Greater stability of carbon capture in species-rich natural forests compared to species-poor plantations

Anand M Osuri,Shahid Naeem,Susan C Cook-Patton,Abhishek Gopal,Ruth Defries,T R Shankar Raman

doi:10.1088/1748-9326/ab5f75

Abstract

Tree plantations and forest restoration are leading strategies for enhancing terrestrial carbon (C) sequestration and mitigating climate change. While it is well established that species-rich natural forests offer superior C sequestering benefits relative to short-rotation commercial monoculture plantations, differences in rates of C capture and storage between longer-lived plantations (commercial or non-commercial) and natural forests remain unclear. Using a natural experiment in the Western Ghats of India, where late-20th century conservation laws prohibited timber extraction from monodominant plantations and natural forests within nature reserves, we assessed forests and plantations for aboveground C storage and the magnitude and temporal stability of rates of photosynthetic C capture (gross primary production). Specifically, we tested the hypothesis that species-rich forests show greater temporal stability of C capture, and are more resistant to drought, than monodominant plantations. Carbon stocks in monodominant teak (Tectona grandis) and Eucalyptus (Eucalyptus spp.) plantations were 30%–50% lower than in natural evergreen forests, but differed little from moist-deciduous forests. Plantations had 4%–9% higher average C capture rates (estimated using the Enhanced Vegetation Index–EVI) than natural forests during wet seasons, but up to 29% lower C capture during dry seasons across the 2000–18 period. In both seasons, the rate of C capture by plantations was less stable across years, and decreased more during drought years (i.e. lower resistance to drought), compared to forests. Thus, even as certain monodominant plantations could match natural forests for C capture and storage potential, plantations are unlikely to match the stability–and hence reliability–of C capture exhibited by forests, particularly in the face of increasing droughts and other climatic perturbations. Promoting natural forest regeneration and/or multi-species native tree plantations instead of plantation monocultures could therefore benefit climate change mitigation efforts, while offering valuable co-benefits for biodiversity conservation and other ecosystem services.

Highlights

Tropical forests harbour over two-thirds of global biodiversity and perform vital ecosystem functions necessary for biodiversity conservation and human well-being (Gardner et al 2009, Costanza et al 2014).These forests annually sequester around 2.0 Pg (1015 g) of carbon (C) from the atmosphere through photosynthesis, and store over 400 Pg C in vegetation and soil pools, thereby strongly regulating atmospheric CO2 concentrations and global climate (Pan et al 2011)
We identified drought years based on the long-term (1988–2018) average and standard deviation of annual precipitation at each plot, based on the CHIRPS dataset, and on the regional Standardized Precipitation Index (SPI) of drought (Aadhar and Mishra 2017)
Aboveground C storage per plot was highest in evergreen forest (12.2 Mg 0.04 ha−1, on average), followed by deciduous forest (9.6 Mg 0.04 ha−1) and teak plantation (7.0 Mg 0.04 ha−1) with statistically similar C stocks, while Eucalyptus plantations had the lowest carbon density (5.5 Mg 0.04 ha−1) of the four forest types (figure 2(c))

Summary

Introduction

Tropical forests harbour over two-thirds of global biodiversity and perform vital ecosystem functions necessary for biodiversity conservation and human well-being (Gardner et al 2009, Costanza et al 2014). These forests annually sequester around 2.0 Pg (1015 g) of carbon (C) from the atmosphere through photosynthesis, and store over 400 Pg C in vegetation and soil pools, thereby strongly regulating atmospheric CO2 concentrations and global climate (Pan et al 2011). Reforestation is promoted by major international agreements and policies such as the Bonn Challenge and Paris Climate Accord, with participating countries committing to increase forest cover by nearly 300 Mha in total by 2030 (United Nations 2015, Lewis et al 2019). Monoculture or monodominant tree plantations—that are widely misclassified as forests—are expanding, while species-rich natural tropical forests continue to be deforested (Puyravaud et al 2010, Payn et al 2015, Hua et al 2016)

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