Measuring the Effects of Disturbance & Climate on the CO2 & Energy Exchange of Ponderosa Pine Forests in the Pacific Northwest: Integration of Eddy Flux, Plant and Soil Measurements Page: 2 of 10
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RESULTS TO DATE:
Objective 1: Investigate the effects of logging and fire on carbon storage and carbon
dioxide and energy exchange in chronosequences of ponderosa pine, using consistent
Heterotrophic respiration two years after wildfire was not significantly different from that
of unburned forests, and there was a consistent relationship between annual soil
respiration and aboveground NPP across burned and unburned forests. Based on
continuous seasonal measurements of soil respiration in a severely burnt plot, in areas
kept free of ground vegetation, soil heterotrophic respiration accounted for 56% of total
soil CO2 efflux, comparable to the values previously reported for two of the unburnt
forest plots. Estimates of total ecosystem heterotrophic respiration (Rh) were not
significantly different between plot types two years after fire. The ratio Rh/NPP averaged
1.85, 1.16 and 0.85 in the severely burnt, moderately burnt and unburnt plots
respectively. Annual soil CO2 efflux was linearly related to above ground net primary
productivity (ANPP) with an increase in soil CO2 efflux of 1.48gC y-1 for every lg
increase in ANPP (p<0.01, r2=0.76). There was no significant difference in this
relationship between the recently burnt and unburnt plots.
Severely burnt forests were a significant source of COz to the atmosphere two years after
wildfire, and carbon pools were significantly lower than unburned plots (Irvine et al. in
review): We measured carbon pools and fluxes in moderate and severely burnt forest
stands two years after a fire in semi-arid ponderosa pine forests near the burnt flux site to
determine the controls on net ecosystem productivity (NEP) and make comparisons with
unburned stands in the same region. Soil carbon to 1 m depth was the largest, most stable
pool, with no significant differences between plot categories. Carbon stored in the bole of
living trees accounted for 37%, 31% and 2% of total ecosystem carbon in the unburned,
moderately burnt and severely burnt plots respectively. Total ecosystem carbon in soil
and live and dead pools in the burnt stands was on average 63% that of unburned stands
(9.3 and 14.7 kgC m-2 respectively, p<0.01). NEP was significantly lower in burnt
compared to unburned stands (p<0.01) with an increasing trend from -143 44 gC m-2 y-i
( 1 SD) in severely burnt plots (stand replacing fire), to -44 96 and +141 111 gC m-2 -
'in moderately burnt and unburned plots respectively. Net primary production NPP (gC
m-2 y-1, 1 SD) of severely burnt plots was 47% of unburned plots (167 76, 346 148,
respectively p<0.05), with forb and grass above ground NPP accounting for 74% and 4%
of total above ground NPP respectively. Contrary to expectations that the magnitude of
NEP two years post fire would be principally driven by the sudden increase in detrital
pools and increased rates of Rh, the data suggests NPP was more important in
determining post-fire NEP (Irvine et al. in review). Based on biometry data, the burnt
flux site was a net carbon loss of 136gC y-1 two years after the fire (Irvine et al. in
review, plot 601). The rates of decomposition of below-ground detritus and standing
charred trees, in conjunction with the time-course of vegetation succession, will play a
vital role in the timing of transition of the burnt flux site from carbon source to sink.
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Law, Beverly E. & Mahrt, Larry. Measuring the Effects of Disturbance & Climate on the CO2 & Energy Exchange of Ponderosa Pine Forests in the Pacific Northwest: Integration of Eddy Flux, Plant and Soil Measurements, report, January 5, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc890287/m1/2/: accessed May 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.