Experimental warming and plant species diversity and composition interactively influence seedling establishment

Background/Question/Methods:

To better predict the effects of global warming on species’ abundances in communities, and ultimately on community composition and species’ ranges, we aim to understand if, how and why the effects of warming on a given species’ abundance depend on the diversity and composition of co-occurring species. Species will colonize newly suitable areas by establishing from seed, and be extirpated from unsuitable areas when regeneration from seed is insufficient to offset mortality, so we focused on seedling establishment. We compared the survival and growth of 25 species’ seedlings in experimental grassland ecosystems receiving factorial temperature (ambient, +1.5 C, +3 C) and species number and composition (1, 4, or 16 species drawn from a pool of 18 species) treatments. We tested the hypothesis that the effects of warming on a species’ survival and growth are modified by other species primarily through competitive or facilitative interactions mediated by shared limiting resources (N, water and light), and that species vary in their responses to warming primarily due to variation in their temperature-dependent requirements for, and impacts upon, these resources. We sought to explain this variation between species in temperature-dependent resource requirements and impacts using measurable functional traits.

Results/Conclusions:

Seedling survival and growth was significantly inhibited by warming for most species. For some species, the negative effect of warming was disproportionately strong in ecosystems with fewer other species, i.e. there was an interaction between the effects of warming and species number. Similarly, surface soil moisture was reduced by warming more strongly in low-diversity communities. Growth of young seedlings is often water-limited due to their shallow root systems, so soil moisture may mediate some of the treatment effects. We explored how various functional traits of adults influenced their temperature-dependent impacts upon resource levels, and how functional traits of seedlings determined their temperature-dependent responses to those resource levels, with the goal of explaining why seedlings of different species responded differently to the treatments. Our findings provide a more mechanistic understanding of how species’ abundances may respond to changes in climate, and can be a basis for predicting how species’ ranges, community composition and plant diversity respond to climate change.