Forests

My interest in studying forests was sparked not just from my college education in the plant sciences but also from the need to understand the real impacts that elephants made on trees. Research on forests in my lab has been anchored since 1988 by the Mudumalai Forest Dynamics Plot, a large 50-hectare area located in the heart of the Mudumalai National Park in the Western Ghats, India. A few years later we set up 19 smaller plots of one hectare each across Mudumalai's sharp rainfall gradient as well as one plot in the montane "shola" forests of the Nilgiris. HS Suresh, HS Dattaraja and CM Bharanaiah, supported by several field assistants from the Kuruba tribal community, are the pillars of this work. Our forest monitoring continues until the present with the aim of understanding growth, mortality, regeneration, invasive plants, impact of fire and elephants, carbon stocks and changes in diversity.
Why do trees die? Trees die due to fires, herbivory by elephants and other animals, water stress, and other natural causes. Fires obviously kill the smaller sized trees but there are differences among species in their susceptibility. Elephants strip the bark of their favourite plants such as Kydia calycina or push them over especially the medium-sized trees. High wind may fell a tree. A host of other natural factors, chiefly lack of water during droughts, seem to kill the large-sized trees. HS Suresh's analysis has also shown that the mortality of the large trees is among the lowest recorded for tropical forests. Interestingly, Rutuja Chitra-Tarak's analysis showed that deep-rooted tree species are more likely to die from a prolonged drought in this forest because the deeper soil layers are depleted of water and take more time to refill when the rains return.


How do trees regenerate and grow? We know surprisingly little about the regeneration of tropical dry forests. Most of the thousands of plants which have regenerated are from existing root stock and a few species. Some of the very common canopy trees have hardly put out new plants. As can be expected trees grow more slowly or even cease growth during periods of drought and water stress. The common tree species differ in their growth rates. While Terminalia crenulata is a slow growing tree it also seems to grow rather consistently under all environmental conditions.
How do fires and elephants impact forests? The work of Narendran Kodandapani and Nandita Mondal has shown that the fires occur most frequently in the dry deciduous forests growing in the intermediate rainfall zone (as compared to the drier thorn forests or the moister deciduous forests) because of sufficient fuel production and flammable conditions. Nandita further showed that seedlings of these dry forest plants grow faster in burnt areas and catch up with those growing in nearby unburnt areas. Elephants initially hammered the populations of Kydia calycina and Helicteres isora; while the former has recovered to some extent the latter has profusely regenerated over the past two decades when fire has been largely absent


How have invasive plants impacted the forests? While many invasive plants are found in Mudumalai, a thorny shrub Lantana camara has shown the most remarkable spread in recent decades. Geetha Ramaswami's work showed that the spread of lantana was related to a stochastic combination of drought and fire, and, further, that birds such as bulbuls feed upon the fruits and disperse their seeds. Lantana can alter the native plant communities. Grasses have been suppressed with the spread and growth of this invasive plant.
How is this forest structured across a rainfall gradient? The sharp east-west rainfall gradient in the Mudumalai landscape has provided us the opportunity to try and understand what factors influence tree species diversity in the seasonally dry tropical forest. We have shown that rainfall and fire frequencies are the major drivers of diversity; while higher rainfall increases diversity the higher fire frequencies in the intermediate zone depresses diversity. Overall, this landscape presents an almost complete turnover of tree species within a short east-west distance of 50 kilometers




What are the fundamental drivers of tropical dry forests? The Mudumalai Forest Dynamics plot has provided novel insights from a theoretical perspective on species coexistence in a tropical dry forest. Not only has environmental stochasticity emerged as the major driver of forest dynamics, the importance of autecological differences between species arising from their evolutionary and biogeographic histories in species co-existence was shown by Sandeep Pulla's analysis. This forest has also shown great resilience to disturbances and continues to sequester carbon until the present.
Contributions from Mudumalai to global understanding of forests: As one of the longest running sites in the global ForestGeo programme coordinated by Smithsonian Institution, the work at Mudumalai has made important contributions to global understanding of many facets such as co-existence of many tree species in tropical forests, spatial patterns in species distribution, estimates of tree species richness in the tropics, metabolic theory and tropical tree sizes, carbon stocks of forests and the important contribution of large trees.
