Grantee Research Project Results
2000 Progress Report: Modeling Ozone Flux to Forests Across an Ozone Concentration Gradient in the Sierra Nevada Mountains, CA.
EPA Grant Number: R826601Title: Modeling Ozone Flux to Forests Across an Ozone Concentration Gradient in the Sierra Nevada Mountains, CA.
Investigators: Goldstein, Allen H. , Panek, Jeanne A.
Institution: University of California - Berkeley
EPA Project Officer: Chung, Serena
Project Period: August 1, 1998 through October 31, 2002
Project Period Covered by this Report: August 1, 1999 through October 31, 2000
Project Amount: $621,367
RFA: Ecological Indicators (1998) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Aquatic Ecosystems
Objective:
Tropospheric ozone is a pollutant that is responsible for forest damage worldwide. Currently, ozone exposure in forests of the United States and Europe is estimated from ozone concentrations, but because ozone must enter the foliage through stomatal pores to cause damage, ozone flux into the foliage?not ambient ozone concentration?is the physiologically relevant ozone metric. We are measuring ozone flux both directly (through eddy covariance methods) and indirectly (from measures of leaf-level stomatal conductance) to ponderosa pine forests in the Sierra Nevada Mts., CA, and developing a model to estimate ozone flux which we hope to adapt for monitoring networks utilizing routinely-measured ozone concentrations and meteorology. We are also exploring the utility of d13C as a proxy for stomatal conductance to estimate ozone flux. The results of this research will contribute to the efficacy of forest health monitoring and enhance the general understanding of the relationship between ozone exposure and forest response in California ponderosa pine ecosystems.Progress Summary:
We have completed 2 years of continuous meteorological measurements, and two full growing seasons of measurements of tree growth, tree water potential, leaf level physiology (stomatal conductance, net photosynthesis, transpiration, dark respiration), and d13C at each of the 4 sites along an ozone concentration gradient transect. We continue to measure fluxes of CO2, water vapor, ozone and energy at the canopy level using eddy covariance at one site. The growing season operation of this facility has been expanded to year-round operation. We have explored the relationships between environmental variables and physiological response that control ozone flux into pine. We have found the expected relationships are very strong and should be straightforward to model. Model development has begun.
Results: Leaf Level. Soil moisture dropped progressively at all sites from May through September. Maximum stomatal conductance also dropped at all sites over that time period. The shape of the diurnal trend in stomatal conductance indicated that during late summer water stress, stomata opened briefly only in the morning. Stomata are the pathway of ozone into foliage. Ozone fluxes were much smaller at the end of the summer when conductances were at a minimum than the beginning, although ozone concentrations were higher, demonstrating a decoupling between ambient ozone concentration and ozone flux (Figure 1).
Figure 1. Ozone flux-calculated from ozone concentration x stomatal conductance to ozone-dropped progressively over the season. Ozone concentration, however, increased over the season. The Sequoia site is shown here as an example, but this decoupling of ozone flux from concentration is the same at all sites.
Environmental moisture controls stomatal conductance, and thus ozone flux, in a predictable manner. We found the expected relationships between soil moisture, tree water potential, and stomatal conductance (Figure 2).
Figure 2. a) Stomatal conductance daily maximum is strongly correlated with predawn water potential. b) Predawn water potential in pine foliage is strongly correlated with soil moisture. Although water potential is difficult to measure, the continuous measurement of soil moisture is easy to automate.
Results: Eddy Covariance. Ozone concentration and deposition at the canopy scale were decoupled seasonally, resulting in substantial ozone deposition in winter even when ozone concentration was relatively low. Cumulative ozone flux (Table 1) was highest in the summer but did not make up the majority of annual ozone deposition. Most studies on ozone deposition have focused on the summer months because that is when ozone concentration is typically the highest. Our work shows that two thirds of annual ozone deposition occurred during non-summer months. Furthermore, during the winter when plant activity is reduced, the trees should have lowered defense systems thus potentially making them more susceptible to ozone damage. While it is likely that this relatively high non-summer ozone deposition resulted from stomatal regulation by the ponderosa pine trees rather than deposition to other surfaces, year-round leaf-level physiological studies are necessary to test this hypothesis.
