Safety of water transport in cold environment
Cold temperatures would be strong selective pressure, leading plants to select vascular anatomy features that prevent the formation or spread of embolism (Brodribb & Holbrook, 2004; Brodersen & McElrone, 2013). When we analyzed the complete dataset of the frequency distribution of xylem vessels diameter and their relative contribution to total hydraulic conductivity, we realized that 50% of hydraulic conductivity is sustained by 11% of xylem vessel (>100 µm) (Fig. 2), so losing the functionality of this small percentage of vessels would have a great impact on the conductivity of the individual. Lianas favor the safety of hydraulic transport in freezing conditions, with the reduction of up to 3-times in vessels diameter with the increase in latitude (Table 2), losing the diameter class responsible for the highest percentage of conductivity (Fig. 2). Also, the selection of a high percentage of narrow vessels (~80%), associated with xylem tissue made up of denser xylem vessels and a slight increase in the density of the wood (Table 2), would favor the recovery of the hydraulic conductivity, since they can remain functional after the freezing events (Sperry & Sullivan, 1992; Davis et al., 1999; Hacke et al., 2001; De Guzman et al., 2016). This could function as a backup system that delivers water to the leaves when large vessels are dysfunctional (Ewers, 1985; Zwieniecki & Holbrook, 2009; Sperry et al., 2006), being functionally relevant at the start of the growth season in temperate environments (Chiu & Ewers, 1992).
The decrease in vessel diameter, as narrow as observed in trees (Jiménez-Castillo & Lusk, 2013), decrease the hydraulic conductivity in such a magnitude that lianas would not be able to sustain a high leaf area/active xylem area ratio or a high gaseous exchange (Davis et al., 1999). This finally would lead them to lose their main competitive advantage, which is their high growth rate. So, this trade-off between safety and efficiency of water transport would make the climbing habit less competitive and would sustain mechanism behind the already observed pattern of less diversity of liana species in cold climates (Lobos-Catalán & Jiménez-Castillo, 2019).
Root pressure would allow lianas species to recover their water transport capacity after freezing-thaw embolism occurs (Tibbets & Ewers, 2000; Sperry et al., 1987; Ewers et al. 1997; Yin et al., 2018), being a trait with adaptive value in cold environments (Jiménez-Castillo & Lusk, 2013). All studied liana species generate root pressure, but we did not observe an increase trend towards higher latitudes (Table 2), and the registered magnitudes would not allow reverting embolism along the whole stem of individuals that reach the forest canopy. Like our results, it has been frequently observed that root pressure is too low for reverting completely the embolism (Ewers et al., 1997; Tibbetts & Ewers, 2000). Only in particular cases, species of genus Cissushave shown root pressure that would dissolve embolism up to 14-20 meter in the plant (Fisher et al., 1997), but we did not observe such pattern on the Cissus species included in our study. The results indicate that the root pressure would be enough only to fill vessels of the root system and the basal part of the sprouted stems (Ewers et al., 1997; Isnard & Silk, 2009). Therefore, root pressure would be a first step to achieve the recovery of conductivity, as it could be associated with other recovery traits such as the increase in vascular tension by transpiration ”throw-away strategy” (Hacke & Sperry, 2001: Nardini et al., 2011), and/or the generation of new vessels in early wood (Sperry & Sullivan, 1992; Tibbetts & Ewers, 2000). Although root pressure would be an important trait for lianas that inhabit cold environments, more studies are needed to estimate their actual contribution to the recovery of hydraulic conductivity.