HR: 09:15h
AN: B41A-06    [PDF]
TI: Seasonal Variation in the Carbon Isotope Ratio of Ecosystem Respiration in Two Coniferous Forests
AU: * McDowell, N G
EM: mcdowell@lanl.gov
AF: Earth and Environmental Sciences, MS-D462 Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Bowling, D
EM: bowling@biology.utah.edu
AF: Department of Biology, University of Utah 257 South 1400 East, Salt Lake City, UT 84112 United States
AU: Schauer, A
EM: schauer@biology.utah.edu
AF: Department of Biology, University of Utah 257 South 1400 East, Salt Lake City, UT 84112 United States
AU: Irvine, J
EM: james.irvine@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AU: Bond, B J
EM: barbara.bond@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AU: Law, B
EM: bev.law@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AU: Ehleringer, J R
EM: ehleringer@biology.utah.edu
AF: Department of Biology, University of Utah 257 South 1400 East, Salt Lake City, UT 84112 United States
AB: We examined weekly variation in the stable carbon isotope signature of ecosystem respiration ($\delta ^{13}$C$_{R}$) using the Keeling plot approach at two forests in Oregon USA: a wet ($>$2300 mm annual precipitation) 20-year-old {\it Pseudotsuga menziesii} plantation located in the Coast Range near the Pacific Ocean, and a dry ($\sim$520 mm annual precipitation) 250-year-old {\it Pinus ponderosa} forest located on the eastern slope of the Cascade Mountains. The two forests experience similar regional weather patterns with wet winters and dry summers, but the coastal site has milder temperatures and greater soil and atmospheric water content. Air was sampled on 51 and 42 separate nights at the {\it Pinus} and {\it Pseudotsuga} forests, respectively, between 2001 and 2002. Both forests exhibited greater intra-annual variation in $\delta ^{13}$C$_{R}$ than has been previously observed in C3 ecosystems ($>$8.0 $\permil$ over the year). Mean annual $\delta ^{13}$C$_{R}$ matched that expected based on annual precipitation, averaging -25.4 $\permil$ at the {\it Pinus} forest and -26.2 $\permil$ at the {\it Pseudotsuga} forest. Variability in $\delta ^{13}$C$_{R}$ at both sites was highest during seasons when rainfall was abundant (autumn, winter and spring) and lowest during summer drought. During the period of drought, soil temperature was positively correlated with $\delta ^{13}$C$_{R}$ at both forests. When all seasons were analyzed, $\delta ^{13}$C$_{R}$ was negatively correlated with soil water content at both forests. The {\it Pseudotsuga} forest was more strongly coupled to soil and atmospheric water content than the {\it Pinus} forest. This difference could be related to availability of a deep water source at the {\it Pinus} forest that buffers it from drought effects. $\delta ^{13}$C$_{R}$ at the {\it Pinus} forest was significantly but weakly related to canopy conductance suggesting that $\delta ^{13}$C$_{R}$ is coupled to canopy gas exchange. $\delta ^{13}$C$_{R}$ was significantly correlated between the two forests after removal of outliers associated with extreme, site-specific meteorological events (i.e. local freezes), demonstrating that $\delta ^{13}$C$_{R}$ is coupled across forests located $>$100 km apart. This coupling is presumably due to shared regional climate.
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2003 Fall Meeting