Hydraulic traits and water use of Eucalyptus on restored versus natural sites in a seasonally dry forest in southwestern Australia

Abstract

Eucalyptus is a globally important genus for reforestation but eucalypt growth in non-native environments has uncertain ecohydrological consequences. This study investigated the hydraulic structure and function of juvenile (2.5–10m height) Eucalyptus marginata on restored mine sites and forest sites in a seasonally dry climate in southwestern Australia. We measured sap flow, leaf area, sapwood area, leaf water potential, stomatal conductance and weather variables and applied a simple model of plant water use to examine hydraulic traits across chronosequences of trees (using height as an index of age) in each environment. Younger trees on restored sites grew three times faster and used four times more water per unit leaf area (0.53kgm−2day−1) than forest trees, consistent with greater light, water and nutrient availability on restored sites. To maintain hydraulic homeostasis, trees on restored sites reduced leaf area/sapwood area ratio by 25%, increased sapwood permeability by 20%, and reduced stomatal conductance and transpiration by 50% to counterbalance fourfold increases in height and leaf area. In contrast, forest trees reduced leaf area/sapwood area ratio by 33% but increased sapwood permeability by 100% and stomatal conductance and transpiration by 50% in response to increased water availability, in addition to increased height and leaf area. A linear relationship between leaf area and water use for forest trees was indicative of hydrological equilibrium, whereas a curvilinear relationship for trees on restored sites suggested transition from high to lower water availability. Young eucalypts are likely to develop traits that are hydraulically compatible with their climate and substrate, but they may also adjust their architecture and physiology over time in response to changes in water availability. Trees in reforested landscapes with modified soils and initially abundant soil moisture may develop hydraulic architectures that are more efficient than found in native forests, which may pose risks if they can only access a fixed volume of soil. The long-term sustainability of such systems is likely to depend on trees eventually developing appropriate hydraulic architectures to cope with regimes of water availability more akin to native forests, especially in drought-prone environments.