SPECIES, DIVERSITY, AND DENSITY AFFECT TREE SEEDLING MORTALITY FROM ARMILLARIA ROOT-ROT

Mortality from Armillaria root rot is a major concern of forest manage ment. Field experiments were conducted in Minnesota to evaluate inters pecific differential susceptibility and to assess whether density or s pecies composition, specifically the proportion of conifers in a plot, influences seedling mortality from Armillaria spp. Seedlings of 10 tr ee species (six conifers and four hardwoods) were planted at four dens ities in several species mixtures on recently logged sites. Species di ffered significantly in susceptibility (p < 0.0001); balsam fir (Abies balsamea (L.) Mill.), tamarack (Larix laricina (Du Roi) K. Koch), and black spruce (Picea mariana (Mill.) BSP) showed the greatest infectio n and mortality. Hardwood species and eastern white pine (Pinus strobu s L.) showed negligible mortality. There was a trend (p = 0.1) toward increased root infection with increasing density of the three species that showed significant mortality. Their mortality rates were 5.6, 8.7 , 10.2, and 10.8% in plots with 2-, 1-, 0.5-, and 0.25-m spacing, resp ectively. Mortality increased significantly (p = 0.001) with an increa se in the proportion of conifers in a plot. Mortality in the three mos t susceptible species was reduced by 75% when grown in aspen-rich rath er than in conifer-rich plots. Thus, seedling mortality was related to species identity, planting density, and proportion of conifers, sugge sting that selected diversity (mixtures of conifers with hardwoods rat her than conifer monocultures) reduces disease impact.