Niels Schwab

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.

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.

Sustainable Disaster Risk Reduction in Mountain Agriculture: Agroforestry Experiences in Kaule, Mid-Hills of Nepal

Modernization as a transformational strategy towards sustainable development has to promote further improvement of mountain farmers’ livelihoods while at the same time ensuring ecological sustainability and inducing social equity. In this context, a multiyear joint project with local farmers was launched in spring 2009 to introduce agroforestry practices in the village Kaule, Nuwakot District, mid-hills of Nepal. Practical components of the project included trainings and workshops on agroforestry, restructuring of terrace fields for conversion to agroforestry, and monthly meetings for open discussions among involved households. The project was accompanied scientifically to analyse socio-economic and ecological impacts. This paper presents scientific findings, summarizes the experiences during the transition to sustainable land management from an interdisciplinary perspective and gives evidence of increased willingness to adopt sustainable agricultural practices and the obtainment of environmental benefits and increased livelihood security. Participation of the farmers in the entire process, beginning with the definition of goals, the envisioning of a desired future and the integration of local knowledge, skills and resources were found to be of key importance for the project success. During the transition process, a diversification of marketable crops and additional income generation further enhanced the willingness to adopt new agricultural practices. After the adoption of agroforestry, soil quality and soil productivity have been significantly ameliorated, with positive effects appearing shortly after the conversion from conventional monocropping systems. We also assessed significantly higher species richness, beta diversity and cover of trees and shrubs in the agroforestry system. We conclude that the transition from conventional terrace cultivation to agroforestry practices has the potential to generate significant environmental and socio-economic benefits, thus contributing to sustainable modernization processes and disaster risk reduction in mountain agriculture.

Decreasing nutrient concentrations in soils and trees with increasing elevation across a treeline ecotone in Rolwaling Himal, Nepal

At a global scale, tree growth in alpine treeline ecotones is limited by low temperatures. At a local scale, however, tree growth at its upper limit depends on multiple interactions of influencing factors and mechanisms. The aim of our research was to understand local scale effects of soil properties and nutrient cycling on tree growth limitation, and their interactions with other abiotic and biotic factors in a near-natural Himalayan treeline ecotone. Soil samples of different soil horizons, litter, decomposition layers, and foliage samples of standing biomass were collected in four altitudinal zones along three slopes, and were analysed for exchangeable cations and nutrient concentrations, respectively. Additionally, soil and air temperature, soil moisture, precipitation, and tree physiognomy patterns were evaluated. Both soil nutrients and foliar macronutrient concentrations of nitrogen (N), magnesium (Mg), potassium (K), and foliar phosphorus (P) decrease significantly with elevation. Foliar manganese (Mn) concentrations, by contrast, are extraordinarily high at high elevation sites. Potential constraining factors on tree growth were identified using multivariate statistical approaches. We propose that tree growth, treeline position and vegetation composition are affected by nutrient limitation, which in turn, is governed by low soil temperatures and influenced by soil moisture conditions.