Summary and Future Directions
We provide strong evidence that the type of climbing mechanism is a central characteristic to understand the functional ecology and distribution of lianas. Our synthesis is the first to jointly assess the global-scale patterns of distribution and functional trait variations of lianas while accounting for their type of climbing mechanism.
It has been shown that the increase in lianas abundance and biomass in recent decades across the tropics is not uniform (Schnitzer & Bongers, 2011). An increase is evident in the Neotropics, while there has been no increase in the Afrotropic and mixed results for Indo-Malay (Schnitzer & Bongers, 2011; Schnitzer, 2018). Our results suggest that these differences might be related to a combination of functional trait differences and factors affecting the distribution among liana species that are related to the climbing mechanisms and growth strategies of lianas. Furthermore, we showed that the relative richness of lianas with active climbing compared to passive climbing differs significantly between the Neotropics and other biogeographical realms (Fig. 4), suggesting that the consistent increase in lianas abundance and biomass across the Neotropics (Schnitzer and Bongers, 2011), compared to other biogeographic regions, could be a result of the preponderance of active climbing species in this region. Future studies should attempt to test this hypothesis, ideally using demographic data and information about the liana climbing mechanisms from different biogeographic regions. Increases in temperature and in forest disturbance (leading to increasing light availability and trees with thinner trunks) are likely to act synergistically to increase the abundance of lianas, particularly species with an aggressive strategy such as the active climbing species whose abundance strongly depends on temperature and forest structure. We suggest that future studies should use demographic data from local and large spatial scales, combined with data on the liana climbing mechanisms and functional traits to test the generality of our findings.
Our finding that there is a biogeographic contingency in the abundance of lianas according to their main type of climbing mechanism also has important implications for modeling the effect of lianas on the forest structure at a large spatial scale. For instance, the first DGVM to include lianas considered them as a single, plant functional type (di Porcia e Brugnera et al., 2020). The implicit assumption is that there is no, or very little, functional variation within the lianas as a group. However, our results show that there is significant functional variation among lianas and that an important part of this variation can be explained by the type of climbing mechanism. The inclusion of lianas in DGVMs is definitely an important step for increasing our ability to understand their effects on forest dynamics, however, our results suggest that representation of lianas as a single, plant functional type in DGVMs may be premature and deserves more investigation in the future.
Conclusions
The evolution of climbing habit and its implication for the life history of lianas is still an understudied topic in ecology, but this information is essential to link a species response to climate change across multiple scales. As we showed in this study, when accounting for the climbing mechanism, lianas clearly diverge in their functional traits and abiotic factors which drive their distribution. We argue that in order to understand the main causes of the increases in liana abundance and biomass that are being observed in tropical forests, it will be crucial to gather data at the species-, population- and plot-levels across forests and to analyze them, taking into account their life-history differences such as the type of climbing mechanism. As these data continue to be generated and become available through databases, such as BIEN and TRY in recent years, we shall be able to understand better the ecology of lianas and their role in tropical forest dynamics.