Abstract:
Net uptake of carbon from the atmosphere (net ecosystem production, NEP) is
dependent on climate, disturbance history, management practices, forest age, and
forest type. To improve understanding of the influence of these factors on forest
carbon flux in the western U.S., a combination of federal inventory data and
supplemental ground measurements was used to estimate several important
components of NEP in forests in Oregon and Northern California during the 1990’s.
The specific components studied were live and dead biomass stores, net primary
productivity (NPP), and mortality. In the semi-arid Northern Basin and mesic Coast
Range, mean total biomass was 4 and 24 Kg C m-2, and mean NPP was 0.28 and 0.78
Kg C m-2 y-1, respectively. These values were obtained using species- and ecoregionspecific
allometric equations and tended to be higher than those obtained from more
generalized approaches. There is strong evidence that stand development patterns of
biomass accumulation, net primary production, and mortality differ due to climate
(ecoregion), management practices (ownership), and forest type. Among those three
factors and across the whole region, maximum NPP and dead biomass stores were
most influenced by climate, while maximum live biomass stores and mortality were
mostly influenced by forest type. Live and dead biomass, NPP, and mortality were
most influenced by forest type. Decrease in NPP with age was not general across
ecoregions, with no marked decline in old stands (>200 years) in some ecoregions,
and in others, the age at which NPP declined was very high (458 years in East
Cascades, 325 in Klamath Mountains, 291 in Sierra Nevada). There is high potential
for increasing total carbon storage by increasing rotation age and reducing harvest
rates in this region. Only 1% of forest plots on private lands were >200 years old,
whereas 41% of the plots were greater than 200 years old on public lands. Total
carbon stocks could increase from 3.2 Pg C to 7.3 Pg C and NPP could increase from
0.109 Pg C y-1 to .168 Pg C y-1 (a 35% increase) if forests were managed for
maximum carbon storage by increasing rotation age.
