Effects of nitrogen addition and above-ground biomass removal on the growth and interactions between species of xerothermic grasses

Aspects of global change, such as increasing atmospheric nitrogen deposition and changes in land-use, promote the increasing dominance of grass in xerothermic grasslands. It is assumed that nutrient addition and land use affect interactions between the expanding Bromus erectus and other co-occurring xerothermic species of grass in Central Germany. The plant-plant interactions are key factors in the regulation of species composition, productivity, structure and dynamics of plant communities as well as the functioning of ecosystems. A competition experiment between B. erectus and the grasses Brachypodium pinnatum, Stipa capillata and Stipa tirsa was carried out using a replacement design, with species cultivated at two densities (9 individuals or 1 individual per pot) and three different species compositions of nine plants in monoculture and mixtures (6:3, 3:6), to study the effect of B. erectus on the other three grasses. Treatments included nutrient addition (nutrient-poor vs. nutrient-rich) and aboveground biomass removal (unclipped vs. clipped). Over two consecutive years, each species’ growth was documented by measuring both vegetative traits (i.e. above-ground biomass, plant height, specific leaf area (SLA), leaf nitrogen concentration (LNC)) and generative traits (panicle length, number of inflorescences, seed mass). Plant-plant interactions were assessed by calculating the relative interaction index (RII). Bromus erectus increases at the detriment of the other three species because of its highest biomass production when nutrients are added. When B. erectus was grown alone, biomass production significantly increased when nutrients were added. The effect of intraspecific competition in B. erectus was slightly more intensive than interspecific competition, in contrast to the other grasses. Also, B. erectus was dominant when nutrients were added, but less so when clipped. As a result, the growth and competitive ability of B. erectus affected S. capillata and S. tirsa more negatively than B. pinnatum, which can result an increasing dominance of B. erectus under future increasing nitrogen enrichment, leading to displacement of the two rare species of Stipa.