Woodland caribou herbivory on The Slate Islands altered the forest landscape and ground flora resulting in the emergence of the rust fungi, Melampsorella caryophyllacearum. This rust alternates between chickweeds and balsam fir, where it causes a broom deformity. My study identified factors influencing the spread of brooms across a remote archipelago and evaluated whether caribou affected the spread. Brooms were sampled in 240 5-m radius plots in eight sites representing two habitat types. In four sites, caribou browsing created open-glade forests, and the other four were closed-canopy, fir-dominated forests with minimal caribou influence. I used generalized linear mixed models to examine the relationship between broom infections and biotic and physiographic variables. The best model included habitat type, distance to chickweeds, their interaction, slope aspect, and total solar radiation. Infections were significantly higher in open forests (mean 15.7%) than in closed (1.7%). Infections were also strongly related to the distance to chickweed in open forests, highlighting the role of local spore dispersal from chickweed to fir. Aspects exposed to prevailing winds had significantly higher infections, suggesting that long-distance spore dispersal from the broom stage may be important at broader scales. Forest composition and proximity to the intermediate host emphasize the indirect effects of herbivory on rust dispersal and host abundance
