Tropical forests are essential components of the earth system and play a critical role for land surface feedbacks to climate change. These forests are currently experiencing large-scale structural changes, of which the most apparent may be the increase in liana abundance and biomass. Lianas, as structural parasites of trees, strongly compete with trees for both above and belowground resources. The strong competition from lianas for resources reduces tree growth, reproductive output and recruitment, increases tree mortality and alters the relative allocation to stem and leaf biomass. Lianas therefore have a strong impact on whole-forest respiration, carbon sequestration and residence time, and ongoing liana proliferation has a potential high impact on the future carbon and water cycle of tropical forests.
State-of-the-art global vegetation models have problems to realistically simulate the carbon cycle of tropical forest. Improvement of these models is of major importance to better inform society and political actors on the impact of climate change. Currently, a major source of uncertainty in the global vegetation models is their poor representation of vegetation demographic processes. We are convinced that modeling the ecosystem demography in tropical forests is only possible by accounting for lianas. Nevertheless, no single terrestrial ecosystem model currently included lianas.
By building the first vegetation models that includes lianas, TREECLIMBERS has generated important insights into the mechanisms by which lianas influence the carbon balance of tropical forest ecosystems. We have made the first integrative study of (1) the contribution of lianas to instantaneous carbon and water fluxes, (2) liana contribution and influence on canopy structure, (3) their role for long term demographic processes, and (4) of their role in forest responses to drought events (i.e. disentangling belowground and aboveground competition between lianas and trees). To reach this challenging objective, TREECLIMBERS has developed the first liana plant functional type (PFT) based on a unique global meta-analysis and integrated this in the Ecosystem Demography model (ED2), a forerunner of the next generation of vegetation models. Moreover, we also implemented a liana PFT in the individual based model Formind. By using a model-data fusion framework TREECLIMBERS has for the first time integrated a large amount of available and newly collected data on liana ecology. New data collection has focused on important knowledge gaps: (1) characterization of belowground competition for water between lianas and trees using stable water isotopic techniques; and (2) characterizing the extent of the contribution of lianas to the vertical canopy structure, using innovative terrestrial LiDAR 3D forest structure measurements, involving an important methodological effort to identify lianas in LiDAR observations. This has resultes in important new insights on the root water uptake of lianas versus trees and on the impact of liana load on tree structure and biomass.
