Hayato Abe, Tomonori Kume, Ayumi Katayama

Overpopulated ungulates reduce the biomass of understory vegetation and promote the expansion of unpalatable plants in world forests. These understory degradations possibly influence sensitivities of soil respiration (Rs) and heterotrophic respiration (Rh) to temperature and moisture. Here, we examined this possibility in a cool-temperate forest in southern Kyushu, Japan. At the study site, the dominant understory vegetation, dwarf bamboo (Sasa; Sasamorpha borealis), has been lost and replaced by an unpalatable shrub, Asebi (Pieris japonica), owing to sika deer feeding. We targeted three understory vegetation types, namely, 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 also evaluated understory conditions such as surface litter amount, fine root biomass, and soil physio-chemical properties to explore factors influencing the temperature sensitivity proxy (Q10) of Rs and Rh. The temporal variation of Rs and Rh was affected strongly by soil temperature and weakly by SVWC for all understory types. Differences in Q10 among SU, NU, and AU were comparable to the differences in Q10 among measurement points within the same understory type. Spatial variation in Q10 of Rs and Rh was explained by fine root biomass and surface litter amount, respectively. There were no differences in fine root biomass and surface litter amount among understory types. The lack of difference in surface litter amount can be explained by the minimal litter runoff associated with the alteration from SU to NU and AU due to the flat topography. Our findings indicate that understory loss and species replacement caused by deer do not affect the sensitivity of Rs or Rh at our site, which is characterized by flat topography.