DURHAM, N.C. -- Duke University researchers meticulously measuring the fate of tree seeds in the Appalachian Mountains have obtained results that call into question a popular theory of why tropical forests show such extraordinary diversity of tree species.
Their findings that, as in the tropics, natural enemies play a role in promoting diversity in temperate forests, suggest that ecologists will need to explore other underlying reasons why tropical forests have many more tree species than temperate ones. And, their findings suggest that preserving tropical diversity might also depend on preserving the natural enemies of trees.
In an article in the June 13, 2002, Nature, ecologists including Professor of Biology James Clark report on studies of the fate of tree seeds and seedlings in research forests in the Appalachian Mountains. Their research is sponsored by the National Science Foundation.
The scientists' aim was to test whether a process called "density-dependent mortality" was at work in temperate forests, just as other researchers had found evidence for the process in tropical forests. If the process was, indeed, functioning in temperate forests, that discovery would call into question scientists' belief that it uniquely contributed to the high diversity found in tropical forests.
"Basically, the idea of density-dependent mortality is that near adult trees of a given species, there is a concentration of natural enemies -- for example pathogens or predators of seeds or seedlings," said Clark. "If the natural enemies of these trees congregate in the neighborhood of adult trees, then there would be disproportionate mortality of seeds and seedlings.
"Thus, even if a given species were a better competitor for light or nutrients than other species, this density-dependent mortality would tend to neutralize the advantage, thus promoting diversity," he said. According to Clark, scientists had theorized that tropical forests had a great number of species-specific natural enemies that would therefore promote tree diversity.
"While this theory has long been viewed as a mechanism that might promote diversity at low latitudes, the process has never really been examined using latitudinal data."
Thus, Clark, lead author Janneke Hillis Ris Lambers and co-author Brian Beckage measured the mortality of seeds from five tree species -- red maple, birch, ash, tulip poplar and oak -- as they progressed through germination to older seedlings. The scientists confirmed that the mortality at each stage depended on how many adult trees of that species were clustered nearby -- demonstrating that density-dependent mortality was at work in a temperate forest, just as in tropical forests. According to Clark, however, showing that density-dependence is at work does not imply that its mechanism is understood.
"We still need to understand the actual factors that cause density-dependence, whether in a temperate or a tropical forest," he said. "We can only say that these data suggest that you can't credit density-dependence for high species diversity in the tropics."
Further understanding of the role and mechanism of density-dependence will mean long-term studies showing how the initial fate of seeds and seedlings impacts the long-term dynamics of forests. Clark and his colleagues are now pursuing such studies, he said.
According to Clark, other theories might explain the basis of tropical diversity, and those bear further study. For example, he said, ice ages have preferentially contributed to species loss at higher latitudes.
"Even though there have been climatic fluctuations in the tropics, there isn't a situation where the biological slate is essentially wiped clean," he said. "Also, it is probably difficult for plants to adapt to seasonal climates, particularly long winters, so species might just evolve slower at higher latitudes because of the demands of such adaptation." Although much research remains to understand the origins of diversity in both temperate and tropical forests, such an understanding will be important to preserving forests, said Clark.
"It may be years before we come up with models that incorporate the many pieces of the puzzle of diversity in forests," said Clark. "But you can imagine that if density-dependence is one of the dominant controls on diversity, and you lose some of those natural enemies that are responsible for this phenomenon, that could have some important impacts on diversity."