Seed dispersal interactions across elevational gradients: Does microclimate at deposition sites match with plant regeneration requirements?

Background/Question/Methods

Three important components have been distinguished that determine distributional ranges of plants: Local abiotic factors, biotic interactions, and the ability of species to disperse. Understanding these processes will be key to comprehend the responses of plants to environmental change. Here we focus on quantifying the dispersal ability of plants, which is particularly challenging since dispersal often depends on plant-animal interactions. We conducted a study to investigate how animal-mediated seed dispersal and local environmental conditions shape the establishment of a plant across its elevational distribution. In particular, we investigated the mutualism between Swiss stone pine (Pinus cembra) and its dispersing bird, the Spotted nutcracker (Nucifraga caryocatactes) to ask how seed deposition by animals matches with the plant’s regeneration niche across its elevational distribution. The degree of matching is important to better understand how animal-mediated seed dispersal shapes plants distributional ranges. We observed seed deposition by nutcrackers for 152 hours in the Swiss Alps at nine elevations across two mountain slopes. Across the same gradients we experimentally deployed 180 artificial seed caches to unveil the pine’s environmental requirements to regenerate. We characterized observed and experimental caching sites by recording environmental variables decisive for pine establishment.

Results/Conclusions

We found a significant mismatch between microsites of high seedling establishment and seed deposition rates. Caching rates by nutcrackers significantly increased with decreasing canopy openness, soil moisture and vegetation cover, respectively. In contrast, we found that seedling establishment rates significantly increased with increasing canopy openness, soil moisture and vegetation cover. According to these results, nutcrackers counteracted pine regeneration by mostly caching seeds at sites that did not meet the optimal requirements of pine seeds to germinate. Such a directed caching behaviour, which seems detrimental for the pine, may severely affect its establishment in the environment. Additional analyses shall verify whether this pattern is consistent across the elevational distribution of the pine. Our study helps to better understand the mechanisms behind animal-mediated seed dispersal and plant establishment across elevational gradients. Quantifying the critical linkage between seed deposition and plant regeneration across environmental gradients will be one step forward in determining the potential range dynamics of plants in the face of climate change.