Landscape dynamics of oak-savanna in the Lower Chippewa River Valley, Wisconsin

Background/Question/Methods The Lower Chippewa River Valley (LCRV) is one of the most unique landscapes in the state of Wisconsin; it remains forested and undeveloped along most of its channel for 65 river miles ending where it meets the Mississippi River on Wisconsin’s west-central border.  Interspersed along the LCRV are patches of prairie, floodplain savanna, woodland, and forest. Within LCRV are large patches of oak-savannah, dominated primarily by Quercus macrocarpa and Quercus ellipsoidalis.  These oak-savannahs are unique because they are located primarily on islands, directly within the floodplain, and occur at the northern extent of oak-savannah distribution in Wisconsin.  The goal of this research is to study vegetation dynamics in the LCRV (180 km²) over the last 70 years.  Specifically the objectives are to: map vegetation and land use cover (Anderson Level II) for 5 individual years spanning 1938-2008; examine changes in each vegetation class for the 5 individual years during the 70-year span; quantify changes in landscape patterns by calculating and comparing standard landscape metrics; and reconstruct the vegetation history of the study area using dendrochronological analysis.  The above results are used to gain insight into the dynamics of oak-savannah within the study area. 
Results/Conclusions

In 1938, oak savannah covered 4.93% of the study area (2,000 acres); by 2008 oak savannah was 3.76% of the study area.   The average patch size of oak savannah increased from 23 acres to over 40 acres in 2008.  A majority of the change is attributable to succession of oak-savannah into either forest (3%), woodland (.55%) or crop (.31%).  While the overall landscape experienced a net loss of oak-savannah some patches of prairie succeeded to savannah (317 acres) and forest reverted to savannah (116 acres).  Preliminary dendrochronological data indicate that many of the oak trees in island patches of oak- savannah established in late 1880’s.  The oldest oak from the island savannah dates to 1877 with a DBH of 48.5 cm.  The largest patch of oak-savannah on Brush Island has been burned by the DNR, every other year for the past 20 years. The predominance of oaks since at least the 1880’s suggests that island sites in the LCRV are prime sites for oak-savannah. Further research into the fire history of the area may indicate whether non-anthropogenic fires were frequent enough to maintain oak savannah or if geomorphology (floodplain dynamics) was the prime disturbance agent.