Facilitation by Azorella selago affects Agrostis magellanica population structure more strongly than abiotic conditions

Abstract

Environmental conditions and plant size can both alter the outcome of interspecific plant-plant interactions, with seedlings generally being facilitated more strongly than larger individuals in stressful habitats. However, the combined impact of plant size and environmental severity on the outcome of interactions is poorly understood. Here, I test explicitly for the first time the hypothesis that ontogenetic shifts in interactions are delayed under increasingly severe conditions by examining the interaction between the two most-widespread vascular plant species on Marion Island (the grass Agrostis magellanica, and the cushion plant Azorella selago) along two severity gradient s. The impact of A. selago on A. magellanica abundance, but not reproductive eff ort, was related to A. magellanica size, with a trend for delayed shifts towards more negative interactions under greater environmental severity. Intermediate-sized individuals were most strongly facilitated, leading to differences in the size-class distribution of A. magellanica on the soil and on A. selago. The A. magellanica size-class distribution was more strongly affected by A. selago than by environmental severity, demonstrating that the plant-plant interaction impacts A. magellanica population structure more strongly than habitat conditions. As ontogenetic shifts in plant-plant interactions cannot be assumed to be constant across severity gradients and may impact species population structure, studies examining the outcome of interactions need to consider the potential for size- or age-related variation in competition and facilitation.