Comparative water use in short-rotation Eucalyptus benthamii and Pinus taeda plantations in the Southeastern United States

Background/Question/Methods

Background/Question/Methods

Short rotation eucalyptus plantations offer great potential for increasing wood-fiber production in the southeastern United States. Eucalyptus plantations can be highly productive (>35 m³ ha^-1 year^-1), but they use a lot of water and this has raised concern about how expansion of eucalyptus plantations will affect water resources. Water requirements of eucalypt plantations in the southeast US are unknown. Will intensively managed eucalyptus plantations use more water than contemporary intensively managed pine (primarily Pinus taeda) plantations or other native speices? Are there important species differences in water use efficiency (WUE)? Water consumption of short-rotation eucalyptus needs to be quantified in order to develop management and silvicultural practices that maximize productivity and WUE without compromising water resources. We compared stand-level water use and WUE over one year in adjacent eight-year-old E. benthamii and Pinus taeda plantations near Ravenel, SC (33^oN). Sapflow was measured continuously using thermal dissipation probes. Species differences in above and belowground biomass, stem growth, stand level transpiration per unit ground (E_t) and leaf (E_l) area, canopy stomatal conductance (G_s), leaf area index (LAI), and leaf to sapwood area ratio (A_L:A_S) were quantified. 

Results/Conclusions

Results/Conclusions

Daily E_t ranged from 0.30 to 2.75 and 0.20 to 2.00 mm day^-1 in eucalyptus and pine stands, respectively. Monthly E_t was 22-34% higher in eucalyptus and differences between species were greatest in the spring and fall. Annual E_t was 515 mm in eucalyptus compared to 361 mm in pine (p=0.04). WUE, defined as annual stem biomass produced (kg ha^-1) per unit water transpired (m³ ha^-1), was greater in eucalyptus (1.83 kg m^-3) than in the pine (1.28 kg m^-3). Seasonal patterns of volumetric soil moisture (Θ) were similar between stands; however, Θ decreased at a more rapid rate in eucalyptus following rainfall events. Pine exhibited a larger seasonal range in LAI.  At low LAI, E_l was 20% greater in eucalyptus compared to pine (1.03 vs 0.82 mm day^-1, SE=0.04, p=0.03). At maximum LAI, eucalyptus E_l was 44% greater in than pine (1.14 vs 0.64 mm day^-1, SE=0.03, p<0001). E_l and G_s were significantly correlated with atmospheric vapor pressure deficit (VPD), although pine exhibited greater stomatal sensitivity to VPD under both low and high LAI. We conclude that eucalyptus used more water than pine at this site, but annual E_t was well below precipitation (1345 mm) and PET (1225 mm).