Changes in the Abundance of Grassland Species in Monocultures versus Mixtures and Their Relation to Biodiversity Effects

et al. (2013) Changes in the Abundance of Grassland Species in Monocultures versus Mixtures and Their Relation to Biodiversity Effects. PLoS ONE 8(9): e75599. doi:10.1371/journal.pone.0075599 Changes in the Abundance of Grassland Species in Monocultures versus Mixtures and Their Relation to Biodiversity Effects Elisabeth Marquard 0 Bernhard Schmid 0 Christiane Roscher 0 Enrica De Luca 0 Karin Nadrowski 0 Wolfgang W. Weisser 0 Alexandra Weigelt 0 Jon Moen, Umea University, Sweden 0 1 UFZ - Helmholtz Centre for Environmental Research, Department of Conservation Biology, Leipzig, Germany, 2 Institute of Evolutionary Biology and Environmental Studies and Zurich-Basel Plant Science Centre, University of Zurich , Zurich , Switzerland , 3 UFZ - Helmholtz Centre for Environmental Research, Department of Community Ecology , Halle, Germany , 4 Institute of Biology, University of Leipzig , Leipzig, Germany , 5 Terrestrial Ecology/Department of Ecology and Ecosystem Management, Technische Universita t M u nchen, Freising-Weihenstephan, Germany, 6 German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig , Leipzig , Germany Numerous studies have reported positive effects of species richness on plant community productivity. Such biodiversity effects are usually quantified by comparing the performance of plant mixtures with reference monocultures. However, several mechanisms, such as the lack of resource complementarity and facilitation or the accumulation of detrimental agents, suggest that monocultures are more likely than mixtures to deteriorate over time. Increasing biodiversity effects over time could therefore result from declining monocultures instead of reflecting increases in the functioning of mixtures. Commonly, the latter is assumed when positive trends in biodiversity effects occur. Here, we analysed the performance of 60 grassland species growing in monocultures and mixtures over 9 years in a biodiversity experiment to clarify whether their temporal biomass dynamics differed and whether a potential decline of monocultures contributed significantly to the positive net biodiversity effect observed. Surprisingly, individual species' populations produced, on average, significantly more biomass per unit area when growing in monoculture than when growing in mixture. Over time, productivity of species decreased at a rate that was, on average, slightly more negative in monocultures than in mixtures. The mean net biodiversity effect across all mixtures was continuously positive and ranged between 64-217 g per m2. Short-term increases in the mean net biodiversity effect were only partly due to deteriorating monocultures and were strongly affected by particular species gaining dominance in mixtures in the respective years. We conclude that our species performed, on average, comparably in monocultures and mixtures; monoculture populations being slightly more productive than mixture populations but this trend decreased over time. This suggested that negative feedbacks had not yet affected monocultures strongly but could potentially become more evident in the future. Positive biodiversity effects on aboveground productivity were heavily driven by a small, but changing, set of species that behaved differently from the average species. - Competing Interests: The authors have declared that no competing interests exist. Numerous biodiversity experiments suggest that, all else being equal, plant communities are more productive when they contain higher numbers of species [1]. Commonly, these studies analysed differences in the performance between high- and low-diversity plant communities and in some experiments these differences were observed over several years [2,3,4,5,6,7]. Monocultures usually provide the baseline for such studies but they have not yet been the focus of interest. However, being the reference for comparisons and analytical tools in the context of biodiversityproductivity relationships, understanding the performance of monocultures over time is of critical importance for interpreting biodiversity effects in plant communities. Long-standing agricultural knowledge suggests that a single plant species is likely to decline in its yield if grown in monoculture at the same site for multiple years [8,9]. Obviously, plants have the potential to influence the biotic or abiotic conditions they experience. For example, plants may change the soil in which they grow for the worse by an imbalanced depletion of resources [10], the release of toxic compounds [11,12] or the accumulation of soil-borne pathogens over time [13,14,15,16,17,18]. Such interactions are also known as negative plantsoil feedbacks [12,13,19,20,21,22,23]. Similarly, host-specific foliar pathogens may accumulate in monocultures if they respond positively to host density [24]. Therefore, it is conceivable that positive plant species richnessproductivity relationships are largely due to negative feedbacks in monocultures and lowdiversity mixtures rather than to complementary resource-use among species in high-diversity mixtures [13,16,18,23,25,26,27]. On the other hand, some plant species have the potential to improve the conditions of their environment, possibly by accumulating beneficial soil biota [28,29]. Such positive plantsoil feedbacks have mainly been studied in the context of plant invasions (see e.g. [30,31,32]). Interestingly, monoculture performance over time has rarely been studied in the context of biodiversityecosystem functioning relationships. Some previous studies conducted in biodiversity experiments compared the performance of particular species, but this was mostly done only at a single point in time (e.g. in monoculture: [33,34], across a diversity gradient: [33,35,36,37,38,39,40,41,42]). Where biodiversity experiments were used to compare the performance of species over multiple years the focus was usually on the biodiversitystability relationship [6,43,44,45]. To our knowledge, no study has so far explicitly addressed the question how much a potential decline of plant monocultures over time contributes to the common phenomenon of overyielding mixtures in biodiversity experiments. Here, we present a detailed analysis of the temporal dynamics in aboveground biomass production (productivity) that occurred over a period of 9 years in monocultures of 60 different grassland species belonging to a large scale biodiversity experiment (Jena Experiment). Given the continuous nutrient export caused by regular mowing, a general decline of plant biomass was expected in the Jena Experiment. Therefore, we could not assess the performance of our monocultures in absolute terms. Instead, we compared the performance of our plant species in monoculture to the performance of populations of the same species within plant mixtures of the same experiment. Specifically, we tested the following hypotheses: 1) Monocultures produce on average less aboveground commu nity biomass than plant mix (...truncated)