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

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). 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.

Study areas

Our study focused on five wildlife reserves in the Western Ghats (8.1–20.1 °N, 73.2–77.6 °E), a 1600 km long mountain chain in peninsular India that forms part of the Western Ghats and Sri Lanka Biodiversity Hotspot (Kumar et al 2004). We conducted a field study in the Anamalai Tiger Reserve (ATR), and an analysis of Landsat satellite imagery across ATR, Parambikulam Tiger Reserve (PTR), Rajiv Gandhi Tiger Reserve (RTR), Wayanad Wildlife Sanctuary (WWS), and Bhadra Tiger Reserve (BTR) (figure 1(a)).

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The study areas were important centres for forestry operations from the colonial period (1800s), and were exploited for timber and for raising plantations of commercially important species such as teak and Eucalyptus spp. in place of native evergreen and moist-deciduous forests (Chandran 1997, Sekar and Ganesan 2003). Strengthening of Indian conservation laws including designation of wildlife reserves during 1950–80 shifted management priorities from forestry toward conservation in these reserves. Presently, these reserves support significant areas (6%–32%) under mature, monodominant teak and Eucalyptus stands–the former being more extensive–raised as plantations before 1980, but not harvested due to the cessation of forestry operations (information from official management plans for the individual reserves). Alongside these plantations grow mixed-species native wet-evergreen and moist-deciduous tropical forests (Pascal 1986) that were selectively logged before 1980, but have since largely been freed from extractive or commercial use.

Our study system thus provides an opportunity for comparing the C sequestering functions of monodominant plantations raised for reforestation/afforestation or long-rotation harvest, and species-rich natural forests that may be secured by averting deforestation, or that could emerge over time through reforestation via natural succession or active restoration. Although not exactly matched for age (some natural forests may be older than plantations), our system nevertheless offers a unique, long-term perspective on the roles of these contrasting types of forests in fighting climate change.

Forest inventory plots

Aboveground C storage

Aboveground biomass (AGB_est, kg) of individual trees was estimated using a general allometric equation from Chave et al (2014):

$$\begin{eqnarray*}&&AG{B}_{{\rm{e}}{\rm{s}}{\rm{t}}}=0.0673\times {(\rho {D}^{2}H)}^{0.976},\end{eqnarray*}$$

where ρ is wood density (g cm⁻³), D is tree diameter at breast height (cm), and H is tree height (m). Species' wood densities were obtained from published sources (Zanne et al 2009, Osuri et al 2014). Trees belonging to species that lacked published wood density estimates (10% of all individuals) were assigned the community-weighted average wood density of their respective plots. The carbon fraction of aboveground biomass was assumed as 47% (Thomas and Martin 2012). Plot-level estimates for aboveground C storage (Mg 0.04 ha⁻¹) were obtained by totalling C stocks across all trees within each plot.

Locations for satellite data analysis

Rates of C capture

The Enhanced Vegetation Index (EVI) is an index of photosynthetically active vegetation derived from remotely sensed reflectance in near-infrared (NIR), red (R) and blue (B) wavelengths (Huete et al 2002). This index is known to correlate positively with field-based estimates of gross primary production (GPP), or C capture via photosynthesis (Rahman et al 2005, Huete et al 2006, Glenn et al 2008, Huete et al 2008).

We calculated EVI of all cloud-free (≤10% cloud cover) scenes spanning the ATR plots from January 2000 to April 2019 using the surface reflectance products of the USGS Landsat-7 and Landsat-8 platforms (USGS 2017).

$$\begin{eqnarray*}&&EVI=2.5\times \displaystyle \frac{(NIR-R)}{(NIR+6\times R-7.5\times B+1)}.\end{eqnarray*}$$

Cloud- and shade-covered pixels, and no-data (Landsat-7 SLC-off) pixels were excluded from subsequent analysis.

Next, taking into consideration the timing of the Indian summer monsoon (June–September) which is the main source of rainfall in the Western Ghats, we calculated median EVIs across all scenes within the post-monsoon wet season (September–December), and within the subsequent dry season (January–April), for each year. Many deciduous species in the region, including teak, shed and regrow leaves over the latter period. Estimates of annual post-wet and dry season median EVIs were based on a median of 1 scene per year (range: 0–3) for post-wet seasons and a median of 3 scenes per year (range: 0–6) for dry seasons. Finally, we extracted year-wise post-wet and dry season median EVIs for the 242 forest plots in ATR and the 96 locations (48 forest-plantation pairs) across the five focal wildlife reserves, by averaging over pixels contained within individual 30 m buffers. See figure S2 for a flowchart of EVI data processing and analysis steps.

