Do plant–microbe interactions support the Stress Gradient Hypothesis?

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. Specifically, we hypothesized that support for the SGH would depend 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 one-half of the species examined, with soil microbes facilitating germination with increasing stress for 5 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.