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.