- Scientists with the Swiss university ETH Zurich used forensic genetics to determine that seed dispersal and seedling establishment rarely occured more than a few hundred meters from the seed tree in their 216-square-kilometer (about 83-square-mile) study area in an agro-forest landscape in India’s Western Ghats.
- The scientists say theirs is the first large-scale, direct estimate of realized seed dispersal of a high-value timber tree — in this case, Dysoxylum malabaricum, or White Cedar, which is listed as Endangered on the IUCN Red List.
- That means that many tropical tree species that are important to humanity and for preserving biodiversity, like Dysoxylum malabaricum, are less likely to recover from logging and habitat degradation than we previously thought, according to Dr. Christopher Kettle of ETH Zürich, a co-author of the study.
Deforestation is estimated to be responsible for about 10 percent of greenhouse gas emissions, which is why mechanisms for reducing forest destruction and promoting the reforestation of already degraded forestlands were included prominently in the Paris Climate Agreement.
Restoring forests and their ability to sequester carbon is not just a relatively simple way to lower the emissions that contribute to global warming, but also a cost-effective measure that has been shown to actually help curb illegal logging in some cases.
But a study published in the journal New Phytologist late last month suggests that tropical trees need a little help in order to find their way back into logged and degraded forests.
Many of the world’s governments have set ambitious targets to enhance forest cover in the tropics, the authors of the study note, and the idea that trees will regenerate on their own, provided the land is protected from further disturbance, is crucial to these efforts.
Clearing large swaths of rainforest can permanently drive away or kill the large tropical birds that feed on tree fruits and are key to the dispersal of seeds, however. A December 2016 study that surveyed 330 forest, farmland, and tree plantation sites in the Brazilian Amazon determined that recovering forests don’t have the diversity of birds needed to ensure their survival.
For the New Phytologist study, scientists with the Swiss university ETH Zurich used forensic genetics to determine that seed dispersal and seedling establishment rarely occured more than a few hundred meters from the seed tree in their 216-square-kilometer (about 83-square-mile) study area in an agro-forest landscape in India’s Western Ghats.
The scientists say theirs is the first large-scale, direct estimate of realized seed dispersal of a high-value timber tree — in this case, Dysoxylum malabaricum, or White Cedar, which is listed as Endangered on the IUCN Red List. “Our estimates found that most realized seed dispersal was within 200 [meters], which is insufficient to effectively bridge the distances between forest patches,” the authors of the study write.
What’s more, the researchers found evidence to suggest that even highly mobile birds like hornbills do not ensure successful movement of seeds between isolated forest fragments, making it extremely unlikely that the species they studied could re-colonize degraded forest patches without help, such as artificial seeding or the planting of seedlings.
That means that many tropical tree species that are important to humanity as well as for preserving biodiversity, like Dysoxylum malabaricum, are less likely to recover from logging and habitat degradation than we previously thought, according to Dr. Christopher Kettle of ETH Zürich, a co-author of the study. “The reality for this high value tree species is that unless we physically put the trees back in the forest from where they have been removed, they will be lost from some forests forever,” Kettle told Mongabay. “We need policies regarding tropical forest restoration that take this reality into account.”
Research released last month found that more than seven percent of the intact forest landscapes that existed in 2000 had disappeared by 2013, mostly due to the timber trade and land clearing for agriculture. And last year, researchers who calculated that the world experienced a net loss of 3.76 million square kilometers of interior forest area between 2000 and 2012 warned that the increasing pace of forest fragmentation across the globe could severely jeopardize the ability of forests to provide critical wildlife habitat and other ecological functions.
At the same time that tropical forests are undergoing increasing fragmentation, they are also subject to high levels of hunting, especially for large birds and mammals that are vital seed dispersers.
Dr. Sascha Ismail of the University of Aberdeen in the UK, the lead author of the New Phytologist study, said that even where large seed dispersers persist despite human impacts on the landscape, his team’s results show that effective seed dispersal can be insufficient to ensure the persistence of some tree species.
Governments and forest managers will have to be more realistic in their expectations of nature’s ability to ensure the recovery of forests in human-modified landscapes, Ismail added. In some cases, artificial seeding and seedling planting may be the only way some tropical tree species will be capable of returning to forests where humans have previously removed them, which will be essential in order to maintain the biodiversity of these degraded landscapes.
“Simply relying on the power of nature to restore the worlds’ forests is an idealization that will leave some tree species in jeopardy,” Ismail told Mongabay. “The biodiversity value of agro-forestry landscapes will disappear if seed dispersal — and consequently the recruitment of some tree species — is disrupted. Both local people and the forest department should be made aware that their most valuable timber resources will not recover spontaneously without some help.”
- Bregman, T. P., Lees, A. C., MacGregor, H. E. A., Darski, B., de Moura, N. G., Aleixo, A., … Tobias, J. A. (2016). Using avian functional traits to assess the impact of land-cover change on ecosystem processes linked to resilience in tropical forests. Proceedings of the Royal Society B: Biological Sciences, 283(1844). doi:10.1098/rspb.2016.1289
- Ismail, S. A., Ghazoul, J., Ravikanth, G., Kushalappa, C. G., Uma Shaanker, R., & Kettle, C. J. (2017). Evaluating realized seed dispersal across fragmented tropical landscapes: a two‐fold approach using parentage analysis and the neighbourhood model. New Phytologist. doi:10.1111/nph.14427
- Potapov, P., Hansen, M. C., Laestadius, L., Turubanova, S., Yaroshenko, A., Thies, C., … Esipova, E. (2017). The last frontiers of wilderness: Tracking loss of intact forest landscapes from 2000 to 2013. Science Advances, 3(1), e1600821. doi:10.1126/sciadv.1600821
- Riitters, K., Wickham, J., Costanza, J. K., & Vogt, P. (2016). A global evaluation of forest interior area dynamics using tree cover data from 2000 to 2012. Landscape Ecology, 31(1), 137-148. doi: 10.1007/s10980-015-0270-9
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