Oxygen-18 Content of Ecosystem Respiration Across a Climatic Gradient in Oregon.
Abstract
The oxygen-18 content of ecosystem respiration (d18OR) was estimated using the Keeling plot approach in 1996, 1997, 2000, and 2001 across a steep climatic gradient in Oregon, USA. Six coniferous forests located from the Pacific coastal region to the east side of the Cascade Mountains were studied, with species dominance by Picea sitchensis (two sites), Pseudotsuga menziesii (two sites), Pinus ponderosa and Juniperus occidentalis (one site each). Precipitation and leaf area index ranged from 2760 mm per year and >10.0 m2 m-2 in a coastal Picea forest to 227 mm per year and <1.0 m2 m-2 in the eastern Juniperus woodland. There were pronounced differences in d18O of water pools and respired carbon dioxide between the west and east side of the Cascade Mountains. Stem water d18O showed a continental pattern consistent with precipitation isotope patterns, being most enriched on the coast and most depleted on the east side of the Cascades. Bulk soil water d18O (top 10 cm) was depleted in the winter and became enriched in the summer, with the degree of enrichment greater on the east side than the west side of the mountains. During summer drought, d18OR was consistently more enriched on the east side (1997 and 2000 mean of 33.87 %) than the west side of the mountains (1997 and 2000 mean of 21.84 %); however, during the rainy period this pattern disappeared (19.88 and 22.89 % for the west and east sides, respectively). The more enriched d18OR on the east side during drought occurs despite the fact that annual precipitation is more negative on the east side, suggesting that greater enrichment of leaf and soil water occurs on the east side of the mountains. Indeed, for the east side sites, d18OR was strongly correlated with the daily average vapor pressure deficit summed (Sum-vpd) since the last rainfall. There was no correlation between d18OR and Sum-vpd on the west side. Possible explanations for the lack of Sum-vpd response on the west side include 1) d18O variability in precipitation may obscure the vpd-d18OR relationship, or 2) leaf-respired d18O, which is probably insensitive to Sum-vpd over multiple days, is a much greater contributor to ecosystem respiration on the west side than the east side of the mountains. In an intensive experiment at the Pinus forest, d18OR was positively correlated with daily vpd and inversely related to soil water content for the first five days after a precipitation event, and then each stabilized for the following eight days. It appears that precipitation control over d18OR is mediated by vpd in hot, dry climates, while in moist, cool climates d18OR may be much more coupled to the d18O of precipitation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.B11A..06M
- Keywords:
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- 0315 Biosphere/atmosphere interactions