Vulnerability and hydraulic segmentations at the stem–leaf transition: coordination across Neotropical trees

Abstract

Hydraulic segmentation at the stem-leaf transition predicts higher hydraulic resistance in leaves than in stems. Vulnerability segmentation, however, predicts lower embolism resistance in leaves. Both mechanisms should theoretically favour runaway embolism in leaves to preserve expensive organs such as stems, and should be tested for any potential coordination. We investigated the theoretical leaf-specific conductivity based on an anatomical approach to quantify the degree of hydraulic segmentation across 21 tropical rainforest tree species. Xylem resistance to embolism in stems (flow-centrifugation technique) and leaves (optical visualization method) was quantified to assess vulnerability segmentation. We found a pervasive hydraulic segmentation across species, but with a strong variability in the degree of segmentation. Despite a clear continuum in the degree of vulnerability segmentation, eight species showed a positive vulnerability segmentation (leaves less resistant to embolism than stems), whereas the remaining species studied exhibited a negative or no vulnerability segmentation. The degree of vulnerability segmentation was positively related to the degree of hydraulic segmentation, such that segmented species promote both mechanisms to hydraulically decouple leaf xylem from stem xylem. To what extent hydraulic and vulnerability segmentation determine drought resistance requires further integration of the leaf-stem transition at the whole-plant level, including both xylem and outer xylem tissue.

Leaf hydraulic safety margin and safety–efficiency trade-off across angiosperm woody species

“…It is likely that these species may rely largely on leaves acting as safety valves (early loss of hydraulic conductance) to maintain the hydraulic functioning of the stem during drought periods [15,20]. Further study on the hydraulic safety margins of the leaves and stems simultaneously would enable accurate assessments of the plant hydraulic risk during drought and improve our understanding of vulnerability segmentation [15,20,34].…”

A semi-automated method for measuring xylem vessel length distribution

“…The convenience to collect and to analyze data is important to allow pioneering studies with a large number of samples such as interbranch or interspecific comparisons, especially in highly diverse plant communities, along with measurements of other anatomical and hydraulic traits. Also, this method may be used for studies on hydraulic segmentation (Tyree and Zimmermann 2002;Levionnois et al 2020), by estimating vessel length distribution for different plant organ connections, such as across nodes, near side branches, stempetiole transitions, fruit pedicels, or petiole-leaf blade transitions.…”

A semi-automated method for measuring xylem vessel length distribution

“…The convenience to collect and to analyze data is important to allow pioneering studies with a large number of samples such as interbranch or interspecific comparisons, especially in highly diverse plant communities, along with measurements of other anatomical and hydraulic traits. Also, this method may be used for studies on hydraulic segmentation (Tyree and Zimmermann 2002;Levionnois et al 2020), by estimating vessel length distribution for different plant organ connections, such as across nodes, near side branches, stem-petiole transitions, fruit pedicels, or petiole-leaf blade transitions.…”