Ecological communities are structured through a combination of niche-based ecological filtering and the chance arrival of propagules from a regional species pool. Understanding how the relative strengths of these mechanisms vary over space and time is an ongoing challenge in ecology. One way to evaluate the presence and strength of ecological filtering is to compare the composition of arriving propagules to the composition of those that establish. In plants, it is possible to go a step further and characterize how ecological filtering influences transitions between life stages, and in doing so, identify which stages affect the composition of mature communities most strongly. In this study, we combine data on seed rain, seed banks, seedling germination, seedling establishment, and the composition of mature communities in a network of 12 alpine grassland sites in southern Norway. We measure the strength of ecological filters over life stages by comparing them to stochastic null models, and then use species trait patterns to uncover potential mechanistic processes at work.
Results/Conclusions
We found strong evidence for non-random trait-based ecological filtering to occur in grassland community transitions between all life stages. In particular, diversity of emerged seedlings was lower than predicted based on the compositions of seed bank and seed rain communities, and diversity of established seedlings was lower than expected based on the composition of emerged seedlings. Species with higher specific leaf area were disproportionately likely to emerge and establish, indicating that the ability to grow quickly when resources are available is an advantageous trait following disturbances in these alpine meadows. We also found smaller seeds to predominate in the seed bank, while larger seeded species were favored to germinate. These findings illustrate a possible tradeoff between the probability of seed and seedling survival. We conclude with a discussion of our findings in light of current climate change projections for southern Norway, highlighting seed dispersal as a process likely to hinder community response to rapid shifts in temperature and precipitation.