Effects of a 30% reduction in precipitation on transpiration and hydraulic properties of 7-year-old loblolly pine

Background/Question/Methods

Loblolly pine (Pinus taeda L.) plantations in the southern U.S. generate more timber than any other country in the world and therefore reductions in net primary productivity associated with climate variability may have significant economic impacts. To understand the effects of climate variability on forest productivity, an interdisciplinary project (PINEMAP) was initiated to better manage loblolly pine for increased carbon sequestration, enhanced fertilizer efficiency, and climate resiliency. As part of that project, the objective of this study is to determine the effects of a 30% reduction in ambient precipitation combined with varying nutrient availability on sap flow and hydraulic properties of 7-year old loblolly pine. We hypothesize that trees will adapt to drought by adopting a conservative water strategy defined by lower stomatal conductance, increased water use efficiency and avoidance of lower leaf water potential. The experiment is located in the Piedmont physiographic region near Washington, GA. The experimental design is a 2x2 factorial combination of fertilization (control and operational) and rain manipulation (ambient and 30% reduction by throughfall exclusion) treatments replicated in four blocks. Sap flow measurements began in November and sap flow is being monitored continuously to determine tree and stand-level water use and canopy-level stomatal conductance.   

Results/Conclusions

Transpiration per until land area (E) and leaf area (E_L) summed by month, and mean midday sap flux density (J_S) were not influenced by throughfall rain exclusion in December. Mean E was 32 mm month^-1 and J_S was 47 g m^-2 s^-1 in December. In November, there was a trend (p<0.100) for reduced E and J_S in response to throughfall rain exclusion. Mean E was 29 and 35 mm month^-1 for rain exclusion and control treatments, respectively, and J_S was 44 g m^-2 s^-1  for rain exclusion compared to 51 g m^-2 s^-1 in the control treatment. Mean midday canopy stomatal conductance averaged across all days in November was 65 mmol m^-2 s^-1 and not influenced by throughfall rain exclusion treatment. The fertilization treatment had no effect on sap flow, although leaf area production was increased. In summer months when soil water is more limiting, treatment effects may be detected.