The influence of land-use intensification and biodiversity on grassland biomass, water use and plant functional traits

Abstract

Agricultural intensification has transformed most grasslands of Central and Western Europe from extensive to highly intensive management during the last 50 years, resulting in a strong increase in biomass production and large reductions of plant species diversity. We investigated the impact of different management intensities and plant diversity on the structure and functioning of temperate pastures and meadows. The study took place within the framework of two projects in the Solling Mountains, Central Germany in the years 2008 to 2010: The BIOMIX project aims at disentangling the effects of plant functional group abundances and different types of livestock on species composition, grassland yields, plant nitrogen status and biomass allocation. The objective of the GrassMan project is to investigate the effects of plant species richness and land-use intensification on the above mentioned parameters and the water balance of meadows. At the BIOMIX site, we investigated the effects of different types of livestock in pastures with different abundances of herbs and grasses on above- and belowground biomass and nitrogen allocation. Pastures were grazed by cattle, sheep, or a mixture of both for two years before we conducted vegetation analyses, biomass sampling and biomass N analyses. Aboveground biomass was reduced up to 80 % by grazing while belowground biomass was not altered. Cattle reduced aboveground biomass to a larger extent than sheep in diverse pastures while sheep grazing tended to do so in grass-dominated pastures. The aboveground N pool was less reduced than biomass, because grazing reduced the C/N ratio of aboveground biomass relative to the ungrazed control. The GrassMan project is conducted in a matrix of meadow plots at an old grown permanent grassland site. Plots differ in fertilization (no vs. NPK-fertilization) and cutting frequency (one vs. three cuttings per season) and a gradient of species richness was created by the application of herbicides either against dicotyledonous or monocotyledonous species. One of our approaches at the GrassMan site aims at analyzing the effects of species richness under altered cutting frequencies with and without fertilization on above- and belowground biomass and biomass production. While fertilization had a strong positive effect on standing aboveground biomass and aboveground biomass production and increased cutting frequency a minor positive effect on aboveground biomass production, the effect of plant species richness on aboveground biomass was insignificant and aboveground biomass production was negatively related to species richness. Root biomass and distribution patterns gave no indication of belowground complementary resource use at this site. Another study, also conducted at the GrassMan site, focused on the effects of different land-use intensities on the water balance of temperate meadows. With small weighable lysimeters, we measured evapotranspiration and infiltration rates in response to different management intensities during the growing season 2009. Aboveground biomass production, belowground biomass, root length density, plant diversity, water use efficiency, and climatic factors were also measured. Fertilization increased aboveground biomass production by 50 70 % and stand evapotranspiration by 10 15 %, while infiltration and groundwater recharge decreased by about 50 %. Consequently, fertilization increased the water use efficiency of the grassland plants by 20 30 %, while increasing the mowing frequency from one to three had no significant effect. We found close relations between aboveground biomass production and evapotranspiration or infiltration and conclude that grassland management intensification influences the water balance primarily through fertilization effects on productivity. The last focus was on plant functional trait responses to climatic conditions and different land-use at the GrassMan site in 2009 and 2010. We measured the specific leaf area (SLA) of eight grassland species and related the values to mass- and area-based nitrogen concentrations of the leaves (Nmass, Narea, respectively). It could be shown that NPK-fertilization led to generally higher SLA, Nmass and Narea values, while the effect of altered cutting frequencies on leaf traits was more species-specific. A dry period followed by a period of high precipitation led to higher values of these traits in 2010 compared to 2009. Differences in species responses to management altered the trait-based species rankings and the SLA-Nmass and SLA-Narea relationships. These results indicate that plant functional traits are not as consistent between management and years as suggested by earlier studies.