Seasonal and annual respiration of a ponderosa pine ecosystem

The net ecosystem exchange of CO2 between forests and the atmosphere, measu red by eddy covariance, is the small difference between two large fluxes of photosynthesis and respiration. Chamber measurements of soil surface CO2 e fflux (F-s), wood respiration (F-w,) and foliage respiration (F-f) help ide ntify the contributions of these individual components to net ecosystem exc hange. Models developed from the chamber data also provide independent esti mates of respiration casts. We measured CO2 efflux with chambers periodical ly in 1996-97 in a ponderosa pine forest in Oregon, scaled these measuremen ts to the ecosystem, and computed annual totals for respiration by componen t. We also compared estimated half-hourly ecosystem respiration at night (F -nc) with eddy covariance measurements. Mean foliage respiration normalized to 10 degrees C was 0.20 mu mol m(-2) (hemi-leaf surface area) s(-1), and reached a maximum of 0.24 mu mol m(-2) HSA s(-1) between days 162 and 208. Mean wood respiration normalized to 10 degrees C was 5.9 mu mol m(-3) sapwo od s(-1), with slightly higher rates in mid-summer, when growth occurs. The re was no significant difference (P > 0.10) between wood respiration of you ng (45 years) and old trees (250 years). Soil surface respiration normalize d to 10 degrees C ranged from 0.7 to 3.0 mu mol m(-2) (ground) s(-1) from d ays 23 to 329, with the lowest rates in winter and highest rates in late sp ring. Annual CO2 flux from soil surface, foliage and wood was 683, 157, and 54 g C m(-2) y(-1), with soil fluxes responsible for 76% of ecosystem resp iration. The ratio of net primary production to gross primary production wa s 0.45, consistent with values for conifer sites in Oregon and Australia, b ut higher than values reported for boreal coniferous forests. Below-ground carbon allocation (root turnover and respiration, estimated as F-s, - litte rfall carbon) consumed 61% of GPP; high ratios such as this are typical of sites with more water and nutrient constraints. The chamber estimates were moderately correlated with change in CO2 storage in the canopy (F-stor) on calm nights (friction velocity u* < 0.25 m s(-1); R-2 = 0.60); F-stor was n ot significantly different from summed chamber estimates. On windy nights ( u* > 0.25 m s(-1)), the sum of turbulent flux measured above the canopy by eddy covariance and Fstor Was only weakly correlated with summed chamber es timates (R-2 = 0.14); the eddy covariance estimates were lower than chamber estimates by 50%.