Increasing water availability strengthens conspecific negative density dependence but weakens heterospecific negative density dependence in Japanese old-growth forests

The relative magnitudes of negative conspecific and heterospecific density dependence (CDD and HDD) have been widely used to explain tree species diversity. However, it remains unclear how the negative effects of CDD and HDD on tree growth are affected by temporal climate variability and the implications of any change for diversity maintenance. We used a repeated-measured forest inventory dataset (19 1-ha plots of old-growth natural forests with 11,560 trees spatially mapped from 2004 to 2018) in Japan to investigate how temporal climate variability affected CDD and HDD. Both water availability and temperatures increased during the study period, and tree growth increased with temporal warming. After accounting for the effects of spatial variation in annual temperature and water availability (precipitation minus potential evapotranspiration) and soil properties, negative CDD increased while negative HDD decreased with the temporal increase in water availability. When assessed by tree growth stages, we found that negative CDD in saplings (diameter at breast height; DBH <10 cm) increased, while negative HDD in adult trees (DBH >10 cm) decreased with temporally increasing water availability. Our findings suggest that increasing water availability strengthened negative CDD in saplings and weakened negative HDD in adults, potentially stabilizing species coexistence. This implies that higher water availability may play an important role in influencing species coexistence dynamics under climate warming.