Age-related changes in production and below-ground carbon allocation in Pinus contorta forests

A decline in wood production and above-ground net primary production (ANPP) following an early maximum is a widely observed feature of forest developm ent. Why should relatively young, seemingly vigorous even-aged forests unde rgo this decrease in production? We measured aboveground net primary produc tion and below-ground carbon allocation in an age sequence of lodgepole pin e forest in south-central Wyoming spanning 260 yr of forest development. AN PP and total root carbon allocation (TRCA) are examined to determine if the re is an increase in TRCA of sufficient magnitude to offset the observed de crease in ANPP in developing lodgepole forests. Also, we examined changes i n canopy structure, leaf area efficiency (E), and above-ground carbon alloc ation patterns to determine their potential role in the age-related decreas e in ANPP. ANPP declined by 50% between peak production of 192 gC m(-2) yr(-1) at age 30 to 92 gC m(-2) yr(-1) at age 260, largely as a decrease in stemwood prod uction. TRCA was similar for forests from 30 to 100 yr old, but was signifi cantly less in 260-yr-old forest at 391 gC m(-2) yr(-1) than in younger lod gepole forests at 516 gC m(-2) yr(-1). Declining ANPP was not due to an inc rease in carbon allocation to below-ground production. Net primary producti on (NPP), the total of aboveground and below-ground NPP, declined by 36% fr om 450 gC m(-2) yr(-1) at age 30 to 288 gC m(-2) yr(-1) at age 260. This pr ogressive decline in NPP was associated with changes in stand structure tha t occur during stand development, including decreased leaf area, shifting c arbon allocation, and decline in leaf area efficiency. Our results suggest that a combination of several factors, acting in concert, produce the obser ved decline in ANPP associated with the development of even-aged lodgepole pine forests.