Drivers of variation in abundance and species richness between climbing mechanisms across the tropics
Our results showed that, across the tropics, climate and forest structure are the main factors controlling liana abundance and species richness, with both factors having different effects and explanatory powers depending on the type of climbing mechanism used by the lianas.
The forest gap index was positively related to the abundance of active climbing species but not to the abundance of passive climbing species, while, contrary to our expectation, canopy height was not significantly related to either active or passive climbing abundance. The influence of the forest structure on liana recruitment largely depends on the climbing mechanisms used by lianas to ascend the canopy. For instance, the host trunk diameter restricts mainly tendril and twiner species (active climbing in Fig. 1), but variation in host size (as indicated by the forest gap index) within a forest may allow them to climb from one tree to the next and, thus, reach the forest canopy. Hooks and scramblers (passive climbing in Fig. 1), in general, do not reach the upper parts of the canopy (Rowe & Speck, 2004; Sandrine & Isnard, 2009), particularly because these species lean on their host and cannot easily climb beyond the host’s height to reach the canopy, while root climbers can attach to any size of trunk but cannot cross between different trunks. It is clear, therefore, that the host size and the climbing behavior of the liana determines the differences in the potential environment that lianas can explore within forests (Ichihashi & Tateno, 2011, 2015). This variation suggests a niche differentiation among lianas; this could be an important mechanism underlying the diversification and coexistence of lianas species across tropical forests.
The species richness of active and passive climbing were both positively associated with the forest gap index, but only the species richness of the passive climbers was negatively related to canopy height. Our results, in part, contrast with a recent study showing that across the Neotropics the liana species richness from the tribe Bignoniae, which in general have a type of active climbing mechanism (tendril-climbers), was positively related to canopy height (Meyer et al., 2020). There are two possible explanations for these differences. Firstly, our study analyzed a wide group of liana species and not only a specific clade and, secondly, our study focused on community plot-level data, while Meyer et al. (2020) focused on grid cells. It is possible that at the plot-level scale, successional status and disturbance, as indicated by the forest gap index, might represent a stronger filter to active climbing species than canopy height by providing more light and suitable supports. Indeed, disturbance and forest structure have been shown to play an important role in the maintenance of liana diversity and density in forests (Schnitzer & Carson, 2000; DeWalt et al., 2000; Schintzer & Bongers, 2002; van der Heijden & Phillips, 2008, 2009). On the other hand, the richness of the passive climbing species in the present study was found to be negatively related to canopy height. It is possible that the passive climbing species have less opportunity to grow towards regions of high light intensity in a tall forest canopy, whereas the active climbing species can forage more effectively and, thus, face no such loss in richness in relation to canopy height.
In our study, the temperature was positively related to the abundance and richness of active climbing species, while precipitation had a positive effect only on the richness of the passive climbing species. These results are broadly consistent with previous studies considering lianas in general (Couvreur et al., 2015; DeWalt et al., 2015; van der Heijden & Phillips, 2009), however, our study revealed that climate contributes differently to explain the abundance and richness of lianas when accounting for the climbing mechanism. The fact that only precipitation was strongly positively related to the passive climbing species richness supports previous results which showed that root climbers (which have a passive climbing mechanism) correlate positively with rainfall across the globe (Durigon et al., 2013). The positive relationship between active climbing abundance and species richness with temperature may also reflect the higher potential of acclimation and phenotypic plasticity of active climbing species. For instance, Carter & Teramura (1991) analyzed the photosynthetic acclimation of lianas differing in climbing mechanisms and found that adhesive discs displayed greater physiological acclimation to low light environments, while tendril climbers were found to have the broadest physiological plasticity to high light environments. Overall, the positive relationship between the forest gap index (significant) and temperature (marginally significant) with the active climbing species richness are compatible with our functional trait analyses of showing that active climbing species have an acquisitive strategy.
The soil variables, at least at the macroecological scale, that were analyzed in the present study were only weakly associated with the abundance of the active and passive climbing species, this is consistent with the soil properties usually being more important predictors of liana distribution at the fine spatial scale (Powers, 2015; van der Heijden & Phillips, 2009). The richness of both active and passive climbing species generally decreased with soil fertility (i.e., with increasing CEC), which was in contrast to our initial expectation that higher nutrient availability should increase liana species richness (Gillman et al., 2015; Pausas & Austin, 2001). This, in part, may be due to there being further differences in the soil parameters between regions that were unaccounted for in our analysis.
We found that the relative richness of active to passive climbing species was significantly higher only in the Neotropics compared to the other biogeographical realms (Appendix S2 – Table S1, Fig. 4). One possible explanation might be related to differences in canopy height and forest history, leading to a strict association between liana ecology and 3D forest structure in some biomes and continents, but not others. For instance, differences in the vegetation structure may promote specific climbing mechanisms (e.g., tendrils) (Lohmann, 2003) and, thus, favors the evolution of alternative lifeforms across these biogeographical realms (e.g., Neotropics versus Afrotropic). In fact, Couvreur et al. (2015) showed a link between the diversification of the climbing life-form in palms and the forest canopy height. Likewise, ancestral area reconstructions suggest that almost all ancestors of the currently recognized genera of the tribe Bignonieae, where most of the species are tendril climbers, inhabited forest habitats (Lohmann et al., 2013). Additional work integrating abundance patterns with a comprehensive species-level phylogeny of lianas would be important to clarify the link between ecology and the evolution of the climbing mechanism.