Impacts of drought and precipitation variability on the C balance of a regenerating semi-arid pine forest

Background/Question/Methods

Climate change can impact forests by altering the frequency, intensity and duration of droughts and fires, particularly in semi-arid forest ecosystems. Young regenerating forest stands may be strongly affected as water access during drought conditions is often limited by shallower rooting compared to mature stands. Moreover, with fire severity increasing these regenerating young forest stands likely become more abundant, yet their response to changed precipitation patterns and increased drought events is still highly uncertain. In this study, we applied the soil-plant-atmosphere model (SPA), a process based model that simulates ecosystem carbon processes and water balance at fine temporal scales, to estimate effects of precipitation variability and drought events on the C balance of a post-fire regenerating ponderosa pine stand in eastern Oregon. We evaluated the sensitivity and seasonality of modeled drought response using ecosystem flux measurements (Eddy Covariance and chamber measurements), and a watering experiment (July–August 2010 and 2011). 

Results/Conclusions

Observed and simulated daily fluxes of heterotrophic respiration (Rh), transpiration (T), gross primary productivity (GPP) and net ecosystem exchange (NEE) were strongly related (R²=0.64–0.87) and closely followed the one-to-one line. Model simulations showed that frequent summer rain events increased Rh more than GPP during both study years which also confirmed the results from the summer watering experiment. However, the net effect on NEE was rather small and variable (between 5% decrease and 7% increase) with a negative effect when water additions were below 200 mm (about +50% of annual precipitation). Sensitivity to vapour pressure deficit (VPD) was tested by increasing and decreasing observed VPD by 50%. VPD affected NEE strongly (between 22% decrease and 30% increase). In summary, our results show that summer precipitation variability is an important driver of both GPP and Rh with the net effect on NEE depending on the intensity and frequency of rain events. Together with the strong control of VPD on stomatal conductance (and thus water loss and C assimilation) this points towards a likely reduction of the net C gain in regenerating ponderosa pine ecosystems under future more extreme climate conditions.