THE HERBACEOUS UNDERSTORY RESPONSE OF A DEGRADED
OAK SAVANNA TO PRESCRIBED BURNING
Ann M. Pierce
Savannas are terrestrial ecosystems dominated by prairie species, a few shrubs, and widely spaced trees (Curtis 1959). Before European settlement oak savannas were one of the most abundant ecosystems in Wisconsin (Curtis 1959). These ecosystems were formed along the prairie forest border from existing prairies and forests in response to frequent fires and to disturbance from browsing and grazing herd animals such as elk and bison (Szeicz and MacDonald 1991; Tester 1989).
As a result of European settlement, much land was cleared for agriculture; bison and elk were eliminated while fire was greatly curtailed. These reduced the abundance of oak savannas. Only a few remnants of the ecosystem remain. Most of the oak savannas that were not destroyed have developed into oak woodlands and mixed forests in the absence of fire (Curtis 1959). As the canopies closed on these savannas, they lost the diversity of prairie species that once were the dominant ground cover. Also lost were the insects and animals that depend on these plants. These areas now are considered degraded oak savannas.
Today savannas are one of the rarest communities in North America. Within The Nature Conservancy's global ecosystem classification, quality oak savannas on good soil are classified as globally endangered. In response to this situation, restoration projects have been started in an attempt to reestablish oak savannas. Most of these restoration projects have been carried out by reintroducing fire through prescribed burning (Cole et al. 1992; Frost 1985; White 1983). In some studies these prescribed burns have resulted in the reestablishment of many prairie species; some appear to come from seeds which have remained in the seed bank of these degraded oak savannas (Tester 1989).
The present study will provide information on the response of herbaceous species in a degraded oak savanna to perturbation by oak wilt and prescribed burning. This project also will investigate the amount and composition of herbaceous germinants from the seedbank of a degraded oak savanna under three ecological factors (light intensity, temperature, and leaf litter cover).
Quincy Bluff and Wetland Preserve is approximately a 1650 acre area owned by The Nature Conservancy. Quincy Bluff is located in Adams County, Wisconsin, five miles east of the Wisconsin River. The vegetation in the upland area of the preserve is predominantly mixed oak and jack pine dominated by black oak with groups of red maple and big tooth aspen scattered throughout (Haney and Aphelbaum, unpublished report.). Soils consist of mainly Plainfield sands and Delton series (Haney and Aphelbaum, unpublished report). Haney concluded that the last fire disturbance at Quincy Bluff was 60 to 70 years ago based on tree age and absence of recently charred wood. Although pastured this area has not been timbered. Scattered remnants of the prairies species that may have once dominated this degraded oak savanna still exist in the understory.
In 1993, the Nature Conservancy began a restoration project on the degraded oak savanna located at Quincy Bluff and Wetland Preserve in 1993. As part of this restoration project, a prescribed burn was conducted on a 16 ha (40 acre) area in the spring of 1993, and on another approximately 8 ha (20 acre) adjacent area in the spring of 1994.
Three research treatment areas were set up in two adjacent burned areas, in an adjacent stand which was not burned, and in four areas affected by oak wilt.
Within the areas of the degraded oak savanna affected by oak wilt a slow decrease in the amount of canopy cover and an increase in the amount of sun reaching the forest floor has resulted. Four oak wilt sites were chosen. Within each site a 4 m x 4 m grid was set up and the ten interior intersections of the grid were inventoried with 1m square quadrats. Ten quadrats were chosen based on the absence of canopy cover from oak trees greater than 2" DBH.
A total of 250 m transects were set up within each treatment area. Along the 250 m transects, one m square quadrats were placed every 25 m; these were placed 1 m west of the transects on the two burned plots and one m north of the transect on the control area, for a total of 10 on each transect. Steel rods were used to permanently mark the quadrat location. The percent cover and occurrence of prairie, savanna, and forest herbaceous species were recorded in each quadrat. The prairie, savanna, and forest species were determined primarily from the categories in Bray (1960). Bray's study surveyed the composition of savanna vegetation in southern Wisconsin. Bray based these categories on percent presence of species in 60 forests, 49 savannas, and 59 prairies. Bray's list was supplemented by the lists in Tester (1989). Tester classified species according to their origin using the categories true prairie, introduced, and native/not prairie. The true prairie species were considered typical savanna species.
