Impact of elevated CO2 on biomass production, forage quality and population dynamics of an extensive managed permanent grassland ecosystem : results from the long-term Giessen Free Air CO2 Enrichment (GiFACE) experiment

Abstract

Future increase in atmospheric CO₂ concentrations will potentially enhance grassland biomass production and shift the functional group composition as well as nutrient concentrations in plant tissues, with consequences for ecosystem functioning. Despite several studies about climate change impact on grassland ecosystems, long-term measurements and observations over more than 10 years continuous monitoring are scarce. To understand effects of elevated atmospheric CO₂ concentration (eCO₂) on biomass production, forage quality and nutrients concentration of the biomass, as well as effects on regeneration ability and longevity of the grassland ecosystem, four different studies were conducted. The studies were carried out at the Giessen Free Air CO₂ Enrichment (GiFACE) experiment, which was set up in 1998. The main objectives were to assess the influence of eCO₂ (+ 20 % above ambient CO₂ concentrations) on yield production, forage quality, biomass nutrient concentration and population dynamics of the ecosystem. The biomass consists of C3 grasses and forbs, with a small proportion of legumes and was harvested twice per year (end of May, beginning of September). The results of the long-term GiFACE experiment revealed effects of eCO₂ concentration on biomass production, forage quality and nutrients concentration, as well as population dynamics in the form of the soil seed bank in different ways. The functional groups grasses and forbs responded differently to eCO₂ through time. In general, total biomass increased under eCO₂ accompanied with a small increase in soil moisture. The forage quality declined due to eCO₂, especially in forbs and most nutrient element concentrations were reduced under eCO₂, especially in grasses. Forbs and legumes showed a positive eCO₂ effect in nutrient yield for single elements. Although the soil seed bank is a system where environmental changes trace only slowly into the seed bank, several significant changes in seed bank compositions and shifts in functional traits were indicated due to eCO₂. The CO₂ effect on biomass yield and forage quality is most pronounced under soil and climatic conditions that did not restrict plant growth under eCO₂. As soon as other conditions and resource limitations limited the CO₂ fertilization effect, factors like water or nutrient availability and factor interactions dominated the biomass and forage quality results under eCO₂. Thus, to obtain reliable evidence of climate change effects on biomass yield, forage quality and nutrients content, as well as the population dynamics, it is important to consider all relevant site conditions. Extreme climate events (i.e. drought, heat waves, heavy rainfall), which are expected to increase in the future, may lead to a decrease of the CO₂ fertilization effect on grassland biomass and also to a negative impact on forage quality. Furthermore, the observed changes in nutrient contents and yields of grassland biomass, accompanied with changes in the soil seed bank composition may have serious impact on ecosystem services, e.g. forage production and grassland management and adjustment of fertilization and harvest regimes might be needed.