Woody Plant Community in the Cross Timbers Over Two Decades of Brush Treatments

Abstract

The closed overstory of fire-adapted forests throughout the world precludes surface fires of sufficient intensity to open the overstory. The cross timbers, an upland oak forest that spans an area from Texas to Missouri, developed with frequent fire, but removal of fire since European settlement has increased canopy cover of dominant overstory trees. To assess response of woody plant species cover and plant community compositional trajectories to brush treatments that included herbicides and fire in the cross timbers, we analyzed a 20-year data set (1982-2001) from untreated pastures, pastures treated with tebuthiuron or triclopyr in 1983, and pastures treated with tebuthiuron or triclopyr in 1983 and burned periodically thereafter (tebuthiuron + fire and triclopyr + fire). We used analysis of variance to determine change in overstory canopy cover and understory canopy cover in response to experimental treatment, and we used detrended correspondence analysis to assess trajectories of species composition over time. Overstory canopy cover was reduced from more than 100% to less than 20% following initial herbicide application, but canopy cover increased subsequently to over 60% as herbicide-resistant woody species increased in all but the tebuthiuron + fire treatment. The most striking change in overstory occurred in the tebuthiuron + fire pastures, which contained almost 20% cover of sumac (Rhus spp.) by 1994, and in the tebuthiuron pastures, which contained > 40% cover of eastern redcedar (Juniperus virginiana L.) by 1994. Tebuthiuron, both with and without fire as a follow-up, influenced composition of the overstory more than did triclopyr. Application of tebuthiuron appears to be a long-term unidirectional disturbance in the cross timbers. Composition also remained distinct from pretreatment condition in triclopyr-treated pastures after almost 20 years of postherbicide succession. In contrast, the understory woody plant community was quite resilient to both herbicides.