Discussion
Our study demonstrates a strong functional divergence associated with
the liana climbing mechanism. Specifically, our results reveal that
species with an active climbing mechanism are characterized by traits
associated with an acquisitive strategy compared to species with a
passive climbing mechanism. We also find that across the tropics the
abundance of active climbing species is best explained by temperature
and forest structure (the tree size distribution), while these factors
were poor predictors of the abundance of passive climbing species.
Collectively, our results indicate that, when accounting for their
climbing mechanism, lianas clearly diverge in terms of functional
strategies and the environmental factors that affect their distribution.
This highlights the need to reconsider the current view of treating
lianas as a single, functional plant type and may have important
implications for the understanding and prediction of the effects of
lianas on tropical forests in the future.
Trait variation
explains the differences between climbing mechanisms of lianas
Most of the current knowledge of liana functional traits comes from
studies that compare lianas to tree species (reviewed in Wyka et al.,
2015; Schintzer, 2018). These studies showed that, compared to trees,
lianas specialize toward the acquisitive end of the global trait
spectra, although there remains a huge trait variability within lianas.
Differently from previous studies, we showed that a significant part of
this trait variability can be explained by the climbing mechanism of
lianas. We found that, compared to passive climbing species, the active
climbing species showed higher values of leaf economic traits (SLA,
Nmass and Amax) that are typically
associated with an acquisitive strategy. The maximal photosynthetic rate
per leaf area (Amax), the amount of leaf area per unit
leaf mass (SLA) and the amount of nitrogen per leaf content
(Nmass) are all related to several important functional
traits, such as leaf longevity, which, in turn, influence performance
and help to account for the differences in the species growth and
survival across gradients (Wright et al., 2004; Sterck et al., 2011;
Laughlin et al., 2020). The growth and potential acclimation of lianas,
inside and across forests, depends on their climbing mechanism (Putz,
1984; DeWalt et al., 2005; Putz & Holbrook, 1991), however, in contrast
to trees, it is the rate of stem elongation, more than the investment in
diameter increase, that plays a central role in the liana’s ability to
find suitable support structures and successful colonization. For
instance, there is evidence that lianas exhibit higher rates of height
growth compared to trees (Schnitzer, 2005) and that AC species have
higher rates of shoot elongation compared to PC species (Teramura et
al., 1991). Although this evidence is still limited, the clear
differences in the economic traits that we found between the AC and PC
species support the idea that the liana vertical growth strategy may be
determined by the type of climbing mechanism.
Wood density did not differ between the AC and PC species suggesting
that the stem construction costs are widely constrained across lianas,
irrespective of their climbing mechanism (Fig. 2). One reason that wood
density poorly represents differences across lianas species may be due
to the limited investment in wood construction (low biomechanical
demand) and the resulting constant replacement of old branches in order
to keep a high photosynthetic to non-photosynthetic organ ratio.
This idea of rapid branch and stem
turnover rates in lianas is supported by previous studies showing that
lianas grow towards the canopy by discharging a large number of branches
(Ichihashi et al., 2010; Ichihashi & Tateno, 2011, 2015), whereas, in
comparison, trees grow higher by accumulating supporting tissue in the
stems to stand upright. For instance, Ichihashi & Tateno (2015) found
that the stem turnover varied according to the liana’s climbing
mechanism, with both stem extension and relative stem losses being the
greatest in the twiner species (Actinidia arguta andCelastrus orbiculatus ) and the least in a root climber species
(Schizophragma hydrangeoides ), probably reflecting the different
demands for finding well-lit sites and new hosts (Selaya and Anten,
2008).
We found that active climbing species have higher values of size-related
traits, specifically seed mass and leaf area, when compared to passive
climbing species. In general, large stature plants tend to have large
leaves and large, heavy diaspores (Diaz et al., 2015). Seed mass is also
a trait associated with larger seedling size and, therefore, higher
changes of recruitment (Moles et al., 2005; Gilbert et al., 2016). Data
related to liana allometry are limited, but there is evidence that
active climbing species tend to reach the canopy and spread among
different host crowns, developing an aggressive strategy in terms of
growth and space occupation compared to passive climbing species
(Ichihashi & Tateno, 2015). This suggests that the larger leaves and
seed mass we found in the active climbing species could be a result of
their faster stem lengthening and growth strategy.
Overall, we found strong evidence for the idea that active climbing
species, compared to passive climbing species, specialize towards a fast
strategy (i.e., rates of resource acquisition and processing) along the
plant global trait spectra (Reich, 2014; Diaz et al., 2015). The fact
that active climbing species do not show a phylogenetic signal
considering the traits analyzed here, suggests that the acquisitive
strategy of active climbing species results from a convergence
ecological strategy. This clear distinction between active climbing and
passive climbing species is contrary to the current view considering
lianas as a uniform, plant functional type and, instead, supports the
hypothesis of niche differentiation among liana species related to the
type of climbing mechanism (Icischi &Tateno, 2011; Wyka et al., 2014).