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

Abstract

AbstractFacilitation 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 useAzorella 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 ofA. magellanicaonA. selagoidentified using our long-term dataset with data collected from a single time period. We hypothesized thatA. selagosize and vitality would be negatively affected byA. magellanicacover and that the effect ofA. magellanicaonA. selagowould become more negative with increasing beneficiary cover and abiotic-severity, due to, e.g., more intense competition for resources. We additionally hypothesized thatA. magellanicacover would increase more on cushion plants with greater dead stem cover, since dead stems do not inhibit grass colonization or growth. The relationship betweenA. magellanicacover andA. selagosize and vitality was not significant in the long-term dataset, and the feedback effect ofA. magellanicaonA. selagodid not vary significantly with altitude or aspect; however, data from a single time period did not consistently identify this same lack of correlation. Moreover,A. selagodead stem cover was not significantly related to an increase inA. magellanicacover over the long term; however, we observed contrasting results from short-term datasets. Long-term datasets may, therefore, be more robust (and practical) for assessing beneficiary feedback effects than conventional approaches, particularly when benefactors are slow-growing. For the first time using a long-term dataset, we show a lack of physical cost to a benefactor species in a facilitative interaction, in contrast to the majority of short-term studies.