Anthropogenic-induced vegetation and fire regime changes in the upper Great Lakes pine ecosystems

Background/Question/Methods

Anthropogenic-mediated changes in fire regimes and vegetation communities have been documented across northern portions of Minnesota. Changes in fire regimes and vegetation at regional scales have been speculated, but there have been very limited empirical studies of these changes.  This study attempts to document the degree to which fire regimes and vegetation have changed since presettlement times. The study area occurs within the northern portion of the Minnesota, with emphasis being placed on two National Forests: the Superior and the Chippewa. Ecological units called subsections were used to subdivide the study area. The results of both the forest fire and vegetation analysis will also be presented using the next smallest ecological unit „Ÿ Landtype Associations.

Six main hypotheses will be addressed: 1) density of trees has changed in response to fire regime change, 2) species composition has changed in response to fire regime change, 3) the average diameter of trees has changed in response to fire regime change, 4) The fire return interval is much longer today compared to presettlement times, 5) the average size of fires today is smaller than during presettlement times, 6) the number of fires per year is less today then in presettlement times.

Results/Conclusions

Density of trees has increased from 0-10 trees/acre during the presettlement era to 81 trees/acre in the current time period. The principal dbh has also changed significantly, with a decrease in dbh seen in most LTAs. Shifts in tree composition have also occurred, but do not follow the anticipated transition from more fire-tolerant species to less fire-tolerant species. Fire return intervals have significantly lengthened from less than 1,000 years to over 4,000 years, and median fire size has decreased for most LTAs by at least 50 acres, and the number of fires have been reduced for most LTAs.

The exclusion of fire from northern Minnesota may have a number of potential effects on vegetation, these include the transition of pine dominated forests to spruce- fir- birch and cedar forest types; the possible near elimination of red and white pine and aspen from the BWCA;  and the increased occurrence of catastrophic windthrow in future forests. A trend towards increased balsam fir/ upland shrub components in one of the subsections can already be seen in the data presented in this study.