OAK MORTALITY IN SAND SAVANNAS FOLLOWING DROUGHT IN EAST-CENTRAL MINNESOTA

Savanna plots that were established and sampled in 1984 at Cedar Creek Natural History Area in east-central Minnesota were resampled in 1989 , towards the end of a major drought period. Tree diameter was measure d and mortality was assessed for all stems greater-than-or-equal-to 10 cm dbh in eleven 50 x 75 m plots. Quercus ellipsoidalis E. J. Hill (n orthern pin oak) and Q. macrocarpa Michx. (bur oak) dominate the savan nas, accounting for > 95% of all stems greater-than-or-equal-to 10 cm dbh. In 1984, diameter distributions and species composition were sign ificantly different on different soil types. Zimmerman soils had a lar ger proportion of stems in the 10-25 cm dbh size classes (62%) than di d the Sartell soils (36-60%), and a significantly larger proportion ov erall of Q. ellipsoidalis stems (93%) than did the Sartells (51-84%). Plots burned during the previous 20 years had fewer stems in the 10-25 cm dbh size class (45%) compared to unburned plots (58%). These diffe rences were not changed by mortality over the 1984-1989 period, nor di d diameter distributions within a plot change significantly over this period. Average five-year mortality rates per plot from 1984-1989 for Q. ellipsoidalis (21.4%, range 0.0-60.0%) were significantly higher th an for Q. macrocarpa (6.1%, range 0.0-33.3%). Five-year mortality rate s of Q. ellipsoidalis declined from 60% to 15% with higher total stem basal area and density. Drought or large moisture changes may have bee n primary factors responsible for oak mortality. Other factors, such a s percent organic matter, depth to water table, and fire frequency wer e not correlated with mortality rates. The inverse relation between mo rtality and stand density suggests that the possible effects of drough t varied with stand structure. Closed savannas or woodlands had lower mortality rates than open savannas. These findings emphasize the role of climate in maintaining sand savannas in the prairie-forest border r egion.