Cientistas estudam a proliferação anormal de lianas (cipós) em florestas latinoamericanas e avaliam relação com mudanças climáticas

COMENTÁRIO DE AXEL GRAEL:


De olho nos cipós

A notícia do trabalho de pesquisadores publicada pela Universidade de Yale traz luz a um problema que é evidente aos olhares mais atentos às nossas florestas: as lianas ou cipós (com caules lenhosos) e trepadeiras herbáceas estão dominando a copa das florestas.

Como engenheiro florestal, com a mania de andar pelas ruas olhando para as árvores e sempre acompanhar as nossas matas com a preocupação com a integridade e saúde das mesmas e com um inevitável olhar de gestão: "o que é preciso fazer..."

Aminha atenção para o problema surgiu na década de 1980, no meu primeiro emprego, no Estaleiro Verolme, em Jacuecanga, Angra dos Reis. Ao observar uma mata num morro no meio da vila de Jacuecanga, constatei que após a implantação de um conjunto de casas e a construção de um ginásio esportivo do Clube Cornelis Verolme, parecia haver o crescimento mais rápido de cipós sobre o dossel da mata que localizava-se bem próxima.

Já naquela época, procurei informações na literatura técnica e científica e não encontrei muita coisa, mas me lembro de um estudo que associava preliminarmente o estímulo ao crescimento a uma maior incidência de luz. Considerei ser esta uma boa explicação para a mata de Jacuecanga a ocorrência do problema na vegetação urbana. Desde então continuo as minhas observações e mantenho a procura por resposta científica. Vale destacar que já é possível encontrar boa literatura acadêmica produzida por cientistas brasileiros também.

O desequilíbrio na presença das lianas, principalmente de algumas espécies bem agressivas de trepadeiras herbáceas, é constatada também nas matas e áreas verdes de Niterói.

Apesar de apresentar caule lenhoso, as lianas não são capazes de sustentar o próprio peso e apoiam-se sobre as árvores na busca pela luz. Acabam sufocando as árvores, competindo por luz. Além disso, por fazerem peso sobre os galhos, acabam por quebra-los. Em certas áreas de reflorestamento, a presença de trepadeiras é considerado uma praga e obriga a rotinas de controle da sua proliferação.

O texto abaixo, publicado ontem, dá destaque ao trabalho de pesquisa do biólogo Stefan Schnitzer, realizado numa floresta do Panamá, traz algumas considerações interessantes sobre a dimensão do problema e as suas preocupantes implicações para a saúde das florestas e os impactos para a capacidade que a florestas terão para fixar carbono.

Os pesquisadores constataram que por motivos ainda não bem conhecidos, a presença de lianas e trepadeiras dobrou nas últimas décadas nas matas da América do Sul e Central. Também, experimentos em que a floresta teve a retirada das lianas e trepadeiras, tiveram a absorção de carbono 75% mais altos do que as áreas que não sofreram o manejo.

E, além dos danos, lianas têm reduzida capacidade de retenção de carbono, estimado em apenas 5% do total fixado por uma floresta tropical. Por outro lado, podem representar uma parte significativa da área foliar de um determinado trecho de floresta.

Schnitzer faz considerações sobre o que pode estar aumentando a proliferação das lianas e cipós. Cogita a possibilidade de estar ligado a uma maior mortalidade das árvores dominantes pela maior incidência e intensidade de tempestades, causadas pelas mudanças climáticas. Outros cientistas consideram a possibilidade de uma maior concentração de carbono na atmosfera estar favorecendo o crescimento das lianas e cipós.

Enfim, aqui temos uma boa leitura para ajudar na compreensão do problema.

Axel Grael



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Ecologist Stefan Schnitzer with a liana, a type of woody jungle vine, on Panama's Barro Colorado Island. Sean Mattson/Smithsonian Tropical Research Institute.

The Strange Case of the Liana Vine and Its Role in Global Warming

Liana and other vines are proliferating in the rainforests of Central and South America, and their spread is impeding the ability of trees to sequester carbon dioxide. Now, researchers are trying to determine the impact of this phenomenon on climate change.

