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1. Temporal controls on crown nonstructural carbohydrates in southwestern US tree species

   https://doi.org/10.1093/treephys/tpaa149  

   Peltier, Drew M ; Guo, Jessica ; Nguyen, Phiyen ; Bangs, Michael ; Gear, Linnea ; Wilson, Michelle ; Jefferys, Stacy ; Samuels-Crow, Kimberly ; Yocom, Larissa L ; Liu, Yao ; et al ( November 2020 , Tree Physiology)

   Meinzer, Frederick (Ed.)

   Abstract In trees, large uncertainties remain in how nonstructural carbohydrates (NSCs) respond to variation in water availability in natural, intact ecosystems. Variation in NSC pools reflects temporal fluctuations in supply and demand, as well as physiological coordination across tree organs in ways that differ across species and NSC fractions (e.g., soluble sugars vs starch). Using landscape-scale crown (leaves and twigs) NSC concentration measurements in three foundation tree species (Populus tremuloides, Pinus edulis, Juniperus osteosperma), we evaluated in situ, seasonal variation in NSC responses to moisture stress on three timescales: short-term (via predawn water potential), seasonal (via leaf δ13C) and annual (via current year’s ring width index). Crown NSC responses to moisture stress appeared to depend on hydraulic strategy, where J. osteosperma appears to regulate osmotic potentials (via higher sugar concentrations), P. edulis NSC responses suggest respiratory depletion and P. tremuloides responses were consistent with direct sink limitations. We also show that overly simplistic models can mask seasonal and tissue variation in NSC responses, as well as strong interactions among moisture stress at different timescales. In general, our results suggest large seasonal variation in crown NSC concentrations reflecting the multiple cofunctions of NSCs in plant tissues, including storage, growth and osmotic regulation of hydraulically vulnerable leaves. We emphasize that crown NSC pool size cannot be viewed as a simple physiological metric of stress; in situ NSC dynamics are complex, varying temporally, across species, among NSC fractions and among tissue types. 

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2. Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

   https://doi.org/10.1111/pce.13543  

   Fu, Xiaoli ; Meinzer, Frederick C. ; Woodruff, David R. ; Liu, Yan‐Yan ; Smith, Duncan D. ; McCulloh, Katherine A. ; Howard, Ava R. ( March 2019 , Plant, Cell & Environment)

   The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.

    

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3. Effects of plant hydraulic traits on the flammability of live fine canopy fuels

   https://doi.org/10.1111/1365-2435.13771  

   Scarff, Fiona R. ; Lenz, Tanja ; Richards, Anna E. ; Zanne, Amy E. ; Wright, Ian J. ; Westoby, Mark ; McCulloh, ed., Katherine ( February 2021 , Functional Ecology)

   Plant species vary in how they regulate moisture and this has implications for their flammability during wildfires. We explored how fuel moisture is shaped by variation within five hydraulic traits: saturated moisture content, cell wall rigidity, cell solute potential, symplastic water fraction and tissue capacitance.

   Using pressure–volume curves, we measured these hydraulic traits in twigs and distal shoots (i.e. twigs + leaves) in 62 plant species across four wooded communities in south‐eastern Australia. Moisture content of fine fuels was then estimated for circumstances typical of fire weather. These projections were made assuming that under the hot, dry, windy conditions typical of large wildfires, leaves and fine twigs would function at internal water pressures close to wilting point (i.e. turgor loss point, TLP). The effect of different moisture contents at TLP on ignition time was then modelled using a fully mechanistic, finite element model of biomass ignition based on standard principles of physical chemistry.

   We also measured predawn water potential, an indication of plant access to soil water that is influenced by root architecture. These data were used to model how root traits influence fuel moisture and ignition time.

   Most variation among species in fuel moisture under fire weather conditions arose from differences in saturated moisture content (3.4‐ to 3.6‐fold variation). Twig capacitance was also an important driver of fuel moisture under these weather conditions (1.9‐ to 2.2‐fold variation in moisture content). A suite of other leaf and root traits influencing how much shoots dry out as they approach wilting point each contributed 1.0‐ to 1.6‐fold variation in projected fuel moisture during fire weather. Fuel moisture variation in turn drove variation in flammability by modifying predicted ignition time.

