CANCELLED - Changes in aboveground biomass following stand-replacing wildfire: Re-measurement of a chronosequence

Background/Question/Methods

Chronosequence, or space-for-time substitution, studies have long been used to study ecological processes occurring over long time scales; however, they are susceptible to misinterpretation if factors other than age vary among sites.  Re-measurement after a suitable interval of elapsed time is a powerful tool for validating the accuracy of chronofunctions derived from one-time chronosequence sampling.  We re-sampled aboveground biomass along a fire chronosequence of northern Michigan jack pine (Pinus banksiana) stands originally sampled in 2002 with two specific objectives: 1) To validate the accuracy of chronosequence-derived estimates of aboveground biomass change for this ecosystem, and 2) To test for evidence of a recent acceleration in forest growth, which has been demonstrated in several high-profile studies across the globe and attributed to rising atmospheric concentrations of carbon dioxide.  We sampled stands after re-measurement intervals of either 5 or 8 years and compared observed increases in aboveground biomass to those predicted by the original chronosequence.

Results/Conclusions

Change in aboveground biomass as a function of age since stand origination was well described by a modified Richard’s function (above ground biomass = 155.941*[1-EXP(-0.027*age)]^1.51, R² = 0.96).  Re-measurement estimates of biomass change conformed reasonably well to predictions generated from one-time sampling (R² = 0.90).  We found no evidence to support the hypothesis of a recent increase in forest growth.  Rather we found that that younger stands (< 25 y) systematically underperformed predictions of biomass accumulation from the original chronosequence by an average of 55% (range from 4 to 93 %). In contrast, aboveground biomass accumulation in older stands was better predicted by the original chronosequence with no systematic bias.  These results demonstrate the usefulness of the chronosequence approach with appropriate site selection to control for potentially confounding factors, and suggest that there has been a recent decrease in young stand productivity in the study area.