Context-dependent effects of deer on aboveground carbon stocks 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 studies, assessing the effects of deer exclusion on aboveground carbon stocks within the seedling and sapling layer (≤3m) 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). Our results show that including species-specific values in calculations had a minimal effect on the estimated impact of deer exclusion on carbon stocks compared to generic values. Our results show that preventing deer herbivory did not have a consistent positive effect on aboveground carbon stocks in the regenerating forest layer, except in sites dominated by conifer saplings and boreal sites. Instead, the among-study 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 for effective forest conservation efforts in response to growing deer populations.