Stress factors associated with forest decline in xeric oak forests of south-central United States

Near total canopy mortality occurred over several tens of hectares of post oak-blackjack oak forests in the Cross Timbers of south-central United States in 2008. This event closely followed extreme drought in 2006 and a region-wide late spring frost in 2007. Oak decline could contribute to the vegetation pattern of the Cross Timbers which is characterized as a mosaic of forest, savanna and grassland. We investigated the roles played by various stress factors involved in this oak decline. The suspected long-term predisposing factors, short-term inciting factors, and contributing factors ultimately responsible for oak decline were examined after the decline using Geographic Information Systems (GIS) analysis, binary logistic regression, climate data and by direct observation of trees affected. Analysis of predisposing factors showed areas affected by oak decline tended to be close to water, on low elevation steep slopes facing northeast, east or southeast. Proximity to water possibly led to underdeveloped root systems that made trees more susceptible to drought. The extreme drought of 2006 was the fourth most severe since 1895 and may have played a role in the decline by making trees more susceptible to other stresses. A false spring in 2007 was suspected to be a major inciting factor, as its potential to cause cavitations and permanent damage to important conducting vessels in early wood could have led to significant stress for trees already weakened by drought. Three weak fungal pathogens that may have contributed to oak decline were isolated and identified: Biscogniauxia mediterranea, Botryosphaeria obtusa and Discula quercina. Global climate change projections suggest increases in intensity and frequency of extremes in precipitation and temperature events such as severe drought and late spring frost. These deviations could increase oak decline and thereby contribute to changes in the vegetation patterns in ecotones such as the Cross Timbers where there is a mosaic of forest, savanna and grassland.