The Stress Gradient Hypothesis (SGH), which predicts increasing ratios of facilitative:competitive interactions with increasing stress, has long been a guiding framework for conceptualizing plant-plant interactions. Recently, there has been a growing recognition of the roles of microbes in mitigating or exacerbating environmental stress for their plant hosts. As such, we might predict based on the SGH that beneficial microbial effects on plant performance should be positively associated with stress. We hypothesized that support for the SGH would also vary depending on the host plant’s habitat specialization such that species that specialize in high stress habitats and thus likely coevolved with the resident microbes would exhibit stronger support for the SGH than non-specialist plant species. We further hypothesized that support for the SGH would vary with germination frequency, since boosting germination of low-frequency germinators is one effective means by which microbes can benefit plant species performance. Here, we explore whether plant-microbial interactions support the SGH, using 12 plant species native to the Florida rosemary scrub. We conducted factorial experiments that manipulated the presence of microbes in nine soils collected along an elevational stress gradient, and recorded germination frequency and biomass. Microbes increased the germination frequency of four species, all of which had relatively low germination rates. Furthermore, we found support for the SGH in nearly half of the species examined, with soil microbes facilitating germination with increasing stress for five of the 12 species tested, and none of the species exhibiting the opposite trend. Support for the SGH was not predicted by either the plant hosts’ habitat specialization or germination frequency. In contrast to germination, biomass results showed little support for the SGH, with four of 12 species refuting and one species supporting SGH predictions. Taken together, our study documents that interactions between the soil microbial community and plant species along a stress gradient can support the SGH, but emphasizes that these effects are life history stage-dependent. This work also identifies a common mechanism (germination facilitation) by which microbes can benefit plant species in stressful habitats.

Data from germination and plant growth experiments in which 12 plant species (specialists: Lechea cernua, Polygonella robusta, P. basiramia, Eryngium cuneifolium, Hypericum cumulicola, Liatris ohlingerae; non-specialists: Chamaecrista fasciculata, Pityopsis graminifolia, Liatris tenuifolia, Balduina angustifolia, Chapmannia floridana, Lechea deckerti) were grown in Live or Sterilized soil collected from 9 different patches of Florida rosemary scrub. We manipulated the presence of soil microbes following David et al. (2019) American Naturalist. Briefly, we inoculated pots (15% soil volume) with Live (unmanipulated, microbially-active) or Sterilized (autoclaved, microbially-sterile) soils, with the remainder of the soil consisting of sterilized soil from the same rosemary scrub patch as the inoculum. All pots (66 mL; Ray Leach Cone-tainer, Stuewe & Sons, Tangent, OR, USA) and sterilized soils were autoclaved at 121°C twice prior to use. For eight species, seeds were sown directly into pots. For logistical reasons, we conducted separate germination experiments for the remaining four species in soil-filled Petri plates using the same 15% soil inoculum. In order to examine later life history stages of these four species, seedlings were either transplanted into Cone-tainers directly from the germination study plates (E. cuneifolium) or germinated from additional seeds on Petri plates with moistened filter paper (L. ohlinerae, L. tenuifoila, C. floridana). Seeds were allowed to germinate under ambient light conditions in the laboratory. Once the germination rate plateaued, pots were placed under fluorescent grow lights (16:8 light:dark conditions), though we continued to monitor for new germinants. At this stage, we thinned germinants as necessary to prevent overcrowding. We harvested each species before plants outgrew their pots, and dried and weighed above- and belowground biomass.