Water limitation to soil CO₂ efflux in a pine forest at the semiarid "timberline"

Warming and drying is predicted for most of the Mediterranean and other regions, and knowledge of this effect on forest carbon dynamics cannot be easily extrapolated from temperate climates. Instead, we provide quantitative information from a 6-year study in a 40-year old pine forest at the dry timberline (280 mm annual rainfall) on soil CO₂ efflux (F _s) and some of its controlling factors. Annual F_s was relatively low (405.9 ± 23.8 g C m^-2 a^-1), but within one standard deviation of a global nonlinear relationship we derived between mean annual precipitation and F_s. in forests. Seasonal variations in F_s were dominated by soil temperature (with Q ₁₀ = 2.45) in the wet season, and by soil moisture in the water-limited seasons, but not by pulse responses to precipitation No temperature sensitivity was observed in the dry season, but there was clear evidence for down regulation of sensitivity to Q₁₀ = 1.18 when soil moisture was kept high by a supplement summer irrigation treatment. Interannual variations in F₅ correlated linearly with cumulative rainwater availability, indicating the combined importance of both precipitation amount and its temporal distribution between the wet (and cool) season and the transitional periods characterized by high evaporative demand. Low rates of soil carbon loss combined with high belowground carbon allocation (41% of canopy CO₂ uptake) might explain the high soil organic carbon accumulation and net ecosystem productivity in this dry forest. Our results indicate that F_S in pine forests may adjust to dry conditions with better carbon economy than estimated from response to episodic drought in more temperate climate.