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Even in stable tropical rainforests, trees dance to the tune of the environment

Forest at Mudumalai (© Wikipedia)

A cross-continental study including over 2 million trees from about 4000 species has found that among the factors that affect how forest communities change over time, variation in environmental conditions like temperature, rainfall and fire is the most important. The study shows for the first time that forest communities respond to environmental variation over a decade or two; not over thousands of years as previously thought.

A study of this kind is bound to be complex. Researchers working in twelve tropical forests across the world have been painstakingly counting trees and watching them grow, for almost three decades in some places. The Center for Tropical Forest Science (CTFS; http://www.ctfs.si.edu/), a global network of forest research plots and scientists dedicated to studying forests, has coordinated this data collection, which uses standard methods in all forests. The network now has 53 plots comprising about 8500 species. In each plot, researchers have been recording the diameter of each tree, typically every 4-5 years; but the Indian plot has been going further in recording all deaths and new recruits every year.

Such long-term data can be used to study “population dynamics” -- short-term and long-term changes in size and composition of populations, and the different processes that cause these changes. In small forests, factors like births and deaths of trees, and competition between individuals of the same species for resources, affect the composition of the tree community.

In large forests, other factors affect what species make up the community.Among these, variation in environmental factors is the most important, concludes a paper published last week in the prestigious Ecology Letters journal. An international team of researchers, including four from India (details below) collected the massive amount of data (2 million trees, as stated above) that went into the study.

Wanting to establish a long-term ecological monitoring programme in the Nilgiris, Professor Raman Sukumar from the Centre for Ecological Sciences in 1988 established a 50-hectare plot at Mudumalai Tiger Reserve in Tamil Nadu. “Mudumalai is a dry forest. Till then, long-term plots were only established in tropical rainforests that receive a lot of rainfall. I thought a dry forest plot would make for an interesting comparison,” he said.

The study showed that being a dry forest, drought was a major environmental factor affecting the tree community at Mudumalai. Another was frequent, severe fires that sometimes burned out the entire study plot. Apart from these factors, Mudumalai had an additional dimension that other long term monitoring plots lacked – a healthy population of elephants.

“Every year, researchers at Mudumalai collected data about what plants were dying and what the cause of death was. It was obvious that plants preferred by elephants sometimes got decimated, but then seem to bounce back,” said Sandeep Pulla, a PhD student in Professor Sukumar’s lab and co-author on the paper.

After ten years of monitoring at Mudumalai, it became clear to Professor Sukumar and his team that environmental variation – fires, drought and herbivory by elephants – was the main factor affecting the forest at Mudumalai. “Being a dry and seasonal forest, I expected this to be the case,” said Sukumar.

“What really surprised me was, even in habitats that apparently did not change much over the years – like tropical rainforests – variation in the external environment was the main factor affecting forest dynamics”, he added.

Diseases, insect infestation and storms have all affected forests in other CTFS plots. A typhoon hit a plot in Palanan in Philippines in 2003, causing trees to fall and letting light into the otherwise dense forest; the following year saw an increase in plant species that need a lot of light to grow. Forests without such conspicuous disturbances were obviously more stable, like the ones in Africa and Pasoh in Malaysia. The analysis showed that a high diversity in the number of species may be the result of a stable environment.

Human-induced environmental changes like climate change, release of excessive nutrients like carbon dioxide and nitrogen into the environment and the hunting of animals and birds that disperse fruits can thus impact tropical forests, both over decades as well as over longer timescales.

About the paper

The paper appeared in Ecology Letters, one of the foremost international journals in the field of Ecology (impact factor of 13). Link: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1461-0248

Author information (Indians)

Professor Raman Sukumar is at the Centre for Ecological Sciences, Indian Institute of Science. Sandeep Pulla is his PhD student. H S Suresh and H S Dattaraja work with Prof Sukumar.

Contact: Website http://ces.iisc.ernet.in/new/?q=user/28; Email rsuku@ces.iisc.ernet.in