The tradeoff of maximizing forest carbon storage and resiliency to disturbance: An assessment using long-term ponderosa pine density management growth data
The increase in atmospheric CO2 and its role in global climate change have encouraged the search for ways to reduce or offset greenhouse-gas emissions. Forests store a substantial amount of carbon. Therefore, we continue to seek forest management strategies that could be used to mitigate climate change by enhancing forest carbon stores and carbon sequestration. One strategy often explored is to maintain high stocking levels to maximize forest carbon storage. However, this strategy ignores several possible consequences such as reduced resiliency to disturbances such as wildfire and insect epidemics, which are projected to increase in frequency as a result of climate change. To explore these tradeoffs, we use data from a ponderosa pine density management study established in the 1960s on the Black Hills Experimental Forest. Six density levels, ranging from 5 to 28 ft2/ac, were established in even-aged 55 to 65 year old ponderosa pine stands. Diameter and height growth and mortality were measured periodically from 1963 to 2010. Using this information, live carbon storage and sequestration was calculated for each stocking level. In addition, we assessed crown fire hazard and mortality susceptibility to wildfire and mountain pine beetle infestation.
Over the 47 years of measurements, carbon storage and sequestration increased with stand density. However, stands with higher density levels had substantially increased crown fire hazard due to the increased canopy bulk density. Furthermore, these higher density stands had trees 10 to 15 cm smaller than the lower density stands. These higher density stands with smaller diameter trees would experience substantial levels of mortality if exposed to wildfire, reducing the amount of live carbon storage. Furthermore, the higher density stands were more susceptible to the mountain pine beetle epidemic that occurred in the area in 2010. These results demonstrate that maximizing carbon storage in high density even-aged stands in ponderosa pine is likely not the best strategy due to the increased mortality risk. Rather, if even-aged stands are to be used for carbon storage, moderate density stands, which are more resilient to crown fire hazard and less susceptibility to wildfire and/or insect mortality, should be used.