A comparison of tree water use in two contiguous vegetation communities of the seasonally dry tropics of northern Australia: the importance of site water budget to tree hydraulics

Tree water use in two contiguous communities (eucalypt open-forest and Melaleuca paperbark forest) wasmeasured in tropical Australia, over a 2-year period. The aims of the study were to (1) quantify daily and seasonal patternsof water use in each community, (2) compare patterns of water use among the communities and (3) compare relationshipsamong tree size, sapwood area and water use within the two contrasting vegetation communities. Access to deep soil waterstores and the effect of run-on from the eucalypt forest resulted in a relatively high pre-dawn water potential throughout theyear, particularly for Melaleuca forest. There were no differences in daily rates of water use, expressed on a sapwood area(Qs) basis, between the two eucalypt species examined (Eucalyptus miniata Cunn. Ex Schauer and E. tetrodonta F.Muell)at any time in the eucalypt forest. For both the eucalypt and Melaleuca forests, there was less seasonal variation in wateruse expressed on a leaf area (Ql) basis than on a Qs basis, and neither year nor season were significant factors in Ql. Inthe mono-specific Melaleuca forest, Qs was not significantly different between years or seasons. Water use on a Ql basiswas similarly not significantly different between years or seasons in the Melaleuca forest. Leaf area index (LAI) of theeucalypt forest was about half of that of the Melaleuca forest throughout the year but sapwood area per hectare was 33%larger in the eucalypt than the Melaleuca forest, despite the basal area of the Melaleuca forest being almost double thatof the eucalypt forest. There was no significant difference in stand water use (mm day−1) between eucalypt and Melaleucaforests during 1998; however, in 1999 Melaleuca stand water use was larger than that of the eucalypt forest. Because ofthe enhanced dry-season availability of water in the Melaleuca forest and its larger LAI, average annual water use of theMelaleuca forest was almost 60% larger than that of the eucalypt forest. Despite differences in Ql, Qs and annual wateruse between forests, the ratio of LAI to stand water use was similar for all seasons in both forests. The applicability of‘universal rules’ linking tree water use and tree hydraulics and the importance of ecosystem location on site water budgetsand plant adaptations are discussed.