Long turnover time and large sequestration potentials in a dry pine forest based on 15-year flux and inventory records

A large terrestrial carbon sink significantly influences the rate of change in atmospheric CO₂ concentrations, but uncertainties associated with its estimate are considerable. Here we combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55-year-old 30 km² pine forest growing at the semi-arid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semi-arid lands, which cover ~18 % of the global land area. Annual integrated carbon accumulation was 145-160 g C m^-2 y^-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^-2 y^-1 indicating 20 % uncertainty in carbon uptake estimates. This carbon uptake reflect high carbon use efficiency NEP/GPP of 29 compared to ~21 in temperate forests, leading to the current ecosystem stocks of 7943 ± 323 g carbon m^-2 and 372 g nitrogen m^-2. In addition, carbon is mostly stored in the soil (~71 % of the current ecosystem C stock), with a long C turnover time of 59 ± 4 y (compared to mean value of 18 years in temperate forests). It is also estimated that soil carbon at the study site constitutes only ~25 % of the estimated soil saturation capacity. Irrespective of un-expected disturbances beyond those observed at the study site, the results support considerable C sink potential in semi-arid soils and forest plantations, and imply that afforestation of even 10 % of semi-arid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C y^-1 over several decades.