Soil respiration and heterotrophic respiration in a cool-temperate forest with sika deer-induced alteration of understory vegetation

Overpopulated ungulates reduce the biomass of understory vegetation and promote the expansion of unpalatable plants in world forests. Such understory degradation may influence soil respiration (Rs) and heterotrophic respiration (Rh). Here, we examined this possibility in one cool-temperate forest in southern Kyushu, Japan. At the study site, the dominant understory vegetation, dwarf bamboo (Sasa; Sasamorpha borealis), has been patchy lost and replaced by an unpalatable shrub, Asebi (Pieris japonica), owing to sika deer feeding. We targeted three understory vegetation types: Sasa understory (SU), no understory (NU), and Asebi understory (AU). The Rs, Rh, soil temperature, and soil volumetric water content (SVWC) were measured at three points in each understory type using an automatic opening/closing chamber system from August 2022 to November 2023. We then evaluated biotic and abiotic factors, including surface litter amount, fine root biomass, and soil physiochemical properties, to examine their effects on the temperature sensitivity proxy (Q10) of Rs and Rh. Annual Rs and Rh were estimated using continuously measured soil temperature data and temperature-Rs and Rh relationships. The temporal variation of Rs and Rh was strongly affected by soil temperature and weakly by SVWC for all understory types. The spatial variation in Q10 of Rs and Rh was explained by fine root biomass and surface litter amount, respectively, regardless of understory type. Differences in annual Rs and Rh among understory types were comparable to those among measurement points in the same understory type. This was due to the similar soil temperature and Q10 of Rs and Rh among understory types. Thus, inner-site Rs and Rh variations could be generated by the spatial variations in soil biotic factors, regardless of understory vegetation type in our study site. This means our study cannot clearly verify whether overbrowsing increases Rs and Rh, highlighting the necessity for future research.