Finding partners in a habitat mosaic: Patch history and size mediate host colonization by arbuscular mycorrhizal fungi

Abstract

Restoration of glades over the past 30 years, involving removal of woody cover and re-establishment of herbaceous plant communities, has created an archipelago of habitat patches varying in age, size, and isolation. Within these glades, habitat varies in quality from edge to core. We investigated impacts of within- and among-glade variation on the frequency of root colonization by arbuscular mycorrhizal fungi (AMF) for two host species, Schizachyrium scoparium and Rudbeckia missouriensis, and the plant community at large. We also conducted a soil analysis to explore the role of nutrient resources in mediating putative habitat effects. We used Akaike's information criterion (AIC) to evaluate a set of a priori models that assessed effects of glade size, isolation, age, and edge effects on AMF colonization. In community samples, AMF colonization increased while variation in AMF colonization decreased with restoration age. Edge effects also reduced plant community AMF colonization. AMF colonization was higher in R. missouriensis than in S. scoparium, but increased for both species in core habitat. Glade size had no direct effect on AMF at the plant scale, but indirectly affected overall AMF occupancy at the landscape scale via its geometric relationship to glade edge:core ratio. Hosts in small glades that are dominated by edge exhibit lower occupancy rates. The first two axes in a principal components analysis (PCA) combining soil nutrient variables explained 74% of sample variance: PC1 correlated positively with organic matter (OM), total nitrogen (N), calcium (Ca), magnesium (Mg), and potassium (K) and decreased from the edge to the core of older glades. PC2, which correlated positively with pH and negatively with available P, increased with time since restoration. While results are consistent with a role of soil properties in explaining edge and age effects, the two PCA dimensions themselves are weakly correlated with AMF colonization. Overall, our findings suggest that environmental filters associated with woody encroachment limit the colonization of otherwise suitable herbaceous hosts and create a landscape mosaic with larger, older glades serving as hotspots or reservoirs of AMF root colonization and smaller, newly restored glades as cold spots or potential sinks.