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

Ni., M. and M. Vellend. 2023.


Many plant species are predicted to migrate to higher elevations or latitudes in response to climate warming, but predictions come mostly from climate-only models, neglecting the influence of non-climatic factors, such as soil properties and dispersal limitation. Macroecological studies rely on soil data at much coarser spatial resolution than that experienced by plants, and for forests, they focus mainly on trees and shrubs, rather than understory herbs, which account for most forest plant diversity. Studies along elevational gradients permit detailed soil data to be collected at the same spatial resolution as occurrence and abundance 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. Synthesis: Our study highlights the importance of soil properties in determining plant range limits along putatively climatic gradients. Soil factors interact strongly with climate to determine the distribution and abundance of these understory herbaceous plants. Unsuitable soils for plants at high elevations or latitudes may represent an important constraint on future plant migration.