Environmental Controls of Rainforest Tree Communities: Climate Change Implications for the Gondwana Rainforests
Melinda J. LaidlawABSTRACT
Questions
A 15‐year study of the Gondwana Rainforest tree community along a transect between 300 and 1100 m elevation was established to answer: (i) What changes have occurred in the community along the elevational transect, including compositional turnover; (ii) What are the environmental controls of these patterns, and have these changed during the study; and (iii) What predictions can be made about the future of the Gondwana Rainforests, and what are the implications for their management and World Heritage status.
Location
Upland subtropical and temperate rainforests of the Tweed Caldera section of the Gondwana Rainforests of Australia World Heritage Area (WHA), southeastern Queensland.
Methods
Species richness, Shannon diversity, evenness, stem counts, size class distributions and basal area are summarised for 20 plots for up to four surveys. Multivariate analyses are used to examine shifts in tree community composition and their associated environmental drivers. Existing compositional dissimilarity mapping and climate projections help to extrapolate the trajectory for tree communities in the Gondwana Rainforests.
Results
Unexpected and persistent recruitment patterns were found in rainforest tree communities along the transect over 15 years. At high elevations, tree communities are recruiting in situ, with the understorey community closely resembling that of the canopy. This pattern is significantly linked with precipitation and moisture variables. At mid‐to‐low elevations (≤ 700 m asl), the community of understorey recruits differed significantly from the composition of the established canopy in both 2006 and 2021, regardless of elevation. This pattern is significantly associated with temperature and radiation variables. The pattern of compositional dissimilarity in the tree community along the IBISCA transect is consistent with modelled turnover projections for 2030 and supports modelling that this pattern will continue under projected climate change scenarios, perhaps towards modelled turnover projections for 2070.
Conclusions
The environmental controls of rainforest floristics in this study are dependent on the elevation of observation. High‐elevation tree communities (≥ 900 m) continue to be associated with low moisture stress driven by precipitation and significant cloud water inputs, whereas mid‐to‐low elevation tree communities (≤ 700 m) continue to be associated with higher moisture stress driven by higher temperatures, evaporation and lower humidity. Floristic change is occurring in the Gondwana Rainforests, particularly at mid‐to‐low elevations (≤ 700 m) where the community of understorey tree recruits differs from the overstorey regardless of elevation. This is particularly pronounced at low elevations (300 m) where unprecedented drought and associated bushfires have resulted in tree community turnover. These changes have not been recorded at high elevations (≥ 900 m), suggesting that floristic change due to moisture stress may be mitigated by cloud water at high elevations, where it provides a cool moist refugium. Climate projections for the Gondwana Rainforests include higher temperatures, lower humidity and a rising cloud base contributing to increased moisture stress and fire risk, potentially posing a considerable threat to the Outstanding Universal Value of the World Heritage Area.