Anthropogenic disturbances have diminished the extent of oak savannas
throughout the Midwest and altered what few remnants remain. Although oak savanna
restoration is of great interest to the public and reserve managers, scientists do not fully
understand the intricate dynamics of the ecotone, leaving land stewards without solid
restoration models. This study examined the age structure and historical fire frequency at
four remnant savannas in Minnesota. A total of 846 tree cores were used to reveal
temporal changes in savanna structure and 42 wedges and cross-sections were cut from
oaks to date fire scars. Northern pin oak (Quercus ellipsoidalis) dominated in the
southeast, grading to bur oak (Quercus macrocarpa) dominance in the northwest. Oaks
were the oldest trees at each site, with relatively recent recruitment of more shadetolerant,
fire-intolerant species. Few oaks predated Euro-American settlement. High bur
oak establishment during the late 1800s-early 1900s was followed by a period of low oak
establishment in the 1930s and 40s. Northern pin oak establishment increased rapidly in
the mid-1900s, while bur oak establishment appears to have decreased, displaying a shift
from bur oak dominated establishment to northern pin oak dominated establishment over
the past 200 years. Whereas bur oak dominated the seedling layer, northern pin oak
dominated the sapling size class. Open and healed fire scars from prescribed burns were
abundant at all sites, but no fire scars predated settlement. These results suggest that
many areas we currently designate as “oak savanna” may not have many (or any) oaks
predating European settlement of the area due to previous land-use, climatic conditions,
or species specific life history characteristics. Nevertheless, the scarcity or absence of
older oaks in these areas (regardless of oak species) does not directly imply that these
areas were not pre-settlement oak savanna. Anthropogenic land-use has heavily shaped
the savanna community composition and structure since European settlement.
Throughout Minnesota in the late 1800s, the implementation of continuous cattle grazing
increased bur oak establishment and survival. Periods of logging have reduced the
presence of old oaks and heavy grazing reduced oak establishment. Canopy cover has
increased at all sites due to fire suppression and the maturation of earlier surges of oak
establishment. The most apparent and, perhaps, threatening trend to savanna structure
and composition, is the recent shift from bur oak dominated savannas to northern pin oak
dominated savannas due to a combination of springtime prescribed burns, fire
suppression, increasing deer populations and squirrels. A conclusive pre-settlement
average fire return interval for Minnesota oak savannas could not be deduced from the
fire history aspect of this study due to an insufficient number of pre-settlement fire scars.
Prescribed burns are probably scarring trees more frequently than historic fires did and
have failed to reduce the number of mesic, fire-intolerant species. This study
demonstrates the variation between and heterogeneity within Minnesota oak savannas,
exemplifying the problems inherent in extrapolating patterns and management
implications from site-specific case studies. Future oak savanna management in
Minnesota should focus on thinning areas before prescribed burning to decrease scarring
frequency, performing summer or fall burns to increase bur oak regeneration, as well as
increasing our knowledge of land-use patterns before determining land management
University of Minnesota Master of Science thesis. December 2009. Major: Conservation Biology. Advisor: Susy S. Ziegler. 1 computer file (PDF); vii, 182 pages.
Margoles, Sarah Speeter.
Perspectives on oak savanna restoration in Minnesota: a dendroecological approach.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.