Lianas the silent competitor
The Tropical Garden, Spring 2015
By Belén Fadrique
When people ask me about my research and I say that I study “lianas,” they usually have no clue what I am talking about. When I tell them that lianas are woody vines, they almost invariably mention or make an impression of Tarzan. While Tarzan has shown us lianas are useful for jumping from one tree to the next, there are a lot of other interesting things about their biology—and they are much more crucial in forest dynamics than most people believe.
Lianas need trees to support their weight and help them climb all the way up to the canopy. But contrary to common perception, lianas are not direct parasites; they uptake water and nutrients from the soil. In fact, the roots of lianas can be up to eight times longer than their visible above-ground stems. Lianas do, however, use the structure that trees provide to climb to the canopy and expose their leaves to the sun. In fact, liana species present distinct climbing mechanisms that can be used to classify them as stem-twiners, branch-twiners, tendril climbers or spiny species. When they arrive at the canopy, lianas spread their leaves on top of the tree crown, partially or totally shading the tree leaves. Next time you are strolling through Fairchild, find a liana and try to follow its full length with your eyes; if you don’t lose it, you will probably see its messy crown lying on top of a tree crown. This is a crucial interaction between lianas and their environment, where, by shading tree leaves and outcompeting them for light, they can decrease tree growth.
I tested this hypothesis during my graduate studies in the south of Ecuador. I studied where lianas occur and their contribution to forest biomass in the Andes, at altitudes from 1,000 meters up to 3,000 meters. In a sample of more than 50 study plots in these “premontane,” “low montane” and “upper montane” forest habitats, I measured liana and tree sizes and recorded which trees supported lianas. As the trees had already been measured years before by other scientists, I was able to compare tree growth of liana-infested trees versus liana-free trees. As predicted, the liana-free trees grew faster than did trees with lianas. Lianas also increased tree mortality due to physical damage, such as broken branches, caused by all the weight they piled on top of the trees. Only a small fraction of the carbon dioxide released into the atmosphere as a result of the decreased tree growth and increased tree mortality is compensated for by the growth of the lianas themselves. Consequently, greater liana abundance leads to reduced carbon storage in these forests.
Many studies have reported that both abundance and size of lianas are increasing in tropical forests worldwide. This increase is probably due to the rise in atmospheric carbon dioxide concentrations and climatic change. This is because higher temperatures, enhanced seasonality and decreased rainfall favor lianas—as they generally perform better than trees do under these dry conditions. Lianas may also be increasing in abundance because of disturbance: they are well-adapted to grow on degraded areas and can quickly colonize and dominate tree fall gaps or other cleared areas, with some liana stems measuring more than 500 meters long.
Given lianas’ increasing abundance and strong effect on forest function, we clearly need to be able to estimate liana abundance in tropical forests and monitor their changes through time. There are several permanent forest-monitoring plots around the world where scientists carry out liana censuses every year—measuring, recording and tagging every liana stem so they can track individual performance and community-wide changes. But monitoring plots can clearly cover only a limited amount of forest. As such, there is a strong emphasis on developing remote-sensing tools that can be used to monitor lianas. One state-of-the-art technology that holds a great deal of promise is based on laser-imaging detection, which makes it possible to obtain very high-resolution aerial images of forested areas from airplanes or even unmanned drones.
I previously helped to develop this new remote sensing system for liana research as part of an internship in Barro Colorado National Monument in Panama. In order to perform an accurate computer analysis of the laser images, we first needed to “train” the computer to distinguish between the different spectrums of light reflected by trees and the light reflected by lianas. With that aim, our team went into the field every day in order to find the trees that had been previously selected from the images. Once they were identified, we estimated liana coverage from the ground by looking with binoculars and calculating the percentage of liana coverage; this estimation was then fed into the computers. This was a demanding job, as looking through binoculars at the crown of 40-meter-high trees quickly strains the neck. Lianas are not an easy system to study.
Due to the difficulties inherent in studying lianas and the general lack of background knowledge about their biology, lianas have generally been ignored in most studies of tropical forests and their place in the carbon cycle. Now that we have realized the important effect that lianas have and the functions that they serve, we are rushing to understand their complex dynamics and their role in determining the future of these rich ecosystems.