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.