Water use in plantations of eucalypts and pines: a discussion paper from a tree breeding perspective

Plantation forests are being established on more marginal, harsh sites to accommodate expansion of agriculture. Because of this, tree breeders and silviculturists are interested in improving drought hardiness of trees. Breeding approaches to improve drought hardiness includes the assessment of trees as dead or alive in trials after severe droughts followed by the selection of the best surviving phenotypes for the establishment of seed orchards or land races. Physiologists estimate water utilization in trees in terms of water use efficiency (WUE). It is a function of biomass per unit of evapotranspiration and is calculated on a leaf or wood scale in clonal plots and stands. The gene complexes that control drought hardiness and WUE are probably different. It one respect, traditional breeding might have a limited impact on changing WUE since it will be difficult to alter rate of evapotranspiration through selection. In another respect, one could argue that selecting trees for high productivity is indirectly selecting for high WUE in some environments. Alternative approaches such as assessing heavy isotopes of carbon and oxygen in leaf tissue are being used by some scientists as a surrogate measure of water use in crops and trees. This paper presents the general findings from catchment studies on water uptake in pines and eucalypts and discusses how tree breeders might use results to further the development of drought hardy varieties grown in seasonally dry environments.