Table 1.
|
Days |
O3 Conc ppb |
O3 Flux umol/m2/hr |
O3 Vd cm/s |
Cum. O3 Flux mmol/m2/h |
Cum. O3 Flux % |
Summer 1999 |
153-243 |
65.5 |
38.3 |
0.47 |
42.35 |
37.21 |
Fall 1999 |
244-334 |
54.9 |
21.0 |
0.33 |
22.73 |
19.97 |
Winter 2000 |
335-60 |
37.0 |
16.2 |
0.35 |
17.73 |
15.58 |
Spring 2000 |
61-151 |
48.6 |
25.9 |
0.41 |
31.00 |
27.24 |
The pattern of ozone deposition changed throughout the year based as a result of changes in environmental variables and phenology. Certain environmental variables were important for controlling deposition at certain times of the year or only in combination with other environmental variables. For example, soil moisture was an important variable controlling ozone flux in the late summer and early fall when the ecosystem was very dry, but not at any other time of year. Vapor Pressure Deficit was found to exert strong control over ozone flux only when the ecosystem was dry, after soil moisture at 10 cm dropped below 12-13 percent. The onset of the rainy season in Fall was accompanied by an immediate decrease in ozone concentration and an increase in both ozone flux and ozone deposition velocity?this occurred at a time that plants in this region are generally assumed to have lower levels of activity. However, our CO2 flux data confirmed that the trees were still active, although at lower levels, and suggests that stomatal conductance increased immediately following rainfall.
To understand the controls on ozone deposition and the effects of ozone on
this and other ecosystems that remain physiologically active in non-summer
months, it is critical to quantify ozone deposition on a year-round basis. To
further investigate the year-round controls on ozone deposition we have set up
sap flow measurements as a means to monitor stomatal conductance (and hence gas
exchange) at the tree level and are beginning to parameterize a biophysical
ecosystem model.
Future Activities:
We will continue leaf-level monitoring along the transect through September 2001. Year-round eddy covariance measurements have shown that trees can remain active through the winter and that winter ozone fluxes are significant. Therefore, we have included winter-time sampling in the transect work as well. We will compare models that estimate stomatal conductance and ozone flux into foliage and evaluate them for their appropriateness in the Sierra Nevada ponderosa pine ecosystem. We will further develop the selected model to apply to our specific needs and compare modeled ozone deposition with direct measurements of canopy scale ozone deposition at Blodgett Forest.Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 19 publications | 10 publications in selected types | All 8 journal articles |
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Type | Citation | ||
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Bauer MR, Hultman NE, Panek JA, Goldstein AH. Ozone deposition to a ponderosa pine plantation in the Sierra Nevada Mountains (CA): a comparison of two different climatic years. Journal of Geophysical Research 2000;105(D17):22,123-22,136. |
R826601 (2000) R826601 (Final) |
Exit Exit |
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Goldstein AH, Hultman NE, Fracheboud JM, Bauer MR, Panek JA, Xu M, Qi Y, Guenther AB, Baugh W. Effects of climate variability on the carbon dioxide, water, and sensible heat fluxes above a ponderosa pine plantation in the Sierra Nevada (CA). Agricultural and Forest Meteorology 2000;101(2-3):113-129. |
R826601 (1999) R826601 (2000) R826601 (Final) |
Exit Exit |
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Panek JA, Goldstein AH. Response of stomatal conductance to drought in ponderosa pine: implications for carbon and ozone uptake. Tree Physiology 2001;21(5):337-344. |
R826601 (1999) R826601 (2000) R826601 (2001) R826601 (Final) |
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Supplemental Keywords:
ozone damage, modeling forest response, carbon isotopes, forest physiological processes, carbon cycling, water cycling, ozone deposition, pollution stress, forest health monitoring., RFA, Scientific Discipline, Air, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Ecology, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, State, Forestry, Ecological Effects - Environmental Exposure & Risk, tropospheric ozone, Ecological Indicators, stressors, meteorology, forest ecosystems, ozone, ecosystem indicators, carbon storage, forests, pine trees, Sierra Nevada Mountains, atmospheric contaminants, California (CA), meteorological fluctuationsRelevant Websites:
http://www-cbe2.ced.berkeley.edu/panek/
http://www.CNR.Berkeley.EDU/~ahg/
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.