Curtis Prairie: World’s oldest restored prairie or newest restoration challenge?

Background/Question/Methods Despite 80 years of work to restore a horse pasture, Curtis Prairie is on a trajectory away from native grassland. Since 1935, the UW-Madison Arboretum has aimed to achieve a sustainable tallgrass prairie on ~72 acres in south central Wisconsin. At age 31 years (1966), the site was a species-rich prairie with 212 native species (census of PZ&JZ), but at age 56 (1991), two native shrubs had increased frequency: Cornus racemosa occupied 56% of 951 1-m² plots in 1991 (15% in 1966), and Rhus glabra occupied 8% (<1% in 1966). The 2002 census found 23 woody plant species, of which two remained frequent (52% C. racemosa and 11% Rhus glabra). Burning experiments indicated that shrubs reduced grass cover (fuel) and further limited the ability of fire to reduce shrubs (a strong feedback). Meanwhile, under the influence of urban runoff, native and exotic invaders reduced native-plant diversity in wetland areas. Harvesting shoots in a 2008 subsample showed differential effects of high biomass in upland plots (no effect on diversity) versus wetland plots: dominance by a tall clonal graminoid (Calamagrostis canadensis, Carex stricta, or Phalaris arundinacea) and reduced species richness (JD&JZ). At age 79, a GPS-based map (polygons of wetland dominants) characterized 22% of Curtis Prairie as wetland. Given woody upland and low-diversity wetland, we ask: Should we abandon the 1934 target or “re-restore” diverse prairie vegetation?

Results/Conclusions   Long-term monitoring and research document compositional shifts in Curtis Prairie, which trace to an urban context that limits burning and augments eutrophication. Without renewed restoration, woody plant cover will continue increasing, and wetland diversity will remain low. Some suggest that we declare the site novel, abandon Leopold’s 1934 vision of pre-settlement prairie, and establish a new target (shrub carr?). We recommend instead establishing large field experiments to test alternative methods to re-restore diverse prairie (adaptive restoration), to fulfill the mission for this global restoration icon.