Research Program Area: Ecosystem & Multimedia Effects
Trees of a commercial cultivar of plum were grown under commercial management protocol including annual pruning to the "open-vase" morphology, as part of a larger ozone exposure study using open-top-chambers (OTCs). Intensive microenvironmental and physiological measurements were made on the trees subjected to ambient (non-filtered and non-ozone enriched) air inside and outside of the OTC, i.e., the Ambient Control and Non-Chamber Control of the larger study. Concentrations of ozone, carbon dioxide, and water vapor were obtained within the canopy by sampling air from each of nine positions, comprising a matrix from interior to exterior and from top to bottom of the active canopy. Leaf and air temperature, photosyntheticalIy active photon flux density, and wind speed were also obtained from this matrix.
Two upper interior canopy positions were averaged, and two upper exterior canopy positions were averaged, inside and outside the OTC to provide four well-characterized canopy zones. Gas exchange measurements were made in these interior and exterior zones of individual trees inside and outside of the chambers. Transpiration by each canopy zone was measured with heat balance sap flow gauges.
Objectives were to 1) map the distribution of microenvironmental parameters within canopies inside and outside the OTC to quantify canopy and OTC effects on exchange with the atmosphere and on cross-canopy gradients, 2) use redundant measurements of transpirational water flux (as vapor and liquid) to determine transport parameters for exchange of gases between the atmosphere and leaves in contrasting positions in the canopies, and 3) develop a protocol to calculate ozone flux (effective dose, uptake) by canopy region to rationalize future and past ozone yield response studies using diverse OTC and non-chamber exposure protocols on a common basis of whole-canopy ozone uptake.
Significant differences in bulk (measured above the canopy) gas concentrations and micro- meteorological parameters were observed between inside and outside the OTC. Significant vertical and horizontial gradients in these parameters were observed within the canopies, despite the open canopy architecture of the pruned trees. The gradients were diminished and often inverted inside, relative to outside, the OTC, due to air distribution at the bottom of the OTC, as opposed to entry from the canopy top outside the OTC. More uniform exposure of leaves, fruiting sites and growing points throughout the canopy, to the ozone concentration entering the canopy, occurred inside the OTC than outside. Transpiration and ozone uptake by interior leaves was greater inside than outside the OTC, though the largest values were observed in the exterior zone of the outside tree. Canopy-averaged stomatal conductance was greater inside than outside the OTC, and accounted for only 59% of the total transport resistance for gas exchange, compared to 79% outside, resulting in greater stomatal control of transpiration and ozone flux in the outside tree. Total ozone uptake by the canopy was greater outside the OTC than inside, due to dominance of the exterior of the outside tree and the reduction of ozone concentrations inside the chamber by distribution losses. Penetration of ambient ozone concentrations to the leaf surfaces would have reversed this pattern.
As suggested by others, expression of ozone exposure as a dose is physiologically more significant, and more process-based for modelling applications, than as an ambient ozone concentration, particularly measured as a bulk parameter outside the canopy. In the present case these techniques unified into a single linear relationship the yield-exposure data from the OTC and Non-Chamber treatments that had previously fallen on separate relationships when expressed as ozone concentration. Specification of transport resistances to several canopy locations should be required of all future OTC exposure studies. A catalog of transport parameters associated with contrasting canopy morphologies and OTC designs could be compiled to reduce the need for expensive micrometeorological measurements in each individual study, and to facilitate post-hoc, meta-analyses of all ozone response studies undertaken to date.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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