Facilitation consequences for reproduction of the benefactor cushion plant Laretia acaulis along an elevational gradient: costs or benefits?

Abstract

Environmental stress may favour facilitative interactions among plants but whether these interactions are positive for the benefactor and how this depends on stress factors, remains to be determined. We studied the effect of beneficiary cover and biomass on reproduction of the benefactor cushion plant Laretia acaulis (Apiaceae) in the central Chilean Andes during three years. Study sites were situated along an elevational gradient at 2600, 2800, 3000 and 3150 m a.s.l. This range comprises a cold-and a drought-stress gradient, with moisture increasing and temperature decreasing with elevation. We studied the effect of natural gradients in beneficiary cover and of experimental cover removal on cushion flower and fruit production. Beneficiary cover had a negative effect on flower production but not on infructescence and fruit densities or fruit weights. A positive effect of beneficiaries on the fraction of flowers converted into fruits was detected for hermaphrodite cushions. The effect of beneficiary cover on flowering was independent of elevation or cushion gender, although these latter factors explained most of the variation. Removing the aboveground parts of the beneficiaries positively affected flowering at 2800 m a.s.l. but not at the other elevations. Our results suggest negative effects of facilitation on L. acaulis flowering, but these are neutralized in fruit production. Surprisingly, this conclusion holds along the entire elevational or stress gradient. This suggests that this system of facilitation is evolutionarily stable and not very sensitive to environmental change. It remains to be tested, however, whether facilitation affects fitness via growth and long-term survival in these slow-growing alpine cushions.

Facilitation costs and benefits function simultaneously on stress gradients for animals

“…In most cases, feedback was neutral at low-stress levels and did not change at higherstress levels, in line with the commensalism hypothesis. These findings support previous studies on cost for cushion plant along altitudinal gradients [11]. In only one case, T. solanivora paid a cost for facilitation at higher-stress levels, supporting the commensalism-parasitism hypothesis ( figure 1c).…”

“…All random effects were non-significant after comparison. studies where negative and neutral feedback effects have most often been reported ( [14,33] but see [1,11] for cases of mutualistic interactions). Importantly our manipulations, together with the conceptual framework (figure 1), further revealed how environmental stress can modulate feedback effects of beneficiaries on benefactors.…”

“…While evidence of net facilitative interactions has been largely documented in a wide range of plant communities [3] and for some animal taxa [6][7][8], we also need to examine the relative importance of the bi-directional nature of interactions, i.e. not only the effects of benefactors on beneficiaries but also the impacts of associated beneficiaries on benefactors [9][10][11]. Bi-directional interactions are likely important factors for many of the drivers of biodiversity and certainly not exclusive to plants.…”

A dynamic model of facilitation on environmental stress gradients

“…Also, model simulations showed that reciprocal competitive interactions of the beneficiary on the facilitator might have a significant effect at a population level. This supports recent experimental work suggesting that costs and benefits of facilitation function simultaneously on environmental gradients for both plants (García et al 2016) and their associated organisms (Dangles et al 2018). Overall, the present study reconciles existing discrepancies in SGH predictions and additionally shed light on the potential mechanisms dictating species interaction outcomes along environmental stress gradients (Soliveres et al 2015).…”

Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

“…A common definition of facilitation found in the literature states that the facilitator should not pay a cost for nursing (Stachowicz, ). However, there is increasing evidence in the literature of nurses that do pay a cost due to nursing (García, Bader, & Cavieres, ; Schöb et al., ). Our results showed that, without grazers, the associated communities increased in height and cover, thereby possibly capturing light at the expense of the nurse, which would be in agreement with these previous studies.…”