The two co-dominant species of the Crosstimbers are post oak (Quercus stellata Wangenh.) and blackjack oak (Quercus marilandica Münchh.), yet little is known about how these two species differ in physiological response to heat, drought, and biotic attack. By determining the variable response to water stress between these two species, we may better explain the higher mortality rates experienced by Q. marilandica and their increased susceptibility to Biscogniauxia atropunctatum. Through a combination of fieldwork and laboratory experimentation, this study intends to elucidate this relationship in order to understand the role of water stress and B. atropunctatum in oak mortality. As a preliminary experiment, we investigated the pathogenicity of B. atropunctatum within ten 500 m2plots at McPherson Preserve in central Oklahoma measuring: DBH, tree height, disease class, canopy position, and health status. As the vegetative season progresses, predawn and midday xylem water potential measurements will be taken from 20 randomly selected healthy individuals of both species from April-October in order to decipher the varying susceptibility to hydraulic failure between the two species.
Our preliminary analysis indicated that B. atropunctatum affects Q. marilandica significantly more than Q. stellata; 76% and 32% of healthy individuals, respectively. Within the ten 500 m2 plots, we found that 46% of the blackjack oaks sampled were dead, while only 10% of post oaks suffered the same fate. Additionally, we found that increased density of a native invasive—Juniperus virginiana—did not result in higher rates of pathogenicity of B. atropunctatum suggesting that variation in drought sensitivity may be the driving force behind pathogenicity. Through further investigation, we may better anticipate the future impacts of warmer temperatures and increased drought frequency on pathogen susceptibility within the Crosstimbers and, in turn, how these factors may impact the community composition.