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Oak decline is a chronic problem in Missouri Ozark forests. Red oak group species are most susceptible and decline is reportedly more severe on droughty, nutrient-poor sites. However, it was not clear whether greater decline severity was caused by poor site conditions or is simply due to the greater abundance of red oak group species found on poorer sites. We conducted this study to determine whether oak decline severity in oakdominated Missouri Ozark forests is related to factors strongly influencing site quality including soil, landform position, and slope-aspect. We monitored the survival of 6606 dominant or co-dominant oak trees for 10 years and surveyed crown dieback on more than 1995 oak trees during a single year to determine if mortality and the presence of decline symptoms were related to site factors. Analysis confirmed that red oak group species had more crown dieback and greater mortality than did white oak group species. We also found greater red oak mortality on upper slope positions and where soils were gravelly and low in base cations. However, the abundance of red oaks was also greater in these same locations. Further analysis showed that if the initial abundance of red oaks was included as a covariate in the model, the site factors no longer were significant effects related to oak mortality. Moreover, we found that frequency of oaks exhibiting crown dieback was the same or sometimes greater on high quality sites. These findings show that red oak mortality is more prevalent on droughty and nutrient-deficient sites because red oak group species are more abundant there. Rather than simply predisposing oaks to decline, droughty and nutrient-deficient site conditions most likely favored the establishment and growth of red oaks following the extensive logging during the early 1900s. The extensive oak decline occurring on droughty and nutrient-deficient soils today appears to due to the high abundance of mature red oak group species on these sites. Published by Elsevier B.V.

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.

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Managers are often concerned about oak mortality in maturing mixed-oak forests, but they often lack explicit information about mortality risk for oaks that differ in species, size, crown class, competitive status, and growth rate. In eastern North America, tree species in the red oak group (Quercus Section Lobatae) are typically shorter lived than those in the white oak group (Section Quercus), and red oak group mortality can be greatly exacerbated during episodes of oak decline. We analyzed factors associated with red oak group mortality for two large data sets from the Ozark Highlands and identified groups of trees that differed significantly in mortality risk. One data set was from a large, upland Ozark forest with more than four decades of uneven-aged management, and the other was representative of relatively undisturbed upland oak forests in the same ecoregion. Neither location experienced significant episodes of oak decline during the period of observation, and our results represent endemic mortality for mature (sawtimber size) forests in the region. We found that mortality rates for trees in the red oak group were four to six times greater than mortality rates for associated trees in the white oak group. We also found that red oak group mortality was significantly related to tree crown class, tree diameter, and basal area of larger competitors. When recent tree dbh growth was known, it significantly increased the ability to predict future mortality for some classes of trees. Annual mortality rates for different classes of trees in the red oak group ranged from 0.4 to 10.7%. Classes of trees in the red oak group that were most likely to die (in rank) were as follows: trees with dbh growth <0. /ha; and trees with annual dbh growth greater than 0.6 cm (assuming recent dbh growth is known). This information can be used when preparing silvicultural prescriptions and/or forest management plans to identify and treat classes of trees that are at high risk of mortality. Published by Elsevier B.V.

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