One of the busiest fields of inquiry at the Smithsonian Tropical Research Institute is about the patterns of dispersal and diversity of tree species in our tropical forests. From this we may get basic science insights into evolution’s workings and we may develop better applied science techniques of forest management. In order to do the research, computerized databases have been and are being developed, and that is also of interest to scientists around the world.
Priya Davidar is one such scientist. A biologist who grew up on the slopes of India’s Western Ghats, she has been studying the flora of that range, which runs parallel to India’s western coast, for about 20 years. She has just spent a year in Panama, primarily for the purpose of using techniques and programs created at the Smithsonian to chart tree distribution at Barro Colorado Island to create her own database of the forests of the Western Ghats.
“It’s easy working here,” she told her colleagues at the start of her May 20 talk at Tupper Auditorium, part of the weekly free lecture series that the Smithsonian presents every Tuesday at noon and to which the public is invited. “If you want to see how the other end of the spectrum works, you should come to Pondicherry.”
Back in India, she explained, fewer resources are dedicated to her kind of research, and the areas in which she works are often battlegrounds between poachers of endangered tigers and elephants and park rangers, so there are often bureaucratic delays just to get permission to enter national parks. And forget about driving to work, or as the Barro Colorado Island researchers do, taking the launch. “You can’t go to these rainforests if you are not willing to walk,” Davidar warned.
Davidar’s database and analysis work here is based upon her observations and records and those taken by colleagues and her students in India, and is funded by grants from the governments of India, France and the United States.
Her lecture at the Smithsonian, accompanied by some beautiful slides of a place that she obviously loves, was to explain the ecosystem of the Western Ghats and how it differs from the forests we know here in the tropical areas of the Western Hemisphere.
The Western Ghats, part of which may include older mountains, were formed at about the same time as the Himalayas in something like the tectonic equivalent of a multi-car pile-up on a foggy highway. The Indian subcontinent crashed into Asia, with its leading edge diving deep and pushing what had been the southern edge of Asia into the roof of the world. Slowed down by that collision, another plate coming off of the Indian Ocean pushed under the western edge of the subcontinent, raising the Western Ghats and the island of Ceylon.
The ecology of the Western Ghats, like the rest of India, is profoundly affected by the monsoons, seasonal rains that sweep up and down the Indian Ocean and the Bay of Bengal. In the southern part, the Western Ghats catch the tail end of the monsoon that sweeps down India’s eastern shore. Another monsoon move northwesterly from the southern tip of India, up the Western Ghats toward Bombay, and then recedes along the track from whence it came. The western slopes of the mountain range catch much of the rain coming off of the Indian Ocean, leaving the eastern slopes much less humid.
“The Western Ghats has one of the strongest climatic gradients in the world,” Davidar explained. Within a very few miles you can go from a rainforest to a dry forest. Higher up the mountains you have grasslands with many unique --- some believe evolutionary relic --- species to be found. In the southern part that gets rain from both monsoons there’s hardly any dry season, while in other parts of the range dry season lasts as long as five months. Some of the places that have long dry seasons nevertheless get hit very hard by the monsoons, so register much greater annual rainfall than other parts of the range with substantially shorter dry seasons. Add to this the variation in climate and wildlife that normally comes at different altitudes and you get an area that challenges some assumptions that could be made if all your assumptions are based upon experiences gathered in the New World.
In the Western Ghats, the dry forests are where you find most of the tigers and elephants, and these have had human populations since prehistoric times and are heavily disturbed from an ecological point of view. The rainforests are “pretty patchy,” according to Davidar. “I think this is because of the climatic gradient.”
One thing that makes a biologist’s field work in the Western Ghats easier than in the tropical forests of the Americas that “the flora is well known and there aren’t many species.” As in fewer than 700 species of trees, compared to tens of thousands in the American tropics.
Davidar’s database includes her own observations of 57 forest plots, most of them a fraction of a hectare, situated along the Western Ghats between 8° and 15° north of the equator. Added to that are observations by others, climatic data over five years, and bioclimatic maps. Species are listed by where they are found, and data about traits such as how species disperse their seeds (for example, by birds, mammals, or the wind) are included. The database sorts by species that are common, widespread or rare, and those that are endemic to the Western Ghats that those that aren’t.
Mostly, she found that the rare species grow in the understory and the widespread ones are canopy trees.
As with the forests we have in the New World, she found that most tree species in the Western Ghats are rare and a few are widespread.
However, one thing that is believed about tropical forests in this hemisphere, that the dispersal of tree species is largely a function of annual rainfall, doesn’t hold in the Western Ghats. There, seasonality is much more important predictor of dispersal. In that part of the world, there is more diversity of tree life in aseasonal forest niches, whether relatively dry or wet, than in places with longer and more distinct dry seasons.
That may leave researchers here with something to do. There is a strong correlation between the length of the dry season and the amount of rainfall in the Western Hemisphere, so it has been easy for scientists to presume that the two factors are one and the same. But the Western Ghats show us that it isn’t necessarily so. The ability to make a more precise distinction between annual rainfall and the length of the dry season around here might lead to new inquiries in both basic and applied science.
Also in this section:
Tree diversity in India's Western Ghats
The WHO's new chronic disease database
