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

Abstract

Eucalyptus dives Schauer,E. mannifera Mudie, and E. rossiiR.Baker & H.G. Smith are sympatric trees that form a canopy functionaltype in dry sclerophyll forests of south-eastern Australia. Although theirranges broadly overlap, distributions of the three species differ subtlywithin an abstract environmental space defined by mean annual temperature andprecipitation. We used a combination of common environment and field studiesto assess the extent to which these eucalypts might respond differentially toenvironmental conditions and to determine whether distributional differencesare related to physiological or morphological attributes of leaves. In threeglasshouse experiments, gas exchange characteristics ofE. dives and E. rossii seedlingswere remarkably similar. However, data indicated thatE. 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 andamong species for leaf nitrogen and phosphorus content, carbon isotopediscrimination, and specific leaf area. Significant site × leaf-variableinteractions showed that the species responded differentially to unique siteconditions. At four of the five sites, 13Cdiscrimination was significantly higher in E. manniferathan in the other two species, and it was significantly higher inE. mannifera than in E. rossii atall five sites. Lower water-use efficiency may restrictE. mannifera to sites on the higher end of theprecipitation gradient, whereas higher water-use efficiency may giveE. rossii an advantage on arid sites. Variation inspecific leaf area across sites was greater inE. mannifera and E. rossii than inE. dives; in contrast, E. diveswas more variable in 13C discrimination. Greaterphysiological plasticity could explain why E. divesoccupies a broader range of habitats than the other two species. Contrary toexpectations, no leaf variable was strongly correlated with the amount ofrainfall received at the sites, but 13C discriminationwas negatively related to mean annual radiation received, which may be abetter index of water availability. Despite similarities in gas exchangephysiology, members of this putative functional type clearly responddifferentially to varying environmental conditions, implying that accurateprediction of responses to environmental change would require knowledge ofattributes of each species. However, differential responses might contributeto functional stability on a site subjected to environmental variability.