Summary Armillaria root disease is a contributing factor to oak decline in the Ozark Mountains of central USA. We have identified Armillaria gallica, Armillaria mellea, and Armillaria tabescens in Quercus‐Carya‐Pinus forests of the region. Presence/absence patterns of each Armillaria species as well as all possible Armillaria species combinations were analysed by contingency tables and/or stepwise logistic multiple regressions with principal characteristics of the studied sites and forest stands, both quantitative and qualitative: geographic land‐type association, bedrock type, landform position, slope direction (aspect), soil type and soil surface stone cover, down woody debris, abundance and basal area of woody vegetation and decline mortality by species. Most decline mortality consisted of two red oak species (section Erythrobalanus, Quercus coccinea and Quercus velutina), which also were most sensitive to Armillaria infection. Site characteristics related to the distributions of Armillaria species and decline mortality were also related to the preponderance of Q. coccinea and Q. velutina, regional vegetation history (i.e. conversion of Pinus echinata stands to hardwoods), and the different strategies of territory acquisition and spread of the Armillaria species involved. The presence of A. gallica may reduce the activity of more virulent Armillaria species.

No abstract

Since the 1970s, oak decline has been a chronic problem throughout the oak-dominated forests of the Missouri Ozarks. Prior research indicates that environmental stress, particularly drought, leads to the onset of oak decline. Consequently, some scientists and managers have advocated thinning and intermediate harvesting to maintain or improve tree vigor and growth, thereby leaving stands less susceptible to pathogens and pests that are frequently the ultimate cause of mortality of declining trees. However, few studies have experimentally evaluated the effectiveness of cutting treatments for mitigating oak decline, and some scientists have cautioned that cutting in declining stands exacerbates the problem. We conducted a replicated, 14-year study in southeastern Missouri to determine if improvement harvests would reduce the severity of oak decline symptoms and increase forest growth compared with untreated stands. Although we found that improvement harvests did not mitigate oak decline, they did not make decline worse and had the benefits of increasing the diameter growth of trees in the residual stand. Even in the absence of the improvement harvests, more than 70% of red oaks that initially exhibited little or no crown dieback remained in the same crown dieback classes during the 14-year study period. We also observed that more than 50% of the red oaks that initially exhibited moderate to severe decline symptoms appeared to improve during the study, regardless of harvest treatment. In contrast, fewer than 18% of declining white oaks improved during the study period.