Feedbacks between forest structure and an opportunistic fungal pathogen

Abiotic stresses, physiological dysfunction, forest stand dynamics, and primary tree attackers (native and non-native) are all recognized as important contributors to both anomalous tree mortality and background tree mortality, and thus as important influences on biogeochemical cycling and habitat for associated terrestrial organisms. Opportunistic and latent tree pathogens and insect pests have largely been left out of this discussion, probably because they are difficult to monitor and their effects sometimes more diffuse, yet they play important roles in tree mortality scenarios.

We know very little about the influence of biotic gradients, e.g., forest dynamics and forest structure, on the occurrence and effects of these opportunistic biotic enemies. To better understand the influence of an important biotic gradient—forest structure as represented by tree density and competition—on the occurrence and distribution of a typical opportunistic pathogen, we analyzed the relationship between tree neighborhood competition and soil-dwelling rhizomorphs of the fungus Armillaria gallica in regenerating (20-yr-old) and mature (80-100-yr-old) hardwood-dominated forest stands in central Missouri following a severe drought.

We detected A. gallica, rhizomorphs in twice as many sampling sites in young (dense) plots as in mature (open) plots, and by contrast, found a twofold occurrence of other saprophytic fungi in the soil of mature plots compared to young plots. Mean tree density and competition index were significantly higher in subplots where A. gallica was found than in those with no A. gallica detections, and the pattern of A. gallica occurrence tracked closely with the occurrence of dense "hotspots" of competition in the plots.

Synthesis. Early stages of forest structure and succession—potentially through initial inputs of disturbance-produced woody biomass, increased host connectivity and competition-influenced host tree susceptibility to fungal colonization—provide important feedbacks to A. gallica foraging, which in turn influences forest development. The recognition that tree density and competition influence A. gallica foraging suggests that in order to achieve a more comprehensive understanding of forest growth, development, and biogeochemical cycling, it may be necessary to increase life-history studies and basic occurrence surveys for opportunistic tree pathogens and insect pests.