Oak mortality risk factors and mortality estimation

Abstract

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 <0.2 cm per year; suppressed trees; trees with intermediate crown class and basal area of larger competitors >14 m 2/ha; dominant and codominant trees >25 cm dbh with basal area of larger competitors <14 m 2/ha; trees with intermediate crown class and basal area of larger competitors <14 m 2/ha; dominant and codominant trees >25 cm dbh with basal area of larger competitors <15 m 2/ha; dominant and codominant trees <25 cm dbh with basal area of larger competitors >15 m 2/ha; dominant and codominant trees <25 cm dbh with basal area of larger competitors <15 m 2/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.