1993 Proceedings of the Midwest Oak Savanna Conferences
LOWLAND OAK SAVANNAS: GROUNDLAYER COMPOSITION AND DISTRIBUTION IN RELATION TO LIGHT AND SOIL MOISTURE
A detailed study of three lowland oak savannas was undertaken to provide baseline data on the composition and differential distribution of groundlayer species along gradients of light and soil moisture. The light environment was quantified using hemispheric photography. As lowland savannas are wetlands, site hydrology determined soil moisture levels. Preliminary results indicate that the groundlayers of lowland savannas are diverse, containing species associated with floodplain forests, sedge meadows, and wet to wetmesic prairies. Sedges were an important component of the groundlayer; species included Carex laeviconica, a rare sedge in Wisconsin. The distributions of individual groundlayer species showed affinities to different regions along gradients of light and soil moisture. The groundlayer of lowland savannas may be diverse because of the complex matrix of microhabitats created by the overlapping light and soil moisture gradients.
Oak savannas occurring in wet to wetmesic areas are dominated by a scattered canopy of swamp white oak (Quercus bicolor ) rather than the characteristic bur oak (Q. macrocarpa ), black oak (Quercus velutina), white oak (Quercus alba), and Hill's oak (Quercus ellipsoidalis) of upland sites (Curtis 1959). The groundlayer composition of lowland savannas, however, remains little known. Bray (1955) did not study the groundlayer of lowland savannas, because such remnants were extremely rare and all seemingly degraded through intensive livestock grazing.
Oak savannas currently cover less than 0.1% of their former extent (Nuzzo 1986) and lowland savanna variants are extremely rare and may be potentially threatened. Lowland savannas are of importance to conservationists both as uncommon savanna variants and as rare wetlands.
Like their upland counterparts, lowland savannas lie at the interface of forest and grasslandin this case, between floodplain forest and wet prairie or sedge meadow. Like other savannas, they are mosaic communities, with patches of lightly and densely shaded areas in the understory. Bray (1955, 1958) identified the importance of the light gradient in relation to groundlayer species distribution in oak savannas, but his measurements of light availability were crude because of the limitations of the technology then available.
As wetlands, lowland savannas occur at the interface of land and water. Lowland savannas are distinguished from other savanna variants by their hydric soils. Curtis (1959) showed that soil moisture influences plant associations. Though Bray and Curtis did not actually study the groundlayer of lowland savannas, they believed that they were analogous to wetmesic prairies (Curtis 1959).
The research summarized below examines the composition and differential distribution of groundlayer species in relation to light and soil moisture in lowland savannas. Light availability was quantified via computer analysis of hemispheric canopy photos (Chazdon and Field 1987).
Preliminary results suggest that the groundlayer of lowland savannas is diverse and includes species associated with floodplain forests, sedge meadows, and wet to wetmesic prairies. Three species of sedgeincluding Carex laeviconica, a rare sedge in Wisconsinmay be unique to lowland savannas. Diversity of the groundlayer of lowland savannas may result from the complex matrix of microhabitats created by the overlapping gradients of light and soil moisture.
The composition and horizontal patterning of lowland savanna groundlayers were studied at Avoca Prairie and Chiwaukee Prairie, two designated state natural areas in Wisconsin. The two preserves are both large, covering 1885 and 350 acres respectively. Avoca Prairie is the largest native tallgrass prairie/savanna complex east of the Mississippi River. It is located on the sandy terrace of the lower Wisconsin River and is owned and managed by the Wisconsin Department of Natural Resources. Because of the size and quality of this preserve, two sites were chosen for study, at the east and west ends respectively. Chiwaukee Prairie, a Nature Conservancy preserve, is located along Lake Michigan and contains the most diverse prairie in Wisconsin. Avoca Prairie contains floodplain forest , while Chiwaukee Prairie does not.
At each site, individual trees were tallied by species. Preliminary data on soil texture and cation concentration were gathered using 3 random samples below tree canopies and 3 in openings for each site.
Sites were sampled for understory compostion three times during the growing season (May, July, September), to allow for differences in phenology among species. Species lists were compared with those compiled by Curtis (1959) for floodplain forest understories, sedge meadows, and wet and wetmesic prairies.
