Partitioning of canopy and soil CO2 fluxes in a pine forest at the dry timberline across a 13-year observation period

Abstract

Partitioning carbon fluxes is key to understanding the process underlying ecosystem response to change. This study used soil and canopy fluxes with stable isotopes (13C) and radiocarbon (14C) measurements in an 18 km2, 50-year-old, dry (287 mm mean annual precipitation; nonirrigated) Pinus halepensis forest plantation in Israel to partition the net ecosystem's CO2 flux into gross primary productivity (GPP) and ecosystem respiration (Re) and (with the aid of isotopic measurements) soil respiration flux (Rs) into autotrophic (Rsa), heterotrophic (Rh), and inorganic (Ri) components. On an annual scale, GPP and Re were 655 and 488 g C m-2, respectively, with a net primary productivity (NPP) of 282 g C m-2 and carbon-use efficiency (CUE D NPP = GPP) of 0.43. Rs made up 60 % of the Re and comprised 24 ± 4 %Rsa, 23 ± 4 %Rh, and 13 ± 1 %Ri. The contribution of root and microbial respiration to Re increased during high productivity periods, and inorganic sources were more significant components when the soil water content was low. Comparing the ratio of the respiration components to Re of our mean 2016 values to those of 2003 (mean for 2001 2006) at the same site indicated a decrease in the autotrophic components (roots, foliage, and wood) by about-13 % and an increase in the heterotrophic component (Rh=Re) by about C18 %, with similar trends for soil respiration (Rsa=Rs decreasing by-19 % and Rh=Rs increasing by C8 %, respectively). The soil respiration sensitivity to temperature (Q10) decreased across the same observation period by 36 % and 9 % in the wet and dry periods, respectively. Low rates of soil carbon loss combined with relatively high belowground carbon allocation (i.e., 38 % of canopy CO2 uptake) and low sensitivity to temperature help explain the high soil organic carbon accumulation and the relatively high ecosystem CUE of the dry forest.