Long-term trends in biomass and tree demography in northern hardwoods: an integrated field and simulation study

Long-term trends in forest biomass have important implications for carbon sequestration and storage, but chronosequence field studies and computer simulations have produced differing conclusions about whether biomass declines in older stands. In this paper, the Bormann-Likens hypothesis of a biomass peak followed by a decline to a steady state with zero net growth is tested against alternative hypotheses of asymptotic or increasing trends with positive net growth even at advanced ages. Trends in aboveground live-tree biomass and large tree density in northern hardwoods were examined from chronosequences and 30-yr permanent plot data from unmanaged forests spanning a wide range of developmental stages, as well as multi-century simulations using an intensively tested individual-tree model ( CANOPY). Both field data and simulations indicated an average decline in aboveground live-tree biomass and number of large trees in the later stages of old growth under environmental conditions of the recent past. This decline was robust to modeling assumptions but occurred to varying degrees on different habitat types and species mixtures tested. The decline began when stand age structure was changing from even-aged to multi-aged, and when underlying size distributions shifted from a unimodal to descending monotonic form. In the majority of cases, net biomass growth of younger secondary cohorts was insufficient to compensate for attrition of the initial even-aged cohort. Incorporating natural disturbances into the simulations lowered the level of the aboveground biomass peak, but a subsequent average decline was still predicted even with addition of the deadwood component. Mean annual net growth of live-tree biomass in old-growth stands with descending monotonic size distributions ( i.e., quasi-steady state) was approximately zero for both the field data and simulations. Results suggested that under recent environmental conditions, the aboveground component of old-growth northern hardwoods in the study areas would be unlikely, on average, to continue accumulating carbon in stands >200 yr old, even when averaging over a larger landscape containing some younger stands with positive net growth.