   Two main life‐history types in fire‐prone habitats are obligate seeders and resprouters. There were no significant differences between these species groups in estimated fuel moisture during fire weather, nor in any measured hydraulic traits.

   Live fuel moisture is an important determinant of wildfire activity. Our data show that variation in tissue saturated moisture content among co‐occurring species represents an important ecological store of variation in flammability in the study communities.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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4. Influence of Vertical Heterogeneities in the Canopy Microenvironment on Interannual Variability of Carbon Uptake in Temperate Deciduous Forests

   https://doi.org/10.1029/2020JG005658  

   Wozniak, M. C. ; Bonan, G. B. ; Keppel‐Aleks, G. ; Steiner, A. L. ( August 2020 , Journal of Geophysical Research: Biogeosciences)

   Vegetation structure and function are key design choices in terrestrial models that affect the relationship between carbon uptake and environmental drivers. Here, we investigate how representing canopy vertical structure in a terrestrial biosphere model—that is, micrometeorological, leaf area, and leaf water profiles—influences carbon uptake at five U.S. temperate deciduous forest sites in July. Specifically, we test whether the interannual variability (IAV) of gross primary productivity (GPP) responds differently to four abiotic environmental drivers—air temperature, relative humidity, incoming shortwave radiation, and soil moisture—using either a Community Land Model multilayer canopy model (CLM‐ml) or a big‐leaf model (CLM4.5/CLM5). We conclude that vertical leaf area and microclimatic profiles (temperature, humidity, and wind) do not impact GPP IAV compared to a single‐layer model when plant hydraulics is excluded. However, with a mechanistic representation of plant hydraulics there is vertically varying water stress in CLM‐ml, and the sensitivity of carbon uptake to particular climate variables changes with height, resulting in dampened canopy‐scale GPP IAV relative to CLM4.5. Dampening is due to both a reduced dependence on soil moisture and opposing climatic forcing on different leaf layers. Such dampening is not evident in the single‐layer representation of plant hydraulic water stress implemented in the recently released CLM5. Overall, both model representations of the canopy fail to accurately simulate observed GPP IAV and this may be related by their inability to capture the upper range of observed hourly GPP and diffuse light‐GPP relationships that cannot be resolved by canopy structure alone.

    

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5. Deconstructing precipitation variability: Rainfall event size and timing uniquely alter ecosystem dynamics

   https://doi.org/10.1111/1365-2745.13724  

   Griffin‐Nolan, Robert J. ; Slette, Ingrid J. ; Knapp, Alan K. ( July 2021 , Journal of Ecology)

   Water‐limited ecosystems are highly sensitive to not only precipitation amount, but also precipitation pattern, particularly variability in the size and timing of growing season rainfall events. Both rainfall event size and timing are expected to be altered by climate change, but the relative responses of dryland ecosystems to changes in rainfall event size versus timing have not been resolved. Here, we disentangle the effects of these different aspects of precipitation pattern on ecosystem dynamics.

   We experimentally assessed how these two aspects of rainfall variability impacted a semi‐arid grassland ecosystem by altering an ambient precipitation pattern to eliminate variability in (a) rainfall event size (all events were made the same size), (b) rainfall event timing (all events were uniformly spaced in time) and (c) both. Total precipitation amount was constant for all treatments. We measured responses of soil moisture, ecosystem carbon flux (e.g. net primary production and soil CO₂flux), plant community composition and physiological responses of the dominant C₄grass,Bouteloua gracilis.

   Removing variability in rainfall event size altered ecosystem dynamics more than a pattern of uniform event timing, but the largest impact occurred when variability in both were removed. Notably, eliminating variability in both event size and timing increased above‐ground net primary productivity by 23%, consistent with reduced water stress in the dominant C₄grass, while also reducing seasonal variability in soil CO₂flux by 35%, reflecting lower seasonal variability in soil moisture.

   Synthesis. Unique responses to different aspects of precipitation variability highlight the complexity of predicting how dryland ecosystems will be affected by climate change‐induced shifts in rainfall patterns. Our results provide novel support for the key roles of rainfall event size and timing, in addition to total precipitation amount, as determinants of ecosystem function.

    

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