Partenaires et organisations internationales
(Anglais)
|
Universität Innsbruck, Institut für Botanik (A), Institute of Terrestrial Ecology, Banchory, Kincardinshire (UK), Institut für Terrestrische Ökosystemforschung (BITÖK), Bayreuth (D), Europäische Akademie Bozen (EAB), Bozen (I), Centro di Ecologia, Monte Bondone, (CEA), Trento (I), Instituto Pirenaico de Ecologia (IPE), Jaca (E), Paul-Scherrer-Institut (PSI), Villigen (CH), Schweiz. Institut für Umweltschutz und Landwirtschaft (IUL), Bern-Liebefeld (CH)
|
Résumé des résultats (Abstract)
(Anglais)
|
Abandonment of grasslands is an important form of land-use change in subalpine regions. It leads to changes in the vegeta-tion cover, which, in turn, may affect the radiative properties of the canopy and exchange processes between the ecosystem and the atmosphere. The first aim of this project was to quantify the differences in net radiation (Rn), albedo (a) and energy balance components between a managed (LM) and an abandoned (LMA) wet field on the northern slope of the Swiss Alps (1400 m a.s.l.), in relationship to biomass and necromass production. Continuous measurements of the energy and radiation balances were carried out during two consecutive seasons (1997/98). LM, which was mown once during the autumn, was characterized by a larger number of species, but in LMA, the dominating species (Ranunculus aconitifolius) was much more abundant. The annual production of aboveground plant biomass was similar in both fields, but litter accumulation on the ground and the amount of standing necromass was larger in the abandoned field. Early in the season, the ground 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 a 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 va-pour flux (evapotranspiration, ET) between the fields was small. Largest reduction in ET occurred during early and late parts of the season, and this was partly due to decreased soil evaporation. Although the resulting effect of abandonment on the cumulative ET for the two measuring periods was small, it was sufficient to increase the soil water content in the abandoned field, particularly during the later parts of the season. It is concluded that abandonment causes a change in canopy structure which leads to reduced ET and increased soil water content.
The effect of abandonment on the net CO2 exchange (NCE) was investigated by continuous measurements using IR gas analysers connected to Bowen-ratio-systems. CO2-release from the soil (RS) was measured independently with cuvettes. In 1997, the managed site was mown in September, but in 1998 no grass cut was possible during the measuring period. Maxi-mum NCE of 30 g CO2 m-2 d-1 were recorded in both fields in 1997 when soil water availability was high. NCE was reduced during 1998 because of drier conditions. RS attained maximum values of 40-50 g CO2 m-2 d-1, and varied during the season according to variations in soil temperature. The PPFD-dependence of NCE remained stable throughout most of the season, indicating the presence of a mixture of plant species with younger and older individuals during most stages of canopy devel-opment. The comparison of C contents in aboveground plant mass and cumulative C assimilation suggested allocation of a large fraction of assimilated C to shoots between early spring and the end of June, and to below ground components later in the season. Translocation below ground was larger in the managed than in the abandoned field. Integration of NCE over the measuring period gave net C acquisitions at LMA of 268 and 99 g C m-2, and 182 and 273 g C m-2 at LM in 1997 and 1998, respectively. These figures suggest that abandonment reduced the capacity of the system to absorb C from the atmosphere before the cutting date at the managed site, whereas after being mown the managed site become a strong net C source and the seasonal NCE was lower than at the abandoned site. The results indicate that the difference in the system C balance between abandoned and managed fields mainly depends on the timing of the grass cut. This suggests that the C balance of wet grasslands could be improved by a late cutting date.
|
