Elevated CO₂ did not affect the hydrological balance of a mature native Eucalyptus woodland

Abstract

Elevated atmospheric CO concentration (eC ) might reduce forest water-use, due to decreased transpiration, following partial stomatal closure, thus enhancing water-use efficiency and productivity at low water availability. If evapotranspiration (E ) is reduced, it may subsequently increase soil water storage (ΔS) or surface runoff (R) and drainage (D ), although these could be offset or even reversed by changes in vegetation structure, mainly increased leaf area index (L). To understand the effect of eC in a water-limited ecosystem, we tested whether 2 years of eC (~40% increase) affected the hydrological partitioning in a mature water-limited Eucalyptus woodland exposed to Free-Air CO Enrichment (FACE). This timeframe allowed us to evaluate whether physiological effects of eC reduced stand water-use irrespective of L, which was unaffected by eC in this timeframe. We hypothesized that eC would reduce tree-canopy transpiration (E ), but excess water from reduced E would be lost via increased soil evaporation and understory transpiration (E ) with no increase in ΔS, R or D . We computed E , ΔS, R and D from measurements of sapflow velocity, L, soil water content (θ), understory micrometeorology, throughfall and stemflow. We found that eC did not affect E , E , ΔS or θ at any depth (to 4.5 m) over the experimental period. We closed the water balance for dry seasons with no differences in the partitioning to R and D between C levels. Soil temperature and θ were the main drivers of E while vapour pressure deficit-controlled E , though eC did not significantly affect any of these relationships. Our results suggest that in the short-term, eC does not significantly affect ecosystem water-use at this site. We conclude that water-savings under eC mediated by either direct effects on plant transpiration or by indirect effects via changes in L or soil moisture availability are unlikely in water-limited mature eucalypt woodlands.

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

“…Therefore, assuming that wood construction costs remain stable between treatments, similar stem growth rates should result in similar growth respiration. Likewise, assuming similar sapwood volume (Gimeno et al, ) and bark thickness (measured at the time of NDIR probe insertion) between treatments, an equal amount of stem living cells might also result in equal maintenance respiration under ambient and eCO 2 , as similarly observed at the leaf level (Crous et al in preparation). On the other hand, a modest increase in R S under eCO 2 could be expected at EucFACE due to (a) enhanced C translocation belowground (Drake et al, ; Hasegawa et al, ) and therefore stimulated phloem function or due to (b) increased soil nutrient availability (Hasegawa et al, ; Ochoa‐Hueso et al, ) from which stem living tissues could intensify their respiratory activity.…”

“…Vertical CO 2 flux through the xylem (F T, μmol CO 2 m −3 s −1 ), the storage flux (ΔS , μmol CO 2 m −3 s −1 ), and total stem respiration (R S , μmol CO 2 m −3 s −1 ) was estimated following the mass balance approach proposed by McGuire and Teskey (): where F H20 is tree sap flow (L s −1 ), Δ[CO 2 * ] (μmol CO 2 L −1 ) is the difference of sap [CO 2 * ] measured above and below the stem collar, V is the sapwood volume (m 3 ) of the stem segment delimited by the NDIR probes (m 3 ), ΔS is the storage flux that accounts for the build‐up of CO 2 within the monitored stem segment, [CO 2 * ] Ti and [CO 2 * ] Ti‐1 is the mean of sap [CO 2 * ] above and below the collar at i and i − 1 moment, respectively, SWC is the stem water content (L m −3 , obtained from woody samples of trees located outside the rings, Gimeno et al, ), t is the elapsed time between consecutive measurements (s), and E A_V is stem CO 2 efflux on a volume basis (μmol CO 2 m −3 s −1 ).…”

“…To obtain continuous E A_V data from discrete E A_S measurements, E A_S was first modelled as a function of temperature as described in Equations (1) and (2). Second, continuous E A_S was expressed on a volume basis according to allometric properties of the tree obtained from cut sections of trees used for sap flow calculations (Gimeno et al, ) following Rodríguez‐Calcerrada et al (): where r t is the tree radius (including heartwood, sapwood, and bark) and r h is heartwood radius.…”

“…A site specific calibration was performed to convert the raw readings of the neutron probe to VWC. Gimeno et al () present further details on access tube installation and VWC calibration.…”

Lower photorespiration in elevated CO2 reduces leaf N concentrations in mature Eucalyptus trees in the field

“…Annual mean precipitation is about 800 mm (1881–2018, BOM; station: 067021). The site was relatively moist in 2012–2013 and between mid‐2014 and mid‐2017 (Gimeno et al, ; Gimeno, McVicar, O'Grady, Tissue, & Ellsworth, ; Pathare et al, ). The soil is a nutrient‐poor loamy sand (Aeric yellow podosol) and phosphorus‐limited, with foliar nutrition described in Crous, Ósvaldsson, and Ellsworth ().…”