Predicting the net effects on the carbon and water balance of semi-arid forests under future conditions depends on ecosystem processes responding to changes in soil and atmospheric drought. Here we apply a combination of field observations and soil-plant-atmosphere modeling (SPA) to study carbon and water dynamics in a regenerating ponderosa pine forest. The effects of soil and atmospheric drought were quantified based on a field irrigation experiment combined with model simulations. To assess future effects of intensifying drought on ecosystem processes, the SPA model was run using temperature and precipitation scenarios for 2040 and 2080.
Experimentally increased summer water availability clearly affected tree hydraulics and enhanced C uptake in both the observations and the model. Simulation results showed that irrigation was sufficient to eliminate soil water limitation and maintaining transpiration rates, but gross primary productivity (GPP) continued to decrease. Observations of stomatal conductance indicated a dominant role of vapor pressure deficit (VPD) in limiting C uptake. This was confirmed by running the simulation under reduced atmospheric drought (VPD of 1 kPa), which largely maintained GPP rates at pre-drought conditions.
The importance of VPD as a dominant driver was underlined by simulations of extreme summer conditions. We found GPP to be affected more by summer temperatures and VPD as predicted for 2080 (-17%) than by reductions in summer precipitation (-9%). Because heterotrophic respiration responded less to heat (-1%) than to reductions in precipitation (-10%), net ecosystem C uptake declined strongest under hotter (-38%) compared to drier summer conditions (-8%).
Considering warming trends across all seasons (September-May: +3 °C and June-August: +4.5 °C), the negative drought effects were largely compensated by an earlier initiation of favorable growing conditions and bud break, enhancing early season GPP and needle biomass. An adverse effect, triggered by changes in early season allocation patterns, was the decline of wood and root biomass. This imbalance may increase water stress over the long term to a threshold at which ponderosa pine may not survive, and highlights the need for an integrated process understanding of the combined effects of trends and extremes.