Analysis

Tree species richness and aboveground carbon storage

In the Anamalai Tiger Reserve (ATR), teak and Eucalyptus plantations had fewer tree species (3 and 6 species on average, respectively) than natural evergreen and deciduous forests (14 and 9 species, respectively) at the plot level (species density: figure 2(a)), and cumulatively fewer species across plots (rarefied species richness: figure 2(b)). Aboveground C storage per plot (carbon density) 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 (average of each group lies within the 95% CIs range of the other) 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)). To test for potential biases arising from variation in mean annual precipitation (MAP) across the four forest types, we re-ran our models with MAP included as an additional predictor. Including MAP as a predictor did not alter model predictions (table S1), suggesting that differences in species and carbon density across forest/plantation types are not simply an artefact of differences in MAP.

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Average rates and stability of C capture

Wet-season C capture rates were 4%–9% higher in teak and Eucalyptus plantations (average EVI: 0.46 and 0.45, on average, respectively) than in evergreen and deciduous forests (average EVI: 0.42 and 0.44, respectively), but dry-season average EVIs of teak and Eucalyptus plantations (0.30 and 0.36, respectively) were 3%–29% lower than those of evergreen and deciduous forests (0.42 and 0.37, respectively) (figures 3(a), (b)). By contrast, temporal stability of C capture in forests–especially evergreen–was consistently higher than plantations over wet and dry seasons (figures 3(c), (d)). The differences in average and stability of C capture across forests and plantations were evident even after accounting for variation explained by MAP (table S1). Moreover, excluding areas that experienced fire during a given year (source: Modis Terra Thermal Anomalies & Fire data accessed via the Earth Engine platform) from the analysis did not alter the results, indicating that our results are not biased by variation in fire occurrence (table S1).

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C capture by plantations appeared less resistant (i.e. decreased more) to drought than that of natural forests. Wet season EVIs of forests during drought years differed less from their corresponding overall averages (EVI_diff = 0.00 – −0.03; 0%–3% decrease) compared to plantations (5%–8% decrease) (figures 4(a); S4). Differences between the responses of forests and plantations to drought were even more pronounced among dry season EVIs (forests: 2%–6% declines; plantations 12%–14% declines) (figures 4(b); S4), and were not biased by variation in MAP or fire (table S1).

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Additionally, for teak plantations in ATR, we explored relationships of C storage and average and stability of C capture with approximate plantation ages (table S2) using pairwise correlations (all Eucalyptus plantations being of similar age, were not included). Teak plantation age correlated positively with aboveground C stocks, negatively with average EVI (dry season), stability of EVI, and EVI_diff (i.e. greater declines of EVI during drought years (relative to long-term average) among older plantations; table 2).

Table 2.  Pearson's correlation coefficients between the age of teak plantations and the different indicators of C sequestration in the Anamalai Tiger Reserve. Correlations with 95% confidence interval estimates not overlapping zero are marked with *.

Indicator	Correlation with plantation age
Aboveground carbon stocks	0.44*
Average EVI (Wet season)	0.01
Average EVI (Dry season)	−0.33*
Stability of EVI (Wet season)	−0.37*
Stability of EVI (Dry season)	−0.39*
EVIdiff (Wet season)	−0.25*
EVIdiff (Dry season)	−0.64*

Multi-site analysis

Patterns of C capture in evergreen and deciduous forests, and teak and Eucalyptus plantations across multiple locations in the Western Ghats generally matched those observed in ATR. The average EVI of forests and plantations did not differ consistently during the wet season (95% CI range of LRR_av includes zero), but plantations had consistently lower EVI than forests during dry seasons (negative 95% CI range of LRR_av; figure 5(a)). These patterns were driven by the DF-TP contrast, which constituted 67% of all edge pairings, while EF-TP and DF-EP showed qualitatively similar responses, but were associated with wider uncertainty estimates (figure 5). Forests showed greater temporal stability of EVI than plantations, on average, in both wet and dry seasons, but with 95% CIs overlapping zero for EF-TP and DF-EP comparisons, possibly due to the limited sample sizes of these types of pairs (figure 5(b)).

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With forest plantations and natural forest regeneration gaining global significance as strategies for mitigating climate change, understanding the long term carbon storage potential of different types of reforestation strategies, and how effectively and reliably they capture carbon from the atmosphere, is critical for reforestation policy and practice. Our study in India's Western Ghats compared multiple facets of C sequestration across mature monodominant plantations and species-rich natural tropical forests. Our findings reveal interesting contrasts across indicators of C sequestration quantity and stability in plantations and natural forests.

