INTRODUCTION
Liana are woody climbing plants (Gentry, 1991) that contribute to the
diversity, structure, and productivity of forests worldwide (Putz, 1991;
Hegarty & Caballe, 1991; Schnitzer & Bonger, 2002, 2011; DeWalt et
al., 2010, 2015). However, their diversity strongly decreases from
tropical to temperate ecosystems (Gentry, 1991; Phillips & Miller,
2002; Schnitzer & Bongers, 2002; Schnitzer et al., 2015; DeWalt et al.,
2010, 2015). The global pattern of liana diversity decreases sharply
toward higher latitudes (Gentry, 1991; Phillips & Miller, 2002;
Schnitzer & Bongers, 2005; Hu et al., 2010; DeWalt et al., 2015;
Schnitzler et al., 2016; Lobos-Catalán & Jiménez-Castillo, 2019). In
tropical forests, lianas represent about 25% of woody plants species
(Gentry, 1991; Chave et al., 2001), while in temperate forests they
represent only 10% of woody flora (Gentry, 1991), representing a
five-fold decrease in liana diversity from tropical to temperate forests
(Gentry, 1991; Schnitzer & Bongers, 2002; Schnitzer, 2018).
From this pattern that we call the “Global Pattern of Liana Diversity”
(hereafter GPLD), it has been proposed that climbing habit is
incompatible with cold environments (Gentry, 1991; Ewers et al., 1997;
Schnitzer & Bongers, 2002; Londré & Schnitzer, 2006; Hu et al., 2010),
where they would have low survival and/or performance (Sperry et al.,
1987; Ewers et al., 1991 b; Schnitzer, 2005; Jimenez-Castillo & Lusk,
2013) and therefore there would be low diversity of liana species. This
called ”cold hypothesis” has been supported by research that show a
negative relationship between liana diversity and cold gradients, the
observed decrease in liana diversity along altitudinal gradients
(Vázquez & Givnish, 1998; Bhattarai & Vetaas, 2003; Schnitzler et al.,
2016) and the parallelism between latitudinal and altitudinal
distribution patterns of lianas species in temperate forests
(Jiménez-Castillo et al., 2007).
The proposed underlying mechanism of GPLD is that lianas’ vascular
system would lose its functionality in cold environments. On one hand,
lianas have significantly higher growth rate than coexistent trees in
tropical ecosystems (Schnitzer, 2005), which is supported by a very
efficient vascular system, formed by wide vessels (Gartner et al., 1990;
Ewers & Fisher, 1991; Holbrook & Putz, 1996; Davis et al., 1999;
Jiménez-Castillo & Lusk, 2013; De Guzman et al., 2016). On the other
hand, in cold environments, wide vessels are highly prone to
freezing-thaw embolism (Sperry et al., 1987; Sperry & Sullivan, 1992;
Ewers et al., 1991a; Tibbetts & Ewers, 2000), which would negatively
affect its performance. So, the same traits that give high efficiency in
water transport to lianas species in tropical forest would become
maladaptive in temperate forest (Sperry et al., 1987; Ewers et al.,
1985, 1991a; Jiménez-Castillo & Lusk, 2013). That is how the
hypothesized mechanism behind GPLD is based on these two lines of
research: first, that the latitudinal and altitudinal gradients of liana
diversity are negative related to cold gradients (Gentry, 1991; Vázquez
& Givnish, 1998; Phillips & Miller, 2002; Jiménez-Castillo et al.,
2007; van der Heijden & Phillips, 2008; DeWalt et al., 2010;
Lobos-Catalán & Jiménez-Castillo, 2019); second, the vascular system of
lianas becomes dysfunctional in cold environments (Sperry et al., 1987;
Ewers et al., 1991, 1997; Davis et al., 1999; Jiménez-Castillo & Lusk,
2013).Although these inferences have supported the hypothesis about the
cold temperature as the environmental factor limiting the liana
ecological patterns, it becomes imperative to test the proposed
mechanism in natural ecosystems.
If cold strongly limits the ecological performance of lianas toward
higher latitudes, how can they even inhabit temperate ecosystems? In
cold environments lianas would confront a trade-off between the
efficiency and safety of the water transport capacity. Since wide
vessels are highly prone to freezing-thaw embolism (Zimmermann, 1983;
Ewers et al., 1997), the selection of narrow vessels would confer safety
in cold environments (Carlquist, 1988; Ewers & Fisher, 1991 a, b).
Another safety strategy is that lianas could reverse embolism by
generating high root pressure (Sperry et al., 1987; Tibbetts & Ewers,
2000) which would be triggered by low temperatures. It has been observed
that only 3 of 29 liana species generates root pressure in tropical
forest (Ewers et al., 1997), while all liana species have twice as high
root pressure than coexisting trees species in southern temperate forest
(Jiménez-Castillo & Lusk, 2013). However, any of these mechanisms would
reduce the performance of liana in cold environments because narrow
vessel decreases the efficiency of water transport and root pressure
implies costs by being osmotically generated (Tibbetts & Ewers, 2000;
Isnard & Silk, 2009; Jiménez-Castillo & Lusk, 2013).
These traits of avoidance and/or
reversal of the embolism would not be excluding, rather both would allow
lianas species to persist under low temperature conditions.
To study the underlying mechanism of ecological patterns, ecologists
have increasingly studied the functional traits to understand species
performance from a mechanistic approach and infer niche-based mechanisms
to explain community patterns (Cadotte et al., 2015). Thus, a set of
quantifiable traits at the level of individual (morphological,
physiological, phenological), would have a direct effect (performance
traits) or an indirect effect (functional traits) on the biological
fitness of an individual (Violle et al., 2007). Allowing the
construction of general models that facilitate the understanding of the
ecological strategies of the species (Lavorel & Garnier, 2002; Reich et
al., 2003;Westoby & Wright, 2006; Poorter et al., 2010; Dwyer &
Laughlin, 2017).
In this context, we pursue to establish the functional mechanism related
to lianas species performance along a latitudinal temperature gradient.
We addressed the following research questions: i) How does liana
performance vary along a latitudinal temperature gradient? Along the
latitudinal temperature gradient we measure the liana species
performance, as the relative apical growth rate. In warmer site, we
would expect a higher liana species performance in relation to species
in higher latitudes because of the more frequent freezing temperatures
ii) How the trade-off between safety and efficiency of water transport
capacity in liana species impacts on their ecological performance? Along
this same gradient, traits associated with the efficiency (wide vessel
selection, low wood density, high hydraulic conductivity, low root
pressure and low PLC) and safety of water transport (narrow vessel
selection, high wood density, low hydraulic conductivity, high root
pressure and high PLC) have been studied. In warmer site we would expect
a higher performance related to efficiency water transport traits while
toward colder site, we would expect a lower liana species performance
associated with tolerance or avoidance traits to cope with freezing-thaw
embolism.