Plant dispersal in a changing climate. A seed-rain study along climate gradients in Southern Norway

Abstract

Seed dispersal is a key event in a plant's life, affecting the outcomes of many ecological processes ranging from species reproduction to population and community dynamics. It is influenced by various environmental and ecological variables and is therefore expected to vary among and within communities. Understanding how dispersal varies in space and time is important, as the effectiveness of dispersal may modify abundance and diversity patterns observed in nature. This work explores seed rain patterns in grasslands in western Norway and how they are influenced by climate. Seed rain was examined during one year throughout twelve grassland sites (ca 4°50' - 8°45' E; 60°20' - 61°50' N) arranged in a climate grid" design where the effects of temperature and precipitation can be decoupled. Mean summer temperature ranges from 5.9oC to 10.8oC primarily driven by elevation and continentality, while annual mean precipitation ranges from 596 mm in the east to 3029 mm in the west. I hypothesized that seed rain, being the primary reflection of dispersal, might: show differences at species, community and landscape scales; be highly influenced by adult vegetation; be constrained by the distance of the dispersal source. About 15 800 seeds from 122 species fell into the seed traps. Temperature appears to be the most important factor limiting seed rain density through the grid, with fewer seeds recorded in colder-climate sites. The results also show that temperature affects correlations between seed and plant abundance within species and restricts dispersal distances through an interaction with precipitation. 98% of the seeds came from the vegetation; the remainder has been assigned to long distance dispersal between communities and this process appears to be regulated by climatic variables. Adult plant abundance and deposited seeds at species level were correlated. Seed rain and vegetation diversity followed broadly the same patterns along the grid while inter and intra-specific variability is more strongly linked to environmental variables and different vegetation composition driven by altitude In general, found relationships appeared to be stronger when zooming up from community-site to landscape-gradient scale. Thus, climate seems to play a role in seed rain variability, affecting dispersal processes on the scale of this study. These results provide a trigger for more detailed, longer-term studies of the effect of climate on seed dispersal.