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Ecology

Entangled in the Jungle

Aunque poco se sabe de ellas, las lianas han atraído la atención de los ecólogos, pues desempeñan un rol fundamental en la dinámica de los bosques tropicales; sin embargo, estudios recientes detectan una proliferación superior a la registrada históricamente que podría llegar a representar una amenaza para combatir el cambio climático.

Text and photos: Javier Pinzón

Anyone who has visited a tropical forest has seen vines; the natural ropes hang from trees and climb, drop, twist, and curl into odd shapes, making it difficult to determine how many vines are actually there. Although vines gained fame as a mode of transport for Tarzan, their distinctiveness has drawn the interest of scientists, including the renowned Charles Darwin, who wrote an essay titled The Movements and Habits of Climbing Plants (1875).

What most interested Darwin was that vines —despite appearances— are not a family of plants, but rather a kind of growth tolerated by various types of plants. They constitute a polyphyletic group, i.e. they evolved independently into different families. This is why vines can belong to families as diverse as Leguminosae (legume), Sapindaceae (soapberry), Bignoniaceae (bignonia), Polygonaceae (buckwheat), and many more.

Although little is known about them, they have recently become of interest to ecologists because they play an essential role in the dynamics of tropical forests, comprising up to 25% of woody plants (including trees and shrubs); young vines comprise from 18% to 22% of upright plants in the understory of neotropical forests. A study carried out by scientist Francis Putz in the jungles of Venezuela showed that vines provide the soil with 4.5% of the forest’s total biomass. The same study counted 1,597 vines per hectare (2.5 acres) in Panama.

Vines act as express corridors between the forest floor and the canopy (the highest part of the forest), as well as serving as bridges between trees. This partially explains why vines are so vital to many aspects of forest dynamics. They also provide animals with travel routes and a valuable food source.

Vines serve the human inhabitants of these ecosystems as well. In the Americas, Africa, and Asia, vines are used in the production of handicrafts (baskets, sculptures), as building material (chairs, fishing nets), for ceremonial and medicinal purposes, and as a source of food (the leaves, fruits, and roots). Some of the compounds found in vines are highly valued by indigenous groups: ayahuasca, yagé or nishi cobin (Banisteriopsis caapi), combined with another plant, the chacruna (Psychotria viridis), is used to make a hallucinogenic drink that is widespread in Amazonia. The medicinal properties of “cat’s claw” (Uncaria tomentosa) are being researched in several countries.

Vine Growth

Seedlings of arboreal species take a direct route to the upper canopy. In contrast, climbers reach for the sun via haphazard paths that can cover long distances horizontally as well as vertically. In this competition for light, they use neighboring plants, allowing them to blanket a large part of a host tree’s canopy. Vines grow faster when there is a clearing in the forest. They begin their life cycle in a shrub-like state and then become climbers, developing in unpredictable ways.

Some species have very slender, low-density stems, with a high proportion of parenchyma (“filler” tissue) and a great deal of water that is safe for human consumption. Others have woody stems as thick as, or thicker than, tree trunks. Some extreme vines are so heavy that they endanger their host trees by putting them at risk of toppling over in strong winds.

Another incredible characteristic of vines is that their roots are among the deepest in the tropical forest. Some Amazonian vines have roots that reach depths of 13 to 20 feet, which is why vines can grow faster during the dry seasons.

How do vines climb trees?

Different climbing mechanisms may be present in the same family. Some vines, known as twiners —the least specialized group— spiral around their host trees. Other vines produce hooks, thorns, adventitious roots (aerial roots, for example), tendrils (specialized stem from leaves, branches, or stalks), and modified leaves, such as touch-sensitive leaves that adhere to another plant when they come in contact, allowing them to piggyback their way to the forest canopy.

A vine’s climbing mechanism determines the environment where it will develop. For example, climbers with tendrils require slender hosts, and are therefore found more often in young forests and environments with abundant sunlight. Twiners, on the other hand, need thicker hosts, so they predominate in mature forests. Since adhesive climbers cannot move from one host to another, they are largely restricted to shady microsites.

Trees and Vines

These elongated plants are rather picky about the hosts they favor, and some trees, especially those with slow-growth patterns, are more susceptible to being invaded or covered than others. Vine-covered trees are a common sight in tropical forests. It has been reported that 86% of trees in Amazonian forests have vines, 50% in Malaysian forests, 63% in Los Tuxtlas (México), and 43% on Barro Colorado Island (Panama). This indicates that trees with vines represent the rule rather than the exception.

However, the overall situation is not necessarily promising. Vines may give forests an artistic look, but they compete with trees for various resources. This competition can have a negative impact on tree regrowth, especially in managed forests. Vines can likewise delay tree recovery (growth after a damaging incident) in tropical forests by occupying clearings and suppressing tree regrowth. They can also affect tree seedlings by overpowering them and competing for sunlight, water, and nutrients.

Vines and Climate Change

The natural dynamics of the forests keep these struggles for light and resources in perfect balance, but this balance can be disrupted if circumstances change. A recent study of the Smithsonian’s ForestGEO network of forest plots reveals an extensive proliferation of vines in some tropical forests, which has been linked to climate change, upheavals, and more severe seasonal droughts. According to the studies, the disproportionate increase in vines will make it more difficult to fight climate change.

A study carried out as part of Panama’s Agua Salud project analyzed data on the growth, mortality, and recruitment of woody plants, including trees, shrubs, and vines, on 84 plots of former pasture where the forest was regenerating. It found that vines reduce arboreal biomass growth by 19% after five years and by 32% after thirty years.

Scientist Stefan Schnitzer believes that if vines continue their current rate of growth, it could reduce the capacity of forests to absorb and store carbon by up to 35%, causing the premature death of trees and accelerating climate change.

The planet needs to recover its equilibrium so that ecosystems can continue to function as efficiently as they have for millions of years.