On one hand, indicators related to quantity, namely aboveground C stocks and rates of photosynthetic C capture (average EVI), did not differ consistently between species-rich natural forests and species-poor plantations in our study. Hardwood teak plantations had similar C stocks as moist-deciduous forests (but substantially lower C stocks than evergreen forests), while fast-growing Eucalyptus plantations had comparatively low C stocks, but similar to greater rates of C capture than both forest types across wet and dry seasons. While these patterns might partly be due to differences in age—i.e. relatively younger plantations store less C but exhibit higher rates of C capture—they nevertheless support the BEF theory prediction that monocultures of certain species could match or surpass species-rich forests for C storage or average rates of C capture (Bonner et al 2013, Hulvey et al 2013, Huang et al 2018). While our study focused on mature, unharvested plantations, harvesting plantations for manufacture of long-lived wood products could effectively increase C capture and storage (Keith et al 2014). However, the climate regulating benefits of the latter approach remain questionable, because harvesting of timber is also likely to release significant amounts of CO₂ to the atmosphere (Keith et al 2014, Lewis et al 2019).

In contrast to C storage and average rates of C capture, and in agreement with BEF theory (Yachi and Loreau 1999, Hooper et al 2005), the stability of C capture rates was consistently higher in species-rich forests (EF > DF) than in monodominant plantations (EP > TP) in our study. Furthermore, our findings indicate that this difference in stability is likely due to plantations showing lower resistance of C capture to drought compared to forests. Moreover, variation with age across teak plantations suggest that these differences in C capture stability are likely to persist, and possibly increase, over time. Collectively, these results suggest that species-rich natural forests may be more reliable than plantations as agents for terrestrial C sequestration (Naeem 2003), especially given ongoing and predicted increases in climatic perturbations such as droughts in south Asia (Singh et al 2014) and several other regions globally (Dai 2012).

We illustrate the above argument with a simple bootstrapping simulation. In ATR, the difference in cumulative C capture (i.e. cumulative average EVI) between teak plantations and deciduous forests across 10 randomly selected dry seasons (sampled with replacement; 100 iterations) increased from 21% lower in teak plantations on average when considering non-drought years alone, to 23% and 27% lower in teak plantations when two and four drought years, respectively, were included in the random samples (figure 6(a)). Similar patterns were observed among other forest-plantation contrasts across dry seasons (figures 6(b)–(d)), and to a lesser extent across wet seasons (figure S5) in our simulations.

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We focused on trees in this study because of their dominant influence on the C cycles of mature forest and plantation ecosystems (Lü et al 2010, Chen et al 2015). Forest/plantation C cycles also comprise several other pools (e.g. coarse woody debris) and fluxes (e.g. soil respiration) (Malhi 2012), but how these differ between forests and plantations, and ultimately shape C capture, storage and release to the atmosphere by these ecosystems, remains poorly studied. Further research assessing these other C pools and fluxes can shed light on how patterns of photosynthetic C capture and storage observed at the level of tree communities translate in terms of overall C sequestration at the ecosystem scale in species-rich natural forests and monodominant plantations.

A strict focus on quantity (potential C capture rates and storage) might suggest equivalence of monoculture/monodominant plantations and high-diversity forests for mitigating climate change (Bonner et al 2013, Hulvey et al 2013). However, our findings suggest that when assessed for both quantity and quality, in terms of reliability of C capture in the face of perturbation such as droughts, forests are superior to, and irreplaceable by, plantations as agents of terrestrial C sequestration. This is consistent with findings from other biomes such as grasslands, which too might be more reliable than plantations for sequestering carbon, and which are facing a significant threat from ongoing and planned expansions of plantations (Bond et al 2019). Policies that facilitate land transitions from natural forests (and other ecosystems) to plantations—e.g. India's compensatory afforestation programme—can therefore have lasting detrimental impacts on terrestrial carbon sequestration, in addition to posing a significant threat to biodiversity (Narain and Maron 2018). Our findings thus underscore the need for policy changes that increase focus on protecting and restoring natural forests and/or mixed plantations of native tree species—instead of promoting low-diversity plantations (Lewis et al 2019)—as a more appropriate strategy for sequestering carbon in an increasingly variable and drought-prone climate. Apart from likely benefits for climate change mitigation, such policy changes would also generate valuable co-benefits for biodiversity conservation and other ecosystem services.

We are grateful to Rajesh, Sathish, Sundar Raj and Moorthy for assistance during fieldwork. We thank Divya Mudappa, Raman Kumar, M D Madhusudan, Girish D V, Chengappa S K, Srinivasan Kasinathan and Sreedhar Vijayakrishnan for sharing expertise and technical inputs. We thank the Tamil Nadu Forest Department for research permits, and appreciate guidance from V Ganesan (IFS), S Thangaraj Panneerselvam. AMO received a fellowship and research funding from the NatureNet Science Fellows Program and the Earth Institute Fellows Program. TRSR acknowledges funding and partnership from Rohini Nilekani Philanthropies, Arvind Datar, and the AMM Murugappa Chettiar Research Centre for long-term rainforest restoration and recovery research in the Anamalais.

The data that support the findings of this study are openly available at https://doi.org/10.25412/iop.11324132.v1.