New metric pinpoints pressures on tropical forests, says S.Ananthanarayanan.

Forest cover is dwindling in the face of human encroachment. Is climate change a silent agent that does as much to cause forests to die out?

Alice E. Haughan, Nathalie Pettorelli, Simon G. Potts and Deepa Senapathi, from the University of Reading, UK, and the Zoological Society of London, describe in the journal, Global Change Biology, a first, comprehensive study of forests in India, over 2001 – 2018. “Understanding the different regional and seasonal relationships between climatic conditions and forest distributions will be key to effective protection of the country's remaining forests as climate change accelerates,” the study says.

And in the same week, the journal Science reports a study that compares the efficacy of reforestation, in the form of tree plantation, that the plantations hope to replace, with the value of natural forests. Fangyuan Hua, L. Adrian Bruijnzeel, Paula Meli, Phillip A. Martin, Jun Zhang, Shinichi Nakagawa, Xinran Miao, Weiyi Wang, Christopher McEvoy, Jorge Luis Peña-Arancibia, Pedro H. S. Brancalion, Pete Smith , David P. Edwards , Andrew Balmford, from Peking and Yunnan Universities, the Universities of Cambridge, Sao Paulo, New South Wales, Aberdeen, Sheffield and La Fontera, in Chile, and King’s College, London, find that in respect of carbon storage, water provisioning, soil erosion control and biodiversity, younger plantations perform poorly, in comparison to native are naturally regenerated forests.

The group writing in Global Change Biology notes that it is direct human action, in the form of agricultural expansion and deforestation, that have been studied as the reason for the alarming decline of tropical forests in India during the last century. As for climate change, although things like global warming, drought and floods are rising and are projected to rise, the potential of climate change in affecting the productivity of forests has not been studied. While changes in land use, arising from demand for food grain and timber, is clearly the main reason for the decline, there is concern now that the effects of climate change “could be eclipsing those of land use change on 60% of the global land surface,” the paper says. And with tropical forests facing the largest decline, there are “implications for biodiversity that is reliant on tropical forests, and the ecosystem services tropical forests provide in the form of carbon capture and the water cycle,” the paper says.

The group took up detailed records of tree cover in India, over the period from 2001 to 2018. The source of records was the Hansen Global Forest Change dataset, a resource attributed to Prof. Matthew Hansen, of the University of Maryland, and uses satellite data to compile a high-resolution global map of forest cover. The data coves the period from 2001 to 2018, and records changes at each pixel of the area of interest, taking into account all trees that are more than 5 m in height. And the data was compiled from this record for all 577 districts (administrative, geographical divisions) in India.

There are several factors, the location and the interplay of climate variables, that affect forests, the paper says. While the temperature has been studied as an important factor of tree mortality, so has the level of rainfall. But the relationship is complex and the effect of climate change on tree growth and survival is still an area that is not understood, the paper says.

Most of the time, the paper says, it the time progression of climate variables that are considered, and not the variation in the physical landscape. To this end, the group considered a new metric, called Climate Velocity, which proposes that “areas where climate is changing quickly, and similar climates are further away, will be at greater risk to climate change.” The idea is that the population of species, and in this case, of tree cover, could be expected to move, during climate change, to a contiguous area where the suitable climate is found. But if such an area is far removed, or the change in climate is too fast, the population is expected to decline. Areas of high velocity are hence of high risk, while those of low velocity areclimate refuges.

The result of the study is that climate change has been a significant factor in the forest loss that India has seen. And the study is first analysis of climate velocity as a measure of risk arising from climate change. The “emerging drying trends and the locations and magnitude of singular high velocities in India's remaining forest strongholds” are matters of concern with regard to future forest loss, the paper says. And while the National Forest Policy, of 1988, is being reviewed, it is crucial that an earnest look be taken at data and the multiplicity of threats face, the paper says.

Reforestation strategy

The paper in the journal, Science, looks into whether the current practice of replacing lost forest cover with comparatively simple, i.e., less heterogenous tree species, would serve the purpose. While the objectives of restoring forests are generally recovery of ecosystem services such as carbon storage, soil erosion control, water provisioning and wood production, the restoration drive seems to assume that “these services can be effectively delivered by forests regardless of their composition,” the paper says. While this assumption is yet to be tested, there is a risk that tree plantation, as opposed to natural restoration, may negatively impact biodiversity and “large-scale forest restoration, may hamper progress toward global commitments to halt and reverse ecosystem degradation.”

The group hence brought together data from the world’s main forest systems, to assess the merits or shortcomings of different reforestation approaches. “By simultaneously considering forests’ performance in carbon, soil, water, and biodiversity (i.e., environmental outcomes), plus in wood production, our study also provides a critical assessment of the trade-offs likely to confront forest restoration decision-makers,” the paper says.

The data collected was farmed into pairs – data that involved tree plantation and data of restoration of matching native forest. The second group consisted of “secondary forests resulting from natural regeneration, as well as actively restored native forests resulting from the planting of a diverse native tree mix,” the paper says.

The result of the pairing was that in respect of recovery of environmental services, the naturally restored forest performed significantly better than tree plantations. Although, in respect of wood production, tree plantation does score higher. “These findings provide evidence that if the goal of forest restoration is to recover environmental services on the land being restored, and if wood production is not a primary concern, native forest restoration should be prioritized, using site-appropriate measures including unassisted and assisted natural regeneration and active planting of diverse native species. Beyond biodiversity, the stakes are especially high for soil erosion control— given its far poorer delivery by tree plantations relative to native forests. Our synthesis refutes the implicit assumptions of ecosystem service-oriented forest restoration initiatives such as China’s Grain-for-Green Program … and a large collection of projects targeting carbon storage, soil conservation, and water provisioning that have focused mostly on establishing (monoculture) tree plantations,” the paper says.

Losseslikely to increase
India’s loss of forest cover during 2019-20 was 38.5 thousand hectares. Despite the government going on record to say that the loss is under control, it is feared that natural forces would drive the decline forward in the coming decades.

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