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Getting richer as you go higher: New Insights on Soil Variability in a Dry Tropical Forest

“Each soil has its own history”, wrote Charles Kellogg in his book The Soils that Support Us (1956). Indeed, there are several processes happening in the background that are responsible for the dynamic nature of the soils that support our forests and agriculture. These processes can give rise to variations in the soils that are evident in as little as a kilometre in the same geographic area, as examined by a team of scientists from the Indian Institute of Science, Bangalore. The variation in the soil, also known as soil heterogeneity, plays an important role in forest ecology. The spatial differences in the soil may ensure that the same substrate can be differentially utilized by multiple species, thus enabling co-existence of diverse plants and maintenance of the community structure.

Prof. Raman Sukumar and his team at the Centre for Ecological Sciences studied the impact of lithology (characteristics of rock), topography (surface features of land), vegetation and fire, on the spatial variation of soil in the Mudumalai Wildlife Sanctuary. Located in Tamil Nadu, this region is a seasonally dry tropical forest. This type of forest is found extensively all over the world and makes up about 42% of all tropical ecosystems. Unfortunately, these forests are threatened everywhere because they tend to occur in regions that are suitable for human use. Dry forests are particularly common in India because of the highly seasonal climate of the subcontinent.

Of the four factors considered in the study, lithology was found to be the most important factor influencing soil variability as well as topography of the region. Vegetation and forest fire were found to have little role in the same. Contrary to the general observation that lower elevation soils are richer in nutrients, in the present study, higher elevation soils had greater stocks of several plant nutrients, moisture, organic matter andclay compared to lower elevation soils.

The findings suggest that it is mainly the chemical composition of the parent rock that determines the soil nutrient availability in the region. “Due to intense erosion events during the Cenozoic era, the primary-mineral richweathered rocks were brought closer to the surface, imparting the same characteristic to the soils formed over them”, explains Dr. Jean Riotte. And in a seasonally dry tropical forest, there isn’t enough rainfall to deplete these nutrients from the soils. Additionally, since different rocks have different weathering rates, the rocks that were most resistant to weathering were left unchanged in the landscape. This phenomenon highlights the importance of local scale lithology in shaping the topographic variation of the region.

The positive correlation of elevation with soil nutrient content as derived from this study can be an important tool to determine soil quality in future restoration projects. “If the same pattern holds good in future studies, topography could be used as a proxy for these soil parameters, which are otherwise labour-intensive and expensive to quantify on a large scale”, says Sandeep Pulla on behalf of the research team. Knowledge of soil variability may be useful when designing restored ecosystems like the choice of tree species and locations to plant them.

About the authors:

Prof. Raman Sukumar is a professor at CES working on wildlife ecology, forest ecology, climate change and conservation biology

Sandeep Pulla is a PhD candidate in Dr. Sukumar’s lab and is presently working on tree communities in Mudumalai

Dr. Jean Riotte is a scientist at the Indo-French Cell for Water Sciences, IISc, Bangalore and Géosciences Environnement Toulouse (IRD-Université Paul Sabatier-CNRS), Toulouse, France

Dr. H.S. Suresh and H. S. Dattaraja are forest botanists at CES working in and around Mudumalai for nearly three decades

About the publication:The paper was published titled: “Controls of Soil Spatial Variability in a Dry Tropical Forest” in PLoS ONE in April, 2016.