Persistent Effects of Herbaceous Species on the Infectious Lethality of Soil for Conifer Seedlings

Abstract

Seeds of the coniferous trees Abies balsamea, Picea mariana, and Pinus strobus were sown in the laboratory in two soils taken from ground‐layer patches differing in species composition, one of which was dominated by Aegopodium podagraria (goutweed). This permitted inference whether herbaceous species may affect the favorableness of the soil for establishment of these trees. Weibull distributions were fitted to the time course of aggregate seedling emergence and post-emergence mortality, enabling seedling lifespan to be inferred without monitoring of each individual. A higher percentage of Abies seeds developed into emerged seedlings in the goutweed soil, likely because of less preemergence mortality incited by pathogens. Picea and Pinus emergence percentages were similar in both soils. Most emerged seedlings died within weeks, with symptoms of diseases incited by soil‐ or seed‐borne fungi. Although the timing of seedling emergence did not differ between soils, seedlings died more quickly on the goutweed soil, largely because of a faster development of post‐emergence damping‐off. Total post-emergence mortality of Picea and Pinus was greater on the goutweed soil. The relative frequencies of the several symptoms exhibited by dying seedlings varied between the two soils, suggesting that the ground‐layer species differentially affected the microbial community’s composition or interactions with the seedlings. Symptom frequencies differed among tree species. Local spatial variation in herbaceous species composition appeared to produce patchy infectious lethality of soil for tree seedlings, an indirect effect that was observed after the herbaceous plants had been removed.