Individual tree growth in jarrah (Eucalyptus marginata) forest is explained by size and distance of neighbouring trees in thinned and non-thinned plots

Growth of an individual tree in a forest depends on multiple factors amongst which size, age and stand density can be of particular importance. The ability to understand how these factors influence individual growth can help understand forest growth and dynamics more generally. The overall objective of this study was to analyse the growth of individual trees of jarrah (Eucalyptus marginata), a wide-spread species having ecological and economic importance in south west Western Australia (SWWA). More specifically, we investigated (1) how well individual tree growth could be explained by considering tree size at the start of the growth period; (2) whether this explanation could be improved by also accounting for competition from neighbouring trees in different ways, (3) how many neighbouring trees or what neighbourhood distance needed to be considered when accounting for competition; and (4) whether neighbouring tree growth could be a useful indication of competition. Tree size was significant in explaining growth. Explanation was improved when competition was included in the models, based on R² (which ranged from 0.31 to 0.68). A new spatial competition index (CI) developed in this study performed better than previously developed CIs. Explanation of diameter growth was optimized by accounting for competition from the 10 to 12 closest competitors or competitors within ~10 m from the base of the subject tree. As competition from neighbours had a significant negative effect on growth, we recommend including neighbourhood competition when explaining/predicting growth and considering thinning treatments to minimize the effect of competition on growth. The modelling approaches developed in this study are likely to be useful tools for understanding and managing individual tree growth of any species in the world where competition among trees restricts individual growth. Novelties include the first model explaining individual jarrah growth; an effective new competition index; an approach to testing how many neighbouring trees or what neighbourhood distance needs to be considered when accounting for competition; and the idea of neighbouring tree growth being a useful indication of competition.