Estimating water/carbon exchange of loblolly pine (Pinus taeda) plantations across a gradient of climates in southeast US

Background/Question/Methods

The pine forests in the southeastern US are responsible for providing ecosystem services, including clean water and CO₂ sequestration as well as fiber and wood for products. However, there is increasing concern regarding their water use and how climate change will affect it.  Most climate predictions indicate that droughts are expected to increase in frequency in this region. We are using the hybrid process model, 3-PG, to understand how these drought events will impact transpiration, productivity and carbon pools of loblolly pine plantations. The model was calibrated and validated using measurements of sap flow and growth at four locations (Georgia, Virginia, Florida and Oklahoma) representing a wide range in edaphic and climatic conditions in the Southeastern region. At each location there were four different treatments (control, fertilization, throughfall reduction and combined throughfall reduction and fertilization). The aim of this work is to predict biomass and water use under different climate scenarios, on order to better understand the production ecology of these forests. 

Results/Conclusions

The biomass predictions from the 3-PG model showed good accuracy for the different locations. The predictions of transpiration were also accurate.  For example, comparing predicted and observed values of transpiration for the control treatment at the Georgia site produced a R2 of 0.94 and RMSE of 0.1443 mm H₂O. At the Virginia site, a comparison of observed and predicted transpiration for the control treatment had a R2 of 0.85 and RMSE of 1.204 mm H₂O. This was consistent at both monthly and yearly time steps. The 3-PG model was also able to capture the seasonal pattern of LAI (leaf area index) well on the two sites tested (Georgia and Florida, control treatment).  Parallel measurements at the VA, OK sites will be used to validate the model predictions of biomass, LAI and transpiration. Simulated drought had significant effects on biomass growth and water use, indicating that climate change may have substantial effects on the water and carbon cycles in these forests.