Interspecific interactions can increase productivity in grass communities

Background/Question/Methods: During the last two decades, studies focused on investigating the relationship between biodiversity and ecosystem function (productivity, stability and other physical and chemical processes occurring within ecosystems). Evidence for effects of biodiversity on plant productivity is accumulating in artificially assembled grass communities. However, the relationship of diversity-productivity remains controversial. Most of these studies have been carried out by directly broadcast sowing or broadcast sowing with soil. In these sowing methods the spatial pattern of each species was prone to aggregated rather than random, so communities offered inadequate opportunities for species interactions. We asked the following questions: First, what is the effect of interspecific interactions on community productivity? Second, how the changes in interspecific interactions affect the diversity-productivity relationship? Here we innovatively maintained the same species composition and just changed species spatial patterns (aggregated pattern and dispersed pattern) to change the interspecific interactions,aiming to directly test the impact of interspecific interactions on the diversity-productivity relationships. Two experiments were established on an arable field in 2009 and 2010 near the Heishiding Nature Reserve, Guangdong Province, China (111°53′E, 23°26′N). Experiment 1 constituted 324 plots distributed over nine replicated blocks with two spatial patterns and the same species composition (8 species). Experiment 2 with 160 plots was designed to test the effect of interspecific interactions with two spatial patterns in different diversity levels(2,4,8 species).

Results/Conclusions: In experiment 1 above-ground biomass, below-ground biomass and total biomass increased from dispersed mixtures to aggregated mixtures to monocultures. The results provided explicit evidence that dispersed pattern which provided adequate opportunities of species interactions had the greatest productivity, which demonstrated that the increases in plant community biomass were caused by increases in interspecific interactions. The ratio of root to shoot of grasslands dropped significantly from monocultures to aggregated mixtures to dispersed mixtures, which indicated interspecific interactions changed the distribution between above-ground biomass and below-ground biomass of photosynthetic products. In experiment 2 above-ground biomass increased linearly with increasing plant diversity in both aggregated pattern and dispersed pattern, while the latter’s positive slope is significantly greater. The enhancement in the positive diversity-productivity relationship within different richness levels was closely related to increases in interspecific interactions.