Hydraulic transport, PLC, and wood anatomy
Water transport capacity (Ks_max) of liana species was much less efficient in the cold extreme of the latitudinal gradient (Aysén) than the warmer extreme (Nahuelbuta) (Table 2). In fact, the maximum specific hydraulic conductivity (Ks_max) in Puyehue and Aysén was 1.9 and 26 times lower than Nahuelbuta, respectively. This loss of hydraulic conductivity is a critical point in liana species performance, because a high hydraulic conductivity is need to supply water to the large foliar area they develop (Schnitzer & Bonger, 2002; Isnard & Silk, 2009) and support the high growth rates that characterize lianas compared to other growth forms (Putz & Mooney, 1991; Schnitzer, 2005). Thus, as shown by our results (Table 2), the loss of efficiency in water transport results in a lower ecological performance along the latitudinal gradient.
The decrease in specific hydraulic conductivity (Ks_max) would be due to a joint effect between the selection of narrow vessel diameter (VD) and the increase in PLC towards higher latitudes (Table 2). Since wide vessel are highly prone to freezing-thaw embolism (Gartner et al., 1990; Ewers et al., 1991; Davis et al., 1999), they would not be functional towards higher latitudes (Jiménez-Castillo & Lusk, 2013), so cold temperatures would be acting as a selection factor on narrow vessels diameters. We observed that liana species in higher latitudes significantly decrease vessel diameter, to confer safety but not efficiency in water transport. The selection for narrow vessel diameter is also related to the observed increase vessel density and wood density (Table 2), a response that has been documented in different growth forms (Chave et al., 2009; Poorter et al., 2010; Martínez-Cabrera et al., 2011). In addition, the percentage of loss conductivity (PLC) increases from 25% to 37% from the warmer to the colder extreme of the latitudinal gradient. The PLC would exacerbate the loss of efficiency in water transport, for example, liana species lose 25% of their hydraulic conductivity in Nahuelbuta due to freezing-thaw embolism (PLC), this reduced their maximum specific hydraulic conductivity (Ks_max) from 28.7 Kg s-1 m-1 MPa-1 to 21.6 Kg s-1 m-1MPa-1 approximately. While liana species lose 37.5% of their hydraulic conductivity in Aysén due to PLC, which reduced the conductivity from 1.1 Kg s-1 m-1MPa-1 to 0.68 Kg s-1m-1 MPa-1. Therefore, the total effect of PLC over Ks_max caused reduction of 2.2 and 32 times in Puyehue and Aysén in relation to Nahuelbuta. This ratified the idea that is the loss of efficiency of water transport, either by reduction of vessels diameter or elevated levels of embolism, which decreases the performance of lianas in cold environments.