Forest ecosystem structure and function along a 100-year chronosequence

Background/Question/Methods

It is well established that aboveground Net Primary Production (NPP) in forests declines strongly with age. However, the mechanisms governing this decline are not well understood despite the central role of forests in the global carbon cycle and the possibility for an age-related decline in forest C-sequestration. We have begun measurements of NPP across a chronosequence of 12 loblolly pine (Pinus taeda) stands ranging from 14- to 115-years old as part of a larger project to investigate the mechanisms driving age-related changes in the C-cycles of these forests. Wood, foliage, and fine root production were measured in all stands.
Results/Conclusions

As found in other studies, wood production by the dominant trees peaked early in development (~14 years) and subsequently declined by ~80%. This was partially off-set by increasing wood and leaf production by colonizing understory hardwood trees. The efficiency of pine wood production declined with age, suggesting that declining leaf-level photosynthesis is partially involved. However, the observed decline in production cannot be entirely explained by reduced growth rates of individual trees; the age-related decline in NPP appears to be an emergent property at the community or ecosystem scale associated with stand thinning and succession. This observation is at odds with the two most frequently invoked hypotheses for the age-related decline, which stress changes in photosynthesis and respiration at the organism scale. Subsequent research will attempt to predict the effects of global change on ecosystem carbon storage across successional time by identifying the mechanisms driving the observed decline in NPP.