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1. Importance of hydraulic strategy trade-offs in structuring response of canopy trees to extreme drought in central Amazon

   https://doi.org/10.1007/s00442-021-04924-9  

   Garcia, Maquelle Neves ; Ferreira, Marciel José ; Ivanov, Valeriy ; dos Santos, Victor Alexandre ; Ceron, João Vitor ; Guedes, Alacimar Viana ; Saleska, Scott Reid ; Oliveira, Rafael Silva ( May 2021 , Oecologia)

   null (Ed.)

   Plant ecophysiological trade-offs between different strategies for tolerating stresses are widely theorized to shape forest functional diversity and vulnerability to climate change. However, trade-offs between hydraulic and stomatal regulation during natural droughts remain under-studied, especially in tropical forests. We investigated eleven mature forest canopy trees in central Amazonia during the strong 2015 El Niño. We found greater xylem embolism resistance (P50 = − 3.3 ± 0.8 MPa) and hydraulic safety margin (HSM = 2.12 ± 0.57 MPa) than previously observed in more precipitation-seasonal rainforests of eastern Amazonia and central America. We also discovered that taller trees exhibited lower embolism resistance and greater stomatal sensitivity, a height-structured trade-off between hydraulic resistance and active stomatal regulation. Such active regulation of tree water status, triggered by the onset of stem embolism, acted as a feedback to avoid further increases in embolism, and also explained declines in photosynthesis and transpiration. These results suggest that canopy trees exhibit a conservative hydraulic strategy to endure drought, with trade-offs between investment in xylem to reduce vulnerability to hydraulic failure, and active stomatal regulation to protect against low water potentials. These findings improve our understanding of strategies in tropical forest canopies and contribute to more accurate prediction of drought responses. 

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2. Hydraulically‐vulnerable trees survive on deep‐water access during droughts in a tropical forest

   https://doi.org/10.1111/nph.17464  

   Chitra‐Tarak, Rutuja ; Xu, Chonggang ; Aguilar, Salomón ; Anderson‐Teixeira, Kristina J. ; Chambers, Jeff ; Detto, Matteo ; Faybishenko, Boris ; Fisher, Rosie A. ; Knox, Ryan G. ; Koven, Charles D. ; et al ( July 2021 , New Phytologist)

   Deep‐water access is arguably the most effective, but under‐studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep‐water access may delay plant dehydration. Here, we tested the role of deep‐water access in enabling survival within a diverse tropical forest community in Panama using a novel data‐model approach.

   We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990–2015) to vapor pressure deficit, water potentials in the whole‐soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water‐access depths.

   Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress‐related mortality risk through deep‐water access.

   The role of deep‐water access in mitigating mortality of hydraulically‐vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

    

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3. Hurricanes increase tropical forest vulnerability to drought

   https://doi.org/10.1111/nph.18175  

   Smith‐Martin, Chris M. ; Muscarella, Robert ; Ankori‐Karlinsky, Roi ; Delzon, Sylvain ; Farrar, Samuel L. ; Salva‐Sauri, Melissa ; Thompson, Jill ; Zimmerman, Jess K. ; Uriarte, María ( May 2022 , New Phytologist)

   Rapid changes in climate and disturbance regimes, including droughts and hurricanes, are likely to influence tropical forests, but our understanding of the compound effects of disturbances on forest ecosystems is extremely limited. Filling this knowledge gap is necessary to elucidate the future of these ecosystems under a changing climate.

   We examined the relationship between hurricane response (damage, mortality, and resilience) and four hydraulic traits of 13 dominant woody species in a wet tropical forest subject to periodic hurricanes.

   Species with high resistance to embolisms (lowP₅₀values) and higher safety margins () were more resistant to immediate hurricane mortality and breakage, whereas species with higher hurricane resilience (rapid post‐hurricane growth) had high capacitance andP₅₀values and low . During 26 yr of post‐hurricane recovery, we found a decrease in community‐weighted mean values for traits associated with greater drought resistance (leaf turgor loss point,P₅₀, ) and an increase in capacitance, which has been linked with lower drought resistance.

   Hurricane damage favors slow‐growing, drought‐tolerant species, whereas post‐hurricane high resource conditions favor acquisitive, fast‐growing but drought‐vulnerable species, increasing forest productivity at the expense of drought tolerance and leading to higher overall forest vulnerability to drought.

    

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4. 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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5. Xylem form and function under extreme nutrient limitation: an example from California's pygmy forest

   https://doi.org/10.1111/nph.16405  

   Cary, Katharine L. ; Ranieri, Gina M. ; Pittermann, Jarmila ( February 2020 , New Phytologist)

   Xylem anatomy and function have large implications for plant growth as well as survival during drought, but the impact of nutrient limitation on xylem is not fully understood. This study examines the pygmy forest in California, a plant community that experiences negligible water stress but is severely stunted by low‐nutrient and acidic soil, to investigate how nutrient limitation affects xylem function.

   Thirteen key anatomical and hydraulic traits of stems of four species were compared between pygmy forest plants and nearby conspecifics growing on richer soil.

   Resistance to cavitation (P₅₀), a critical trait for predicting survival during drought, had highly species‐specific responses: in one species, pygmy plants had a 26% decrease in cavitation resistance compared to higher‐nutrient conspecifics, while in another species, pygmy plants had a 56% increase in cavitation resistance. Other traits responded to nutrient limitation more consistently: pygmy plants had smaller xylem conduits and higher leaf‐specific conductivity (K_L) than conspecific controls.

   Edaphic stress, even in the absence of water stress, altered xylem structure and thus had substantial impacts on water transport. Importantly, nutrient limitation shifted cavitation resistance in a species‐specific and unpredictable manner; this finding has implications for the assessment of cavitation resistance in other natural systems.

    

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