Carbon and energy fluxes were measured with the eddy covariance technique
above two semi-arid ecosystems, ponderosa pine and juniper/sagebrush, located in
central Oregon. The two ecosystems have low LAI and a very open canopy structure.
The energy closure was ~70-80% at both ecosystems, equivalent to an imbalance of
150-250 W m⁻² on cloudless summer days, when net radiation (R[subscript n]) was ~600-700 W
m⁻². The lack of closure cannot be explained by the uncertainty of an estimate of
available energy due to a single R[subscript n] sensor location. At the more open
juniper/sagebrush ecosystem, a numerical model showed that spatial variation in R[subscript n],
even for large differences in surface radiation temperature and reflection coefficient
between ecosystem components (soil and vegetation), is less than 10% of measured
R[subscript n]. The uncertainty in R[subscript n] at the two-layered ponderosa pine ecosystem with patches
of young and old-growth trees is expected to be smaller than at the juniper ecosystem.
Net carbon exchange (NEE) at the pine site strongly depends on environmental
factors effecting carbon assimilation (A[subscript c]) and ecosystem respiration (R[subscript e]). A more
detailed analysis of the carbon budget showed a strong negative response of carbon
uptake to large vapor pressure deficits (VPD), whereas water vapor exchange (LE)
was less affected. At large VPD the vegetation maintains a sustainable water flow
through the soil-plant system by stomatal control of transpiration. The stomatal
closure leads to limitation in A[subscript c], but LE is subject to a positive feedback from higher
evaporative demand.
Annual NEE of the ponderosa pine forest (200-300 gC m⁻²) was in the mid-range
of reported NEE of temperate forest ecosystems, though, unusually, much of the
annual carbon gain occurred during the fall through spring, because the relatively mild
winters allowed carbon assimilation to occur and R[subscript e] rates were low.
The information gathered at our ponderosa pine site during two years with
contrasting climate suggests that the carbon uptake of the ponderosa pine ecosystem
will be more sensitive to global climate change than the water vapor exchange. / Graduation date: 2000
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28576 |
| Date | 20 October 1999 |
| Creators | Anthoni, Peter M. |
| Contributors | Unsworth, Michael H. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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