Do facilitative interactions with neighboring plants assist the growth of seedlings at high altitudes in alpine Australia?

During the early life-history stages, plants are especially susceptible to the abiotic conditions present in high mountain environments. At high altitudes, facilitative interactions between close neighboring plants may buffer seedlings from these abiotic pressures by providing shelter from frosts and winds. At lower altitudes, seedlings may not be so limited by the abiotic environment, and may therefore compete for resources with close neighboring plants. Using four alpine sites at different elevations (representing an abiotic stress gradient), we investigated how the presence of close neighboring plants influences seedling growth in their first growing season. We experimentally cleared above-ground vegetation and transplanted seedlings of three species into cleared and control plots. We quantified the stress gradient of abiotic conditions across sites by measuring ambient and soil temperatures, soil moisture, and soil frost heave. We used the "Relative Neighbor Effect" index to show the direction of the interaction between transplanted seedlings and their close neighboring plants. Aciphylla glacialis seedlings showed neutral interactions across the gradient of alpine sites, with undetectable change across the growing season, compared with Brachyscome rigidula seedlings which showed positive interactions with neighbors across the growing season at most sites. Trisetum spicatum seedlings showed mostly neutral interactions with neighbors at the higher elevations, and often negative interactions at the lower elevations, particularly midway through the growing season. Our findings highlight the importance of spatial and temporal plantplant interactions with regard to seedling performance across altitudinal stress gradients.