Large herbivores affect the stoichiometry of nutrients in a regenerating woodland ecosystem

Previous studies on the effects of herbivores on nutrient cycling have given little consideration to the relationship between soil nitrogen (N) and phosphorus (P) availability. Here we examined how browsing by red deer influences the relative availability of N and P in a regenerating woodland ecosystem. We found that removal of browsing by fencing for 14 years led to a shift from N toward P limitation of the dominant tree species Betula pubescens. This was evidenced by a significant increase in foliar N:P ratio of B. pubescens as a result of removal of browsing; mean N:P ratio of foliage from browsed areas was 13.2 suggesting that trees growing in browsed areas were N limited, whereas foliage from unbrowsed areas had a mean N:P ratio of 15.8, suggesting that these areas were more P limited. Further evidence of a shift toward P limitation in unbrowsed areas came from the finding that root uptake of labelled P-32 was significantly greater in roots collected from unbrowsed than browsed trees. Soil phosphatase activity did not significantly differ between browsed and unbrowsed areas. Our data indicate therefore that herbivores have the potential to significantly affect the stoichiometry of N and P in forest ecosystems.