Lianas reduce tree height across the tropics

Technical Summary: Lianas reduce tree height across the tropics

Problem and Context
Tree height is a fundamental dimension of forest structure, mediating trade-offs in light access, mechanical stability, and hydraulic transport, while influencing ecosystem processes such as carbon sequestration and resistance to disturbance. While tree height–diameter allometries are known to vary across biogeographic and environmental gradients, the drivers of within-species variation in these allometries remain poorly understood, particularly regarding biotic interactions. In tropical forests, lianas (woody vines) are a defining component that compete intensely with trees for resources. Although previous studies suggest lianas reduce tree growth and survival, their specific influence on tree architecture—particularly height—has remained unresolved, with evidence limited to fragmentary and localized observations. This study addresses the gap in understanding how liana infestation alters tree height allometries across the pantropics and how these effects interact with climate.

Methodology
The authors compiled the largest pantropical dataset to date, comprising 50,473 individual tree observations across 66 sites. The dataset includes coincident measurements of diameter at breast height (DBH, range 10–255 cm), tree height, and a standardized liana crown occupancy index (COI). To ensure robustness, the COI was aggregated into three levels: no occupancy, moderate occupancy (1–50% crown coverage), and heavy occupancy (>50% crown coverage). The analysis focused on trees with DBH ≥ 10 cm to target canopy individuals.

The study employed hierarchical Bayesian allometric models to quantify the effect of liana occupancy on tree height. The authors tested 20 model formulations spanning three functional forms (generalized Michaelis–Menten, Weibull, and power functions) with and without liana effects on shape parameters. Model selection was based on the Watanabe-Akaike Information Criterion (WAIC). The best-performing model (a Weibull function with liana effects on shape parameters b and k) was used to estimate the relative change in predicted tree height for liana-occupied trees compared to liana-free trees of the same DBH.

To investigate environmental drivers, the authors restricted upscaling and projection analyses to lowland old-growth forests (40 sites, 30,154 observations) to avoid biases from disturbance and elevation. They related site-level effect sizes to WorldClim bioclimatic variables using Principal Component Analysis (PCA) and projection predictive variable selection. Finally, these climate-liana relationships were applied to CMIP6 climate projections under four Shared Socioeconomic Pathways (SSPs) to estimate future shifts in liana-induced height suppression.

Key Results

1. Pantropical Height Suppression: The study provides robust evidence that liana-hosting trees are consistently shorter than individuals of the same stem diameter that do not bear lianas. At the pantropical level, moderate liana occupancy reduced tree height by approximately 5.4% (89% CI: -6.4% to -4.4%), while heavy occupancy reduced height by 10.2% (89% CI: -12.1% to -8.4%). For a 50 cm DBH tree, this corresponds to absolute height reductions of -1.5 m and -2.8 m, respectively.
2. Scaling with Occupancy and Size: The suppression effect scaled with liana occupancy; heavily infested trees were suppressed roughly twice as much as moderately infested ones. The relative suppression was strongest in smaller trees (up to ~50 cm DBH) and gradually weakened as DBH increased.
3. Site and Species Variability: While the negative effect was consistent, its magnitude varied significantly among sites and species. At the site level, heavy occupancy resulted in height reductions ranging from -39.3% in Queensland (Australia) to +4.0% in Brazil, though positive effects were rare and often statistically indistinguishable from zero. Similarly, among abundant tree species, most exhibited negative height effects, with fast-growing species (characterized by lower wood density and leaf mass per area) appearing more susceptible, though trait-based models explained little variance.
4. Climate Drivers: The magnitude of liana-induced height suppression was strongly modulated by climate. Suppression was more pronounced in warmer sites and weaker in wetter conditions. Specifically, the effect size was positively correlated with temperature regimes (PCA Axis 1) and negatively correlated with precipitation regimes (PCA Axis 4).
5. Future Projections: Under all future climate scenarios (SSP1-2.6 to SSP5-8.5), liana-induced tree height suppression is projected to intensify. By the end of the century (2081–2100), the pantropical average suppression for heavy occupancy is expected to increase from -8.4% to between -10.2% and -15.3%, driven primarily by rising temperatures and changing precipitation seasonality.

Significance and Claims
The authors claim this study provides the first pantropical evidence that lianas are associated with a reduction in tree height, a structural effect that cascades through forest function. By reducing tree height, lianas likely decrease light interception and competitive position in the canopy, potentially lowering individual tree carbon stocks without being offset by liana biomass. This suggests that liana-induced height suppression contributes to weaker forest carbon sinks.

The paper emphasizes that these structural effects are not uniform; they vary with host species traits and are modulated by climate. The finding that suppression intensifies under warmer and more seasonal conditions implies that climate change may exacerbate the negative impacts of lianas on forest structure. Consequently, ignoring liana-tree interactions in allometric models may lead to systematic overestimations of tropical forest biomass and carbon storage. The authors conclude that understanding these interactions is essential for predicting how tropical forests will reorganize under ongoing climate change and disturbance.