Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Accueil du site → Projets de développement → Projets de recherche pour le Développement → 2014 → RESOLVING THE STRATEGIES FOR MAINTAINING TREE HYDRAULIC FUNCTION DURING DROUGHT

United States Department of Agriculture (USDA) 2014


Tree Drought

United States Department of Agriculture (USDA) Research, Education & Economics Information System (REEIS)


Identification : IDAZ-MS-0106

Pays : Etats Unis

Durée : Sep 15, 2014 à Aug 17, 2017

Mots clés : climate change ; drought ; forestry ; tree mortality


The major goals of this proposal are 1) to understand the components of dramatically contrasting strategies for the maintenance of tree hydraulic function during daily and seasonal cycles of water stress across woody species, 2) to link the strategies employed during normal daily cycles of water stress with the mechanisms of coping with severe drought stress, and 3) to characterize the trade-offs implied in reliance on different suites of traits that maintain hydraulic function. There is emerging evidence that the contrasting strategies for maintaining hydraulic function (repair versus avoidance of embolism) and the highly disparate patterns of hydraulic safety margins (wide versus narrow) observed across species are directly related, and that their resolution would represent a significant advancement in understanding how trees cope with the range of water stress from daily cycles to severe drought.The overarching hypothesis for this project is that across species, there is a continuum of strategies to maintain hydraulic function in leaves, stems and roots ranging from daily or seasonal cycles of hydraulic loss and recovery to complete avoidance of hydraulic failure during daily or seasonal cycles of water stress. Each of these strategies has associated trade-offs : current theories on embolism refilling suggest it requires metabolic energy, and embolism avoidance necessitates either stomatal closure (resulting in reduced carbon gain), structural features in the xylem that enable the tree or organ to prevent air-seeding of embolisms, or sufficient hydraulic capacitance (C) to prevent daily maximum xylem tension from exceeding the embolism threshold.

Twenty commercially- and ecologically-important western US tree species having a wide range of xylem parenchyma abundance and wood densities, and likely associated sapwood capacitance, will be selected for this study. We will also select several species that are known to lose and recover leaf and stem hydraulic function daily and several species that do not. In addition, we will select species known to occupy different positions on the iso- vs. anisohydry (i.e. stomatal sensitivity to drought) spectrum and we will characterize this behavior for species we select based on other criteria.For each species, hydraulic vulnerability curves will be determined on leaves (Sack et al. 2002 ; Scoffoni et al. 2012), branches, trunks and roots (Sperry and Saliendra 1994) from adult trees. Following determination of vulnerability curves, well-watered plants (saplings ; there is good correspondence between hydraulic parameters of saplings and adult trees of the same species) will be monitored as described below to determine organ-specific hydraulic safety margins. Water will then be withheld from groups of plants until they reach at least three species-dependent levels of stem percent loss of hydraulic conductivity (PLC) without causing mortality (e.g. 25, 50, 88%). Root and leaf PLCs corresponding to each level of stem PLC will also be determined. Following these measurements, plants will be re-watered to field capacity and recovery of hydraulic capacity will be evaluated after 12, 24 and 48 h. Loss and recovery of hydraulic capacity will be assessed with both non-invasive and invasive techniques to detect potential excision-induced artifacts. For non-invasive monitoring, sap flow and stem water potential (psychrometers, Scholz et al. 2007) will be measured continuously along with periodic measurements of water potential of transpiring and covered, non-transpiring leaves. These concurrent measurements of water flux and water potential at key points along the hydraulic continuum will allow us to partition loss and recovery of hydraulic conductance between roots, stems and leaves. For invasive monitoring, native PLC will be determined in excised roots, stems and leaves of subsets of plants using established techniques. Stem and root parenchyma volume fractions and diurnal fluctuations in root and stem non-structural carbohydrate content (Woodruff and Meinzer 2011) will also be measured. To determine whether stomatal control regulates stem water potentials over a range that maximizes the use of C, we will measure sapwood moisture release curves (Meinzer et al. 2003) to determine sapwood C over a range of water potentials. We will also measure leaf and stem water potentials using pressure chambers and stem psychrometry, whole-tree transpiration using sap flow probes, and stomatal conductance using a porometer.

Présentation : USDA

Page publiée le 23 octobre 2015, mise à jour le 25 octobre 2017