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.