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Greater stability of carbon capture in species-rich natural forests compared to species-poor plantations [1]

['Anand M Osuri', 'The Earth Institute', 'Columbia University', 'New York', 'Ny', 'United States Of America', 'The Nature Conservancy', 'Arlington', 'Va', 'Http']

Date: 2023-08

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 CO 2 concentrations and global climate (Pan et al 2011). With the majority of tropical forests having been lost or degraded by anthropogenic activity (Watson et al 2018), reforestation has emerged as a leading strategy for conserving biodiversity and mitigating climate change (Griscom et al 2017, Lewis et al 2019).

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). However, even as tree cover has shown an increasing global trend in recent decades (Song et al 2018), this trend conceals critical shifts in tree species composition. Specifically, 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).

An assessment of international commitments toward climate-focused reforestation has revealed that while over 50% of such commitments are for short-rotation (10–20 years) commercial plantations, certain countries (e.g. India) have also committed significant areas to restoring natural forests (Lewis et al 2019). However, ongoing reforestation efforts in India predominantly employ non-commercial monoculture/monodominant tree plantations (Seidler and Bawa 2016, Narain and Maron 2018), comprising substantially lower tree diversity than native forests (e.g. dry-deciduous to wet-evergreen forests in India's Western Ghats region harbour 49 species ha−1, on average (Ramesh et al 2010)). According to the Indian Government's CAMPA program, which channels payments from projects responsible for deforestation towards compensatory afforestation efforts, plantations of five or fewer species constitute 53% of the 2 35 000 ha planted for reforestation during 2015–18 (data from http://egreenwatch.nic.in/; figure S1, available online at stacks.iop.org/ERL/15/034011/mmedia).

It is well established that species-rich natural forests better support plant and animal biodiversity than monodominant plantations (Gibson et al 2011). It is also clear that short-rotation plantations do not directly sequester as much carbon as uncut natural forests (Lewis et al 2019). It remains unclear, however, whether and how mature or long-rotation (e.g. > 50 y) monodominant plantations differ from species-rich naturally regenerating forests in C sequestering functions, including magnitude and temporal stability of C capture from the atmosphere via photosynthesis, and long-term C storage.

Biodiversity and ecosystem function (BEF) theory predicts that diversity promotes efficient resource use, and increases the likelihood of functionally high-performing species occurring within communities (Cardinale et al 2012). Species-rich tree communities are therefore expected to exhibit higher rates of primary production or atmospheric C capture, and potentially accumulate larger C stocks over time, than average monocultures (Huang et al 2018). However, studies have shown that monocultures of highly productive tree species—such as those commonly used in commercial plantations—could match or exceed C capture rates of more species-rich communities (Bonner et al 2013, Huang et al 2018). Likewise, monocultures of hardwood timber species could accumulate similar or larger C stocks over time than more diverse communities comprising hardwood and softwood species (Bunker et al 2005, Hulvey et al 2013). The theory thus suggests that differences in C capture rates and C storage between natural forests and monodominant plantations would vary by plantation species and forest type. This highlights the need for empirical studies making comparisons of C capture and storage between plantation monocultures typically used in reforestation programmes, and natural forests.

A second BEF prediction is that diversity increases temporal stability of ecosystem functions, because larger pools of species are more likely to contain species tolerant to different types of perturbations (Hooper et al 2005). The theory suggests that species-rich tree communities would exhibit greater temporal stability of C capture rates that would additionally offer higher resistance (i.e. be affected less by) perturbations such as droughts (Jucker et al 2014), than monodominant plantations. However, the prediction that species-rich forests would therefore offer more stable (Hulvey et al 2013)—and hence reliable (Naeem 2003)—C capture than monodominant plantations remains untested.

This study examines the above predictions of BEF theory in the context of C capture rates and C storage by species-rich natural forests and monodominant tree plantations. The study is based on a unique natural experiment in India's Western Ghats mountains, where the cessation of timber management activities within newly established wildlife reserves during the mid–late 20th century has resulted in mature monodominant plantations (>40 y old) growing alongside naturally regenerating native tropical forests. First, we compare aboveground C stocks, and rates of photosynthetic C capture over the 2000–18 period [indexed using the satellite-derived Enhanced Vegetation Index (EVI)], across mature monodominant teak (Tectona grandis) and Eucalyptus (Eucalyptus spp.) plantations, and evergreen and moist-deciduous natural tropical forests. In line with the first BEF theory prediction, we expect no consistent differences in C storage and average rates of C capture between species-rich forests and monodominant plantations. Next, we examine inter-annual variation in rates of C capture, and test the second BEF prediction that species-rich natural forests exhibit greater stability of C capture across years, and are less sensitive to droughts, than monodominant plantations.

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[1] Url: https://iopscience.iop.org/article/10.1088/1748-9326/ab5f75

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