DOI: 10.5061/dryad.34tmpg4pp
title: Long-term spatially-replicated data show no physical cost to a benefactor species in a facilitative plant-plant interaction

description: 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.
description: Following Raath-Krüger et al., the outcome of
 the interaction between <em>Azorella selago </em>and
 <em>Agrostis magellanica </em>was inferred from changes in
 <em>Azorella</em> size and dead stem cover and
 <em>Agrostis </em>cover through time. Twelve long-term
 monitoring plots (“Plot” in the dataset), which were established in 2003
 at three altitudes <em>(“</em><em>Altitude” in the
 dataset</em>) on the island’s eastern and western
 aspects (“Aspect” in the dataset), were resurveyed in
 2016. Fifty <em>Azorella</em>
 individuals were surveyed from each of the plots. Each <em>Azorella </em>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
 <em>Azorella</em> individual was photographed again in the
 summer of 2016 using the same methods. To assess how <em>Azorella</em> size
 and stem mortality changed in relation to <em>Agrostis
 </em>cover, the photographs for each year were analysed in Fiji
 ImageJ and Adobe Photoshop. <em>Azorella</em> size
 (“AzoArea03” and “AzoArea16”)  and dead stem area (“DS03” and “DS16”) were
 measured using the polygon area selection tool, the wand tracing tool and
 by adjusting colour thresholds in Image J. <em>Azorella
 </em>size was defined as the total horizontal surface area of the
 cushion plants (including live stems, dead stems enclosed by live stems,
 dead stems contiguous with live stems, and parts of the cushion plants
 that were covered by other plants) as observed from directly above in the
 photographs. The extent of dead stems was used as a measure of plant
 vitality, where higher cover of dead stems on a
 plant was assumed to represent lower vitality.
 The area of
 <em>Agrostis </em><em>“AgrArea03” and
 “AgrArea16”)</em>, other vascular plants (“V03” and
 “V16”) and non-vascular plants (“NV03” and “NV16”) were measured using the
 same methods. Where other vascular and non-vascular plant species were
 growing on the edge of cushion plants and the edges of the cushion plants
 were not directly visible, we interpolated the plant edge based on the
 shape of the cushion plants. Similarly, we interpolated the cover of other
 vascular plant species and mosses when their cover was obscured by
 <em>Agrostis</em> cover; however, this was rare. In almost all
 cases, the vascular and non-vascular plants measured on the
 <em>Azorella</em> individuals were alive. Generally, very few
 dead individuals of other species are observed growing on
 <em>Azorella</em>, likely due to strong wind on Marion Island
 rapidly removing dead individuals. Dead stems were defined as any portions
 of <em>Azorella</em> individuals that were black or grey
 (and/or had very low stem densities) and/or if
 <em>Azorella</em> stem tips consisted entirely of brown
 leaves. The <em>Agrostis </em>area, area of
 other vascular plants and mosses, and dead stem area variables that  were
 measured in ImageJ were then converted into cover values (%). The cover
 values were used for the analyses. The cover values are labelled as
 follows in the dataset: ·         PerAgr03: Cover (%) of <em>A. magellanica</em> on <em>A. selago</em> calculated as: AgrArea03/AzoArea03) x 100. ·         PerCO03: Combined cover (%) of other vascular plants and mosses on <em>A. selago</em> in 2003, calculated as: (CoverOther03/AzoArea03) x 100.           ·         PerDS03: Dead stem cover (%) on each <em>A. selago</em> individual in 2003 calculated as: (DS03/AzoArea03) x 100.      ·         PerDS16: Dead stem cover (%) on each <em>A. selago</em> individual in 2016 calculated as: (DS16/AzoArea16) x 100.                  ·         PerAgr16: Cover (%) of <em>A. magellanica</em> on <em>A. selago</em> calculated as: (AgrArea16/AzoArea16) x 100. ·         PerCO016: Combined cover (%) of other vascular plants and mosses on <em>A. selago</em> in 2016, calculated as: (CoverOther16/AzoArea16) x 100.