Exploring within-plant hydraulic trait variation: A test of the vulnerability segmentation hypothesis

Abstract

Observations show vulnerability segmentation between stems and leaves is highly variable within and between environments. While a number of species exhibit conventional vulnerability segmentation (stem $P_{50}<$ leaf $P_{50}$ ), others exhibit no vulnerability segmentation and others reverse vulnerability segmentation (stem $P_{50}>$ leaf $P_{50}$ ). We developed a hydraulic model to test hypotheses about vulnerability segmentation and how it interacts with other traits to impact plant conductance. We do this using a series of experiments across a broad parameter space and with a case study of two species with contrasting vulnerability segmentation patterns: Quercus douglasii and Populus trichocarpa. We found that while conventional vulnerability segmentation helps to preserve conductance in stem tissues, reverse vulnerability segmentation can better maintain conductance across the combined stem-leaf hydraulic pathway, particularly when plants have more vulnerable $P_{50}$ s and have hydraulic segmentation with greater resistance in the leaves. These findings show that the impacts of vulnerability segmentation are dependent upon other plant traits, notably hydraulic segmentation, a finding that could assist in the interpretation of variable observations of vulnerability segmentation. Further study is needed to examine how vulnerability segmentation impacts transpiration rates and recovery from water stress.

Keywords: cavitation; hydraulic model; hydraulic segmentation; plant hydraulics; vulnerability segmentation.