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

Abstract

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 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.

A robust mixed‐effects parametric quantile regression model for continuous proportions: Quantifying the constraints to vitality in cushion plants

“…For certain plots, it is evident that outliers (relative to the relationship between the final vs. initial dead stem cover) are primarily associated with the previously excluded data. Aside from the previously excluded data due to catastrophic loss, we note that dead stem cover on a few plants (included in the previous analysis of Raath-Krüger et al, 2022) changed considerably during the monitoring period, thus also resulting in data outliers. Figure S1a,b in Section A of Data S1 gives a photograph example of the dead stem cover that remained similar on the vast majority of individual plants over the 13-year monitoring period, not resulting in an outlier.…”

“…1. Given that A. selago vitality could be negatively affected by the cover of A. magellanica and other vascular and nonvascular plant species (see le Roux et al, 2005;Owen, 1995;Raath-Krüger et al, 2022), we ask: Is the limit to A. selago vitality constrained by the cover of vascular and nonvascular plant species growing on A. selago? We model the 0.95th quantile of dead stem cover to address this research question.…”

“…We refer the reader to Quintero and Lesaffre (2018) to appropriately choose the number of replications M and L. We drew M = 10,000 and L = 1000 replicate samples of the random effects to calculate the mDIC statistic, which is efficient for relatively small datasets containing in the range of 500 observations or less such as the dataset of Raath-Krüger et al (2022). Note that the model with the smallest mDIC is favored.…”

“…Raath‐Krüger et al (2022) compiled a long‐term dataset of repeated measures to examine the impact of the grass species Agrostis magellanica on the cushion‐forming plant, Azorella selago , using sub‐Antarctic Marion Island as a model system. These two species are the dominant vascular plants in the sub‐Antarctic, and A. magellanica is the most common vascular plant species growing on A. selago (Huntley, 1972).…”

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

“…Data are available from the Dryad Digital Repository: https:// doi.org/10.5061/dryad.34tmpg4pp (Raath-Krüger et al 2022).…”