Can faster growth compensate for increased mortality in subtropical dry forest fragments?
Abstract
Capturing the effects of fragmentation and ongoing changing climate on the population dynamics of long-lived trees requires long-term datasets, but these are uncommon in rainforests and dry forests outside of the tropics. This study capitalised on nine 0.04-ha permanent plots established in 1982 within corridors of old-growth subtropical dry forest (Araucarian vine forest) retained as fire breaks within forestry plantations in Imbil (southern Queensland, Australia). Tree diameter growth and survival were censused in 1997 and 2021, resulting in two monitoring periods. The most recent period was associated with an increasing trend in vapour pressure deficit (VPD). Consistent with this trend, we found that survival was substantially lower across all size classes in the second period. Mortality-induced reductions in stem density were associated with faster growth rates in all but the largest stems in the second period. Growth was also moderately faster in plots near forest edges in the second period. The richness of obligate understory species declined significantly by an average of 1.44 species over the 40-year study period. Synthesis and applications: Overall, our results are consistent with increasing tree mortality rates reported recently for the Australian wet tropics and suggest widespread and alarming impacts of increasing VPD on rainforest and dry forest community dynamics. To increase forest resilience in a changing climate, we recommend the retention of a buffer of plantation trees adjacent to old-growth forest corridors; widening the forest corridors using faster-growing species identified in this study and maintaining connections between scrub breaks and larger tracts of forest for species dispersal.