Transpiration and canopy conductance of loblolly pine with fertilization and throughfall exclusion: Early results from PINEMAP

Background/Question/Methods

Many predictions of the climate change include decreased precipitation for the Southeast United States.  To study the effects of such changes on the physiology, development and productivity of forests, the Pine Integrated Network: Education, Mitigation, and Adaptation Project (PINEMAP) includes factorial experiments of fertilization and throughfall displacement (~30%) in four loblolly pine (Pinus taeda) stands over multiple growing seasons.  In this poster we present a brief overview of the PINEMAP project and present early findings on the effect of these treatments on transpiration (E_C) and stomatal conductance (G_S) of one of these sites in Buckingham Co., VA.   Water use and carbon uptake are intimately linked in plants, as both plant-atmosphere exchanges of water vapor and CO₂ occur through the stomata of leaves.  Thus, insights about stomatal regulation of plant water use also inform us about carbon assimilation, productivity and growth.  We investigated E_C and canopy-averaged G_S using a network of 80 sap flux probes.

Results/Conclusions

In the first year of treatment, we found that fertilization led to increased E_C (25%) and G_S (26%) relative to the control in the early growing season (June-July), but this effect had disappeared by September.  Fertilization is known to increase the leaf area of loblolly pine at many sites, but due its evergreen nature and roughly 18-month life-span, this difference may take multiple growing seasons to establish.  The expected increased leaf area following fertilization will result in higher E_C unless trees reduce G_S in future months.  On the other hand, fertilization in combination with throughfall displacement led to reduced E_C (12%) and G_S (16%) relative to the control in September through February of the following year, showing that such tightened regulation is already occurring in this treatment.  These results, when viewed in the context of canopy development, offer insights into how management and climate may interact in determining the water use and productivity of Southeastern pine forests under predicted future climatic conditions.