Long-term spatially-replicated data show no physical cost to a benefactor species in a facilitative plant-plant interaction

Facilitation is an interaction where one species (the benefactor) positively impacts another (the beneficiary). However, the reciprocal effects of beneficiaries on their benefactors are typically only documented using short-term datasets. We use Azorella selago, a cushion plant species and benefactor, and a co-occurring grass species, Agrostis magellanica, on sub-Antarctic Marion Island, comparing cushion plants and the grasses growing on them over a 13-year period using a correlative approach. We additionally compare the feedback effect of A. magellanica on A. selago identified using our long-term dataset with data collected from a single time period. We hypothesized that A. selago size and vitality would be negatively affected by A. magellanica cover and that the effect of A. magellanica on A. selago would become more negative with increasing beneficiary cover and abiotic-severity, due to, e.g., more intense competition for resources. We additionally hypothesized that A. magellanica c..., Following Raath-Krüger et al., the outcome of the interaction between Azorella selago and Agrostis magellanica was inferred from changes in Azorella size and dead stem cover and Agrostis cover through time. Twelve long-term monitoring plots (“Plot” in the dataset), which were established in 2003 at three altitudes (“Altitude” in the dataset) on the island’s eastern and western aspects (“Aspect” in the dataset), were resurveyed in 2016. Fifty Azorella individuals were surveyed from each of the plots. Each Azorella individual was photographed in the summer of 2002/2003 from directly above at a height of 1.5 m, with a scale bar included within each photograph. Each Azorella individual was photographed again in the summer of 2016 using the same methods.
To assess how Azorella size and stem mortality changed in relation to Agrostis cover, the photographs for each year were analysed in Fiji ImageJ and Adobe Photoshop. Azorella size (“AzoArea03” and “AzoArea16”)  and dead stem area (“DS03” and...,