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 cover would increase more on cushion plants with greater dead
stem cover, since dead stems do not inhibit grass colonization or growth.
The relationship between A. magellanica cover and A. selago size and
vitality was not significant in the long-term dataset, and the feedback
effect of A. magellanica on A. selago did 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. selago
dead stem cover was not significantly related to an increase in A.
magellanica cover 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.