Carol A. Wessman

Multiple forest disturbances, their interactions, and cumulative effects on carbon dynamics in forest ecosystems

Disturbances have a strong role in the carbon balance of many ecosystems, and the cycle of vegetation growth, disturbance, and recovery is very important in determining the net carbon balance of terrestrial biomes. Despite potentially large losses of carbon in disturbance events (due to combustion, decomposition, or other mechanisms), resilient systems can recover that balance to remain neutral over the disturbance return interval. Conversely, lack of recovery may initiate alternate regimes with a different carbon balance. Compound disturbances are phenomena of growing concern and can further impact ecosystems in novel ways, altering disturbance intensity, severity, and recovery trajectories.

This research focuses on carbon stocks in a compound disturbance environment, with special attention on black carbon (charcoal). Black carbon is a potential source of long-term carbon sequestration, as it is very resistant to decomposition, formed in fires, and has numerous other benefits such as increasing soil fertility. We extensively surveyed a compound disturbance event (wind, logging, and fire) in a Colorado subalpine forest for impacts on carbon stocks, black carbon, and regeneration. We considered all major pools, including organic and mineral soil, and contrasted them with neighboring undisturbed forests as a reference.

The disturbances had an additive effect on carbon loss, with increasing numbers of disturbances resulting in progressively decreasing carbon/black carbon stocks. We interpret this as resulting from substrate availability and fire intensity, and there was no significant difference between unburned and burned plots in terms of total black carbon. Given literature-derived decay rates, it appears that high-intensity fires may actually reduce net black carbon in these forests over the entire fire return interval, with additional disturbances compounding the loss.

Overall, high-intensity disturbances remove a large amount of carbon, and multiple disturbances compound this effect. Differences in regeneration have a small but significant difference on current carbon stocks, a difference that likely will get larger as time progresses due to differential species characteristics. Disturbances, and their interactions, will have long-lasting legacies for carbon, black carbon, and ecosystem structure and function.