Due to decades of fire suppression, much of the Upper Midwest savanna habitat has converted to oak woodland.  In efforts to restore oak savanna habitat, fire has been re-introduced in many of these woodlands.  A primary purpose of these burns is to kill the fire-sensitive mesophytic tree species, which had established themselves during the decades of fire suppression, reduce the number of understory trees, and preserve the larger more widely spaced oaks.  It is clear from ongoing efforts that restoring oak savannas will require frequent fires over decades.  But frequent fires over the long term can also threaten the desirable oaks.  Long-term demographic studies at savanna restoration sites experiencing frequent fires are necessary to determine the extent to the frequent burns are supporting and/or confounding restoration goals.  Results presented here are from a twenty-five-year demographic study of an Upper Midwest bur oak (Quercus macrocarpa) savanna/woodland experiencing frequent fire, during which both the survival and growth of more than 9,000 trees were documented.  Survival was assessed annually and growth every five years.  In the face of frequent fires, stem survival was found to be strongly associated with tree species, stem size, and stem growth.  In turn, stem growth was found to be related to tree species and stem size.  Decades of frequent burning in this oak woodland have substantially reduced the abundance of unwanted trees, specifically mesophytic species and Q. ellipsoidalis, the latter which outcompetes Q. macrocarpa in the absence of fire.  While Q. macrocarpa mid-sized (10-25 cm dbh) and large (> 25 cm dbh) trees are quite resistant to fire and now dominate the savanna landscape, they are not immune from fire-induced mortality.  It is recommended that the number and density of these trees should be re-evaluated every few years to ensure that desirable numbers remain.  If necessary, fires should be suspended for a period of time.  This will give smaller Q. macrocarpa trees time to grow larger and become more fire-resistant, thereby ensuring successive generations of Q. macrocarpa. 

Field observations were conducted at the Cedar Creek Ecosystem Science Reserve (CCESR) located in east-central Minnesota (Latitude: 45.401, Longitude -93.201).  During 1988, I initiated the GLADES (Grid for Landscape Analysis and DEmographic Study) project with the establishment of a square 16 ha grid (consisting of 1600 10 x 10 m cells) in a portion of Cedar Creek’s oak woodland/savanna habitat.  This grid was set up in a north-south configuration, using 1.5 m x 0.95 cm diam iron rebars to mark the grid nodes. A Cartesian coordinate system was used to map tree stems in the grid.  Each cell was identified by the x-y coordinates in the southwest corner of the cell.  

In 1995, numbered aluminum tags were nailed into stems > 10 cm.  In 1996, the tags were nailed into stems 5-9.9 cm dbh and attached with wire to stems 2-4.9 cm dbh.  The dbh of these stems was measured and recorded when initially tagged.  In addition, the cell in which the stem was located was documented, as well as the stem’s x-y location within the cell (determined visually).  A burn program was instituted in the grid area in 1987, and the study grid contains three different burn units. Burn unit 1 (6.17 ha) was burned four times between 1987 and 1995 (prior to this study) and eleven times during the twenty-five-year study (1995-2020).  Burn unit 2 (5.98 ha) was burned once prior to the study (1992) and nine times between 1995-2020; burn unit 3 (3.25 ha) was burned for the first time in 2000 and eight times after that.

All stems were visited annually and their statuses (live, dead standing (snag), or dead fallen) recorded. The dbh of live stems was measured every five years.  To facilitate some analyses involving stem size (dbh), Quercus stems were grouped into three size categories based on their dbh in 1995 or 1996 (2-9.9 cm dbh, 10-24.9 cm, and >25 cm).  Survival and growth rates of the two Quercus species and eight mesophytic species were analyzed using JMP Pro 15.1 (SAS Institute, Inc.).

Number of variable (columns): 39

Number of tree stems (rows): 9314

Variable Name; Variable Description

ID_ ; unique ID number for each stem

SP, Species (B=bur oak (Quercus macrocarpa), P=pin oak (Quercus ellipsoidalis), AE=American elm

              (Ulmus americana), BC= black cherry (Prunus serotina), BE=boxelder (Acer negundo), GA=green

              ash (Fraxinus pennsylvanica), H=hawthorn (Craetagus sp.), QA=quaking aspen (Populus

              tremuloides), PB=paper birch (Betula papyrifera), RM=red maple (Acer rubrum))

X; x coordinate of the 10 x 10 meter cell in which the stem is located, e.g., X=20 means the southwest

              corner of the cell is 200 meters to the right of the y axis 

Y; y coordinate of the 10 x 10 meter cell in which the stem is located, e.g., Y=32 means the southwest

              corner of the cell is 320 meters above the x axis

XX; x coordinate within the 10 x 10 m cell, meters

YY; y coordinate within the 10 x 10 m cell, meters

DBH9596; stem diameter at breast height (cm); stems with dbh > 10 cm were measured in 1995; stems

                with dbh between 2 and 9.9 cm were measured in 1996

DBH GROUP; stems were assigned to one of three groups based on their 1995/1996 dbh: SMALL (includes

                 stems with a 1996 dbh 2 to 9.9 cm), MEDIUM (includes stems with a 1995 dbh 10-24.9 cm), LARGE

                 (includes stems with a 1995 dbh greater or equal to 25 cm).

DBH0001; stem diameter (cm) measured at breast height in 2000 or 2001; stems with dbh > 10 cm were

                 measured in 2000; stems with dbh between 2 and 9.9 cm were measured in 2001

DBH05; stem diameter (cm) measured at breast height in 2005

DBH10; stem diameter (cm) measured at breast height in 2010

DBH15; stem diameter (cm) measured at breast height in 2015

DBH20; stem diameter (cm) measured at breast height in 2020

Y95; status of stem in 1995 (since only live stems were tagged in 1995 all stems were listed as live, L)

Y96; status of stem in 1996, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y97; status of stem in 1997, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y98; status of stem in 1998, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y99; status of stem in 1999, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y00; status of stem in 2000, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y01; status of stem in 2001, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y02; status of stem in 2002, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y03; status of stem in 2003, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y04; status of stem in 2004, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y05; status of stem in 2005, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y06; status of stem in 2006, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y07; status of stem in 2007, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y08; status of stem in 2008, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y09; status of stem in 2009, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y10; status of stem in 2010, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y11; status of stem in 2011, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y12; status of stem in 2012, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y13; status of stem in 2013, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y14; status of stem in 2014, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y15; status of stem in 2015, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y16; status of stem in 2016, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y17; status of stem in 2017, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y18; status of stem in 2018, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y19; status of stem in 2019, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)

Y20; status of stem in 2020, L=live, D=dead standing, DR=dead stem that is resprouting, DF=dead fallen)