Biomass and CWD pools show complex multi-decade dynamics in old-growth northern hardwood forests

Background/Question/Methods

Old-growth forests can support very high biomass densities. However, because long-term, repeated-measures studies of such forests are rare, temporal dynamics of biomass reservoirs are poorly understood. Because dead wood in old-growth forests can be large and slow to decay, coarse woody debris (CWD) can be an important component of biomass pools. I use nearly five decades of remeasurements of permanent plots in a MI old-growth northern hardwood forest to assess trends in aboveground living biomass and input rates to CWD pools. Combined with a recent inventory of CWD, these results allow some assessment and projection of overall dynamics in aboveground biomass. Specifically, I ask whether living and dead biomass pools appear to be aggrading, stable, or declining, and whether biomass dynamics are consistent with already reported long-term community changes. I used standard allometric equations to estimate living biomass for each measurement date. CWD volume and mass was estimated by mapping and measuring all dead wood > 10 cm in diameter by decay class, using geometric calculations to estimate volume, and applying volume:mass relationships from published sources.

Results/Conclusions

Aboveground biomass ranged from 350-450 Mg/ha, among the highest reported for northeastern North American forests. Biomass densities increased modestly over 50 years, but intervening fluctuations were much larger than net overall changes. The proportion of biomass in trees > 70 cm dbh approximately doubled. CWD mass in 2007 averaged about 12% of aboveground living biomass. CWD inputs due to tree mortality increased by about 60% between the first 27 years and the last 20 years of the study. No major disturbances have affected the study plots over the last 50 years, and tree cores indicated that most stands are significantly older than 300 years. These results show old-growth forests with no recent disturbance to be dynamic in terms of biomass/carbon status; this is consistent with earlier studies showing systematic changes in community patterns. Biomass and carbon reserves in old-growth forests show complex long-term trends; total biomass reserves may experience long-term increases due largely to shifts in CWD inputs and volume with time since significant disturbances, driven in part by shifts in tree size distributions. However, the effects of rare intermediate disturbances on both living biomass and CWD input are likely very important and remain little assessed.