Abstract
Lianas are a key growth form in tropical forests. They are believed to be strong competitors for water, thanks to their presumed efficient vascular systems. However, despite being a large polyphyletic group, they are currently often considered as a functionally homogeneous entity. In this study, we challenged this assumption by estimating the variability in hydraulic traits of two common, co-occurring liana species in a water-limited environment, namely, a seasonally dry tropical forest in Costa Rica. We measured vulnerability to embolism at the leaf and branch levels using two different methods (optical and acoustic vulnerability) and found that both species had very different hydraulic properties. Compared to reported P50values in literature, we found two extreme P50values: a low value forBignonia diversifolia(−4.30 ± 0.54 MPa at the leaf level; −7.42 ± 0.54 MPa at the branch level) and a high value forCissus microcarpa(−1.07 ± 0.14 at the leaf level; −1.20 ± 0.05 MPa at the branch level). Furthermore,B. diversifoliahad a higher apparent modulus of elasticity in the radial direction (556.6 ± 401.0 MPa) and a variable midday water potential. On the other hand,C. microcarpahad a low apparent modulus of elasticity in the radial direction (37.8 ± 26.3 MPa) and a high branch water content, which enabled the species to keep its water potential stable during the dehydration experiments and during a drought period in the field. This mechanism may enable this species to coexist with species that are more resistant to drought-induced embolisms such asB. diversifolia. Although only two species were studied, considerable overlap was found between the range of hydraulic properties of trees growing in the same location and trees and lianas growing in two forests in Panama. These findings demonstrate that lianas cannot be considered as a homogeneous group and call for further research into the intra-growth form diversity of liana properties.
Highlights
Tropical forests are one of the most important ecosystems on Earth
Predawn; md, midday; early dry, period from 07/08 to 21/08; dry, period from 22/08 to 13/09; wet, period from 14/09 to 30/09; Optical Vulnerability (OV), optical vulnerability; AV, acoustic vulnerability; MVL, maximum vessel length; P50,OV, water potential at 50% embolized vessel area; P50,AV, water potential at 50% of cumulative acoustic emissions; min minimum stem water potential measured in the field on 1 day; HSMAV, hydraulic safety margin at the branch level; ax50, slope of the vulnerability curve at P50; P12−P88, difference between onset and ending of embolism formation; WSG, wood specific gravity; Er,1, apparent modulus of elasticity in the radial direction of the first regression segment
We showed that liana species found in the same environment can have contrasting hydraulic traits, which refutes our hypothesis that lianas from a single site would exhibit similar hydraulic traits due to evolutionary convergence
Summary
Tropical forests are one of the most important ecosystems on Earth They are responsible for almost one third of the carbon uptake by terrestrial plants (Beer et al, 2010), they store more than half of the terrestrial biomass (Pan et al, 2011) and support approximately 50% of the described species (Dirzo and Raven, 2003). Dry forests make up a significant part of tropical forests as they encompass 42% of the tropical forest region (Miles et al, 2006) By definition, they are characterized by at least 3 months with less than 100 mm precipitation and 50% drought-deciduous species (Sánchez-Azofeifa et al, 2005). Unlike most other growth forms, liana density increases with increasing seasonality and with decreasing mean annual precipitation, which makes them more abundant in tropical dry forests (Schnitzer, 2005). While the mechanistic understanding of this proliferation remains unclear, it could be related to the water competition between growth forms (Schnitzer, 2005, 2018; Smith-Martin et al, 2019)
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