Open forest ecosystems: An excluded state

Once dominant but now largely excluded from eastern North America, open forests of savannas to woodlands occupy the ecosystem gradient between grasslands and closed forests. These fire-maintained systems differ in structure, processes, and species from closed canopy, succession-driven forests that currently dominate this region. In functional open forest ecosystems, frequent, low to mixed severity and intensity surface fires limit tree regeneration, depending on factors such as overstory tree density, resulting in relatively stable structure where overstory trees co-exist with a largely herbaceous understory. Reduced and spatially variable tree densities in open forests result in unique environmental conditions and function. Trees in open forests typically represent a small fraction of the biodiversity, which instead resides in the rich herbaceous ground layer. Rather than being constrained by overstory disturbances, succession, and biological legacies, the permanently open structure and herbaceous communities of open forests support invertebrate and vertebrate species throughout their lifetimes. Transition from open to closed forests across most of eastern North America during the past century produced a "new normal," in which excluded open forests remain largely unrecognized at considerable conservation costs, particularly loss of key processes and wildlife species associated with a matrix of co-dominant tree and herbaceous layers. Management for open forests emphasizes the understory herbaceous plant community, similar to ephemeral seral stages of successional forest, rather than tree regeneration to produce an alternative outcome in structure, function, and support for biodiversity.