Effects of abandonment on the energy balance and evapotranspiration of wet subalpine grassland

Abandonment of grasslands is an important form of land-use change in subalpine regions. It leads to marked changes in the vegetation cover, which, in turn, may affect the radiative properties of the canopy and exchange processes between the ecosystem and the atmosphere. The aim of this study was to quantify the differences in net radiation (Rn), albedo (alpha) and energy balance components between a managed and an abandoned wet grassland field in relationship to biomass and necromass production. The study in the Rotenbach watershed on the northern slope of the Swiss Alps (1400 m above sea level) was based on continuous measurements of the energy and radiation balances during two consecutive seasons. The managed site, which was mown once during the autumn, was characterized by a larger number of species, but in the abandoned field, the dominating species (Ranunculus aconitifolius) was much more abundant. Annual production of aboveground plant biomass was similar in both fields, but litter accumulation and the amount of standing necromass was larger in the abandoned field. Early in the season, the litter layer at the abandoned site had only a minor influence on Rn, but the larger amount of standing necromass in the course of the season decreased alpha and increased Rn. This effect was most pronounced during mid-season and compensated for the higher ratio of sensible to latent heat fluxes, i.e., the Bowen ratio of the abandoned field. The resulting difference in total water vapour flux (evapotranspiration, ET) between the fields was small. Largest reduction in ET occurred early and late in the season, and this was partly due to decreased soil evaporation. Although the effect of abandonment on the cumulative ET during the two measuring periods was small, it was sufficient to increase the soil water content in the abandoned field, particularly during the later part of the season. It is concluded that abandonment causes a change in canopy structure which leads to reduced ET and temporarily increased soil water content.