Impacts of liana infestation on tropical forest demography and functioning: lessons learned from the implementation of a lianescent growth form in a vegetation model

Despite their low contribution to forest carbon stocks, lianas (woody vines) play an important role in the dynamics of tropical forests where they act as structural parasites and compete with free-standing plants for below- and above-ground resources. Doing so, they negatively impact individual tree growth, as well as the net productivity and the long-term carbon storage of the ecosystem. In tropical forests, lianas contribute up to 25 percent of the woody plant species and up to 40 percent of the woody stems. However, they remain largely ignored in plot-scale studies and vegetation models and therefore their impact on forest biogeochemical cycles is very uncertain.

To better understand liana-tree interactions, we developed for the first time a mechanistic representation of the lianescent growth form into a dynamic global vegetation model (the ecosystem demography model, version 2). We implemented several liana-specific processes in the modelling framework, and integrated liana-specific parameters according to data from multiple studies in order to account for differences of functional and structural traits between lianas and trees.

Baseline runs in multiple tropical sites characterized by different rainfall regimes and levels of liana abundance successfully reproduced ecosystem gas exchange fluxes (GPP and latent heat), forest structural features (LAI, AGB), and several other benchmarking aspects of ecosystem and liana-tree interactions functioning. In those simulations, lianas negatively reduced forest productivity and total carbon storage, by increasing tree mortality (+ 30% on average) and decreasing tree growth (-35%). The inclusion of lianas in the simulations reduced the forest net productivity by up to 0.5 tC ha^-1 year^-1 . Taking lianas into account resulted in significantly reduced above ground biomass by up to 20 tC ha^-1 in regrowth forests. The negative impact of lianas on carbon storage almost disappeared in wetter or old-growth forest sites. These findings (higher impact in regrowth forests and importance of the water competition during the dry season) are expected to lead to a reinforcement of the negative impact of lianas on forest productivity under future aggravated forest disturbance and warmer climate conditions.