Last tree standing

30/08/1998
Down To Earth

Last tree standing TROPICAL forests have been disappearing at alarming rates for the past three decades. Farmers, ranchers and timber industries have denuded millions of acres, and only in the past few years has the resultant damage to the ecosystem has become agonisingly clear.

A new study by botanists at the University of Georgia, USA, now shows that trees left standing in pastures can actually dominate the reproduction in nearby remnant forests, creating what scientists are calling a "genetic bottle-neck". According to the research, the survival of these tropical forests could be far more complex than what was thought earlier. New approaches to conservation strategies may be needed to protect these endangered forests.

"The key is to understand how much genetic movement there is between fragments of a forest," said James Hamrick, who conducted the study along with his graduate student Preston Aldrich. "When we lose fragments of a forest, we lose genetic diversity. Gene exchange between fragments helps to maintain this diversity."

Genetic diversity is vital for plants and animals alike. Farmers have, for hundreds of years, bred crop plants as well as farm animals to maintain a healthy diversity of what were, before the 20th century, called traits. Now, with advanced techniques to determine the exact genetic make-up of individuals, scientists have a considerably better understanding of how the genes drift through populations. Aldrich and Hamrick studied a tree species called Symphonia globulifera (S globulifera) in a little-examined rain forest area in southern Costa Rica.

S. globulifera is a shade-tolerant canopy tree with bright red flowers which are pollinated primarily by hummingbirds and occasionally by bats that disperse the seeds by eating fruits and then pass them on through excreta at their resting sites. Like many areas in the tropics, the study focused on an area of fragmented forest with a number of S. globulifera trees scattered in the open pasture land nearby. There were neither seedlings or saplings of S. globulifera in the pastures, suggesting poor habitat quality for both germination and growth. The question was simple; which trees are the genetic 'parents' of the seedlings growing in these forest fragments? Finding the answer would have been nearly impossible even a decade ago. But the sensitive techniques that are now available allowed researchers to determine the exact genetic make-up of individual plants in the ecosystem.

Even then, the problem was daunting since there were more than 800 possible parent pairs. "In trying to tell who the parents are, we had to use the same techniques used in forensic analysis to determine a child's parents," said Hamrick. "We were able to do this only because Preston was able to develop the techniques for our specific genetic analysis." The breakthrough came in using segments of DNA called microsatellites as specific markers for S. globulifera. These markers allowed Aldrich and Hamrick to determine the pedigree for a number of seedlings and saplings in a one-hectare (ha) forest fragment on their 38.5-ha research area.

The scientists knew the genetic composition of all the adults, 42 individuals, in the study area. The results of the genetic analysis were startling. Out of nearly 250 seedlings studied from a single forest fragment, some 68 per cent were produced by adults in pastures - not from adults within the fragments themselves. Moreover, of the seedling produced by pasture trees, 77 per cent came from only two trees. Adults left in the fragment produced less than 5 per cent of the seedlings in their own patches.

The importance of the discovery lies in the fact that the genetic diversity of seedlings in forest fragment may be relatively small indeed. "If you looked at the number of seedlings superficially, you might say that this is a healthy rate of regrowth," said Hamrick. "But in truth, the effect is ecologically unhealthy due to the potential for inbreeding in subsequent generations."

As scientists tell us, inbreeding can be harmful. They have several theories as to why the pasture trees have such an overwhelming impact on gene flow. There is little competition for the pasture trees for sunlight and nutrients, giving them superior abilities to flower and fruit. And then, the abundance of flowers may attract more humming-birds for pollination. Finally, bats have easy pickings of the fruit and take them from the pasture trees to the forest fragment, where they eat and then pass seeds back to the soil.

The study has important implications for conservation and forest restoration and it shows that the impact of deforestation has been far more devastating that the simple removal of individual trees. At least in this species, fragmentation has resulted in the possibility of a serious loss of genetic diversity in this test area. Thus areas that look healthy in terms of regrowth may not be healthy at all over the long term. They may be facing serious future problems due to problems with genetic drift and inbreeding. American farmers already know of the problems this can cause, since they barely avoided a near-disaster because of over-planting of corn with too little genetic diversity in the 1970s.

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