Experimental tests of the mechanisms explaining lags in plant elevational range shifts

Theory predicts that species’ upper range limits are constrained by cool temperatures, and should expand upwards under climate warming. However, recent evidence reveals that many plant species are not moving fast enough to track changing climate. Alternative potential limiting factors are dispersal limitation or competition with resident high-elevation species. To understand the mechanisms explaining range shift lags, we transplanted ten lowland plants to a site above their current elevation limit in the eastern Swiss Alps, with or without the existing vegetation. We tracked individual level vital rates of these species across three growing seasons to construct integral projection models (IPMs) to predict population growth rates in the different experimental treatments.All species were predicted to successfully establish above their current elevation limit when the existing vegetation was removed, indicating that the high elevation site is within the climate niche of the species. Population growth rates of four species were below one when growing with competitors, suggesting that their absense from the high elevation sites can be explained by competitive exclusion by the resident community. While dispersal limitation can not be ruled out for these species, this result highlights the potential for species interactions to constrain range expansion in response to climate change. The remaining six species were predicted to successfully establish above their current range edge even in the face of interactions with the existing vegetation. This indicates that their absence from the high elevation site today can be explained by dispersal limitation. While this is surprising given the very short dispersal distances involved, simulations of the IPMs revealed that in some cases, hundreds of thousands of seeds would be needed to support rapid population establishment.These findings challenge assumptions that high-elevation communities contain weak competitors, and demonstrate that competition and dispersal limitation may often impede expansion into newly climatically suitable habitat. This suggests that even widespread species might require active management to support climate tracking as climates continue to warm rapidly.