Context-dependent effects of deer on aboveground carbon stocks in the regenerative tree layer in temperate and boreal forests: A meta-analysis

Herbivores, including deer (Cervidae), influence ecosystem functions and carbon cycling by affecting vegetation structure and composition. Given the increase in deer populations in Europe and North America, there is growing interest in their impact on carbon dynamics in temperate and boreal forests. We investigated the effects of deer on carbon stored in the seedling and sapling layer through two mechanisms: (1) deer affecting the overall aboveground woody biomass in these strata and (2) deer changing the composition of tree species in these strata, because species differ in wood density and carbon content. We performed a meta-analysis of 17 datasets from 12 cases, assessing the effects of deer exclusion on aboveground carbon stocks within the seedling and sapling layer (≤ 3 m) under two scenarios of carbon calculation, where we used either: (1) species-specific carbon content and wood density values (mean scenario) and (2) generic carbon content and wood density values (neutral scenario). Most cases reported inconclusive effects of deer exclusion on aboveground carbon stocks, regardless of species-specific carbon content or wood density values. A total of 35 % of cases showed positive outcomes, indicating that an increase in carbon stocks within deer exclosures. Despite the short duration of cases, and high variation in effect sizes between them, we still found a small, significant difference between coniferous- and deciduous-dominated forest systems. The among-case variability in effects suggests a complex interaction between the impact of deer browsing and forest carbon. Understanding these interactions is vital to assess wildlife-carbon relations and to develop appropriate forest conservation approaches in response to growing deer populations.