COS 254-5 - Increased forest productivity gives lianas a demographic advantage over trees.

Abstract: A trend of increasing Neotropical liana abundance was first observed two decades ago, and ever since corroborated by various studies. Despite many putative mechanisms have been postulated, the underlying mechanisms of liana proliferation remain elusive. Moreover, it is unclear how physiological responses may translate to ecosystem-level responses (i.e., a demographic advantage), and to what extend a differential response between lianas and trees is needed to support a demographic advantage for lianas. It could be possible that liana proliferation manifest following uniform or marginally differential responses, a phenomenon which might be overlooked by comparative liana physiological assessments.

Using a novel and mathematically tractable host-parasite model that couples plant physiology, carbon allocation, and forest demography, we evaluated current liana proliferation hypotheses in the context of changing forests’ vital rates and environmental conditions. The model was parametrized and benchmarked using extensive physiological and demographic data collected in Panamanian tropical forests, including one of the largest longitudinal surveys of lianas and trees.

Our model suggests that liana canopy prevalence predominantly depends on the interactions between liana infestation rate and forest structure. Drivers promoting forest productivity (i.e., CO₂ and nutrient fertilization) caused a pronounced demographic advantage for lianas driven by an increase of lianas virulence. Contrarily, drivers decreasing forest productivity (i.e., drought and reduced light conditions) produced a decline in liana prevalence in the model. Compared to this productivity effects, elevated forest mortality and increased allocation to reproduction only have a modest impact on liana prevalence. Importantly, these results did not need a differential response between lianas and trees at the physiological level

In conclusion, the observed increase of productivity in Neotropical forests likely supported liana proliferation. Similarly, the absence of a liana abundance trend in African tropical forest and the decreasing liana abundance trend with elevation may in fact parallel the lack of changes in the African forest productivity and the reduced productivity of light-limited mountain forests, respectively.