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1. Lack of vulnerability segmentation among woody species in a diverse dry sclerophyll woodland community

   https://doi.org/10.1111/1365-2435.13519  

   Smith‐Martin, Chris M. ; Skelton, Robert Paul ; Johnson, Kate M. ; Lucani, Christopher ; Brodribb, Timothy J. ; Sala, ed., Anna ( February 2020 , Functional Ecology)

   Recent findings suggest that tree mortality and post‐drought recovery of gas exchange can be predicted from loss of function within the water transport system. Understanding the susceptibility of plants to hydraulic damage requires knowledge about the vulnerability of different plant organs to stress‐induced hydraulic dysfunction. This is particularly important in the context of vulnerability segmentation between plant tissues which is believed to protect more energetically ‘costly’ tissues, such as woody stems, by sacrificing ‘cheaper’ leaves early under drought conditions.

   Differences in vulnerability segmentation between co‐occurring plant species could explain divergent behaviours during drought, yet there are few studies considering how this characteristic may vary within a plant community. Here we investigated community‐wide vulnerability segmentation by comparing leaf/shoot and stem vulnerability in all coexistent dominant canopy and understory woody species in a diverse dry sclerophyll woodland community, including multiple angiosperms and one gymnosperm.

   Previously published terminal leaf/shoot vulnerability to loss of water transport capacity was compared with stem xylem vulnerability to embolism measured on the same species at the same site. We calculated hydraulic safety margins for stems to determine variation in the risk of hydraulic failure during drought among species.

   The xylem of all species was found to be highly resistant to hydraulic dysfunction, with only two of the eight species exhibiting significantly different vulnerability to the overall mean. No evidence of vulnerability segmentation between shoots/leaves and stems was found in seven of the eight species.

   Phylogenetically diverse canopy and understory species in this evergreen sclerophyll woodland appear to have evolved similar strategies of drought resistance, including low xylem vulnerability to embolism and general lack of vulnerability segmentation. This convergence in hydraulic safety indicates a lack of hydraulic niche partitioning in this woodland community.

   A freeplain language summarycan be found within the Supporting Information of this article.

    

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2. Stem and leaf xylem of angiosperm trees experiences minimal embolism in temperate forests during two consecutive summers with moderate drought

   https://doi.org/10.1111/plb.13384  

   Guan, X. ; Werner, J. ; Cao, K.‐F. ; Pereira, L. ; Kaack, L. ; McAdam, S. A. ; Jansen, S. ( December 2022 , Plant Biology)

   Drought events may increase the likelihood that the plant water transport system becomes interrupted by embolism. Yet our knowledge about the temporal frequency of xylem embolism in the field is frequently lacking, as it requires detailed, long‐term measurements.

   We measured xylem embolism resistance and midday xylem water potentials during the consecutive summers of 2019 and 2020 to estimate maximum levels of embolism in leaf and stem xylem of ten temperate angiosperm tree species. We also studied vessel and pit membrane characteristics based on light and electron microscopy to corroborate potential differences in embolism resistance between leaves and stems.

   Apart fromA.pseudoplatanusandQ.petraea, eight species experienced minimum xylem water potentials that were close to or below those required to initiate embolism. Water potentials corresponding to ca. 12% loss of hydraulic conductivity (PLC) could occur in six species, while considerable levels of embolism around 50% PLC were limited toB.pendulaandC.avellana. There was a general agreement in embolism resistance between stems and leaves, with leaves being equally or more resistant than stems. Also, xylem embolism resistance was significantly correlated to intervessel pit membrane thickness (T_PM) for stems, but not to vessel diameter and total intervessel pit membrane surface area of a vessel.

   Our data indicate that low amounts of embolism occur in most species during moderate summer drought, and that considerable levels of embolism are uncommon. Moreover, our experimental andT_PMdata show that leaf xylem is generally no more vulnerable than stem xylem.

