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 carbo n storage in forest ecosystems, because R-a increases with temperature . We estimated annual R-a from chamber measurements in aspen, black sp ruce, and jack pine forests in Canada for 1994. Mean foliage respirati on at 10 degrees C for expanded leaves was 0.21-0.95 mu mol m(-2) (lea f surface) s(-1) for all species and differed little from May to Septe mber. Wood respiration at 15 degrees C (0.2-1 mu mol m(-2) (stem surfa ce) s(-1) for all species) was strongly seasonal, with high rates in m idsummer 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 f or 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 pi ne (old) in northern Manitoba and 600, 480, and 310 g C m(-2) yr(-1) f or aspen, black spruce, and jack pine (old) in central Saskatchewan. C arbon 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 j ack pine (old) for all tissues and 0.61, 0.36, and 0.44 for abovegroun d tissues. Differences in CUE between the northern and the southern si tes were small for all species, and CUE did not vary with stand biomas s. Species differences in CUE suggest that models assuming a constant CUE across species may poorly estimate production and carbon balance f or any given site.