diversity, while effects on beta diversity and cover were inconsistent. Our results suggest that species diversity alone may not adequately reflect the shifts in vegetation structure that occur in response to increased grazing intensity in the dryland biomes of southern Africa. Compositional and structural changes of the vegetation are better reflected by trait-based diversity measures. In particular, measures of plant functional diversity that include evenness represent a promising tool to detect and quantify disturbance effects on ecosystems.

How do soil properties affect alpine treelines? General principles in a global perspective and novel findings from Rolwaling Himal, Nepal

Little is known about how soil properties control tree growth at its upper limit. This paper reviews the state of knowledge and discusses the results specifically related to ecozones, to the scale-dependent importance of single factors, and to new findings from a near-natural treeline ecotone in Rolwaling Himal, Nepal. This paper identifies gaps in literature and shows where new research is needed, both conceptual and geographical. The review shows that at a global scale and throughout diverse ecozones, growing season soil temperature is considered a key factor for tree growth. Soil temperatures differ greatly at a local scale, and are mainly determined by local climatic, edaphic, and topographic conditions. Our result of 7.6 ± 0.6°C for growing season mean soil temperature at treeline in Rolwaling is 1.2 K higher compared to the postulated 6.4 ± 0.7°C for alpine treelines. We suggest a broadening of the ±0.7°C error term to cover the wide range at a local scale. The role of major soil nutrients and soil moisture for treeline shift has been underestimated by far. In Rolwaling, significantly decreasing nutrient availability (N, K, Mg) in soils and foliage with elevation might explain why treeline shift and global warming are decoupled. Further, soil moisture deficits early in the year impede seedling and sapling establishment, which could be an important mechanism that controls treeline position. These findings question previous results which argue that alpine treelines are unaffected by soil nutrient availability and soil moisture. We assume that specific combinations of soil properties as well as single soil properties limit tree growth even below climatic treelines.

Soil Temperature and Soil Moisture Patterns in a Himalayan Alpine Treeline Ecotone

ABSTRACT Soil properties in alpine treeline ecotones are insufficiently explored. In particular, an extensive monitoring of soil moisture conditions over a longer period of time is rare, and the effects of soil moisture variability on alpine treelines have not received adequate attention yet. Soil temperature patterns are generally well documented, and soil temperature is considered a key factor in limiting tree growth at both global and local scales. We performed a 2½-year monitoring in a near-natural treeline ecotone in Rolwaling Himal, Nepal. In this paper, we present new findings on spatiotemporal soil temperature and moisture variability in relation to topographical features and vegetation patterns (variations in stand structures and tree physiognomy). Our results show a growing season mean soil temperature of 7.5 ± 0.6 °C at 10 cm depth at the Rolwaling treeline. Multivariate statistical analyses yield a significant relation between soil temperatures and the variability in tree height, crown length, crown width, and leaf area index (LAI). In turn, soil temperature variability is controlled by the tree physiognomy itself. Soil moisture conditions (available water capacity, 0–10 cm) appear to be less substantial for current stand structures and tree physiognomy. In turn, tree physiognomy patterns control soil moisture, which additionally is affected by snow cover. In Rolwaling, shallow and coarse-grained soils cause low water-holding capacities, and thus a remarkable amount of water percolates from topsoils to subsoils. In the alpine tundra with missing forest canopy, year-round lowest available water capacities are additionally caused by high solar radiation, wind, and thus high evaporation. We assume low soil moisture availability causing largely prevented tree regeneration especially in the alpine tundra. We conclude that soil temperature and moisture patterns reflect tree physiognomy patterns. The latter cause disparities in soil temperature and moisture conditions inside and outside of the closed forest by shading effects and differences in leaf fall.

Towards an urban roughness parameterisation using interferometric SAR data taking the Metropolitan Region of Hamburg as an example

Increased surface roughness and the enhanced drag effect of the urban surface are important alterations in urban boundary layer dynamics. An enhanced roughness mapping would be beneficial for several modelling applications since state of the art roughness parameters derived from land use data do not represent the heterogeneity of urban surfaces at higher resolutions. Today most morphometric methods of urban roughness estimation are based on empirically derived functions of obstacle geometry parameters, which have conceptual and practical shortcomings. In this study we propose a new approach based on the topology, statistics and texture of overall roughness elements as represented by a Digital Height Model generated from side-looking Interferometric Synthetic Aperture Radar. These state of the art remotely sensed high resolution data are readily available for Western Europe, the US, as well as parts of Asia and the Caribbean. A set of new angular dependent urban morphology parameters is developed and tested for suitability for the mapping of roughness characteristics by way of an improved methodology. The results are very convincing and consistent over a wide range of surface characteristics.

