Midwest oak savannas are among the rarest terrestrial ecosystems globally and contain some of the highest plant diversity in North America. Understanding diversity patterns within these ecosystems is crucial for setting management and restoration goals. This study aimed to understand the role of canopy cover and heterogeneity in canopy cover in influencing plant alpha and beta diversity within oak savanna ecosystems. Conducted in the black oak savannas of the Indiana Dunes along the southern shores of Lake Michigan in NW Indiana, USA, we sampled twenty-nine 1000 m plots with gradients in canopy cover and canopy heterogeneity to examine their influence on plant diversity and composition. Woody plant alpha diversity was highest in areas with high canopy cover (>75%). C3 graminoids, C4 graminoids, and sedge alpha diversity were highest in areas with low canopy cover (<25%). Forb and legume alpha diversity were highest with low to intermediate canopy cover (25–50% and 20–35%, respectively). Overall, plant alpha diversity was highest at intermediate canopy cover. High canopy heterogeneity was associated with high alpha diversity. There was no relationship between canopy cover or canopy heterogeneity and beta diversity. However, beta diversity was exceptionally high within all sites. An average canopy cover of 25–50% is optimal for promoting high plant diversity across many functional groups. Still, both low and high canopy cover sites are important for maximizing the diversity of some functional groups. Therefore, we recommend managing oak savannas for 25–50% canopy cover to maximize diversity. However, where opportunities exist in large remnants of oak savanna/woodlands, maintaining some areas with few trees and other areas with closed canopies can maximize diversity on a landscape scale.
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