- New research on the rapid growth of lianas – native woody vines – on the artificial reservoir islands of the Balbina dam in the Amazon finds that forest communities there underwent a transformation as a result of severe habitat fragmentation, resulting in the altering of the carbon sequestration and emission balance.
- Some tree species are severely impacted by this extreme form of habitat fragmentation and die, while native lianas — woody vines that climb to reach the forest canopy — thrive and rapidly fill the biological niche left by failing trees.
- Trees, with their greater biomass, store more carbon in trunks and branches than lianas, so the carbon balance shifts as lianas dominate. Rather than sequestering carbon, these dam-created islands end up emitting carbon as the trees die.
- The rapid growth of lianas further contributes to the degradation of remnant tree communities challenged by fragmentation. Amazon dam environmental impact assessments don’t currently evaluate increased reservoir island carbon emissions.
The environmental impacts of Amazon hydropower projects are many and varied, from inundating primary rainforest, driving deforestation, and releasing the potent greenhouse gas methane, to interrupting long-distance aquatic migrations, altering sediment flows and attenuating flood cycles. New research now suggests that one more effect may be added to the list: carbon emissions from islands created by hydropower reservoirs.
A new study of the Balbina mega-dam in Brazil, one of the oldest in the Amazon, found that plant communities on reservoir islands underwent a transformation as a result of becoming isolated. Lianas — woody vines that climb trees to reach the forest canopy — continued to thrive, but the trees suffered. Because trees store more carbon in their trunks and branches than lianas, the carbon balance shifts as lianas dominate. Rather than sequestering carbon, these dam-created islands end up emitting carbon as the trees die.
“A quick ‘back of the envelope’ calculation […suggests] that annual carbon emissions associated with forest islands in Balbina could be in the region of 10,000 tons (equivalent to emissions from ~230,000 barrels of oil),” said the study’s lead researcher Isabel Jones, of the University of Stirling, UK.
With hundreds of dams currently planned and under construction all across the Amazon basin, failing to take these increased carbon emissions into account in environmental impact assessments and cost-benefit analyses of hydropower developments is a serious oversight, the scientists warn.
Research at one of the Amazon’s oldest mega-dams
Balbina was built in the 1980s, flooding 3,129 square kilometers (1,208 square miles) and creating 3,546 islands with a combined area of 1,180 square kilometers (456 square miles). Jones and her team studied tree and liana communities in 77 study plots scattered across 36 of those islands. They observed 12 further plots on the mainland for comparison.
The islands form a part of the Uatumã Biological Reserve, and have not experienced logging or other human disturbance other than a fire in 1997 which started outside of the reserve and spread to a few of the islands.
Despite their inaccessibility, these artificial islands are an extreme example of habitat fragmentation, a process that causes ecological degradation. As pockets of forest habitat are isolated, their edges are exposed to harsher, brighter, drier conditions and plant and animal dispersal between habitat fragments is made more difficult; in addition, habitat fragments may be too small to sustain populations of some species.
This has already been observed in the tree communities on Balbina’s islands, which have rapidly degraded since their isolation, Jones said, “but what we didn’t know is whether the remaining island tree communities might be facing additional pressure from increasing competition from lianas — we needed to know how lianas were responding to this landscape-scale habitat fragmentation compared to trees, to better understand the potential long-term trajectories of these forest islands for biodiversity and carbon storage.”
Looking at lianas
Jones designed her study to explore the effect of island size, distance from the mainland, the amount of surrounding forest cover, and varying degrees of fire damage on island plant communities.
In the study plots, the team surveyed both sapling and mature lianas and trees, noting particular details about the composition and abundance of the liana communities and whether they were dispersed by animals or by the wind.
In contrast to what is already known about the tree communities, Jones found no evidence of degradation of the island liana communities when she analyzed the data.
“We were surprised that [they] appeared to be remarkably robust to the negative impacts of landscape-scale habitat fragmentation caused by reservoir creation,” she noted, “and saw no difference in the liana communities on islands compared to the mainland.”
