Inbreeding among trees and higher rates of offspring deaths may see some tree species in isolated forest patches die out slowly. In perhaps the first elaborate study of its kind in the Western Ghats, a team of researchers from across the globe has found that breaking up forests by even 200 metres can lead to the gradual death of tree species.
The results of the study — published recently in the journal New Phytologist — are disconcerting and show that less than 10% of germinated seeds survive in such patches owing to genetic faults, when compared to over 50% in contiguous forests.
Researchers from ETH Zurich in Switzerland, University of Aberdeen in the United Kingdom, and College of Forestry in Ponnampet, Kodagu, along with Ashoka Trust for Research in Ecology and the Environment (ATREE) and the University of Agricultural Sciences — both in Bengaluru — studied the seed dispersal patterns of Dysoxylum malabaricum (white cedar) which harbours the Malabar grey hornbill. More than 35 sacred groves of Kodagu, protected and worshipped by the local community, were studied across a 216 sq.km landscape where these forest patches are islands surrounded by paddy or coffee plantations.
The team used a combination of GPS and genotyping (DNA sequencing) to find out the lineage of the offspring in these isolated forests. Of the 321 trees that were sequenced, just 0.3% had emerged from “parents” that were in different patches. A staggering 267 trees (83%) had both their “parents” from the same forest patch.
“The seedlings are often mutated and completely devoid of chlorophyll (that is, their leaves have no colour), and these die out in a few months. This has affected forest regeneration,” said G. Ravikanth from ATREE.
The researchers also found that 95% of the seeds were dispersed within 200 m of the parent tree, but the sacred groves are usually situated at least 400 m and even 24.5 km apart; this leaves inbreeding as the only option. “The growth of areca and coffee plantations over three decades has isolated these forests and we are now seeing the effects of inbreeding depression. When the older trees die, we will start to see these forests disappear entirely,” said Mr. Ravikanth.
C.G. Kushalappa, Dean of the College of Forestry, said that while wind-dispersed species could perhaps overcome the gaps in forests, species that rely on birds or insects for pollination will struggle to get genetic diversity to survive.
Re-establishing tree corridors
The disappearance of plant species owing to inbreeding can be contained either through artificial dispersal of seeds or by setting up tree corridors, say researchers.
The research comes at a time when the lush forests of the Western Ghats are fragmented by reservoirs, hydroelectric projects, large highways, industries and plantations. At present, the team is researching on dispersal rates of wild nutmeg (an important tree for the hornbill population), the wild Amla tree in M.M. Hills and B.R.T. Reserve, and other plants in order to study the effect of fragmentation.
While the concept of corridors for elephants and tigers is well established in the country, the need for a similar approach to ensure constant gene flow for trees has been ignored. “We are trying to establish tree corridors to connect broken-up patches. Near Tiruchirappalli, we are restoring the connections of Myristica swamps. These corridors will provide genetic flow between forest patches,” said G. Ravikanth of the Ashoka Trust for Research in Ecology and the Environment.
In terms of conservation, the disappearance of plants and trees will have an adverse effect on animals that are symbiotically linked. For instance, hornbills and butterflies prefer only certain species of trees or shrubs.
