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.
