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 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 ...

No abstract

Long-term legacy data reveal that the pace of plant community change in remnant prairies has accelerated.