Foliar respiration in an old-growth Pseudotsuga-Tsuga forest

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 source–sink 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 Pseudotsuga–Tsuga 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 var...