Together with Functional Ecology and Journal of Ecology, we’re showcasing some of the top research recently published in the journals on the topic of lianas and global environmental change. Here Ainhoa Magrach brings the papers together and highlights the important roles lianas play in tropical forests.
Lianas are a common feature of tropical forests, contributing up to 25% of woody stems. However, these important tropical forest elements have largely been neglected because surveying them is particularly labour-intensive, oftentimes done by ground surveys. A new study recently published in Journal of Applied Ecology, shows that lianas can be studied using light-weight unmanned aerial vehicles, or drones, allowing researchers to confidently measure liana infestation levels from above the canopy using aerial pictures. This will allow for more studies on the distribution and abundance of lianas, a hot topic given the large increases in their prevalence that have been observed across the globe.
Indeed, global change and different forms of forest disturbance have led to an increase in the abundance and biomass represented by lianas. Recent research for example shows that artificially high densities of feral pigs have a positive effect on liana abundance. Similarly, lianas seem to be positively affected by forest fragmentation. A study in the endangered Brazilian Atlantic forest showed that liana abundance was higher in small fragments and near forest edges as a consequence of the microclimatic conditions found there (e.g., drier and hotter conditions). These microclimatic conditions are similar to those found in canopy gaps, where lianas have been observed to aggregate.
This increase in liana abundance has important implications for biodiversity and ecosystem services. Competition between lianas and trees leads to a decreased reproductive success of many canopy species, with consequences for seedling density and tree seedling recruitment. Further, forest plots with higher liana densities have lower average wood densities, and above-ground biomass levels, all of which could affect the ability of forests to recover from disturbance and their ability to contribute to carbon storage. A mechanistic explanation of this finding is brought by two contributions by the same research team to Journal of Ecology. In these two papers Visser et al (2017, 2018), find that liana prevalence is driven by the ability of different tree species to shed and/or tolerate lianas. These authors explain the greater presence of lianas on shade-tolerant trees as a combination of their inability to shed them and their ability to tolerate them. Conversely, they find that fast-growing species are able to shed lianas, yet their growth rates are reduced to a greater extent by them, and thus the individuals of fast-growing species we observe are those that survived and do not bear lianas. One of the mechanisms by which lianas outcompete trees, is their efficient water transport and physiological regulation. Indeed, a study published in Functional Ecology, suggests that the importance of lianas for forest water dynamics might have been undervalued up to now.
A method that is widely used in response to liana proliferation is liana cutting. Liana cutting is also extensively used as a method to increase tree productivity. It has been particularly successful in the case of commercial non-timber forest products, with production of Brazil nut trees being tripled following complete liana removal. But, the success of liana cutting practices might be modulated by other factors such as droughts. A recent study shows that drought might temporarily weaken the benefits of liana cutting for tree growth. These preliminary findings, based on a single drought event, however show that more studies across additional tropical regions are needed to evaluate the benefits of liana cutting across different conditions.
All of this research contributed to British Ecological Society journals shows that our understanding of liana dynamics within tropical forests is increasing. However, most of this research comes from Neotropical forests, with fewer studies coming from other tropical regions (e.g., only one study from SE Asia, OBrien et al 2019, and none from African forests). We thus seem to be extracting all of our conclusions from a rather geographically-biased set of studies. Yet understanding the consequences of management practices in response to liana growth requires that studies be conducted across more regions and under different environmental conditions. We now have new tools to study lianas using aerial images and models to understand their impacts. The time is thus ripe for us to continue with this interesting line of research, which will become more necessary as tropical forests are exposed to further disturbances in the future.