Liana performance, cold environment, and embolism
There was a strong decrease in liana performance toward the cold extreme of the latitudinal gradient, which was a consequence of the loss in efficiency in water transport due to important levels of freezing-thaw embolism (Fig. 1). Different studies in tropical forests have shown that the competitive advantage of lianas relies on a high growth rate, which allows them to outcompete trees and other growth forms (Putz & Mooney, 1991, Schnitzer & Bongers, 2002; Isnard & Silk, 2009; Schnitzer, 2018). However, in cold environment this advantage is lost or at least it is significantly reduced, as growth rate was 5 times lower in lianas at the colder site (Aysén) than lianas at the warm end of the gradient (Nahuelbuta) (Fig. 1; Table 2). Moreover, growth rate was inversely correlated with PLC along the gradient (Fig. 1), demonstrating that the reduction in performance is related to the vulnerability of their vascular system to freezing-thaw embolism.
Freezing-thaw embolism would restrict lianas’ performance in two ways: first, it would generate a delay in the growth period, because liana species would need to recover water transport capacity of the wood anatomy before they start phenological processes or growth period in early spring (Ewers et al., 1997; Nardini et al., 2011; Christensen & Tyree, 2014). Therefore, liana species would need to invest more carbon on reverting embolism than allocating it on growth (e.g., via root pressure which is osmotically generated), in relation to species that have avoided or have not suffered freezing-thaw embolism. Second, species that lose a high percentage of hydraulic conductivity due to embolism could permanently lose the functioning of their vascular system. In this case, lianas would depend on the generation of new xylem (Sperry et al., 1987; Ewers et al., 1991a, b, 1997; Améglio et al., 2002), which again, would limit the resource allocation to growth. Either way, freezing-thaw embolism would severely limit the growth capacity of lianas species, because of limited water transport to the leaves and therefore a decrease in gas exchange and photosynthesis (McDowell et al., 2019). So, it would imply losses in plant performance and productivity in the long term.
Our results support the hypothesis that climbing habits are incompatible with cold environments. Although, several authors have proposed this hypothesis in the last decades (Supplementary information Table S1), our results show evidence in natural environments of a performance reduction in lianas species because of their high vulnerability to embolism in cold environments.