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 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 longterm 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.

Sex-specific facilitation and reproduction of the gynodioecious cushion plant Arenaria polytrichoides on the Himalaya-Hengduan mountains, SW China

“…We had hundreds of selected cushion individuals (plots) in this study ( Table 1 ) which is quite hard to do investigation on species composition, abundance and/or richness for each individual. Considering that vegetation cover could be positively correlated with plant diversity (e.g., Wiesmair et al., 2017 ), for the purpose of this study, we took the total beneficiary cover ratio (i.e., beneficiary covering area/total cushion individual surface area × %) on the cushion surface as the predictor of the facilitation intensity of cushion plants (also see Raath-Krüger et al., 2023 ). For this, we took the same protocol of assessing flower cover ratio ( Fig.…”

Fine‐scale variation in the effect of the cushion plant Azorella selago on vascular plants, mosses, hepatics and lichens in the sub‐Antarctic

“…For A. selago specifically, a very slow vertical growth rate of 0.43 ± 0.01 cm/year (mean ± SE) has been reported for Marion Island (see le Roux & McGeoch, 2004). In contrast, using a long‐term data set of repeated measures to examine the outcome of plant–plant interactions, Raath‐Krüger et al (2022) reported that A. selago diameter increases more quickly (i.e., by 1.98 ± 0.07 cm/year). Due to the longevity of A. selago plants (le Roux & McGeoch, 2004), the ability of the species to create a microhabitat unique from that of the surrounding matrix (e.g., Nyakatya & McGeoch, 2008; le Roux & McGeoch, 2008b; Haussmann et al, 2009), and with minimal variation in abiotic conditions at the scale of 1 m on the landform, a paired sampling approach was considered well suited for the study.…”

“…Cushion plants are often long‐lived and some species can survive for several hundreds of years (Morris & Doak, 1998). However, given that it is impossible to study these plants over a lifetime, examining the performance of these plants from long‐term data sets may provide deeper insights into the long‐term balance of plant interactions (see, e.g., Raath‐Krüger et al, 2022). For A. selago specifically, a very slow vertical growth rate of 0.43 ± 0.01 cm/year (mean ± SE) has been reported for Marion Island (see le Roux & McGeoch, 2004).…”