    

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3. Aridity drives coordinated trait shifts but not decreased trait variance across the geographic range of eight Australian trees

   https://doi.org/10.1111/nph.16795  

   Anderegg, Leander D. L. ; Loy, Xingwen ; Markham, Ian P. ; Elmer, Christina M. ; Hovenden, Mark J. ; HilleRisLambers, Janneke ; Mayfield, Margaret M. ( August 2020 , New Phytologist)

   Large intraspecific functional trait variation strongly impacts many aspects of communities and ecosystems, and is the medium upon which evolution works. Yet intraspecific trait variation is inconsistent and hard to predict across traits, species and locations.

   We measured within‐species variation in leaf mass per area (LMA), leaf dry matter content (LDMC), branch wood density (WD), and allocation to stem area vs leaf area in branches (branch Huber value (HV)) across the aridity range of seven Australian eucalypts and a co‐occurringAcaciaspecies to explore how traits and their variances change with aridity.

   Within species, we found consistent increases in LMA, LDMC and WD and HV with increasing aridity, resulting in consistent trait coordination across leaves and branches. However, this coordination only emerged across sites with large climate differences. Unlike trait means, patterns of trait variance with aridity were mixed across populations and species. Only LDMC showed constrained trait variation in more xeric species and drier populations that could indicate limits to plasticity or heritable trait variation.

   Our results highlight that climate can drive consistent within‐species trait patterns, but that patterns might often be obscured by the complex nature of morphological traits, sampling incomplete species ranges or sampling confounded stress gradients.

    

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4. Exploring within‐plant hydraulic trait variation: A test of the vulnerability segmentation hypothesis

   https://doi.org/10.1111/pce.14649  

   Wilkening, Jean V. ; Skelton, Robert P. ; Feng, Xue ; Dawson, Todd E. ; Thompson, Sally E. ( September 2023 , Plant, Cell & Environment)

   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 leaf ), others exhibit no vulnerability segmentation and others reverse vulnerability segmentation (stem leaf ). 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 douglasiiandPopulus 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 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.

    

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5. Plasticity drives geographic variation and trait coordination in blue oak drought physiology

   https://doi.org/10.1101/2023.08.20.553748  

   Anderegg, Leander DL ; Skelton, Robert P ; Diaz, Jessica ; Papper, Prahlad ; Ackerly, David D ; Dawson, Todd E ( August 2023 , bioRxiv)

   Two decades of widespread drought-induced forest mortality events on every forested continent have raised the specter of future unpredictable, rapid ecosystem changes in 21^stcentury forests. Yet our ability to predict drought stress, much less drought-induced mortality across the landscape remains limited. This uncertainty stems at least in part from an incomplete understanding of within-species variation in hydraulic physiology, which reflects the interaction of genetic differentiation among populations (ecotypic variation) and phenotypic plasticity in response to growth environment. We examined among-population genetic differentiation in a number of morphological and hydraulic traits in California blue oak (Quercus douglasii) using a 30 year old common garden. We then compared this genetic trait differentiation and trait-trait integration to wild phenotypes in the field from the original source populations. We found remarkably limited among-population genetic differentiation in all traits in the common garden, but considerable site-to-site variation in the field. However, it was difficult to explain trait variation in the field using site climate variables, suggesting that gridded climate data does not capture the drivers of plasticity in drought physiology in this species. Trait-trait relationships were also considerably stronger in the field than in the garden, particularly links between leaf morphology, leaf hydraulic efficiency and stem hydraulic efficiency. Indeed, while twelve of 45 potential trait-trait relationships showed significant wild phenotypic correlations, only four relationships showed both genetic and phenotypic correlations, and five relationships showed significantly different genetic and phenotypic correlations. Collectively, our results demonstrate limited ecotypic variation in drought-related physiology but considerable geographic variation in physiology and phenotypic integration in the wild, both driven largely by plasticity.

    

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