Effects of wildfire and Paulownia tomentosa on native tree regeneration

A wildfire occurred in Shawnee State Forest located in southern Ohio that consumed 1215 hectares. Based on earlier forest inventories it was known that paulownia (Paulownia tomentosa), a non-native invasive tree species, occurred in the forest. The objective of this study was to determine if paulownia heavily colonized areas two years after the fire where the burn occurred, and if its presence had a negative impact on the regeneration (<137 cm height) of native species—red and white oaks (Quercus sp.), red maple (Acer rubrum), and yellow-poplar (Liriodendron tulipifera). Two years after the fire, paulownia had invaded the burned areas but not at significantly higher densities than occurred in the unburned areas. Fire significantly reduced the number of regenerating stems of white oak and red maple two years after the fire, whereas the number of regenerating stems of red oak increased slightly and that of yellow-poplar increased significantly. In areas where paulownia occurred that experienced wildfire, all species studied displayed a reduction in the number of regenerating stems compared to paulownia’s absence in the burn areas. Where paulownia occurred in areas not affected by the wildfire, all the native species studied displayed a reduction in the number of regenerating stems. The average heights of red oak, white oak, and red maple were significantly taller when growing in areas affected by the wildfire due to a more open canopy. However, there was no significant change in the average heights of yellow-poplar. The presence of paulownia in both the burned and unburned areas reduced the number of regenerating stems of the native species studied.

Methods
Paulownia specimens for this study were first located in the burned areas off of hiking trails, logging trails, and dozer lines both in the burned and unburned areas. Each located specimen was tagged, and its GPS location saved and logged for future reference. Paulownia specimens in unburned areas were located along the outer boundaries of the burned area to try to ensure similar soil, topographic, geologic, and moisture characteristics for proper comparison between burned and unburned areas. Of the more than 200 paulownia specimens that were located in both the burned and unburned areas, a total of 61 specimens were randomly selected—28 and 33 specimens in the burned and unburned areas, respectively. Each of the randomly selected paulownia stems served as the plot center, and the minimum possible density of paulownia within a plot was one stem. Plots without paulownia trees were established and paired with the plots containing paulownia trees within 75 m of the paulownia plots in the respective burned and unburned areas, having similar slope steepness, aspect, elevation, and soil type as the paired paulownia plot. The 61 plots containing paulownia trees and the corresponding paired plots without paulownia trees provided a total of 122 sample plots. Accordingly, three treatments were evaluated: fire and paulownia present (F-P), fire and no paulownia present (F-NP), and no fire and paulownia present (NF-P). Plots established in unburned areas with no paulownia present (NF-NP) served as a control.

Sample plots were 10 X 4 m in size and were divided into 5 X 2 m quadrants to facilitate measurements. Within each plot, the total height (to the nearest 1 cm) of each paulownia stem was recorded with the use of a telescoping height pole. Paulownia stems that split below 137 cm height were counted as multiple stems. Canopy closure was measured at the center of each plot with the use of a densiometer.

For this study four important native species or species groups were evaluated. These species were red maple (Acer rubrum), yellow-poplar (Liriodendron tulipifera), the red oak group (Q. marilandica, Q. rubra, Q. velutina), and the white oak group (Q. alba, Q. montana). The focus of this study was on the early regeneration of these native species, which was defined as all stems less than 137 cm in height. A count of each species or species group was performed in each quadrant and summed for the total plot. Three seedlings per species or species group were randomly selected from each quadrant, providing 12 stems per species per plot, and the total height to the nearest 1 cm was measured and recorded. The recorded heights from each quadrant were combined to determine an average height for the plot by species or species groups.