Across Victoria, Australia there exists a steep spatial gradient in water availability, as quantified by the ratio of mean annual precipitation (P) to pan evaporation (Ep). The genus Eucalyptus has diversified and adapted into many species across this gradient ranging from short multi-stemmed mallee at the dry end to some of the world's tallest angiosperms at the wet end. Most plants are subject to the unavoidable tradeoff where substantial water is lost water to the atmosphere in exchange for a comparatively small amount of carbon. A plant's ability to gain carbon can therefore be attributed to its capacity to transport water. We asked 1) what hydraulic traits have evolved to give Eucalyptus its success and 2) is there plasticity in these traits that may explain Eucalyptus diversification.
To answer these questions we selected 10 species of Eucalyptus from across Victoria's moisture gradient (P/Ep 0.19 to 1.14 mm mm^-1) and grew them in four common gardens across the gradient (P/Ep 0.16 to 1.25 mm mm^-1). After one year of growth we measured gas exchange in situ and collected stems for measurement of hydraulic conductivity (K) and wood density. We hypothesized that within sites, species native to wetter climates would show greater hydraulic conductivity, lower wood density and lower stomatal conductance. The same trend would be expected across sites when each species was grown at wetter sites. However, the plasticity seen within species would be far less than the differences between species. Data were analyzed with multiple linear regression.
Results/Conclusions
At each site, K was greater in species from wetter climates while across sites, species tended to have greater K at wetter sites. Dry adapted species showed a muted response to moisture availability compared to those from wetter climates. Trends in K were mirrored by patterns in wood density, which increased as moisture availability declined both across species and across sites. In all sites stomatal conductance declined with increasing native P/Ep. This decline was stronger in the two wetter sites. However, within species, conductance increased as site moisture increased. Together this meant that dry-adapted species showed greater variability in stomatal conductance than wet-adapted species, contrary to the trend in K. Our results point to contrasting hydraulic strategies across these species. Further study at these common gardens will show how these hydraulic traits fit into broader questions of adaptation including survival, growth, allocation and competition.