Disturbance-Induced Changes in the Oxygen Isotopic Composition of Atmospheric CO2 at High Northern LatitudesEPA Grant Number: U916000
Title: Disturbance-Induced Changes in the Oxygen Isotopic Composition of Atmospheric CO2 at High Northern Latitudes
Investigators: Welp, Lisa R.
Institution: California Institute of Technology
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 2001 through January 1, 2004
Project Amount: $102,000
RFA: STAR Graduate Fellowships (2001) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Air Quality and Air Toxics , Fellowship - Atmospheric Sciences
At high northern latitudes, interannual variation in the seasonal cycle of atmospheric C18OO exceeds that observed for 13CO2 or CO2. Shifts in species composition or climate may contribute to this variability in C18OO by influencing the timing of photosynthesis and respiration and the cycling of meteoric water. The objective of this research project is to investigate the hypothesis that increased forest fire frequency contributes to variability in C18OO by changing surface-energy partitioning, fluxes of water and CO2, and the structure and function of vegetation.
We measured the oxygen isotopic composition of ecosystem water pools (leaf, stem, soil, water vapor, and precipitation) in a fire chronosequence in interior Alaska in 2002 and 2003. Our study sites included an 80-year-old Picea mariana stand, which is close to steady state in terms of CO2 exchange, a 15-year-old Populus tremuloides stand, which currently is a significant CO2 sink, and a 3-year-old recently burned area, which is a CO2 source to the atmosphere. We predicted differences in diurnal and seasonal C18OO fluxes (isofluxes) between sites by incorporating net CO2 eddy flux and micrometeorological measurements with oxygen isotopic signatures of ecosystem water pools, local background atmospheric CO2, and nighttime ecosystem respiration. Using estimates of these seasonal isofluxes at each site, a simple atmospheric two-box model was constructed to determine the extent of depletion of atmospheric C18OO (either increased or decreased from the mature control case) and changes in the shape of the seasonal cycle of C18OO that would accompany shifts in the disturbance regime. Initial results have shown that although much smaller gross CO2 fluxes are observed at the 3-year-old recent burn than the fast growing 15-year intermediate aspen stand, and one is a net source while the other is a net sink, both forests have a similar effect on the cumulative annual isoflux to the atmosphere. Both disturbed forests led to a decreased depletion of C18OO compared to the 80-year-old mature spruce forest, a delay in the phase of the CO2 seasonal cycle, and an advance in the C18OO maximum.