Effects of Water-Availability on Carbon and Water Dynamics in a Semi-Arid Pine Forest

Changes in the hydrological cycle, as predicted and currently observed, will significantly impact the water and carbon balance of water-limited forest ecosystems. However, differences in the water-sensitivity of component processes make carbon balance predictions challenging. To examine responses of ecosystem components to water limitations, we conducted a study of tree, soil and ecosystem processes in a young ponderosa pine stand under natural summer drought (control) and increased soil water conditions (watered). Weekly-averaged tree transpiration (T), net ecosystem photosynthesis (Pn_eco) and soil CO₂ efflux (Rs) were related with soil water content (SWC) and declined rapidly when relative extractable soil water (REW) was below 50%. The control of vapor pressure deficit over daily variations in canopy conductance (G_s) was subordinated to SWC (R²=0.97; logarithmic function), decreasing at REW below 50%. Watering maintained REW at about 70% in July and August but positively affected tree carbon and water dynamics only at the end of summer when fluxes in the control treatment were strongly water-limited. A tight coupling of above- and belowground fluxes became apparent; while root-rhizosphere respiration (Rr) in the control treatment decreased along with Pn_eco and T (R²=0.58) as drought progressed, watering maintained Rr, T and G_s, which were significantly higher than those of the control trees in late summer. In contrast, microbial respiration (Rm) responded instantaneously and strongly to the watering compared to the control treatment. The net effect was increased soil water availability negatively affected the short-term ecosystem C balance due to a larger increase in decomposition than photosynthesis. This clearly highlights that understanding the dissimilar response of tree dynamics and soil decomposition to water availability to be a key component in predicting future C sequestration in water-limited forest ecosystems. Continued watering over multiple years will identify longer-term effects, such as changes in above- and belowground carbon allocation.