Hybrid poplar is a particularly promising source of biomass for energy, as well as raw material for other manufacturing industries. The economic and ecological functioning of short rotation poplar plantations is directly tied to their water-use efficiencies and tolerance to changes in climate such as multi-year droughts and tropical storms. We measured tree water use, water use efficiency (WUE), and the relationship between WUE and growth four-year-old Populus deltoides and hybrid clones. Nine genotypes, six pure P. deltoides and three hybrids (two P. trichocarpa x deltoides and one P. deltoides x maximowiczii ) clones, were selected base on two year stem biomass accumulation and represented genotypes with high, average, and low growth potential. We asked: are there differences among genotypes in water use and water use efficiency (WUE) and does WUE correlate with growth? Tree sap flux was measured continuously during the fourth growing season using thermal dissipation probes.
Maximum daily transpiration (Et) ranged from 5 to 10 mm day-1. Growing season water use (m3 H2O tree-1) was correlated strongly with aboveground wood biomass (stem and branches) growth. Tree level WUE ranged between 1.7 and 3.9 kg biomass m3 H2O-1 and there were significant differences among clones (P < 0.001); although, tree WUE was only weakly correlated with growth R2 = 0.14, P = 0.03) and not correlated with total water use (P = 0.81). However, there was significant clone variation (P < 0.001) in leaf δ13C composition. Leaf WUE inferred from Δ13C was correlated positively with tree WUE and wood biomass growth. Strong positive relationship between Δ13C and growth and the differential family response in tree WUE and Δ13C suggest that genotype variation in WUE is due to both stomatal conductance and photosynthetic capacity. Leaf Δ13C may be useful for examining physiological and morphological determinants of growth among genotypes in Poplar.