Abstract
A remarkable increase in vapor pressure deficit (VPD) has been recorded in the last decades in relation to global warming. Higher VPD generally leads to stomatal closure and limitations to leaf carbon uptake. Assessing tree conductance responses to VPD is a key step for modeling plant performances and productivity under future environmental conditions, especially when trees are cultivated well outside their native range as for hazelnut (Corylus spp.). Our main aim is to assess the stand-level surface canopy conductance (Gsurf) responses to VPD in hazelnut across different continents to provide a proxy for potential productivity. Tree sap flow (Fd) was measured by Thermal dissipation probes (TDP) probes (six per sites) in eight hazelnut orchards in France, Italy, Georgia, Australia, and Chile during three growing seasons since 2016, together with the main meteorological parameters. We extracted diurnal Fd to estimate the canopy conductance Gsurf.. In all the sites, the maximum Gsurf occurred at low values of VPD (on average 0.57 kPa) showing that hazelnut promptly avoids leaf dehydration and that maximum leaf gas exchange is limited at relatively low VPD (i.e., often less than 1 kPa). The sensitivity of the conductance vs. VPD (i.e., -dG/dlnVPD) resulted much lower (average m = −0.36) compared to other tree species, with little differences among sites. We identified a range of suboptimal VPD conditions for Gsurf maximization (Gsurf > 80% compared to maximum) in each site, named “VPD80,” which multiplied by the mean Gsurf might be used as a proxy for assessing the maximum gas exchange of the orchard with a specific management and site. Potential gas exchange appeared relatively constant in most of the sites except in France (much higher) and in the driest Australian site (much lower). This study assessed the sensitivity of hazelnut to VPD and proposed a simple proxy for predicting the potential gas exchange in different areas. Our results can be used for defining suitability maps based on average VPD conditions, thus facilitating correct identification of the potentially most productive sites.
Highlights
The role of the leaf to air vapor pressure deficit (VPD) is increasingly recognized as a leading limiting factor to determine plant gas exchange (Novick et al, 2016; Grossiord et al, 2020), while the influence of climate change on VPD rise becomes more and more evident
We explored the relationship between VPD and whole tree conductance Gsurf as a contribution to understanding the limitations of potential productivity in eight commercial hazelnut orchards distributed in different countries
The remaining difference can be explained by the soil texture, which is, for example, high in sandy soil in site A1
Summary
The role of the leaf to air vapor pressure deficit (VPD) is increasingly recognized as a leading limiting factor to determine plant gas exchange (Novick et al, 2016; Grossiord et al, 2020), while the influence of climate change on VPD rise becomes more and more evident. Higher VPD is in general detrimental for plant productivity because of the negative exponential response of stomatal conductance gs to increasing VPD (Schulze et al, 1972; Jones, 1992; Oren et al, 1999). This response limits plant dehydration, but, at the same time, reduces carbon assimilation once stomata are partially closed. This explains why VPD was identified as a climatic factor related to tree mortality (McDowell et al, 2013) and limits crop productivity (Eamus et al, 2013). Because plants are at the base of human life, the impact of changing climate and VPD rise on food provided by plants are a main issue in the upcoming future
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