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.