Benjamin M. Delory, Emanuela W.A. Weidlich, Philipp von Gillhaussen, Vicky M. Temperton
Numerous studies have shown the existence of a positive relationship between plant species richness and functional richness (i.e. the number of different types of plants) and the amount of biomass (i.e., yield) produced in grassland ecosystems. In fact, the yield achieved by a mixture of species is often greater than the average of the monoculture yields for the component species. This positive effect of biodiversity on grassland productivity is also called overyielding. Both species complementarity (complementarity effect) and dominance of high performing species (dominance effect) can lead to grassland overyielding.
The functioning of grassland ecosystems, however, is not only affected by plant species richness. Indeed, both the order and timing of plant species arrival during community assembly can have long-lasting effects on the composition and functioning of ecosystems. This phenomenon is a biotic component of assembly history and is referred to as a priority effect. Depending on whether the establishment of late-arriving species is favoured or inhibited by early-arriving species, priority effects can be either positive or negative.
In this study, we investigated if plant order of arrival during assembly affected grassland overyielding and its drivers (complementarity and dominance effects). We also investigated if the magnitude of biodiversity effects was dependent on the strength and direction of priority effects.
To do so, we used plant biomass data collected from a grassland experiment located in Germany (Jülich Priority Effect Experiment). In this experiment, the order of arrival of three plant functional groups (forbs, grasses, and legumes) was manipulated by sowing one functional group either six weeks earlier or at the same time as the two others. For each plant order of arrival, we then quantified both biodiversity and priority effects.
Our results showed that the mechanisms driving grassland overyielding (i.e., complementarity and dominance effects), but not grassland overyielding per se, were affected by plant order of arrival. In addition, our analyses showed that moving from negative to positive priority effects increased grassland overyielding, mainly via stronger complementarity effects.
This study highlights the strong interlinkage of biodiversity and order of arrival effects, illustrating the need to combine both biodiversity and assembly approaches in order to improve our understanding of the functioning of grassland ecosystems and increase the predictive power of community ecology.