Stand development and the competitive ability of oak ( Quercus spp.) following silvicultural clearcutting on the Hoosier National Forest

Justin Travis Swaim, Purdue University

Abstract

The current failure of oak (Quercus) regeneration across much of the Central Hardwood Region (CHR) is a cause for great concern because of the economic and ecological importance of the genus. Oak species are failing to establish and adequately compete under contemporary disturbance regimes despite their long history of eastern forest dominance. Silvicultural clearcutting was heavily implemented in the 1960s–1980s in an attempt to successfully regenerate oak stands, but its effectiveness is now questioned as many clearcut stands have transitioned into later stages of stem exclusion dominated by other species. In 1988, a study was implemented by the USDA Forest Service North-Central Forest Experiment Station on the Hoosier National Forest in southern Indiana to examine the fate of oak species and their competitors within developing stands following clearcut harvests. Permanent overstory plots were established in six harvest units using a nested design to capture a variety of topographic and edaphic conditions. All stands were characterized as an oak-hickory (Quercus-Carya) forest type at the time of canopy removal. Pre-harvest overstory plot data were collected in 1988 and plots were resampled in 2011, allowing documentation of shifts in species composition over the 23 year period (1988-2011). Aerial photos from the 1930's were used to determine canopy cover and likely historic land-use of each stand prior to incorporation into the HNF. In order to precisely characterize edaphic conditions, soil samples were collected and analyzed in 2011. Non-metric multidimensional scaling (NMS) was used to examine species composition along topo-edaphic and historic canopy cover gradients. Three permanent reproduction plots were established within each overstory plot to investigate the influence of site and seedling physical attributes on height growth and establishment success of oak reproduction in post-harvest stands. Prior to harvest, an array of physical characteristics were documented for individual stems of advance reproduction. The same characteristics were remeasured in years 6, 12, and 23 for all reproduction types (advance reproduction, stump sprouts, and new seedlings). Random Forest (RF) analysis and linear mixed-effects models were used to determine the best environmental and physical predictors of height growth for oak species and their competitors in developing stands. NMS displayed drastic declines in the importance of white oak ( Quercus alba), black oak (Quercus velutina), and chestnut oak (Quercus prinus) across all stands following harvest. During the same time period, I observed large increases in the importance of other species, with red maple (Acer rubrum) showing large increases on the driest sites and yellow-poplar (Liriodendron tulipifera ) displaying the greatest increase across all sites. My results suggest that silvicultural clearcutting has driven stand demographics towards species favored by infrequent large-scale disturbance events. In pre-harvest stands, chestnut oak was confined to the poorest sites and displayed the strongest association with the historically closed canopies on sites where forests were not cleared due to poor soils and steep terrain. In post-harvest stands, yellow-poplar and black cherry (Prunus serotina) were strongly associated with more nutrient-rich sites, while bigtooth aspen (Populus grandidentata ) and red maple were associated with more xeric sites with lower nutrient levels. Random Forest analysis showed that advance regeneration of oak species fared poorly following clearcut harvests. Sprout-origin oak stems proved stronger competitors in developing stands, although their abundance was quite low. Advance and sprout origin maples (Acer spp.), along with new seedlings of black cherry and yellow-poplar, quickly overtopped advance oak stems in the developing canopy. The height of stems during prior sampling periods was the best overall predictor of stem height. Species was also an important predictor, but environmental variables were generally poor predictors of height growth in individual stems throughout the study. Of all environmental variables, percent slope was the best predictor of stem height, a finding that was also supported by analysis with linear mixed-effects models. My results suggest that overstory removal has driven stand demographics towards species favored by infrequent large-scale disturbance events such as clearcutting. Without secondary treatments such as prescribed burning or herbicide to control competitors, the few oak stems that successfully regenerate following harvest are highly unlikely to persist in developing stands. The successful regeneration of oak species will require multiple treatments and should be viewed as a long-term commitment to promoting both biological and functional diversity in these stands.

Degree

M.S.

Advisors

Jenkins, Purdue University.

Subject Area

Ecology|Forestry

COinS