Search

We examined changes in plant species composition at Faville Prairie in southeastern Wisconsin, based on surveys in permanently-marked, 2 X 2 m quadrats in 1946-1948, 1976-1978 and 9005. Permanent quadrats were distributed across a wide soil moisture gradient. During settlement the site was never cultivated but was mowed for hay for several decades prior to 1946, and burned most years during the dormant season after 1946. Total species richness increased 47% between 1946-1948 and 2005, while species density per quadrat doubled. Only nine non-native species were present in 2005 and most were uncommon. Species density of habitat generalists increased nearly fourfold; moderate specialists increased 43%; and remnant specialists increased nearly twofold. Species density of endangered, threatened and special concern species (collectively conservation-dependent species) increased ninefold. Species density of C3 graminoids, C4 grasses, legumes, woody plants and forbs all increased through time. The majority of species density increases were attained by the 1976-1978 sampling period and remained high since. Elsewhere in Wisconsin, prairie habitat specialists have been declining. Our findings run counter to general trends throughout the state. It is likely that the change from annual mowing to annual prescribed burns accounts for the increases in species density., INTRODUCTION The tallgrass prairie has experienced more destruction and fragmentation than any other major ecosystem in North America (Samson and Knopf, 1994). Less than 0.1% of the original prairie within [...]

Prairies covered 800,000 ha in Wisconsin before European settlement, but today they occupy less than 0.1% of their former extent and are mainly restricted to small, isolated remnants in a [...], Recensuses of 54 Wisconsin prairie remnants showed that 8 to 60 percent of the original plant species were lost from individual remnants over a 32- to 52-year period. The pattern of species loss was consistent with the proposed effects of fire suppression caused by landscape fragmentation. Short, small-seeded, or nitrogen-fixing plants showed the heaviest losses, as did species growing in the wettest, most productive environments. The interruption of landscape-scale processes (such as wildfire) by fragmentation is an often overlooked mechanism that may be eroding biodiversity in many habitats around the world.

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.sup.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.sup.4] grasses were common only in bright, sandy microsites; [C.sup.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 ground-layer 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 x 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 ± 1.3 species/m: (mean ± 1 SD), compared with 11.4 ± 2.7 for prairies and 8.2 ± 2.5 for forests. At slightly larger scales, savannas showed high spatial turnover in ground-layer composition: 89.3 ± 12.0 species/20 [m.sup.2], compared with 41.5 ± 8.3 for prairies and 42.8 ± 8.0 for forests. At large spatial scales, a survey of 22 savanna remnants (42 ha) showed extensive floristic differentiation: 507 native plant species, [approximately]27% of Wisconsin's indigenous vascular flora ([approximately]14 x [10.sup.6] ha). Contrary to previous reports, Midwest oak savannas are forb dominated, except on the sandiest or sunniest microsites. Release of forbs from competition with [C.sup.4] grasses under partly shaded conditions may help to account for the high diversity of savanna ground layers relative to prairies. Divergent distributions of plants with different adaptations for energy capture, together with large variation within sites in ground-layer light regime and among sites in soil texture, suggest that partitioning of light and soil gradients is important for maintaining the high plant diversity of oak savannas. Mass effects (involving dispersal subsidies from favorable microsites) may also play a role in maintaining diversity within these mosaic communities. Key words: Aster; beta diversity; community composition; community structure; environmental gradients; forb; leaf height: nitrogen fixation; oak savanna: Quercus; Solidago; species richness; Syrjala test., INTRODUCTION Savannas with a scattered canopy of oaks covered millions of hectares in the North American Midwest soon after settlement (Gleason 1913, Anderson 1983, Nuzzo 1986, Leach and Ross 1995). [...]