Community-wide phenological shifts along a soil resource gradient reveal the role of phenology in grassland community assembly

Background/Question/Methods

The timing of growth and reproduction have a major impact on plant fitness within communities, not only by providing adaptations to seasonal abiotic constraints, but also by promoting or mitigating biotic interactions. Within communities, reduced phenological overlap among plants may facilitate their coexistence by providing a temporal release from competitive interactions. Surprisingly, few studies have quantified the phenological structure of plant communities along environmental gradients. This study investigates how the timing and overlap of both vegetative and reproductive phenologies among coexisting species may structure natural plant community assembly in response to a gradient in soil resources.

The phenology of 90 plant species distributed within 12 communities was surveyed weekly along a gradient of soil depth and resource availability in calcareous Mediterranean rangelands of the South of France. In these rangelands, plant growth is constrained by severe summer drought and cold winters. The onset and duration of key phenological phase (leaf budding, flower budding, flowering, fructification, seed dispersal, whole-plant summer desiccation) were compared among communities in terms of mean and variance, with or without taking species abundances into account. Using a null model approach, we investigated the degree of phenological divergence and overlap among species within communities.

Results/Conclusions

We found that communities on shallow resource-poor soils had an overall earlier phenology than communities on the deeper and richer soils, which was a result of both intraspecific variability and species turnover in response to earlier summer drought and abiotic constraints on the shallower soils. Moreover, shallow soils harboured a larger diversity of vegetative phenological strategies in terms of vegetative response to the dry season (i.e. onset of summer wilting). By contrast, the timing of reproductive phases tended to be generally convergent and to overlap among species during the peak of biomass in spring, except towards the deeper soils. Flowering overlap within communities decreased significantly towards the deeper soils, revealing a temporal partitioning of reproduction possibly promoting species coexistence.

Our results show how a simple gradient in local soil resource availability may involve important community-wide shifts in both vegetative and reproductive phenology, mainly through species replacement, and less through intraspecific variations. This strong response of community phenological structure to environmental conditions highlights the major role played by plant phenology in community assembly. These findings have important implications for conservation issues in relation to plant invasions and community reassembly in a changing environment.