OZONE UPTAKE IN RELATION TO WATER AVAILABILITY IN PONDEROSA PINE FORESTS: MEASUREMENTS, MODELING, AND REMOTE-SENSING

Panek, Jeanne; Ustin, Susan

Ozone is a ubiquitous and toxic air pollutant which threatens forest ecosystem health throughout the United States and much of the rest of the industrialized world. To cause injury, ozone must enter leaves through stomatal pores, which open and close under plant control to regulate gas exchange and water loss. High stomatal conductance, and thus high ozone uptake rates, have been shown recently to be strongly correlated with high soil moisture in drought-adapted forest ecosystems such as those in California. Currently little is understood about temporal and spatial variability in stomatal conductance in these systems and how this variability affects ozone uptake, especially in response to the heterogeneous distribution of water across the landscape. In this sense, remote sensing can help in detecting vegetation physiological parameters to place the spatial variability of water stress and physiological conditions and canopy water content into a spatial context. In this study, physiological response of pine in mesic riparian locations having year round access to water was compared with response of pine in upslope dry locations which are drought-stressed during the growing season, repeated in 2 locations – Yosemite Valley and King’s Canyon, CA. Differences in transpiration and stomatal conductance, and the consequent differences in ozone uptake were measured. Sap velocity techniques were used to measure whole-tree-level responses to changes in soilwater availability. A portable photosynthesis system was used to measure leaf-level response. A physiological model was tested for adequacy in extrapolating to the landscape. From the “in situ” field studies, while all sites showed a decline in transpiration, stomatal conductance, and ozone uptake over the growing season drought period, the mesic riparian site showed the least decline and thus the greatest ozone uptake of all the sites, as hypothesized. Soil moisture measurements at 50 cm were found to be inadequate at measuring water accessible to pine, because pine can tap into water at greater depths than can be monitored. Water potential measurements showed that the mesic, riparian Yosemite site was the only unstressed site throughout the growing season drought. The riparian site in King’s Canyon, although close to the King’s River, became drought stressed by the end of the growing season. Thus close proximity to water is not a useful indicator of year-round access to water. Because of the discrepancy between measured and actual soil water availability, canopy water stress must be assessed by either direct water potential measurement or through remote sensing techniques, not by monitoring soil moisture. The reduction in stomatal conductance and ozone uptake over the season due to drought stress was significant at the dry upslope sites, but greatest at the King’s Canyon site. The effect of drought on the King’s Canyon pine was to cause a reduction in ozone uptake of 80-90% over the growing season. The effect of drought on dry upslope Yosemite pine was a reduction in ozone uptake of around 50%. Total annual ozone uptake at the mesic Yosemite site was 33% greater than the dry upslope site. In the late summer, daily peak ozone uptake at the mesic Yosemite site was 46% higher than uptake at the dry site. STOMATA, a physiological model of gas exchange developed for Sierra Nevada pine forests, predicted annual and diurnal variability with reasonable accuracy at the Yosemite sites, once soil moisture at the lower site was modified upwards to better represent actual available soil moisture there. Thus, it can be used as a tool for extrapolating physiological response to drought across the Sierra Nevada. Remote sensing analyses were conducted using data from NASA’s Advanced Visible Infrared Imaging Spectrometer (AVIRIS).

OZONE UPTAKE IN RELATION TO WATER AVAILABILITY IN PONDEROSA PINE FORESTS: MEASUREMENTS, MODELING, AND REMOTE-SENSING

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