Light and Species Distributions
Species distributions were examined using quadrats stratified along the gradient of light availability. Single belt transects radiating from the boles of 12 randomly selected trees were randomly oriented toward one of four compass points (north, south, east, west). Each transect extended 1.5 times the canopy radius beyond the dripline into an open area. Five 1 m2 quadrats were placed at equal intervals along each transect, for a total of sixty 1 m2 quadrats at each of the three sites. The quadrats were used to calculate percent species cover and frequency of rooted plants.
The light regime of each quadrat was quantified using computer analysis of fisheye canopy photographs. Such analysis provides data on the number, daily timing, and duration of sunflecks expected at different times of year, and on the estimated total photosynthetically active irradiance. Research by Pearcy (1988) has shown that such measurements have considerable ecological significance in terms of plant growth. Because irradiance at a given microsite varies by orders of magnitude with time of day, time of year, and degree of cloudiness, spot measurements from light meters, such as those employed by Bray (1959), are much less meaningful as environmental measurements. Fisheye canopy photographs were taken above all 60 quadrats at each of the three sites for a total of 180 photographs. For species of interest lying outside of quadrats, additional photos were taken above a statistically appropriate number of individuals of each species. Photographs were digitized and analyzed, and estimated light availability data were correlated with rooted frequency data to examine the relationship between irradiance and species distribution. Trends in species richness (a diversity), or the number of species per quadrat, were also related to the light gradient.
Soil Moisture and Species Distributions
The elevated water tables in lowland savannas offer unique research opportunities unavailable in upland sites. Though the detailed hydrology of Chiwaukee Prairie is not known, a regional study by Hutchinson (1970) shows that the water table in the Lake Michigan drainage basin tends to be flat and 03 feet below soil surface. Hydrologic studies of the Lower Wisconsin River terrace indicate a similar tendency. While the water tables at both sites tend to be flat the ground surface is undulating. Elevation relative to site was used as an approximation of depth to water table or an indirect measurement of soil moisture, which fluctuates seasonally with floods, rainfall, and evaporation. The sixty sampling quadrats at each site were surveyed to determine relative elevation using trigonometic leveling and Theodolite equipment.
The lowland savanna remnants are dynamic and complex systems. High river levels flooded Avoca Prairie in the spring. Chiwaukee was not flooded, but depressions were inundated by onsite spring runoff. By midsummer, standing water remained in depressions at both sites, but slightly elevated areas became dry. The groundlayer was composed of both terrestrial and semiaquatic plants, and species diversity was high. Individual species favored specific areas along the light and soil moisture gradients.
The soils at all three sites were sandy, containing 16% organic matter. The soils at the Avoca Prairie sites were strongly to moderately acid with pH values ranging from 5.46.3. The soils at Chiwaukee Prairie, in contrast, were neutral with a mean pH of 6.8.
Swamp white oak were found to be most common at the west Avoca site, while a few river birch (Betulanigra) were also present. Swamp white oak also dominated the east Avoca site, but bur oak and river birch were observed as well. Chiwaukee Prairie supported mostly bur oak, but white oak (Quercus alba) and willow (Salix sp.) were also present.
Groundlayer Composition and Diversity
The groundlayer at all three sites was diverse. At the two Avoca sites, 55 to 83 species were identified, while 100 species were observed at the Chiwaukee site. The three sites contained 165 species in aggregate. The most common forbs (frequency of occurrence > 24%) and graminoids (frequency of occurrence > 16%) are listed (Table 1). Alpha diversity ranged from 2 to 26 species/m2, with 10 species/m2 as the mean value. Mean a diversity was higher under oak canopies than in open areas, but showed a closer relationship with distance from tree (Fig. 1) than with light availability (Fig. 2).
Light and Species Distributions
Several species showed affinity for a specific light regime. Wood nettle (Laportea canadensis) and lanceleaved violet (Viola lanceolata) or arrowleaved violet (Viola sagittata) favored shade (Fig. 3). Several other species also showed affinity for shade (Table 2). Meadow anemone (Anemone canadensis) and sensitive fern (Onoclea sensibilis) favored moderate shade (Fig. 4), as did other forbs (Table 2). Field mint (Mentha arvensis), cord grass (Spartina pectinata), and other species favored sun (Fig. 5, Table 2).
