Stand development moderates effects of ungulate exclusion on foliar traits in the forests of New Zealand

Abstract

Summary 1. Ungulate herbivores may greatly alter the functional composition of plant communities through their selection of leaves with high nutrient concentrations and low structural or chemical defences. These impacts may have broader implications for ecosystem functions via changing rates of litter decomposition and the relative abundance of foliar traits. However, it is unlikely that ungulate impacts will be consistent across different vegetation types and different environmental conditions. 2. We compared shifts in abundance‐weighted trait means and functional diversity indices in 88 pairs of plots fenced to exclude ungulate herbivores (exclosures) and adjacent control plots for eight foliar traits related to ungulate palatability in forest ecosystems. The stem density and basal area per species were both used to weight trait means and functional diversity indices, since these are proportional to recruitment and litter production, respectively. We also regressed exclosure effects on environmental and stand development variables to test whether context influenced the impact of ungulate exclusion. 3. There was a consistent, significant trend across seven of the eight leaf traits examined for increased palatability inside exclosure plots relative to control plots when trait means were weighted by the number of stems per species, but not when weighted by basal area. This suggests that ungulate exclosure consistently increased recruitment of palatable species, but is unlikely to have yet affected mean litter quality. Exclosure effects were linked to stand development variables, with effects being greatest in disturbed stands (where total basal area decreased and the number of stems increased), and least in stands undergoing competitive thinning. 4. Synthesis. The consistency of exclosure effects indicates that ungulate impacts on plant communities in New Zealand’s indigenous forest may be reversible at zero deer density, while their dependence on stand development suggests that the ungulate control impacts may be greatest in seral shrubland or forest communities.