By Daniel Grossman • April 13, 2017
On a hot, buggy morning in January, Stefan Schnitzer, a Marquette University biologist, spritzed insect repellant on his forearms, tucked khakis into knee-high rubber boots, and hiked three miles from his lab at The Smithsonian Tropical Research Institute to a study plot in the Panamanian jungle. Ticks and, on occasion, dengue fever-carrying mosquitos infest Schnitzer’s parcel, on Barro Colorado Island in Panama’s Lake Gatun. The site also harbors bullet ants that inflict one of the world’s most painful stings, and a host of palm-size golden orb spiders. Schnitzer picked up a PVC pipe the length of a walking stick for swatting spider webs strung across his route.

Schnitzer strode nimbly up a steep slope, where workers had carved steps into the hillside and laid cement pavers to ease access to the forest. The path leveled, the stepping-stones petered out, and the track narrowed. A howler monkey in the distance barked a warning to would-be intruders.

Reaching his study site after two hours of hiking, Schnitzer, one of the world’s leading experts on the ecology of rainforest vines, paused and studied a tangle of knotted liana vines six feet high and twice as wide, composed mostly of loops of the species Coccoloba excelsa. These are the most abundant lianas on the island, and they promiscuously sprout shoots that root in the soil, sending up a profusion of new stems. Dozens of dancing blaze-orange streamers flagged the thicket’s coils.

This was all part of Schnitzer’s research into a little-understood but important phenomenon that could impact the pace of climate change: In the jungles of Central and South America, vines are becoming more common, and as they proliferate, they are impeding the ability of tropical forests to soak up carbon dioxide and sequester it as wood.

"For reasons that are not entirely clear, the abundance of liana vines has doubled in recent decades".

For reasons that are not entirely clear, the abundance of liana vines has doubled in recent decades, according to research by Schnitzer and an earlier study. As a result, these rapidly expanding woody vines are increasingly shading and choking rainforest trees, reducing the amount of carbon they sequester from the atmosphere. In his jungle study plot, Schnitzer is endeavoring to understand why vines proliferate faster than the trees that abut them, and how much carbon liana vines store compared to trees. Trees and other plants absorb about 25 percent of the CO2 that humans release from tailpipes and smokestacks.

The Smithsonian has managed the Barro Colorado forest, halfway across the Isthmus of Panama, since the 1920s. The tropical jungle there is among the most intensively studied rainforests on earth. In 1980, two ecology pioneers, Stephen Hubbell and Robin Foster, laid out what was then the world’s most ambitious tropical forest plot for intensive, long-term research. It contains roughly 125 acres and includes about 250,000 trees, each marked with a numbered aluminum tag. Several thousand are too broad for a pair of adults holding hands to reach around and touch. With a team of helpers, Hubbell and Foster have measured the diameter of each tree. They re-census the site every five years, adding “recruits” — trees that have grown big enough to be included in their study — and removing trees that have fallen.

Tropical vines, such as bauhinia (above), are increasingly outcompeting trees, reducing the amount of carbon rainforests sequester from the atmosphere. Beth King/Smithsonian Tropical Research Institute

For years, Hubble and Robbins did not bother surveying lianas. “Lianas are such a small part of what you see when you look around,” says Schnitzer, noting that they contain less than 5 percent of a tropical forest’s standing carbon.

But ecologists have begun to wonder if lianas might play an outsized role in the forest. Schnitzer is particularly interested in how vines colonize gaps in the canopy left by tree falls. In 2007, he and Hubbell, by then his mentor, began a liana census of the Barro Colorado plot. It became, and remains, the largest survey of lianas in a single tract. It took 14 people working full-time for 12 months to tag, measure, and record 65,000 vines. A botanist identified each specimen, encompassing 162 species.