Cardinal flower (Lobelia cardinalis), bottle gentian (Gentiana andrewsii), and downy gentian (Gentiana puberula) occurred outside of sampling quadrats, and were chosen as the species whose light environments would be quantified with additional canopy photographs. Cardinal flower occurred primarily in shade (18 moles/m2/day); bottle gentian, in moderate shade (31 moles/m2/day); and downy gentian, in sun (47 moles/m2/day) (Fig. 6).
Soil Moisture and Species Distributions
Several species also showed affinity for a specific regions along the soil moisture elevational gradient. Carex bicknelii and lanceleaved violet (Viola lanceolata) or arrowleaved violet (Viola sagittata) favored wet microsites (Fig. 7). Several other species favored wet microsites as well (Table 3). Grassleaved goldenrod (Euthamia graminifolia) and blueeyed grass (Sisyrinchium campestre) favored wetmesic microsites (Fig. 8), as did other forbs (Table 3). Wild rose (Rosa sp.), meadow anemone (Anemone canadensis), and other forbs favored mesic microsites (Fig. 9, Table 3).
Correlation tests showed that light and soil moisture were not significantly related across quadrats within each site, indicating that they were independent environmental variables. Several species showed affinities for the microhabitats created by the overlapping of the light and soil moisture gradients (Table 4). Many species that showed an affinity for one gradient did not show an affinity for the other, and thus dropped out of this analysis.
The contrasting soil pH values between the two preserves is related to underlying regional substrates and coresponds with the difference in species richness between the two preserves. The acidic soils of Avoca Prairie lie over Cambrium sandstone, while neutral soils of Chiwaukee Prairie were deposited over glacial drift and Silurian dolomite (Martin 1932). The relatively high pH of the soils at Chiwaukee Prairie support “limeloving species”, which contribute to the increased diversity of the preserve.
Bray and Curtis note that swamp white oak was the major tree of lowland savannas, but do not list the nondominant tree species (Curtis 1959). The presence of fireintolerant trees (Betula nigra, Salix sp.) at all three sites may be a result of low fire frequency or intensity. Standing water in lowland savannas probably blocks and retards wildfire.
The groundlayer composition of lowland savanna remnants did include a large number of species identified by Curtis (1959) as modal and prevalent in wetmesic prairies, but they also contained a significant number of species associated with other communities. Study sites contained 1/3 of Curtis' modal or indicator species for wetmesic prairie and 2/3 of species prevalent in that community. Remnants also contained species indicative of floodplain forests, sedge meadows, and wet prairies. Study sites included 1/3 of Curtis' indicator species for southern wet forest(floodplain forest), 1/6 of his indicator species for southern sedge meadow, and 1/2 of his indicator species for wet prairie. These preliminary results suggest that lowland savanna groundlayers are not actually analogous to those of wetmesic prairies.
Sedges were an important component of the groundlayer. They accounted for a significant number of the common graminoids. Three of the common sedgesCarex conoidea, Carex haydenii, Carex laeviconicawere not modal in any of the communities defined by Curtis. Though Carex laeviconica was abundant at one site, it is generally rare in Wisconsin, as here it reaches the northern limit of its range. This suggests that at least these three sedges are unique to lowland savanna communities. Other uncommon sedges may be located in high quality lowland savanna remnants throughout the Midwest.
Alpha diversity may be closely associated with distance from trees rather than light availability because of the associated factors of leaf litter, precipitation, and vapor pressure. Litter accumulation may result in higher soil organic matter and nutrients in canopycovered soils than in adjacent open areas (Zinke 1962). Leaf litter also effects soil moisture (Lodhi and Johnson 1989, Kucera 1952), as do precipitation and vapor pressure (Kittredge 1948, Kucera 1952). In lowland savannas, however, the influence of these factors on soil moisture is likely insignificant compared to that of site hydrology.
A significant number of species did show affinities for specific points along the independent gradients of light and soil moisture, which overlap to form a complex matrix of microsites. The variety of microsites within lowland savannas may account for the overall species richness of their groundlayers.
I thank Thomas Givnish and Eric Kruger of the University of Wisconsin for their guidance in research and their assistance in preparing this manuscript. I also thank Jeb Barzen, Mark Leach, Richard Beilfuss, Nancy Braker, and Li Fengshan for their ongoing support. Theodore Cochrane kindly assisted me in identifying sedges, and Robert Kowal helped with other taxa. This research was supported by a grant from The Nature Conservancy.
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