Inferring controls on the epidemiology of beech bark disease from spatial patterning of disease organisms

Abstract

1 Spatial pattern in the distribution and abundance of organisms is an emergent property of collective rates of reproduction, survival and movement of individuals in a heterogeneous environment. 2 The form, intensity and scale of spatial patterning can be used to test hypotheses regarding the relative importance of candidate processes to population dynamics. 3 Using 84 plots across eastern North America, we studied populations of two associated plant parasites, the invasive felted beech scale Cryptococcus fagisuga Lind. and the native Neonectria fungi, which together cause beech bark disease (BBD). 4 We evaluated spatial patterns at the scales of trees within stands, stands within the forest and forests within the landscape to examine four hypothetically important factors in the ecology of the disease: (i) local contagion within stands; (ii) regional contagion, or among patch infection–reinfection dynamics; (iii) variation in host susceptibility linked to genetic and/or environmental heterogeneity; and (iv) climate effects on population growth of BBD organisms. 5 Analyses revealed an unexpected lack of spatial aggregation in BBD populations among trees, stands and forests. This implies that propagule pressure is generally sufficiently high throughout the infested region of North America such that neither trees nor stands are spared from the disease by dispersal limitations of the disease agents. Furthermore, variation in tree and stand level susceptibility has minimal impact on BBD dynamics and climate is not a conspicuous driver of abundance within the core range of BBD.