ASSESSING PRESCRIBED FIRE EFFECTS ON OAK REGENERATION, WOODY COMPETITION, AND WILDLIFE FORAGE QUALITY AMONG THREE DIFFERENT IRREGULAR SHELTERWOOD TREATMENTS IN CENTRAL WISCONSIN

Abstract

Oak woodlands and savannas are among the most fire-dependent ecosystems in Wisconsin. Fire exclusion and suppression over the past 100 years has significantly decreased the extent of fire-dependent ecosystems resulting in conversion to closed canopy mixed hardwood stands with excessive understory shading that suppresses oak regeneration. Oak woodland restoration commonly employs several variants of shelterwood systems in combination with prescribed fire. Shelterwood cuts retain variable oak crown cover favoring oak regeneration, while low intensity prescribed fire reduces woody competition. Based on the combined positive effects of shelterwood cuts and prescribed fire application, our study sought to assess the effects of prescribed fire in irregular shelterwood systems of varying retention levels. Five dry, nutrient-poor research sites were selected in central Wisconsin due to the prevalence of sufficient oak overstory composition. Our aim was to assess fire effects on oak regeneration and competing woody vegetation based on varying tree regeneration height classes and basal diameters within four shelterwood overstory retention levels (45%, 30%, 15%, and 0%). Sites were harvested between 2007-2009. Time constraints associated with weather and burn crew logistics only allowed for one research site to be burned in May 2024. The burned site had substantially greater regeneration of white oak (217.4%), red oak (87.8%), and black cherry (19.1%) compared to the adjacent unburned site, while red maple regeneration was 91.2% lower. Logistic regression was used to identify species-specific survivability trends based on basal diameter and six, 2-foot tree height classes. Basal diameter was the strongest predictor of avoiding top kill, with white and red oaks surviving at smaller diameters than black cherry, with a 50% survival rate at 69.1-mm, 71.7-mm, and 88.2-mm, respectively. Top killed individuals sent up more resprouting shoots than trees that were not top killed, with oaks having the greatest amount of resprouting stems. Strip-head firing operations yielded in-situ fire intensities ranging from 67.4-436.0 BTU ft•sec-1, which exhibited a decreasing trend based on increasing % canopy retention, with the 45% and 0% retention treatments yielding the least and greatest intensity, respectively. Ground level and variable height (6-, 12-, 18-inch) heat sensitive paints exceeded the lethal temperature of 60°C in almost all fire behavior plots. However, observed fire intensity remained variable across much of the research unit resulting in no discernable survivability trends across treatments. Oak advance regeneration survivability was virtually absent below the 10-foot height class, with trees greater than 10-feet tall more likely to avoid top kill. In sum, our data suggests that oak regeneration is highly favored by prescribed fire, especially with white oaks. Furthermore, delayed fire treatments after an overstory harvest in dry, nutrient-poor oak woodlands outside of the widely accepted 5–7-year interval, in this case 15-17 years, may still yield positive results in terms of the ecosystem pyrogenicity and sufficient oak regeneration size classes capable of surviving fire or vigorously resprouting if top killed.