Effects of abandonment on the energy balance and evapotranspiration of wetsubalpine grassland

Abandonment of grasslands is an important form of land-use change in subalp ine regions. It leads to marked changes in the vegetation cover, which, in turn, may affect the radiative properties of the canopy and exchange proces ses between the ecosystem and the atmosphere. The aim of this study was to quantify the differences in net radiation (R-n), albedo (alpha) and energy balance components between a managed and an abandoned wet grassland field i n relationship to biomass and necromass production. The study in the Rotenb ach watershed on the northern slope of the Swiss Alps (1400 m above sea lev el) was based on continuous measurements of the energy and radiation balanc es during two consecutive seasons. The managed site, which was mown once du ring the autumn, was characterized by a larger number of species, but in th e abandoned field, the dominating species (Ranunculus aconitifolius) was mu ch more abundant. Annual production of aboveground plant biomass was simila r in both fields, but litter accumulation and the amount of standing necrom ass was larger in the abandoned field. Early in the season, the litter laye r at the abandoned site had only a minor influence on R-n, but the larger a mount of standing necromass in the course of the season decreased alpha and increased R-n. This effect was most pronounced during mid-season and compe nsated for the higher ratio of sensible to latent heat fluxes, i.e., the Bo wen ratio of the abandoned field. The resulting difference in total water v apour flux (evapotranspiration, ET) between the fields was small. Largest r eduction in ET occurred early and late in the season, and this was partly d ue to decreased soil evaporation. Although the effect of abandonment on the cumulative ET during the two measuring periods was small, it was sufficien t to increase the soil water content in the abandoned field, particularly d uring the later part of the season. It is concluded that abandonment causes a change in canopy structure which leads to reduced ET and temporarily inc reased soil water content. (C) 2001 Elsevier Science B.V. All rights reserv ed.