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.