Old-growth forest ecosystems accrue carbon at small mean rates and may function as carbon sinks in some years and as carbon sources in others. Foliar respiration is a large component of stand carbon balance and could be variable enough to substantially affect sourcesink behaviors. However, foliar respiration has not been well studied in old-growth canopies. We examined seasonal, interannual, spatial, and interspecific variation of foliar respiration in an old-growth PseudotsugaTsuga stand in Washington, USA, with measurements made on three species at 3-month intervals, for 4+ years. There were strong seasonal differences, with rates being much larger in June than in December. Rates in March were significantly (p ≤ 0.0001) larger than expected for all species. For data pooled across seasons, the exponential respiration-temperature relationship indicated that a seasonal temperature increase of 10 °C caused rates to increase by 1.78 times. For respiration based on leaf area, but not on leaf mass, rates varied strongly with canopy position (p ≤ 0.0001). Temperature-corrected rates were compared among four consecutive years and declined from 1999 to 2001. Correlation with eddy covariance measurements suggests that interannual changes in foliar respiration did not cause the decline in net ecosystem productivity observed at the site through the same period, but may have instead partially offset a trend toward decreasing net ecosystem productivity caused by other factors.
Changes in net ecosystem productivity with forest age following clearcutting of a coastal Douglas-fir forest: testing a mathematical model with eddy covariance measurements along a forest chronosequence