Modelling the impact of lianas on the biogeochemical cycles of tropical forests

<p>Despite their low contribution to forest carbon stocks, lianas (woody vines) play an important role in the carbon dynamics of tropical forests where they 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.</p><p>However, lianas remain largely ignored in field-scale studies as well as modelling forecasts. Therefore, their exact impact on tropical forest biogeochemical cycles is very uncertain. In particular, it is unclear which resource (light, water) is the most competed for between growth forms and so is is the future impact of lianas on forests in a global climate change context in which brighter, drier and CO2-enriched conditions are expected in the Tropics.</p><p>To answer those burning questions, we incorporated for the very first time a plant functional type accounting for the lianescent growth form into a dynamic global vegetation model (ED2). We implemented several liana-specific processes in the modelling framework (climbing, resprouting, height limitation due to lack of self-supporting tissues etc.), and integrated liana-specific parameters according to data from multiple studies in order to account for significant differences of functional and structural traits between lianas and trees. These parameters included (but were not limited to) leaf biochemical and photosynthesis properties, stem hydraulic traits, root distribution, and allometric relationships.</p><p>Baseline runs successfully reproduced ecosystem gas exchange fluxes (GPP and latent heat), forest structural features (LAI, AGB), and several other benchmarking observations in multiple tropical sites characterized by different rainfall regimes and levels of liana abundance. 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<sup>−1</sup> year<sup>−1</sup>, which resulted in significantly reduced accumulated above‐ground biomass by up to 20 tC/ha in regrowth forests. The negative impact of lianas on carbon storage almost disappeared in wetter, old-growth forest sites. Model uncertainty analyses also revealed that water limitation was the dominant factor driving competition between trees and lianas, even in sites with a short dry season.</p><p>These two-key findings (higher impact in regrowth forests and water-dominated competition) 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. The modelling workflow also allowed to identify key liana traits (quantum efficiency, stomatal regulation parameters, allometric relationships) and processes (water use, respiration, climbing) driving the overall model uncertainty. They should be considered as priorities for future data acquisition and model development to improve predictions of liana-infested forest carbon dynamics.</p>