Evaluating the interacting effects of forest management and spruce budworm outbreaks on broad-scale, long-term forest conditions in the Northern Forest of the northeastern U.S

Background/Question/Methods

The forest resources of the Atlantic Northern Forest are coming under increasing pressure from a host of natural and anthropogenic disturbances. Species diversity within this extensive forest, which spans nearly 11 million ha of the northeastern U.S., is high across both animal and plant taxa as the hardwood forests of the southern U.S. transition into the softwood forests of the northern boreal region. As a result, interactions between disturbances and their cumulative effects are complex. We used the forest landscape model LANDIS-II to simulate future forest conditions (2010-2110) across 4 million ha in Maine, an area that encompasses the majority of Maine’s commercial forestland and the largest block of private commercial forestland in the nation. Maine is a major timber producer in the region; however, concerns over the long-term sustainability of its forest resources have arisen given current harvesting regimes. Maine is also expecting an outbreak of eastern spruce budworm (Choristoneura fumiferana), which will cause widespread defoliation, growth reduction, and mortality of balsam fir (Abies balsamea) and spruce (Picea spp.) trees. We designed simulations that emulated harvest-succession dynamics across more than 200 individual landowners, and integrated the potential impacts of future outbreaks of spruce budworm on those dynamics. 

Results/Conclusions

In the absence of future budworm outbreaks, our results indicate that aboveground live biomass would be higher and more mature forest left intact over the next 100 years if rates of partial harvesting were reduced and clearcutting increased. Clearcut rates in Maine declined sharply from 40% of total acres harvested to <5% following the last budworm outbreak (1972-1986) and implementation of Maine’s first clearcut regulations in 1991. Partial harvests remove less volume per acre, and, thus, require a larger footprint to achieve the same total timber volume as a clearcut. Spruce-fir biomass is also likely to be higher in a future with fewer partial harvests, which tend to favor recruitment of hardwood species unless a forest stand is well-stocked with established softwood seedlings. The next outbreak of spruce budworm will spur a short-term increase in clearcuts as landowners attempt to salvage or pre-salvage infested spruce-fir stands. In the pre-settlement forest, budworm-induced mortality of mature trees perpetuated the same forest type through natural regeneration dynamics; our results suggest, however, that without intensive intervention (e.g., planting followed by herbicide application) premature removal of overstory trees during the next outbreak is likely to exacerbate the conversion of spruce-fir to a deciduous-dominant forest type.