Red deer mediate spatial and temporal plant heterogeneity in boreal forests
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Selective herbivory can influence both spatial and temporal vegetation heterogeneity. For example, many northern European populations of free-ranging ungulates have reached unprecedented levels, which can influence plant species turnover, long-term maintenance of biodiversity and the subsequent stability of boreal ecosystems. However, the mechanisms by which large herbivores affect spatial and temporal vegetation heterogeneity remain poorly understood. Here, we combined a 10-year exclusion experiment with a herbivore intensity gradient to investigate how red deer (Cervus elaphus) acts as a driver of temporal and spatial heterogeneity in the understory of a boreal forest. We measured the two dimensions of heterogeneity as temporal and spatial species turnover. We found that temporal heterogeneity was positively related to herbivory intensity, and we found a similar trend for spatial heterogeneity. Removing red deer (exclosure) from our study system caused a distinct shift in species composition, both spatially (slow response) and temporally (quick response). Vegetation from which red deer had been excluded for 10 years showed the highest spatial heterogeneity, suggesting that the most stable forest understory will occur where there are no large herbivores. However, excluding red deer resulted in lower species diversity and greater dominance by a low number of plant species. If both stable but species rich ecosystems are the management goal, these findings suggest that naturally fluctuating, but moderate red deer densities should be sustained.
KeywordsBiodiversity Cervids Ecosystem stability Herbivory intensity Plant communities
This study was supported by the Norwegian Research Council under the Miljø 2015 programme (project number 204403/E40) and the Norwegian Environment Agency. Thanks to Norwegian Red Deer Centre and K.-K. Berget, P. Greve, I. G. Harstad, M. Knagenhjelm, T. R. Paulsen, T. Seldal, F. Solheim, T. Stokke, S. Vatne, V. Veiberg, T. Wiberg, K. F. Øi and H. Øyrehagen for their valuable help in the field. Sam Steyaert commented on drafts of the manuscript, and Peter Frost did copy editing. Thanks also to Mark Gillespie language help. We would like to thank the two anonymous reviewers for comments that helped improving the manuscript.
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