Spatial dispersion in agents of forest disease: Populations of beech bark disease-associated organisms in space and time

Beech bark disease in its endemic phase is a decline disease of American beech (Fagus grandifolia) arising from the association of an introduced scale insect (Cryptococcus fagisuga) and one of several species of ascomycete fungi of the genus Neonectria.  We used data from 7 spatially-mapped, 0.26 hectare forest plots to assess spatial aggregation of scale insects and fungal pathogen with respect to host trees, employing a resampling approach to test for departure from randomness in the dispersion of disease populations .  We found no evidence for spatial structure at the stand scale in 6 of 7 sites.  This implies that the distribution of disease agents is not strongly influenced by local extinction and reinfection dynamics or by the distribution of susceptible tree genotypes within a stand.  Alternatively, we found strong evidence for spatial aggregation among stands distributed from Maine to West Virginia, using data from 30 sites monitored for scale and fungal populations between 1979-1992 (and again in 2005-06).  The presence of coarse spatial structure suggests the importance of landscape-level characteristics such as stand size, percent beech, forest connectivity, stand history, and/or metapopulation dynamics among disease agents in determining the incidence and severity of beech bark disease.