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A team of earth scientists studying the changes and movements of organic carbon in the soil in the Western Ghats came upon a curious disparity in the tree cover distribution on their site. Looking in one direction, there was more tree cover, and the canopy was taller. In another direction, the tree cover was less with a shorter canopy.
The research team quickly realised that the north-facing (N) slopes on their site in this mountain range had higher tree cover than the south-facing (S) slopes, an example of aspect-related asymmetry, where there is an unequal distribution of vegetation across opposing terrain aspects (the compass direction or orientation of a slope). Eventually, they confirmed a similar West-East (W-E) asymmetry on-site.
It was when they also looked at other protected areas in the Western Ghats that a larger pattern emerged. Using satellite imagery and digital elevation models, scientists from the Indian Institute of Science Education and Research, Pune, analysed tree cover and canopy height in 25 protected areas in the Western Ghats across different aspect categories (N, NE, E, SE, S, SW, W, NW). The study revealed that across all protected areas, tree cover and canopy height were the highest in northwest-facing (NW) slopes and the lowest in southeast-facing (SE) slopes.
With these findings, the scientists could infer that the aspect of a slope, whether facing north, south, east, or west, has a significant impact on the tree cover at a local landscape scale.
According to the research paper, while the slope aspect effects are well-known in the mid-latitudes they are less understood in low-latitude regions that are closer to the equator, such as the Western Ghats. Based on their findings, the authors suggest that it is crucial to consider slope aspect when planning biodiversity conservation and afforestation in the Western Ghats.
Slope aspect and tree cover
The variation in tree cover distribution at a regional scale in the Western Ghats is broadly known. “The dry season lasts longer in the northern part of the Western Ghats than in the southern part, which naturally controls the regional distribution of vegetation,” said Shreyas Managve, Associate Professor at the Department of Earth and Climate Sciences, IISER, Pune, and the lead author of the paper. “What our study demonstrates is that even at a local scale, within a single protected area or watershed in the Western Ghats, there is vegetation asymmetry, which is governed by the direction a given slope faces.”
The study identified two modes of tree cover distribution in the Western Ghats, according to Managave. “West-facing slopes have denser tree cover and taller canopies than east-facing slopes. Similarly, north-facing slopes have denser tree cover and taller canopy than south-facing slopes,” he said. While the N-S asymmetry is attributed to differential heating, with the south-facing slopes receiving more sunlight and hence having drier soils, the W-E asymmetry is linked to orographic rainfall and cloud cover dynamics, with the west-facing slopes receiving more rainfall and higher cloud cover, leading to higher soil moisture retention. “The N-S and W-E asymmetries interact to create a situation where northwest-facing slopes have the most optimum conditions for tree growth and southeast-facing slopes tend to be the least conducive to tree growth,” Managave added.
While the relationship with slope and aspect was qualitatively known to many field ecologists, the new research paper is the first to validate it quantitatively, said Meghna Krishnadas, Assistant Professor at the Department of Ecology and Evolution, National Centre for Biological Sciences, who is not a part of the study. “By and large, the findings reflect basic ecological expectations of wetter, warmer sites supporting denser canopies and the resulting shade can promote species to grow taller, alongside more productive conditions,” she added.
Restoration practitioners
One of the main takeaways from the study is how slope aspect should be a crucial consideration when planning restoration programs in the Western Ghats. “Typically, when people look at heterogeneous forest cover with aspect-related asymmetries, they misunderstand slopes with more tree growth to be a pristine forest, and the slopes with lesser tree growth as degraded,” Managave said. “But our work suggests that S and SE slopes are not favourable for tree growth, whereas N and NW slopes are favourable. So this should be a major consideration when planning for afforestation.”
Divya Mudappa, a scientist, field biologist, and restoration practitioner in the Western Ghats, sees slope aspect as “a novel way to assess the success of restoration efforts”. She also realises the potential for it to be used in the planning phase of a new restoration project. While Mudappa and team have not used slope aspect to plan their restoration efforts to date, she is keen to analyse their restoration outcomes through an aspect lens. However, Mudappa thinks that it makes more sense to consider slope aspect at a landscape level rather than for small restoration patches.
According to Krishnadas, field studies need to ground truth the inferences made in the study. “Since this is a remote sensing study, inferences regarding optimum conditions for growth can be tricky without accounting for differences in individual species and on-ground data on growth and biomass accumulation,” she said.
“We need field studies to understand the mechanisms underlying the patterns, especially when drawing implications for conservation and management.” Mudappa adds, “It may be important to see if [slope aspect] would apply to all species of plants or only a subset of them.”
This article was first published on Mongabay.
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