Stand structure, composition, and regeneration dynamics following removal of encroaching woody vegetation from Midwestern oak savannas

Abstract

Woody encroachment has altered oak savannas throughout much of the Midwestern United States. To help understand restoration options, we assessed the impacts of mechanically removing encroaching mesophytic trees and shrubs on structure, composition, and regeneration dynamics in Quercus alba dominated oak savannas in Iowa ( n = 4), relative to control sites ( n = 4). We monitored stand structure and species composition for the seedling, shrub, sapling, and overstory tree strata for 1 year prior to and 3 years following mechanical removal of encroaching woody vegetation. There was no evidence for differences between treatment and control sites for any study variable prior to treatment implementation. The removal treatment resulted in increased cover by understory vegetation and concomitant reductions in cover by leaf litter and bare ground. Treatment altered overstory tree species composition (assessed by multi-response permutation procedure [MRPP]) in favor of oak species in all 3 years following removal, while shrub and sapling compositions were not statistically different from control sites until the third year following the removal treatment. Seedling composition was unaffected by treatments. We observed a recruitment pulse, with treated sites displaying increased density of shrubs 2 years after and saplings 3 years after removal of encroaching vegetation. Advanced regeneration (saplings size class) was dominated by two species: Ostrya virginiana and Cornus racemosa, which we attribute to vigorous vegetative reestablishment by stump resprouting. Seedlings of Q. alba increased in density throughout the course of this study; however, Q. alba shrub and sapling size classes were unaffected by treatment. We suggest that the encroached savannas in this study represent an alternative stable state, whereby regeneration is dominated by encroaching species even shortly after removal treatments. Continued research during future stages of restoration, which will include prescribed fire, may help to identify effective management options for controlling encroaching woody vegetation and promoting oak regeneration.