Perspectives on oak savanna restoration in Minnesota: a dendroecological approach

Abstract

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 objectives.