Abstract
Abstract. 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.
Highlights
Drought events are characterized by a continuous decline of soil water content and an increase in evaporative demand
Gross primary productivity is generally found to decrease more than respiration during drought conditions (Schwalm et al, 2009; Ruehr et al, 2012), because photosynthesis is limited by both soil drought and high temperatures, while soil moisture constraints on heterotrophic respiration may be partially compensated by temperature (Irvine et al, 2008; Ruehr et al, 2012)
Because our study focuses on ecosystem C dynamics, we define net ecosystem productivity as NEP = −net ecosystem exchange (NEE), with positive fluxes meaning C uptake and negative fluxes C loss from the ecosystem
Summary
Drought events are characterized by a continuous decline of soil water content (soil drought) and an increase in evaporative demand (atmospheric drought). Ruehr et al.: Heat and drought impacts on semi-arid forest ability to predict future effects on forest ecosystems is limited by uncertainty regarding the relative roles of evaporative demand/temperature and precipitation in triggering drought stress (Williams et al, 2013), and by understanding their corresponding effects on component processes like photosynthesis and heterotrophic respiration (Ruehr et al, 2012). Gross primary productivity is generally found to decrease more than respiration during drought conditions (Schwalm et al, 2009; Ruehr et al, 2012), because photosynthesis is limited by both soil drought and high temperatures, while soil moisture constraints on heterotrophic respiration may be partially compensated by temperature (Irvine et al, 2008; Ruehr et al, 2012)
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