Eucalypts forming a canopy functional type in dry sclerophyll forests respond differentially to environment

Eucalyptus dives Schauer, E. mannifera Mudie, and E. rossii R. Baker & H.G. Smith are sympatric trees that form a canopy functional type in dry sclerophyll forests of south-eastern Australia. Although their ranges broadly overlap, distributions of the three species differ subtly within an abstract environmental space defined by mean annual temperature and precipitation. We used a combination of common environment and field studies to assess the extent to which these eucalypts might respond differentially to environmental conditions and to determine whether distributional differences are related to physiological or morphological attributes of leaves. In three glasshouse experiments, gas exchange characteristics of E. dives and E. rossii seedlings were remarkably similar. However, data indicated that E. rossii makes more efficient use of nutrients and, under some conditions, has higher water-use efficiency than the other species. In five natural stands, there were significant differences among sites and among species for leaf nitrogen and phosphorus content, carbon isotope discrimination, and specific leaf area. Significant site x leaf-variable interactions showed that the species responded differentially to unique site conditions. At four of the five sites, ¹³C discrimination was significantly higher in E. mannifera than in the other two species, and it was significantly higher in E. mannifera than in E. rossii at all five sites. Lower water-use efficiency may restrict E. mannifera to sites on the higher end of the precipitation gradient, whereas higher water-use efficiency may give E. rossii an advantage on arid sites. Variation in specific leaf area across sites was greater in E. mannifera and E. rossii than in E. dives; in contrast, E. dives was more variable in ¹³C discrimination. Greater physiological plasticity could explain why E. dives occupies a broader range of habitats than the other two species. Contrary to expectations, no leaf variable was strongly correlated with the amount of rainfall received at the sites, but ¹³C discrimination was negatively related to mean annual radiation received, which may be a better index of water availability. Despite similarities in gas exchange physiology, members of this putative functional type clearly respond differentially to varying environmental conditions, implying that accurate prediction of responses to environmental change would require knowledge of attributes of each species. However, differential responses might contribute to functional stability on a site subjected to environmental variability.