Examining the influences of site conditions and disturbance on rainforest structure through tree ring analyses in two Araucariaceae species

• Tree ring annularity in A. Robusta and A. cunninghamii N. Queensland, Australia confirmed.
• Ring-based ages and growth trajectories varied for forest sites with differing stand structure.
• Tree rings are key to understanding effects of site conditions and disturbance on forest structure.

Tree growth is central to the dynamics of forest ecosystems. Patterns in tree diameter growth from tree rings can yield an understanding of the growth trajectories of trees and how they vary with site conditions but detailed studies have been conducted on relatively few species in tropical forests. Furthermore, tree rings provide information on temporal patterns of tree establishment and thus the influences of disturbance and abiotic conditions on forest structure. Here, we confirm the annularity of growth rings in Agathis robusta and Araucaria cunninghamii from North Queensland, Australia. We then examine tree growth trajectories in two contrasting natural forests (Downfall Creek and Gillies Range) and a plantation. Growth in plantation over 70 years was high and growth trajectories were similar among trees within each species. In natural forests age and diameter were only weakly correlated for both species. Growth trajectories were similar and homogeneous for both Agathis and Araucaria in the plantation, whereas there was a great deal of variation in tree ages and growth trajectories observed for the two forest sites. These differences are likely related to dynamics driven by climate and soil that modulate boundaries between sclerophyll and rainforest over the long-term. Downfall Creek (a ridge with poor shallow soils) is likely recently invaded (past couple of hundred years) sclerophyll woodland. The alternative hypothesis—that structural and compositional characteristics result from local disturbance resulting from World War II training activities—was not strongly supported by Agathis establishment dates. The study of tree rings in tropical trees is underappreciated and can provide valuable information on the influences on tree growth and disturbance in tropical forests.