Even on the small, severely fire damaged islands, liana sapling abundance was comparable to the mainland despite challenging conditions. In contrast, tree saplings performed so poorly that lianas dominated the sapling layer of vegetation.
Jones had anticipated that the lianas wouldn’t be as affected as tree communities. Their ability to grow rapidly and “to exploit the high-light and low moisture conditions associated with forest fragment edges and canopy gaps, much faster than even disturbance-adapted tree species, gives them a competitive advantage over trees” she explained.
But even so, “we were still expecting to see some effects associated with isolation within a water matrix because, for example, liana seed dispersal across water may have been affected by the severe reduction in vertebrate dispersers seen across the archipelago,” said Jones. However, lianas have another adaptive trick to help overcome this limitation. “Lianas can also reproduce clonally and vegetatively,” so can “rapidly colonize newly-disturbed habitat,” Jones revealed.
Less biomass, more carbon
But what do thriving liana communities mean for Balbina in the long-term, and for the Amazon in general, as hydropower development rapidly expands across the basin?
For reservoir islands, “a persistent liana community will likely exacerbate the degradation of remnant tree communities,” the scientists warn. This will have a knock-on effect on the animal species that inhabit these islands, as well as having serious implications for the carbon balance of hydropower dams relative to other sources of energy.
“A forest with more lianas has less woody biomass (long term carbon storage), tree growth is reduced and tree mortality is increasing,” said Hans Verbeeck of Ghent University. Verbeeck, who was not involved with the study, but examined the role of lianas in tropical forest dynamics and carbon cycling. “However, only one experimental study — a large liana removal experiment in Panama, has proven lower carbon uptake in forest with lianas versus forests where lianas have been removed.”
That particular experiment was carried out by Stefan Schnitzer, of Marquette University, as part of a major liana ecology research project. Schnitzer, who was also not involved in Jones’ research, agrees that “lianas are extremely important” in influencing carbon storage in tropical forests.
“According to Jones’ work, and combined with previously published studies, additional dams are going to have a positive effect on lianas and a detrimental effect on long-term carbon storage,” Schnitzer concluded, although he emphasized that lianas are not inherently ‘bad’ but “are an important component of tropical forests with a number of positive effects on the ecosystem.”
The impact of dams on tree and liana composition “will for sure have an impact on carbon sequestration,” agreed Verbeeck. “But it is difficult to quantify that impact.”
Lianas’ resilience to water scarcity and fire damage may also play to their advantage in a warming Amazon, with increasing droughts and more frequent fires predicted for the region as climate change escalates. Research into potential feedbacks between liana growth, carbon sequestration, and climate change is ongoing, Jones said.
Lost forests, lasting impacts
The low-lying, undulating topography of the Amazon means islands are likely to be created in at least some of the reservoirs that will accompany the dams currently planned and being built, Jones noted. And of great concern is the fact that “islands are not counted as part of area calculations for land impacted by dam construction.”
Jones’ work adds to a growing body of evidence that reservoir islands should no longer go unaccounted for. “I want to see the legal framework surrounding dam construction, and developers, properly accounting for the long-term impacts versus energy outputs of dams.”
“There is a wealth of experts in their fields producing evidence, year after year, on the extreme negative consequences of dam construction; I am yet to see any robust evidence that the benefits of dams can outweigh these consequences,” she concluded.
“Protecting tropical forests from disturbance is one of our best means to reduce atmospheric CO2 — yet ~10 million hectares [38,610 square miles] of forests are predicted to be flooded for reservoirs in Amazonia for ‘green energy’ production.”
Jones, I. L., Peres, C. A., Benchimol, M., Bunnefeld, L. and Dent, D. H. (2017) Woody lianas increase in dominance and maintain compositional integrity across an Amazonian dam-induced fragmented landscape. PLoS ONE 12(10): e0185527. https://doi.org/10.1371/journal.pone.0185527
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