The effect of prescribed fire on Biscogniauxia infection and δ13C in an upland oak-pine forest

Incidents of Quercus decline have been driven by complicated interactions among factors, such as temperature, drought, pathogenic fungi, insect pests, and stand structure, which operate at multiple time scales. Past incidents of elevated tree mortality during drought in the upland forests of Oklahoma were associated with biotic factors; specifically, the infection of Quercus species by canker-causing fungi in the genus Biscogniauxia. Biscogniauxia canker and dieback have been previously associated with water stress but little is known about the interactions between forest densification, prescribed fire, water stress, and canker incidence. We investigated the effects of prescribed fire and reduced basal area on Biscogniauxia canker incidence in Quercus species at the Pushmataha Wildlife Management Area (PWMA) in southeastern Oklahoma. The PWMA is an ongoing forest management experiment established in 1984 with stands maintained to the present day with prescribed fire return intervals of 34 + yr, 4-yr, and 1-yr. Through field surveys we found that the incidence of Biscogniauxia canker was lowest among units with a fire return interval of 1-4 years (p < 0.002). Additionally, greater canopy dieback was associated with greater frequency of stem cankers. Management of these forests reduces basal area, alters water stress, and may reduce pathogen incidence due to both greater host resilience and reduction of sporulation sources. To evaluate associations among these factors, we assessed Biscogniauxia canker incidence and leaf stable C isotope composition (delta C-13) an indicator of stomatal closure from drought stress-from Quercus marilandica (blackjack oak) and Quercus stellata (post oak) trees in units experiencing 34 + yr, 4-yr, and 1-yr fire return intervals. We found that leaf delta C-13 was highest in units experiencing 4-yr and 1-yr fire return intervals, indicating that these trees more frequently had closed stomata, counter to our expectation that the reduced basal area associated with frequent fire would decrease delta C-13 due to decreased water stress. Instead, our results suggest that trees in units experiencing reduced basal area encounter additional stressors (e.g., wind, vapor pressure deficit, and high soil evaporation) that lead to stomatal closure. Additionally, higher delta C-13 in units receiving prescribed fire applications may be a response to frequent disturbance via direct effects of fire on tree water relations. Further investigation is needed to understand the interactions of fire regimes with water stress in these ecosystems. Although, we did not find a clear link to tree water status, basal area reduction decreased the incidence of Biscogniauxia canker, demonstrating that prescribed fire can be used to promote forest health.