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 development. Why should relatively young, seemingly vigorous even-aged forests undergo this decrease in production? We measured above-ground net primary production and below-ground carbon allocation in an age sequence of lodgepole pine forest in south-central Wyoming spanning 260 yr of forest development. ANPP and total root carbon allocation (TRCA) are examined to determine if there is an increase in TRCA of sufficient magnitude to offset the observed decrease in ANPP in developing lodgepole forests. Also, we examined changes in canopy structure, leaf area efficiency (E), and above-ground carbon allocation patterns to determine their potential role in the age-related decrease 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 production. TRCA was similar for forests from 30 to 100 yr old, but was significantly less in 260-yr-old forest at 391 gC m(-2)yr(-1) than in younger lodgepole forests at 516 gC m(-2)yr(-1). Declining ANPP was not due to an increase in carbon allocation to below-ground production. Net primary production (NPP), the total of above-ground and below-ground NPP, declined by 36% from 450 gC m(-2)yr(-1) at age 30 to 288 gC m(-2)yr(-1) at age 260. This progressive decline in NPP was associated with changes in stand structure that occur during stand development, including decreased leaf area, shifting carbon allocation, and decline in leaf area efficiency. Our results suggest that a combination of several factors, acting in concert, produce the observed decline in ANPP associated with the development of even-aged lodgepole pine forests.