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.