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Modelling lianas as key drivers of tropical forest responses to climate change

Periodic Reporting for period 4 - TREECLIMBERS (Modelling lianas as key drivers of tropical forest responses to climate change)

Reporting period: 2019-10-01 to 2020-03-31

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.
The liana PFT was developed based on a meta-analysis of existing data and collection of targeted new data. This meta-analysis is summarized in a synthesis study that is currently submitted for publication.

Several extensive field campaigns have been performed in French Guiana and Panama. Some additional field data were collecte in Ecuador, DR Congo, Costa Rica and Rwanda.
Data has been collected in view of model parameterization and model validation: Liana inventories have been made at key sites, including quantification of their interaction with trees. Liana leaf traits have been measured, including nutrient compositions and leaf gas exchange measurements. Sap flow sensors have been installed in the footprint of a flux tower to quantify contribution of lianas to whole-forest respiration. Moreover, hydrulic traits were measured as well.
Specific data collection has been made to gain a mechanistic understanding of important processes: (1) Dual stable water isotopic measurements have been performed and have led to important new insights in the below-ground competition for water between lianas and trees. These results have been published in Tree Physiology and multiple follow-up papers are in the pipeline. (2) Terrestrial LIDAR scans (TLS) have been collected at several sites, including a liana removal experimental site in Panama, and have led to methodological advances on the use of TLS for liana extraction. Multiple papers have been published on this TLS work, and a key high-impact study is on its way based on the last extende field campaign in BCI Panama 2019.

The first vegetation model including lianas (ED2) that we developed was published in 2019 in Global Change Biology. Multiple follow-up studies using that model are submitted or in the pipeline. This includes studies in which we use the model to disentangle above and belowground competition between lianas and trees and a study were we use the model to study the impact of lianas on radiative transfer in tropicla forests. In addition a specific study on the implementation of lianas in the individual based model Formind has been submitted for publication.

Multiple smaller related studies have been published as well. e.g. a study on liana abunadance in DR Congo in Biotropica, and a studty on hydraulic traits of lianas in French Guiana in MDPI Forests.
- Lianas have for the first time been implemented in two dynamic global vegetation models. We have now a fully operational model including lianas (ED2) within the PEcAn framework, which is serving as an integration framework for liana data at multiple scales. With the model we have made the first quantitative assessment of the impact of lianas on the forest carbon and water cycle.

- Important new insights have been gained in belowground competition for water resources based on our stable water isotope study. These results have been explored in the model as well and the initial Liana model as published in Global Change Biology has now been integrated with a full plant hydraulics model.

- Important methodological advances have been made for extracting lianas from terrestrial LIDAR scans. We have been able to quantify the influence of lianas on the ecosystem canopy structure and biomass based on these LIDAR data.