Ecophysiology of Acacia species in wet-dry tropical plantations

Selected tropical Acacia species are used extensively for short-rotation plantation forestry in many parts of Asia and, to a limited degree, in Australia. We explored leaf-level photosynthetic activity and leaf water potential (Ψ_leaf) of three field-grown Acacia tree species (aged between 7 and 18 months) in contrasting wet–dry tropical plantations in southern Vietnam and northern Australia. Light-saturated photosynthetic rate (A₁₅₀₀) declined throughout the morning and early afternoon in the dry season; in the wet season, levels remained high and relatively constant throughout most of the day. Maximum daily A₁₅₀₀ at 09:00 ranged from 22.2 µmol m⁻² s⁻¹ in the wet to 10.4 µmol m⁻² s⁻¹ in the dry season. At both locations, trees were able to extract soil water such that pre-dawn leaf water potential (Ψ_pd) remained > −1.5 MPa even at the end of the dry season. Stomatal conductance to water vapour (g_s) did not respond to decreasing Ψ_leaf during the wet season but was sensitive to changes in Ψ_leaf in the dry season. Species comparisons of the relationships between A₁₅₀₀ and Ψ_leaf revealed different strategies to balance carbon uptake and water loss in a wet–dry environment. Acacia crassicarpa and A. mangium regulated Ψ_leaf to a greater extent than the A. mangium × A. auriculiformis hybrid such that ▵Ψ_leaf (determined as Ψ_pd – midday Ψ_leaf) was unaffected by season. This result suggests that the hydraulic regulation of tree water status varies amongst young tropical Acacia species. From a management perspective, for Acacia species that tend to strongly regulate water loss in environments with an extended dry season, overall productivity at the end of a rotation may be less than for species that prioritise carbon gain.