Savannas of the prairie-forest ecotone in central North America were once common, but have declined dramatically. Agricultural conversion and fire suppression have left <1% of this system in its historic from. Although fire suppression has converted open canopied savannas to closed canopy woodland, this woodland state presents opportunities for restoration. In 2002, we initiated a large-scale restoration experiment, manipulating overstory tree density and prescribed fire, to understand restoration of fire-suppressed white oak (Quercus alba) savannas that have been encroached by mesophytic/fire intolerant tree species (e.g., Ulmus, Fraxinus, Acer) in Central Iowa. Here, we draw on studies of vegetation and ecosystem processes to explore restoration outcomes and challenges, with particular focus on plant biodiversity and carbon storage.
Results/Conclusions
These studies demonstrate complex and evolving dynamics in savannas under modified fire regimes and subsequent restoration. Restoration resulted in rapid changes to biodiversity and above-ground carbon storage; however, these changes were largely in opposite directions. Although restored sites support greater plant biodiversity, woody encroachment represents a major carbon pool, which is lost during restoration. Fire suppression results in positive net changes to above-ground carbon due to woody encroachment, which in turn results in reduced growth rates of residual savanna oak trees. Below-ground carbon dynamics, such as soil respiration and soil carbon, have changed more subtly or not at all, to date. Elevated understory light and vegetation densities following restoration suggest that below-ground carbon changes will occur; however, it remains an open question as to over which time periods this will happen. These studies suggest the need to consider tradeoffs between carbon sequestration and biodiversity conservation goals during Midwestern oak savanna restoration. Perhaps more philosophical than ecological in nature, this is an important consideration throughout the Midwestern savanna region – and beyond – as we plan for restoration of woody encroached systems in the face of global change.
