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: 3 of 10
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Objective 2: Determine key environmental factors controlling carbon storage and
carbon dioxide and energy exchange in these forests through a combination of
measurements and process modeling.
Inter-annual variability in NEP and GPP with climate (Schwarz et al. 2004): Improving
our understanding of the mechanisms underlying inter-annual variability in carbon
exchange is vital to carbon cycle modeling. We investigated the relative importance of
climatic versus biotic controls on gross primary production (GPP) and water vapor fluxes
in seasonally drought-affected ponderosa pine forests using flux and sapflow
measurements, and a process model, SPA. The study was conducted in young (YS),
mature (MS) and old stands (OS) over four years at the AmeriFlux Metolius sites. Model
simulations showed that interannual variation of GPP did not follow the same trends as
precipitation, and effects of climatic variation were smallest at the OS (< 10%), largest at
the MS (> 50%), and intermediate at the YS (< 20%). In the young, developing stand,
interannual variation in leaf area has larger effects on fluxes than climate, although leaf
area is a function of climate in that climate can interact with age-related shifts in carbon
allocation and affect whole-tree hydraulic conductance. Older forests, with well-
established root systems, appear to be better buffered from effects of seasonal drought
and interannual climatic variation. Interannual variation of net ecosystem exchange
(NEE) was also lowest at the OS, where NEE is controlled more by interannual variation
of ecosystem respiration, 70% of which is from soil, than by the variation of GPP,
whereas variation in GPP is the primary reason for inter-annual changes in NEE at the
YS and MS. Across spatially heterogeneous landscapes with high frequency of younger
stands resulting from natural and anthropogenic disturbances, interannual climatic
variation and change in leaf area are likely to result in large interannual variation in GPP
Inter-annual variation in soil CO2 efflux and thus ecosystem respiration can be large and
play a critical role in interannual variation in NEP (Irvine et al. 2004). Based on
automated soil chamber data, we found that over four years, soil CO2 efflux varied by
217 g C m-2 y-i at the mature pine site, which amounts to 40 % of the mean annual soil
efflux (CV=18 %). Much of this inter-annual variability appears driven by climate, in
particular the timing and intensity of rain events. It is important to stress the value of
long-term automated soil efflux measurements as periodic manual measurements haven't
shown clear patterns.
Trends in NEP following disturbance from harvest (Law et al. 2003): Earlier simulations
of NEP using the process model Biome-BGC showed a shift from net carbon source to
net sink (on an annual basis) 10 to 20 years after stand replacing disturbance from
logging or wildfire. Our biometric estimates of NEP at 12 plots along a harvest
chronosequence that includes the three aged flux sites showed that the ponderosa pine
forests growing in the East Cascades first attain positive NEP about 30 years following
disturbance (Campbell et al. 2004, Law et al. 2003). Comparing successional trends after
logging along a precipitation gradient from the coast to the East Cascades, we found that
the most pronounced successional trends in NPP occur at the East Cascade site where
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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/m1/3/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.