Gradients in the Composition, Structure, and Diversity of Remnant Oak Savannas in Southern Wisconsin

Abstract

Savannas with scattered oak canopies, once the most widespread communities in southern Wisconsin, now are among the most endangered. Surprisingly little is known about the composition, structure, and horizontal patterning of their species-rich ground layers. This study relates the distribution and ecological characteristics of 417 ground-layer species to local and regional gradients in soil composition and light regime, based on an analysis of 722 1-m 2 quadrats in 12 remnant savannas. Our findings have important implications for efforts to restore/conserve midwestern oak savannas.Ground-layer composition was strongly related to among-site differences in soil texture and within-site differences in light availability, with variation in sand vs. silt content accounting for twice as much turnover in species composition as that accounted for by direct photon flux density (PFD) estimated from hemispherical photographs. Most species reached peak coverage under sunny or partly shaded conditions. Flowering/fruiting was often skewed toward sunnier microsites.Absolute forb cover increased with silt content and declined with PFD. Graminoid cover showed a curvilinear relationship to soil texture and light, being highest in well-lit, moderately sandy microsites. Total ground-layer cover increased with silt content at a given irradiance; it increased with PFD on silty sites and decreased with PFD on sandy sites. Forb cover increased regularly with PFD and sand content. When quadrats were stratified by sand content and PFD, species richness of forbs and graminoids increased linearly with coverage by each group, with far more forb species present at a given coverage.Among graminoids, C 4 grasses were common only in bright, sandy microsites; C 3 grasses and sedges had broader ecological distributions. Among forbs, leaf width increased and leaf inclination became more horizontal toward shadier and siltier microsites; tall herbs were generally found in silty areas with dense ground-layer cover. Plants with N-fixing symbioses were found mostly on sandy, well-lit microsites, although climbing species occured on shadier and/ or siltier microsites, where N-fixation might be expected to be less advantageous. Most groundlayer species were perennial (88%), with few annuals (6%) or biennials (6%).For the 85 most common species, breadth of distribution across savanna microhabitats (4 soil ϫ 4 light) was significantly correlated with presence across 34 Wisconsin community types, suggesting that similar factors help to constrain distributions at local and regional scales. Species in the two largest genera (Aster and Solidago) differed significantly in distribution according to the Syrjala test, supporting habitat partitioning as a mechanism of coexistence.Oak savannas are unusually diverse. At small spatial scales, there were 16.1 Ϯ 1.3 species/m 2 (mean Ϯ 1 SD), compared with 11.4 Ϯ 2.7 for prairies and 8.2 Ϯ 2.5 for forests. At slightly larger scales, savannas showed high spatial turnover in ground-layer composition: 89.3 Ϯ 12.0 species/20 ...

Relationships among arbuscular mycorrhizal fungi, vascular plants and environmental conditions in oak savannas

“…Given that AMF and plants might respond directly to light and soil gradients, or indirectly to effects of the latter on partners or enemies of a given fungus or plant, there is the potential for complex patterns of association among AMF, plants, and environment. Since oak savannas are among the most endangered communities in the upper Midwest (Curtis, 1959;Nuzzo, 1986;Leach & Givnish, 1999), the results of this study may have fundamental implications for the role of mycorrhizal diversity and composition in their conservation and restoration.…”

“…We conducted studies in three remnant oak savannas in southern Wisconsin, USA, that span the range of soil textures found in the 12 such savannas investigated by Leach & Givnish (1999). Marshall is a privately owned, remnant wet savanna/sedge meadow on loamy soils in the floodplain of the Maunesha River (43 ° 10 ′ N, 89 ° 10 ′ W ).…”

“…One survey found 417 plant species in 722 m 2 across 12 remnant Wisconsin savannas ( Leach & Givnish, 1999). This represents about 22% of the state's vascular flora, and c .…”

“…To elucidate the relationships among plant communities, AMF communities, and the physical environment, we surveyed three remnant oak savannas in southern Wisconsin that vary internally in light regime and collectively span the range of a previously investigated soil-texture gradient ( Leach & Givnish, 1999). We used the resulting data to determine whether plant and AMF species richness and composition are correlated with each other and /or light availability and soil texture and fertility.…”

“…Studies of Wisconsin oak savannas have shown that the understory composition correlates with gradients in soil texture/fertility and light availability. The soil gradient involves collinear increases in per cent clay and silt, Kjeldahl N content, exchangeable calcium (Ca), magnesium (Mg) and per cent organic matter, and explains roughly twice as much variation in plant species composition as light availability, which varies from full sun in openings to deep shade near tree boles (Leach, 1996;Leach & Givnish, 1999;Meisel et al ., 2002). The AMF response to such gradients and/or vascular plant composition is, however, almost unknown, with just one study relating AMF composition to the single factor of light availability across a natural prairie-savanna-forest gradient in Illinois ( Benjamin et al ., 1989).…”