Annual carbon cost of autotrophic respiration in boreal forest ecosystems in relation to species and climate

Autotrophic respiration (R-a) in forest ecosystems can be >50% of the carbon fixed in photosynthesis and may regulate productivity and carbon storage in forest ecosystems, because R-a increases with temperature. We estimated annual R-a from chamber measurements in aspen, black spruce, and jack pine forests in Canada for 1994. Mean foliage respiration at 10 degrees C for expanded leaves was 0.21-0.95 mu mol m(-2) (leaf surface) s(-1) for all species and differed little from May to September. Wood respiration at 15 degrees C (0.2-1 mu mol m(-2) (stem surface) s(-1) for all species) was strongly seasonal, with high rates in midsummer that coincided with wood growth. Fine root respiration at 10 degrees C was 2.5-7.7 mu mol kg(-1) s(-1) for all species and declined throughout the growing season for the conifers. Annual costs of R-a for foliage, wood, and roots (overstory and understory) were 490, 610, and 450 g C m(-2) (ground) yr(-1) for aspen, black spruce, and jack pine (old) in northern Manitoba and 600, 480, and 310 g C m(-2) yr(-1) for aspen, black spruce, and jack pine (old) in central Saskatchewan. Carbon use efficiency (CUE), the ratio of net production to production plus R-a, averaged 0.34, 0.34, and 0.39 for aspen, black spruce, and jack pine (old) for all tissues and 0.61, 0.36, and 0.44 for aboveground tissues. Differences in CUE between the northern and the southern sites were small for all species, and CUE did not vary with stand biomass. Species differences in CUE suggest that models assuming a constant CUE across species may poorly estimate production and carbon balance for any given site.