Foresters have traditionally calculated the number of planks they can obtain from trees using allometry, the science of how an object’s dimensions, such as linear size and cubic volume, relate to each other. Biologists adopted the same technique for calculating the carbon stored in trees and lianas. The process is straightforward for trees. Researchers plug a single measurement — by convention, the trunk diameter at roughly four feet above the ground — into a simple formula worked out through years of research.

But quantifying the volume of carbon stored in liana vines is more challenging. In his study plot, Schnitzer approached a tree-sized growth with waxy leaves that would pass as a tree for all but trained experts. It fact it was a sapling-like vine, Connarus panamensis. “It grows like a tree until it’s quite tall,” before grabbing branches and trunks for support as a normal vine does, Schnitzer says. He notes that unlike trees, which tend to grow straight up into the canopy, lianas follow a more erratic and looping path around the understory. Such factors, along with the relative paucity of studies of liana measurements, have compounded the difficulty of vine research and carbon storage. Nevertheless, Schnitzer has developed protocols — now widely adopted — for measuring lianas.

In 2015, Schnitzer published a paper in the Proceedings of the National Academy of Sciences revealing new evidence that vines could reduce the tropical forest’s ability to absorb CO2 and slow global warming. Wielding machetes, he and a crew of five had completely removed every liana from eight forest tracts on a spit of land near Barro Colorado. It took a week. They tallied up how much carbon was contained in wood and leaves in the vine-free jungle and in a corresponding set of normal, viney areas.

Forests cleared of vines had absorbed 75 percent more carbon than control areas where vines grew freely.

Schnitzer had predicted that, freed from shading and strangling vines, trees would grow more vigorously. Because trees support their crowns with sturdy, carbon-rich trunks that lianas don’t need, Schnitzer hypothesized that the vine-free jungle might contain far more carbon than the control forest. After three years, a re-census confirmed that. The pruned tracts had absorbed 75 percent more carbon per year than the control areas where lianas and other vines were allowed to grow freely. “It was stunning,” says Schnitzer.

In a response to Schnitzer’s paper reporting these findings, Hans Verbeeck, a biologist at Ghent University in Belgium, wrote in a subsequent issue of the Proceedings of the National Academy of Sciences that that “liana proliferation has a potential high impact on the future carbon cycle of tropical forests.” He called for climate researchers to include the changing prevalence of lianas in future climate models, a task he has begun himself.

Despite solid evidence that vines have become more common, Schnitzer says he’s unsure if vine proliferation will continue to increase in the world’s tropical forests. A forecast would require an explanation — so far lacking — for what’s behind the increase in vines that he and others have observed. Schnitzer suspects that one culprit might be increased treefall frequency and mortality due to such changes as enhanced storm intensity. Some scientists believe that the increased CO2 in the atmosphere might favor lianas over trees. But at least one study, by David Marvin, an ecologist at The Nature Conservancy and a former student of Schnitzer’s, suggests otherwise. Marvin raised tree and liana seedlings together in chambers with air containing twice the normal concentration of CO2. Lianas grew faster than controls — but so did trees. There was virtually no difference. However, the experiment tested only a small number of species and lasted less than a year. A longer test, with a larger group of varieties, might have turned out differently.

Schnitzer’s 125-acre plot contains so many lianas that, after the re-census is completed, he should be able to determine which particular species are most responsible for the observed increase in liana biomass. Then, he plans to investigate what characteristics give these vines their advantage. But even with his experiments, he says, making such determinations is difficult. In tropical forests, the huge diversity of species and the boundless variation in how they interact can frustrate even the most robust scientific experiments.

What if his study comes up empty-handed?

“We’ll have the most rigorous non-finding in ecological history,” he replies.

VIEW A VIDEO REPORT ON THE LIANA RESEARCH:

Para assistir ao vídeo, clique em https://vimeo.com/212958318

Fonte: e360 Yale




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OUTRAS PUBLICAÇÕES SOBRE LIANAS:

Engel, Fonseca e Oliveira: Ecologia de lianas e o manejo de fragmentos florestais
Putz: Ecologia das trepadeiras