Plant functional traits are attributes that allow plants to survive under certain environmental conditions, for example the stressful conditions associated with drought. However, independent traits usually do not single-handedly determine levels of drought resistance; it is more likely that a suite of functional traits determines how plants survive drought. In order to assess the generality of trait correlations across a rainfall gradient, we measured four traits related to drought resistance. Those traits were specific leaf area (SLA), which is an index of sclerophylly, a widely recognized drought trait, leaf angle, which can reduce excess solar radiation, stem density (SD), which is correlated with xylem cavitation resistance, and wood density (WD), which is negatively correlated with cavitation resistance and negatively correlated with water storage. In addition, leaf phenology plays an important role in drought resistance, as deciduous trees are able to reduce water loss by dropping leaves, while evergreen trees must resist drought. Correlations between these traits were examined in trees of the dry semi-evergreen vine thicket, 750-800 mm rainfall pa, and wet Type 5b ‘Mabi’ rainforest, 1300-1600 mm rainfall pa, of northeast Queensland, Australia. Both forest types occur on high nutrient, basalt-derived soils.
Results/Conclusions
SLA was significantly negatively correlated with leaf angle, though no significant difference in this relationship was found between evergreen and deciduous species. SLA was significantly negatively correlated with SD regardless of leaf phenology. When examining SLA and WD, a significant positive correlation was found in deciduous species, while a significant negative correlation was found in evergreens. This suggests that deciduous species are able to survive with only one drought resistant trait due to the flexibility that deciduousness allows, while evergreen species have developed multiple drought tolerant traits in order to survive. Because of this, evergreen trees in the wet rainforest with low SLA and high WD, and all deciduous trees, would be most likely to survive the increasing aridity that may be associated with global warming, while high-SLA, low-WD wet rainforest evergreens would be the least likely to successfully survive in their new environments, unless they are able to adapt by shifting to traits associated with drought resistance. There is some evidence of such adaptation having occurred; some species which occur in both environments have traits indicative of greater drought resistance in their low rainfall populations, although there is no evidence regarding the timeframe over which this occurred.
