| Since European settlement, clearing for agriculture, changes in hydrology, and urbanization have reduced the coverage of bottomland forest (BLF) in the Upper Mississippi Valley (UMV) by 46%. Recently, emphasis has been placed on restoring BLF, which provides vital ecosystem services (e.g., enhanced water quality, nutrient cycling, and wildlife habitat). Beginning in 1998, the US Army Corps of Engineers has restored BLF on many sites in the UMV, ranging in area from less than 1 ha to 120 ha. Root production method (RPMRTM) seedlings of three species of bottomland oaks and pecan have been planted to rapidly establish large-seeded species that are not regenerating under current conditions, with the expectation that light-seeded species (e.g., silver maple, green ash, eastern cottonwood, elm) will colonize passively. A chronosequence of nine restoration sites, ranging in age from 1 to 23 yr since planting, and two mature BLF reference sites was used to assess restoration success. Five 0.1-ha circular plots were randomly located at each site. Planted trees and natural recruits with a diameter at breast height (DBH) greater than or equal to 2.5 cm were identified, tagged, and their basal diameter, DBH, and height were measured. Density of shrubs was assessed in belt transects with a total area of 100 m2 and cover of herbaceous species was estimated in twenty 0.5 m2 quadrats. Tree variables by species (mean basal diameter, mean height, density, and dominance) and community variables (richness and Simpson diversity of each stratum, total tree dominance, total shrub density, total herbaceous cover and the percent exotic herbaceous species cover) were calculated at the plot scale. Trajectories of change in tree size and community structure were examined using generalized linear modeling, relative to their values in reference sites. Tree height and diameter increased with time since restoration for all species. Quercus palustris, Q. macrocarpa, and Q. bicolor are all on track to achieve dimensions typical of mature BLF within 27 to 37 yr since planting. However, the dominance models for these trees show general declines which may indicate decreasing survivorship among planted trees and no recruitment of new seedlings. Pecan has suffered high mortality and without replanting it will be underrepresented in the restored forest. Tree dominance, richness, and diversity peaked and then decreased. Both total shrub density and total herbaceous cover showed no trend with time, although diversity of both shrubs and herbaceous vegetation slightly increased. Comparison with reference sites suggest that the shrub density, although there is no trend with time, is still in line with reference plot values. Exotic cover peaked between 10 to 15 years and began to decline to levels similar to reference plots. Overall, the results indicate some restoration success (tree growth rates, shrub diversity, herbaceous richness and diversity, and declines in exotic species cover) but suggest that replanting will be necessary in most sites to overcome mortality due to prolonged flooding and other factors (e.g. white-tailed deer browsing, inhibition of tree recruits by dense grass cover). Accurate mortality data for planted tree species is necessary to evaluate and improve the success of future USACE restorations. Ideally, a subset of trees should be tagged immediately after planting and these trees should be monitored at regular intervals. Frequently updated records will allow the USACE to make site-to-site management adjustments. |
