Carbon sequestration potential across a successional gradient of tropical dry forest in Northwestern Mexico

The tropical dry forest in the state of secondary succession is a common landscape worldwide, mostly due to the abandonment of the agricultural and livestock activities. We investigated the differences of net ecosystem production (NEP), and its components; ecosystem respiration (Reco) and gross ecosystem productivity (GEP), at sites across a successional gradient of a tropical dry forest in Northwestern Mexico. We measured the CO₂ and water vapor fluxes between the ecosystem and the atmosphere (during 2015-2017), in an early secondary succession (ES), a secondary forest (SF), and an old-growth forest (OG) with recovery ages of 10, 35 40 and more than 100 years respectively, using the eddy covariance technique. We found that the secondary succession sites were net carbon sinks throughout the study period, the NEP in the ES was 145.2 in 2016 and 160.4 gC m^-2y^-1 in 2017, for SF it was 298.3, 312.7 and 354.4 gC m^-2y^-1 for 2015, 2016, 2017 respectively, even when the precipitation varied from a wet year above the historic annual mean (2015; 804 mm), a mean year (2016, 720 mm y^-1) to one year below the average (2017; 577 mm y^-1). In contrast, the OG showed more variability; in 2016 it was a net carbon source of -209.7 gC m^-2 y^-1, and switched to a net carbon sink of 101.8 gC m^-2 y^-1 in 2017. These differences are partially explained by the contrasting effects of decreasing precipitation at the sites, since Reco was very similar from year to year at the ES and SF sites, but was reduced by nearly 30% at the OG site during the driest year. In contrast, GEP was less variable within sites. We conclude that Reco and GEP are controlled by different drivers at each site constraining the magnitude of the net sequestration potential of TDF during succession.