Multiple resource limitation interacts with plant functional traits to drive productivity and diversity responses to climate
Predicting variation in ecosystem and community responses to alterations in climate is an essential goal of global change ecology. Low-productivity systems around the world have been suggested to exhibit greater resistance to changing climatic conditions than productive systems, but the detailed patterns and mechanisms remain little understood. We asked whether low-productivity systems were resistant to alterations in climate either because of nutrient limitation or because of the prevalence of plants with resource-conservative, slow-growing traits. We conducted a 5-year factorial experimental rainfall and nutrient addition in an annual grassland system where water availability limits plants growth and soil heterogeneity creates strong productivity gradients. We replicated our manipulations in three grassland habitats differing in their original productivity. We compared how these habitats responded to the resource addition treatments in terms of community biomass, diversity, and functional trait composition (i.e., height, specific leaf area [SLA], C:N ratio, leaf water content [LWC]).
We found that the impact of rainfall addition was much greater when nutrient limitation was relaxed. This synergistic impact of water and nutrients was strongest in the least productive grassland habitat, where biomass increased almost to the level of the most productive habitat, diversity declined sharply, and species and trait composition showed near-complete turnover. Initial community trait composition did not affect responsiveness to treatments, probably because of the rapid change in species composition. However, SLA and height did predict individual species’ probabilities of declining or increasing in response to watering and fertilization, demonstrating that species turnover and diversity loss consisted of the replacement of plants with resource-conservative traits (low SLA, small stature) by those with resource-acquisitive traits (high SLA, tall stature). Our results highlight that colimitation by soil nutrients and water is an essential force maintaining high diversity and the dominance of resource-conservative species in low-productivity conditions.