Research on forest ecology in areas of the tropics has been held back for many years, sometimes simply because of the great diversity of plant and animal species influencing each other's niches. One of eight magnificent relict forests have been studied in today's paper in the Royal Society B: Biological Sciences, including this last piece of primary forest in the SW of Sri Lanka, Sinharaja Reserve.
A forest community can depend on a monoculture such as the larch forests of the Siberian taiga. It can also have so many plants and animals involved that human nature tends to give up after counting the first million! While the ups and downs of hills and valleys tend to be an obvious influence, the soil resources have now been studied together with topography to give the fullest picture yet of, "how nature works."
Claire Baldeck of the University of Illinois, together with co-workers from almost every continent, compiled the giant set of results from eight large tropical forest plots set up for the long term. Trees larger than 1cm were all mapped while soil characteristics and the rest of the vegetation were equally examined with a fine tooth comb. (Don't take that literally, I mean "microscopically.") Elevations, as well as slopes and convexity were all added to the mix.
Credit: © Royal Society B: Biological Sciences
Above: Diversity maps and elevation maps (to the right) for six of the eight study sites: in elevation maps, the colour scheme moves from dark green (low elevation) to white (high elevation), just like normal contour maps. The colours of the community map (left) have no absolute meaning. Only colour differences between locations in the same study site are meaningful. NB. Sinharaja in Sri Lanka is shown on the bottom right with the elevated land to the right, associated with blue/green coloration to the right of the diversity map.
The way in which plant and animal niches are divided up forever interests ecologists. Non-random associations point to certain relationships, while seed dispersal, or its local lack of dispersal, often leads to aggregations of plant species. Environmental factors locally produced results for tree distribution that matched a previous 30-40% non-random arrangement (relevant to soil nutrient variations).
The composition of the plant community was measured in terms of its variation, rather than simply measuring one species at a time. Colours were used to illustrate the variation in an original way. What happened was that the variation due to environmental variables was doubled compared to the viewable amount from simple topography. The rest of the potential variation rests on light and light availability, soil moisture and drainage, etc., which have yet to be included. This fairly complete study represents the peak of investigation at the moment, however.
Elevation at Sinharaja was one of the most significant single variables ever found, with 14.7% of the variation. The local features contribute most of this. A variable with say 3% of the variation would be more subtle, while elevation here is obviously of supreme significance (around 15%). The subtle variations do however have the possibility of showing up important signals about the actual structure of the community.