Combined effects of gap creation and deer exclusion on restoration of belowground systems of secondary woodlands: A field experiment in warm-temperate monsoon Asia

Ground-layer vegetation in abandoned woodlands of the Northern Hemisphere is rapidly declining because of overshading and an overabundance of ungulate herbivores; these changes may be affecting belowground ecosystems. An effective countermeasure is to restore ground-layer vegetation by establishing deer exclosures immediately after creating canopy gaps. However, such measures disturb belowground processes (e.g., decelerate decomposition rates and decrease soil water content) and may cause further disintegration of the whole system. To estimate the combined effects of gap creation and exclosure installation, we established a 2 × 2-factorial experiment (including gap creation, exclosure installation, both methods, and a control) in warm-temperate secondary woodlands. After five years, we studied ground vegetation, litter cover, soil-water physics, abundance of litter-dwelling invertebrates, and litter decomposition in each experimental plot. A significant increase in ground vegetation was found in gap-and-exclosure plots and gap-and-deer plots, although the increase in the latter was limited to approximately half of that in the former; in contrast, no significant increase was found in the control or canopy-and-exclosure plots. The abundance of litter-dwelling invertebrates was significantly reduced by gap creation, although this effect was offset by installation of exclosures in gaps. Fine porosity and water holding capacity of topsoil were reduced by gap creation, but these negative effects were mitigated by exclosures installed in gaps. Meanwhile, coarse porosity and water permeability of topsoil were not affected by gap creation; they tended to be increased by exclosure installation both in gaps and under the canopy. Litter decomposition tended to be delayed in the gap-and-deer plot, although this effect was temporary; exclosure installation slightly increased decomposition rates, but this effect was limited. These results show the possibility of restoring belowground dynamics in abandoned forests by creating small canopy gaps and establishing ungulate exclosures. This mixture of interventions should increase habitat heterogeneity in these forests and help prevent biological homogenization while mitigating long-term negative effects of gap creation on belowground ecosystems.