How reliable are long-lived plantations composed of a few species for carbon capture, when compared with natural tropical forests that comprise many species?
Fighting climate change through reforestation activities, such as large-scale plantations, has gained global traction over recent years. To reduce carbon emissions, international efforts such as the Bonn Challenge and Paris Climate Accord have promoted tapping into the power of trees that suck in and sequester carbon in multiple ways.
Researchers conducted a study in one of India’s biodiversity hotspots, the Western Ghats. They compared carbon storage and rates of carbon capture of mature mono-dominant plantations, with those of neighbouring natural forests harbouring a diverse mix of native species.
Although mono-dominant plantations could match natural biodiverse forests in terms of carbon capture and storage potential, the latter were more stable and hence more reliable in their ability to capture carbon over the years, particularly in the face of droughts.
“Previous studies have shown that while India’s estimated forest cover might be increasing, this trend conceals widespread losses of biodiversity-rich natural forests being offset by the expansion of biodiversity-poor plantations,” said Anand Osuri, a postdoctoral fellow at the Earth Institute and the Nature Conservancy, and lead author of the study.
“Our findings show that such a trend would erode the climate-mitigating potential of India’s forests, even if the overall extent were to increase, because plantations cannot match the climate-regulating benefits offered by natural forests.”
According to Tom Crowther, a professor at ETH Zurich in Switzerland, “this excellent study provides clear evidence that diverse mixtures of native species are vastly more important for long-term carbon storage than monoculture plantations.”
Crowther, who was not connected to the study, added that “this supports other findings from many different regions around the world. Facilitating the natural regeneration of diverse, resilient ecosystems has to be a priority as we aim to improve biodiversity and carbon storage.”
Natural, biodiverse forests versus mono-dominant plantations
For the study, the researchers focused on former teak and eucalyptus plantations (over 40 years old) growing alongside naturally regenerating native tropical forests in five wildlife reserves. During the colonial era, these areas were important for commercial timber plantations, but during 1950-80 management shifted from forestry to conservation and hence these plantations remain uncut.
In one of the sites, Anamalai Tiger Reserve, the team assessed the forest structure, tree species richness and above ground carbon storage in the form of woody vegetation biomass in mono-dominant teak and eucalyptus plantations, as well as in neighbouring evergreen and moist-deciduous forests. Above ground carbon storage of the trees was calculated by measuring tree height and girth within plots. For all reserves, they also estimated the rates of photosynthetic carbon capture over the period from 2000 to 2018 by using satellite-derived Enhanced Vegetation Index of vegetation that is photosynthetically active.
Teak and eucalyptus plantations in Anamalai Tiger Reserve had on average 3 to 6 tree species while natural evergreen and deciduous forests had 14 and 9 species respectively. Both plantations stored 30 to 50 percent lower carbon than natural evergreen forests but similar when compared with moist-deciduous forests.
Carbon capture rates of the plantations during the wet seasons were 4-9% higher than in evergreen and moist-deciduous forests. But differences emerged when they looked at the dry seasons over the years: the carbon capture rate for the plantations was 3-29% lower than natural forests during the dry seasons.
From 2000 to 2018, the rate of carbon capture by natural forests varied less compared to plantations, said Osuri, adding that they also “showed relatively smaller decreases in carbon capture during drought years.”
Only focusing on ‘quantity’ measured by parameters such as carbon storage and capture rates may suggest that mono-dominant plantations are equivalent to high-diversity forests in mitigating climate, stated the authors. But when also considering ‘quality’ in terms of the reliability of carbon capture in the face of climate perturbations, especially droughts, natural forests are superior to plantations in fighting climate change. Additionally, in future climate scenarios, droughts and excess rainfall events are expected to become more frequent.
Protect natural forests, prioritise a mix of native species for plantations
A diverse mix of native species in the natural forests allows them to adapt to a wider range of conditions, explained Osuri, giving the analogy of a cricket team. Having a diversity of bowlers such as fast bowlers and spinners enables the team to better adapt to varying playing conditions than a team with only one type of bowler, he elaborated.
The findings, Osuri said, highlight the importance of protecting and restoring natural forests. But active restoration should only be adopted in areas that are incapable of recovering on their own, he added.
Additionally, the team suggests re-examining “those laws and policies that operate under the assumption that the destruction of natural forests at one location can be compensated by raising tree plantations at another.”
For example, the Indian government’s CAMPA (Compensatory Afforestation Fund Management and Planning Authority) programme directs payments from deforestation that arises from certain projects towards compensatory afforestation efforts. During 2015 to 2018, plantations of five or fewer species comprised 53% of the 235,000 hectares planted for reforestation, calculated the authors using data obtained from e-Green Watch portal, of the Ministry of Environment, Forests and Climate Change.
The Western Ghats harbours an incredible array of ecosystems and tree plantations may not be appropriate in some of them such as in open or grassy ecosystems, according to Osuri. But in the locations that are suitable, he said: “one must consider conditions such as elevation, rainfall, soil and topography, as well as past disturbances, in identifying a diverse mix of native species most likely to grow well at those locations.”
When planting at any location, Osuri stressed that it is “always better to prioritise native species” but in some cases, he observed that certain hardy non-native species could be helpful for facilitating recovery, particularly in areas with severely degraded soils, as shown in some studies.
This article was originally published on Mongabay and is republished under a Creative Commons license.