Experimental scale and precipitation modify effects of nitrogen addition on a plant pathogen

Abstract

Summary We examined how the interaction between the parasitic fungus Valdensia heterodoxa and its host plant Vaccinium myrtillus was affected by nitrogen (N) additions over 5 years in a boreal forest in northern Sweden. To examine whether the N effect was scale‐dependent we fertilized different sized plots (1, 10, 100, 1000 and 5000 m2 in area) with N corresponding to 12.5 or 50 kg N ha−1 year−1. We also examined how the N effect varied with the amount of summer precipitation. Disease incidence of the parasite increased following N addition and the effect was, on average, stronger in large than in small plots. The effect of plot size was significant for both N addition levels during the final year, but only marginally significant for the entire experimental period. Dispersal distances of ascospores and conidia were short, suggesting that high net emigration probabilities of propagules from small plots could explain the lower disease incidence in such plots and thereby the scale dependence of the disease. Disease incidence was also positively correlated with precipitation. High summer precipitation enhanced the N effect, suggesting that precipitation may modify the effects of N deposition on plant–parasite interactions. This may complicate predictions of future effects of N deposition as precipitation patterns are expected to change as a result of climate change. Our results suggest that small‐scale fertilization experiments may underestimate future large‐scale effects of N deposition, and indicate the need for increased awareness of the problems associated with scaling results from experiments using small‐scale manipulation of environmental variables.