Development of a soil-vegetation-atmosphere-transfer-model

Since pre-industrial times the C02 partial pressure in the atmosphere has been steadily increasing due to a largely anthropogenically influenced imbalance between C02 sources and sinks. The most important primary negative consequences of this phenomena is the heating-up of the atmosphere, observed as a global increase in air temperature. At the same time, in the mountain regions of Europe land-use changes in agriculture and forestry are occurring rapidly. They cause long-lasting changes in the spatial structure of plant canopies, species composition and interactions, soil organic matter status and turnover, and nutrient pools and fluxes. Land-use changes are thus likely to affect the fluxes of sensible and latent heat, C02 and other trace gases between the vegetation and the atmosphere. Hence feedback effects between changes in land use and global changes of climate, which affect the sink capacity of ecosystems for C02 , may be expected. Aim of the proposed project is to develop a soil-vegetation-atmosphere-transfer model for simulating mass and energy exchange in mountain grassland ecosystems in order to quantify their sink capacity for C02 and to analyse the way their sink capacity might be affected by land-use changes. The objective of these simulations is a detailed analysis of the processes governing the exchange of mass and energy, rather than pure predictions of these fluxes, hence a detailed approach, using a multi-layer soil-vegetation-atmosphere-transfer (SVAT) model is followed. The multi-layer SVAT model shall integrate current biogeochemical, micrometeorological and eco-physiological theory and will be specifically adapted for the application in mountain grassland ecosystems, with special emphasise on the following points: * Mountain grassland ecosystems are highly diverse ecosystems being composed by a large number of plant species of differing physiology. Leaf model parameterisation hence deserves particular attention, since the sink capacity of the investigated ecosystems is closely related to leaf physiology. * Mountain grassland ecosystems experience higher amounts of precipitation as compared to low elevation grasslands. One central question to be answered by the proposed project thus will be i) whether and when C02 uptake of mountain grasslands is limited by soil water availability, ii) to which extent do these limitations affect their sink capacity and iii) how are these effects influenced by the type and intensity of land use. * When meadows and pastures are abandoned, plant biomass formerly removed by humans (mowing) or grazing animals, respectively, accumulates close to the soil surface forming a thick layer characteristic for abandoned areas. This dead plant material does not contribute to canopy carbon gain, but strongly affects the canopy energy balance due to the absorption, reflection and emission of radiation and the convection of sensible and, if wetted, and latent heat. To investigate the significance of these processes for canopy mass and energy exchange is one of the aims of the proposed project. The proposed project is concerned with modelling only, data from field measurements necessary to parameterise, validate and run the proposed SVAT model are almost entirely derived from the EU-TERI-project ECOMONT, which, co-ordinated by the proposer, has been studying the effects of land-use changes in the major mountain regions of Europe. In order to gather possibly missing validation data or to specifically study some model hypothesis, additional field measurements will be conducted.

A model has been developed which allows the prediction of the exchange processes of carbon dioxide, water vapour and heat between the soil/vegetation and the atmosphere. The model consists of a below-ground and a above-ground compartment: The below-ground compartment simulates the movement of heat and water within the soil and at the soil surface and release of carbon dioxide to the atmosphere by roots and soil microbes (ie. fungi and bacteria). The above-ground compartment simulates the distribution of the environmental drivers of plant photosynthesis and transpiration (ie. light intensity, air temperature, humidity and carbon dioxide concentration, wind speed, precipitation) and their influence on these processes within the canopy. The total exchange of carbon dioxide, water vapour and heat to/from the atmosphere is given as the sum of the respective below- and above- ground contributions. Using this model is was possible to demonstrate that the decrease in canopy net photosynthesis observed after abandonment of mountain meadows and pastures is not due to the accumulation of dead plant matter typical for abandoned mountain grasslands. Rather it could be shown that this decrease is due to a reduction in bulk photosynthetic capacity as compared to meadows and pastures, resulting from a change in species composition and nutrient availability following abandonment. These results are a first indication that the continuation of traditional mountain farming practises may help to abate the increasing accumulation of carbon dioxide in the atmosphere, the so-called greenhouse effect, and the anticipated associated negative consequences.