DOI: 10.5061/dryad.qfttdz0d5
title: Data from: Do plant-microbe interactions support the Stress Gradient Hypothesis?

description: 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.
description: Data from germination and plant growth experiments in which 12
 plant species (specialists: <i>Lechea cernua</i>,
 <i>Polygonella robusta</i>, <i>P. basiramia</i>,
 <i>Eryngium cuneifolium</i>, <i>Hypericum
 cumulicola</i>, <i>Liatris ohlingerae;
 </i>non-specialists: <i>Chamaecrista fasciculata</i>,
 <i>Pityopsis graminifolia</i>, <i>Liatris
 tenuifolia</i>, <i>Balduina angustifolia</i>,
 <i>Chapmannia floridana</i>, <i>Lechea
 deckerti</i>)<i> </i>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 (<i>E. cuneifolium)</i> or germinated
 from additional seeds on Petri plates with moistened filter paper
 (<i>L. ohlinerae, L. tenuifoila, C. floridana</i>). 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.