LB 19-197 - Strong and persistent carbon sink over three decades in a seasonally dry tropical forest in spite of disturbances

Background/Question/Methods:

The land carbon sink, particularly in tropical vegetation, has been an important process which partly mitigates the rise of atmospheric carbon dioxide from anthropogenic emissions. In recent years, however, there have been concerns about the weakening tropical carbon sink in major biomes such as the tropical moist forests of the Amazon and Congo. We present a three-decadal record of carbon stock change in vegetation and soil of a seasonally dry tropical forest (SDTF) in southern India subject to recurring disturbances such as drought, fire, herbivory by elephants and marginal harvest by people. This record is based on regular measurement and monitoring of >70,000 woody plants (≥1cm dbh) in a large permanent plot of 50 hectares (1260mm annual rainfall) and nineteen 1-hectare plots across a moisture gradient (700-1700mm annual rainfall) in the protected forests of Mudumalai National Park. Live aboveground biomass (AGB and C stocks) was estimated following the method of Chave et al. (2014, Global Change Biology).

Results/Conclusions:

The large plot has averaged a net C sink of 0.76 Mg ha^-1 yr^-1 during 1988-2020 with a net negative sink only during a single 4-year period (2000-04) of prolonged drought. The 19 plots across the Mudumalai landscape have averaged a net C sink of 0.50 Mg ha^-1 yr^-1 during 1994-2019 with strong sequestration in areas >1200 mm rainfall and weaker or negative C sink in areas of lower rainfall (which may also be related to harvest of biomass by people or herbivory by elephants). The net carbon sink in AGB in this SDTF is comparable to or higher than the 3-decadal value for tropical moist forests of 0.57 Mg ha^-1 yr^-1 in the Congo basin, the pan-tropical value of 0.47 Mg ha^-1 yr^-1 or, indeed, the value of 0.42 Mg ha^-1 yr^-1 in the Amazon which is experiencing a declining C sink in recent times (Hubau et al. 2020, Nature Climate Change). Soil organic carbon also increased across the Mudumalai landscape during 1994-2019. These results warrant a closer examination of C sink potential of the major tropical dry forest and savanna biomes for climate mitigation.