Reduction in forest carbon stocks by sika deer-induced stand structural alterations

Abstract

Ununiform, excessive, and prolonged understory vegetation feeding by overpopulated ungulates (over-browsing) leads to heterogeneous stand structural alterations at the landscape level. However, it is not yet clear how changes in various types of stand structural alterations affect total carbon (C) stocks in forest ecosystems as the sum of aboveground C stocks (AGC) and belowground C stocks (BGC). We aimed to fill this gap in a cool temperate broadleaf–conifer mixed forest in southern Kyushu, Japan, where over-browsing of sika deer has occurred since 1980. Four replicate survey plots were established in four stand types, namely, stands with presence of understory vegetation (PU), stands with no understory vegetation (NU), stands dominated by unpalatable shrubs (SR), and stands with canopy gap areas with a lack of regeneration (CG). Using PU as a baseline, alterations in stand structure (e.g., stem density) and C stocks were evaluated for each stand type. We measured C stocks of overstory trees (height > 2 m), understory vegetation, leaf litter, fine woody debris (FWD), and coarse woody debris (CWD) as AGC. C stocks of fine roots, coarse roots, and soil organic matter (SOM) at 0–30 cm depths were measured as BGC. We also separated overstory trees according to the palatability of the deer diet. The alteration from PU to NU did not reduce AGC, BGC, or total C stocks. The alteration from PU to SR and CG was found to potentially reduce total C stocks and AGC by up to 49% and 59%, respectively. These reductions in SR and CG were mainly driven by the decreased C stock of palatable overstory trees. There were dense unpalatable shrubs in SR and large amounts of CWD in CG. However, neither was sufficient to offset the loss of palatable overstory trees in SR and CG. We also found a reduction in the C stock of leaf litter and SOM at 0–10 cm depth in NU, SR, and CG. These reductions were likely caused by reduced litterfall and increased soil erosion, implying future reductions in BGC. Our results suggest that stand structure conservation is a key guideline for C stock retention when implementing forest management strategies to prevent over-browsing. The failure to conserve palatable juvenile trees could lead to a lack of more palatable overstory trees in the future, causing irreversible changes in AGC and BGC.

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