Challenges of measuring forest floor organic matter dynamics: Repeated measures from a chronosequence

Accurate estimates of the retention of carbon in forest soils following forest disturbances are essential to predictions of global carbon cycling. The belief that 50% of soil carbon is lost in the first 20 years after clearcutting is largely based on a chronosequence study of forest floors in New Hampshire northern hardwoods (Covington, 1981). We resampled forest floors in 13 stands in a similar chronosequence after an interval of 15 years. The three youngest stands, which were predicted to lose organic matter over this time, did not exhibit the 40-50% losses predicted by the original chronosequence. The oldest stands had about twice as much organic mass in the forest floor as those cut recently, but this pattern could be explained equally well by historical changes in the nature of forest harvest as by the age of the stands. For example, mechanized logging probably causes more mechanical disturbance to the forest floor than horse logging, burying more organic matter into the mineral soil. Markets for forest products and the intensity of harvest removals have also changed over time, possibly contributing to lower organic matter in the forest floor in young stands. In any chronosequence study, effects of change in the nature of the treatment over time can easily be misinterpreted as change with time since treatment. Repeated sampling of the chronosequence provides controls for some of these effects. In the case of forest floor organic matter, however, high spatial variation makes it difficult to distinguish whether the observed variation is more likely due to changes in treatment over time or to differences in time since treatment. Because of the large amounts of carbon involved, small changes in rates of soil organic matter storage may be quite important in global climate change, but they will remain difficult to detect, even with very intensive sampling.