Future increases in the duration and intensity of drought are expected to significantly impact forest biomes by altering tree growth and potentially causing widespread mortality. The impacts of such broad-scale climatic changes may be exacerbated by biotic interactions among co-occurring trees that further increase the water deficit at the individual tree level. As such, it is critical to understand the relative strength of intra- and inter-specific competition for water to accurately predict competitive outcomes in future climates.
Using an experimental forest in south-east Tasmania, Australia, where the species composition has been manipulated, but abiotic variables (precipitation, soil type, temperature) remain constant, we ask how tree size and intra- and inter-specific competition for water influence Eucalyptus delegatensis and E. regnans canopy damage during a significant summer drought. We assess canopy damage and size of ~1300 trees along 15 transects and determined species-specific densities of surrounding competitors.
Results/Conclusions
Both eucalypts sustained significant levels of damage, with E. delegatensis trees experiencing the highest levels of damage. For both species, we show that size was negatively correlated with the level of damage, such that smaller trees were more likely to sustain high levels of drought damage than larger trees. Furthermore, a tree’s neighbourhood strongly influenced the level of damage and species-specific densities were better predictors than total density for both species. We did, however, find variation in which neighbourhood variables had the strongest effect on damage of the two eucalypts. E. delegatensis drought damage had a strong, positive correlation with Pomaderris apetala density. The effect of neighbours was less clear for E. regnans trees but again showed the same positive relationship with P. apetala density, as well as with E. delegatensis density. Importantly, we did not find conspecific density to have a strong effect on drought damage for either species but did detect a weak relationship for E. regnans whereby drought damage decreased with increasing conspecific density. Our findings demonstrate that canopy damage patterns associated with drought can be explained, at least in part, by neighbour interactions and smaller trees are far more likely to be affected than larger trees. We highlight the importance of interspecific competition for water and its role in coexistence dynamics in a changing climate. These findings call for the consideration of the composition and structure of forest communities to accurately predict their fate under future climate scenarios.