Trade-off between the efficiency and safety of water transport, overview
Our results show that lianas sustain a high growth rate in habitats with infrequent frost events, sustained by the efficiency of their water transport systems, result of a set of traits as wide xylem vessels, low vessel density, and high specific hydraulic conductivity (Fig. 2, Fig 3). Conversely, in habitats with frequent frost events, at higher latitudes, lianas species suffer a strong decrease in their growth rate given by elevated levels of PLC, narrow xylem vessel diameter, high vessel density, high wood density and (Fig. 2, Fig 3). These functional strategies that associate performance with the efficiency and safety of water transport are strong enough to functionally differentiate the liana communities along the latitudinal gradient (Fig. 2). These functional strategies, explain the variation in the performance of liana species along this latitudinal temperature gradients, and provides evidence for the “cold hypothesis” in natural environments and support the functional mechanism that underlies the lower diversity of lianas species in temperate ecosystems (Lobos-Catalán & Jiménez-Castillo, 2019).
Other studies in tropical ecosystems have shown that the trade-off in efficiency and safety of water transport in lianas is decoupled when the environmental pressure is due to drought embolism (De Guzman et al., 2016; van der Sande et al., 2019). Unlike trees, lianas only favor efficiency and have not changed in safety traits. So, this trade-off between efficiency and safety of water transport would change depending on the environmental pressure or ecosystem where lianas habit. Moreover, different authors show that it is impossible to combine high hydraulic efficiency and high safety (van der Sande et al., 2019). Species with high efficiency in water transport also have high gas exchange rates, which contributes to fast resource acquisition and growth. Species with high safety in water transport, have low gas exchange rates and other traits related to a conservative life history strategy of resource conservation and slow growth (Santiago et al., 2004). These last functional strategies would be those observed in liana species studied at the extremes of the latitudinal gradient.
We recognize that our study is local, which addresses a variation in liana species performance along a latitudinal temperature gradient and it‘s relation with hydraulic functional traits, but still contributes to the understanding of the different mechanisms that explain the liana diversity patterns. Once the objective has been achieved, we want to propose the idea that mixed mechanisms would explain the liana diversity patterns. On one side, it has been documented that the high growth rate is the main competitive ability of lianas in tropical ecosystems, and that this is exacerbated in low rainfall and seasonal sites (DeWalt et al., 2000, 2006, 2015; Schnitzer & Bongers, 2002, 2011; Schnitzer, 2005). On the other hand, temperature is the main factor modeling the patterns of liana richness and abundance in temperate ecosystems (Jiménez-Castillo y Lusk, 2007; Lobos-Catalán & Jiménez-Castillo, 2019) as consequence of their effect on performance reduction. Consequently, the liana diversity patterns would be result of the differential influence of environmental factors, and mixed explanatory mechanisms. However, more studies are needed to prove this proposal, and especially studies on the functional mechanisms behind the diversity patterns of lianas in different ecosystems.
Through a functional approach, it has been linked the performance of the liana species and the underlying mechanisms involved based on functional hydraulic strategies. These results allow us to explain how temperature models liana diversity patterns in a latitudinal gradient of the temperate rainforest in South America. We also contribute to the representation of this group of plants in functional trait biology, which is a tool that can be applied to conservation, forests dynamics and response to global climate change.