Soil properties constrain forest understory plant distributions along a climatic elevational gradient

Predictions of plant migration under climate warming come mostly from models including only climate variables, neglecting the influence of non-climatic factors, such as soil properties and dispersal limitation. Soil properties might have a stronger effect on plant distributions in colder environments, where plant nutrient absorption capacity is inhibited, but this has rarely been tested. Macroecological studies of range dynamics rely on soil data at much coarser spatial resolution than that experienced by plants. In contrast, field studies along elevational gradients permit detailed soil data, while still covering a wide climatic gradient. Here, we first report an intensive field survey of four spring forest herbs and soil properties along an elevational gradient in southern Québec, Canada, testing the hypothesis that soil properties contribute to defining upper elevational range limits. We then report a seven-year transplant experiment with one species, Trillium erectum , testing the hypothesis that climate warming has already created suitable sites at high elevation, with its near-absence explained by dispersal limitation. In our field survey, soil properties had substantial impacts on the occurrence or abundance of all four species, and soil effects were more pronounced at higher elevations. For two species, T. erectum and Claytonia caroliniana , very infrequent occurrences at high elevation (>950m) were strongly associated with rare microsites with high pH or nutrients. After transplantation to high-elevation sites, T. erectum individuals grew to much smaller size and with very low probability of flowering (<10%) compared to individuals at low or mid-elevations (>60% flowering), suggesting that environmental factors rather than dispersal limitation constrain the species’ upper elevational range limit. Our study highlights that soil factors interact strongly with climate to determine plant range limits along climatic gradients. Unsuitable soils for plants at high elevations or latitudes may represent an important constraint on future plant migration.