WHOLE-TREE TRANSPIRATION AND WATER-USE PARTITIONING BETWEEN EUCALYPTUS NITENS AND ACACIA-DEALBATA WEEDS IN A SHORT-ROTATION PLANTATION IN NORTHEASTERN TASMANIA

Whole-tree water use in 4- and 8-year-old plantations of Eucalyptus ni tens Deane and Maiden (es Maiden) in the presence and absence of Acaci a dealbata Link. weeds was estimated by the heat pulse velocity techni que during a six-week summer period. Maximum sap velocities were recor ded between 5 and 15 mm under the cambium for both eucalypt and acacia trees, and marked radial and axial variations in sap velocity were ob served. The latter source of variation was most pronounced in mixed st ands where crowns were asymmetrical. Mean daily sap flux ranged from 1 .4 to 103.6 l day(-1) for eucalypts and from < 0.1 to 8.4 l day(-1) fo r acacias. Stem diameter explained 98% of the variation in sapwood are a for E. nitens and 89% for A. dealbata, and was determined to be a su itable parameter for scaling water use from the tree to stand level. P lot transpiration varied from 1.4 to 2.8 mm day(-1) in mixed 8-year-ol d plots and was 0.85 mm day(-1) in a mixed 4-year-old plot. The degree of A. dealbata infestation was associated with absolute plot water us e and regression models predicted that, in the absence of acacia compe tition, plot water use for the 8-year-old stand would approach 5-6 mm day(-1) during the growing season.