Treeline Responsiveness to Climate Warming: Insights from a Krummholz Treeline in Rolwaling Himal, Nepal

At a global scale, the elevational position of natural upper treelines is determined by low temperatures during growing season. Thus, climate warming is expected to induce treelines to advance to higher elevations. Empirical studies in diverse mountain ranges, however, give evidence of both advancing alpine treelines as well as rather insignificant responses. Himalayan treeline ecotones show considerable differences in altitudinal position as well as in physiognomy and species composition. To assess the sensitivity of a near-natural treeline to climate warming at local scale, we analysed the relations between changes of growth parameters and temperature gradients along the elevational gradient in the treeline ecotone in Rolwaling valley, Nepal, by a multispecies approach. We observed species-specific transition patterns (diameter at breast height, height, tree and recruit densities) and varying degrees of abruptness of these transitions across the treeline ecotone resulting in a complex stand structure. Soil temperatures are associated with physiognomic transitions, treeline position and spatial regeneration patterns. In conclusion, treeline tree species have the potential to migrate upslope in future. Upslope migration, however, is controlled by a dense krummholz belt of Rhododendron campanulatum. Currently, the treeline is rather stable; however we found a prolific regeneration as well as signs of stand densification. Given the spatial heterogeneity of Himalayan treeline ecotones, further studies are needed to fully understand the complex conditions for the establishment and development of tree seedlings and the responsiveness of Himalayan treeline ecotones to climate change.

Climate Change and Treeline Dynamics in the Himalaya

Treelines are sensitive to changing climatic conditions, in particular to temperature increases, and the majority of global alpine treelines has shown a response to recent climate change. High temperature trends in the Himalaya suggest a treeline advance to higher elevations; it is largely unknown, however, how broader-scale climate inputs interact with local-scale factors and processes to govern treeline response patterns. This paper reviews and synthesizes the current state of knowledge regarding sensitivity and response of Himalayan treelines to climate warming, based on extensive field observations, published results in the widely scattered literature and novel data from ongoing research of the present authors.

Recent Climate Change over High Asia

Though elevated regions have generally been spotted as climate change hotspots due to amplified signal of change observed over recent decades, such evidence for the Tibetan Plateau and its neighboring regions is supported only by a sparse observational network, less representative for the high-altitude regions. Using a larger database of widely used gridded observations (CRU and UDEL) and reanalysis datasets (NCEP-CFSR, ERA-Interim, and its downscaled variant ERA-WRF) along with high-quality homogeneous station observations, we report recent changes in mainly the mean monthly near-surface air temperature and its elevation dependence, as well as changes in precipitation over the Tibetan Plateau, its neighboring mountain ranges, and the basins of major rivers originating from them. Our station-based analysis suggests a well-agreed warming over and around the Tibetan Plateau, which is more pronounced mainly during winter and spring months and generally in agreement but higher in magnitude than that of previously reported. We found a varying skillset of considered gridded and reanalysis datasets in terms of suggesting robust spatial and elevation-dependent patterns of trends and their magnitudes. The UDEL, ERA-Interim, and CRU datasets, respectively, exhibit high- to medium-level agreement with the station observations in terms of their trend magnitudes, which are generally underestimated. We found that all datasets agree with station observations as well as among each other for a strongest warming and drying in March over the northwestern region, for wet conditions in May over the southeastern Tibetan Plateau and Myanmar regions, as well as for the general warming pattern. Similarly, a strongest EDW rate per 1000 m elevation found in January is well agreed qualitatively among all datasets, except ERA-WRF. We also confirm high inter-dataset agreement for higher warming rates for highlands (above 2000 m asl) as compared to lowlands in December and January and with a mild agreement during the growing season (April–September). Except for winter months, NCEP-CFSR reanalysis largely contradicts the elevation-dependent warming signal. Our findings suggest that well-agreed likely changes in the prevailing climate will severely impact the geo-ecosystems of the High Asia and will have substantial influence on almost all dimensions of life in the region.

Implications of tree species – environment relationships for the responsiveness of Himalayan krummholz treelines to climate change

Climate warming is expected to advance treelines to higher elevations. However, empirical studies in diverse mountain ranges give evidence of both advancing alpine treelines as well as rather insignificant responses. In this context, we aim at investigating the sensitivity and responsiveness of the near-natural treeline ecotone in Rolwaling Himal, Nepal, to climate warming. We analysed population densities of tree species along the treeline ecotone from closed forest stands via the krummholz belt to alpine dwarf shrub heaths (3700-4200 m) at 50 plots in 2013 and 2014. We quantified species - environment relationships, i.e. the change of environmental conditions (e.g., nutrient and thermal deficits, plant interactions) across the ecotone by means of redundancy analyses, variation partitioning and distance-based Morans eigenvector maps. In particular, we focus on explaining the high competitiveness of Rhododendron campanulatum forming a dense krummholz belt and on the implications for the responsiveness of Himalayan krummholz treelines to climate change. Results indicate that treeline trees in the ecotone show species-specific responses to the influence of environmental parameters, and that juvenile and adult tree responses are modulated by environmental constraints in differing intensity. Moreover, the species - environment relationships suggest that the investigated krummholz belt will largely prevent the upward migration of other tree species and thus constrain the future response of Himalayan krummholz treelines to climate warming.