Spatial aggregations in annual wild plant communities: Competition, Performance, and Coexistence

Abstract

Biodiversity in agroecosystems has been declining at an alarming rate. Already in 1982, 15 of the 93 arable wild plant species listed in the German Red Data Book were extinct, which equalled 25% of all extinct plant species in Germany. In the European Community agri-environment schemes including incentives to create species-rich field margin strips have been established to improve the conservation of plant communities. However, establishing such communities is often not successful due to a few common and very competitive species. Competition is known to determine the composition of plant communities. Individual plant performance depends on the distance, size, and identity of neighbouring plants. In our experiments, we studied the effect of intra- and interspecific competition on the performance of annual arable wild plant species. We investigated whether a certain spatial seeding pattern, namely intraspecific aggregation, enables competitively weak species to establish and to propagate. Moreover, we studied whether the effect of intraspecific aggregation will change depending on the spatial scale, species combination, and nutrient regime. In the present work, we analysed two field and two pot experiments of varying species mixtures with grasses, forbs and legumes in different combinations and with different treatments. We showed that intraspecific aggregation had a positive effect on the performance of competitively weak species, irrespective of species identity and experimental setup. Increasing the scale of intraspecific aggregation enhanced especially the biomass production of the legume species Trifolium campestre and Medicago lupulina. Poa annua and Bromus mollis responded to the large scale of aggregation: Poa annua yielded more biomass, while the biomass of Bromus mollis decreased. Moreover, increasing the scale of intraspecific aggregation in a small-scale pot experiment showed that the effects of intra- and interspecific competition were species-specific and scale-dependent. The decreasing performance of the competitively strong species was detectable at a large scale of interspecific aggregation, whereas, the enhanced performance of the competitively weak species was already detectable at a small scale of interspecific aggregation. Hence, intraspecific aggregation may already at a small scale of spatial aggregation reduce the dominance of competitively strong species. Functional group identity of the species in mixtures had a strong effect on the performance of a single species. Grass species were positively affected by the presence of legumes, whereas the forb species did not respond. The performance of legume species was enhanced in the only-legumes mixture, but, as soon as grass or forb species appeared as competitors, their biomass decreased due to the higher (interspecific) competition. In contrast, the competitively strong forbs Calendula arvensis and Centaurea cyanus performed better in multi-species mixtures with species of different functional groups, while they were suppressed in the only-forbs mixtures. Nutrient availability was crucial for the performance of all species. Grass and forb species suffered from nutrient deficiency while the legume species produced relatively more biomass in nutrient poor plots. The positive effect of intraspecific aggregation was most pronounced in nutrient rich plots for the competitively poor legumes. It was as well pronounced in species rich mixtures: In six-species mixtures yielded Medicago lupulina only individuals when cultivated intraspecifically aggregated. Competition in the randomly dispersed plots was so strong that all individuals died until harvest. These results highlight the importance of intraspecific aggregation for plant survival and plant-diversity conservation especially when competition is severe. In conclusion, we provide strong and experimental evidence that the performance of competitively weak plant species can be enhanced by intraspecific aggregation. Competitively weak and therefore often particularly rare and endangered plant species can be tter establish and propagate when cultivated intraspecifically aggregated and might, in the long run, establish sustainable populations in the local species pool. Hence, sowings with spatial seeding patterns can successfully contribute to conservation measures by enabling the coexistence of plant species with different competitiveness and thereby, enhance overall plant biodiversity.