Average percent cover and occurrence of each species and group (prairie and savanna versus forest) in each treatment were calculated to determine the effects of fire and canopy cover on the herbaceous understory.
Trees > 1" DBH and > 1.0 m in height were inventoried using a 10 factor prism. Diameter and species were recorded for all "in" trees at 6 points along each transect. Trees < 1" DBH or < 1.0 m in height were inventoried with the 1.0 m square quadrats.
To evaluate the germination responses of understory herbaceous species, 5 cm deep soil samples and litter samples were taken from the treated stands. Two samples were taken every ten meters, resulting in 50 samples from each 250 m transect.
Litter samples and soil samples were stratified at 0°C for 3 months. Soil samples were combined and half of the sample placed in the greenhouse under varying light intensities (100 percent light, 40 percent light, and 16 percent light). Light intensities were determined by measuring the light intensities in full sun light, in a restored savanna and in a degraded savanna, using the Licor light meter. In the green house the two lower light intensities were obtained using shade cloth.
The other half of the composite sample was placed in growth chambers under varying temperatures (25°C, 29°C, and 35°C). These temperatures were selected because: 1) 29°C was the average daily high temperature in full sunlight in the Quincy Bluff area for July 1993, 2) Daubenmire (1968) found an average increase in the ground surface temperature of 6°C after grassland fires, and, 3) Lee (1978) estimated that the temperature decreased by 4°C under the canopy of a forest.
To determine the effect nutrients released by the burn had on species response, each of the three light regimes and temperature regimes were placed under burned leaf litter taken from the 1993 burn, under unburned leaf litter, and with no leaf litter. Each of these treatments was placed in 20 cm (8 in) x 20 cm (8 in) trays. Because of space limitations in the growth chamber, tray size was reduced to 13 cm (5 in x 20 cm. All trays were watered every other day. Occurrence of germinating species was recorded as the response variable to determine whether plants were responding to the factors of increasing light, temperature, and litter cover or the interaction of treatments.
The results at this time are preliminary. Final results will be completed after the second field season of this project. Limited tests were run on the available data.
Paired t-test were run on the percent cover data for the herbaceous species. The total percent cover for the spring of 1993 and the spring of 1994 were compared as were as the percent cover of prairie and savanna species for the spring of 1993 and the spring of 1994. There was a significantly (a=0.05) higher total percent cover for the spring of 1994 in the site burned in the spring of 1993 (t = -3.35, P = .009). The percent cover of the prairie and savanna species was also significantly (a= 0.05) higher for the spring of 1994 in the site burned in the spring of 1993 (t = -3.28, P = .009). All other treatment areas were found not to have any significant difference between the spring of 1993 and the spring of 1994 for both the total percent cover and the prairie and savanna species percent cover. The predominant tree on all treatment sites was found to be Quercus velutina. The predominant herbaceous cover was Carex pensylvanica for all treatments and oak wilt sites; the average percent cover of this species for all sites was 76%.
The only treatment completed at this time is the first temperature (35°C) with three cover regimes (burned leaf litter, unburned leaf litter, and no leaf litter). Data collected from this treatment was analyzed using an ANOVA. There were significantly (a = 0.05) more germinates in the trays with no leaf litter than there were in either the trays with burned leaf litter and unburned leaf litter (F = 3.62, P = .049). There were no significant differences between the number of germinates in the trays with unburned leaf litter and trays with burned leaf litter.
At this time it is impossible to draw any firm general conclusions. The results stated compare only the data from the spring of 1993 and 1994. Due to lack of data from the fall of 1994 comparison on two complete field seasons is impossible. Therefore any inferences here may not apply once the second field season is completed. However, if trends continue to show significantly more prairie and savanna species from the 1994 season compared to the 1993 season at the burned site it could be determined that the burning of a degraded oak savanna did significantly affect the understory herbaceous species by increasing the percent cover of the prairie and savanna species.
The soil samples may contain seeds that were produced by historic understory species of the oak savanna, and the litter samples contain seeds from plants that exist on the sites now, and seeds that may have blown in that year. The trays with no leaf litter had significantly more germinates. This increase was determined to be the result of the increase in light reaching the germinating seeds. It was also noted that the soil in the trays with burned leaf litter and unburned leaf litter retained water longer than the trays with no leaf litter. The decreased moisture may have also played a role in the increase in seed germination. More interactions will be examined once all temperature and light regimes are completed.
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