Climate manipulations differentially affect plant population dynamics within versus beyond northern range limits

Predicting species’ range shifts under future climate is a central goal of
conservation ecology. Studying populations within and beyond multiple
species’ current ranges can help identify whether demographic responses to
climate change exhibit directionality, indicative of range shifts, and
whether responses are uniform across a suite of species. We quantified the
demographic responses of six native perennial prairie species planted
within and, for two species, beyond their northern range limits to a
three-year experimental manipulation of temperature and precipitation at
three sites spanning a latitudinal climate gradient in the Pacific
Northwest, USA. We estimated population growth rates (λ) using integral
projection models and tested for opposing responses to climate in
different demographic vital rates (demographic compensation). Where
species successfully established reproductive populations, warming
negatively affected λ at sites within species’ current ranges. Contrarily,
warming and drought positively affected λ for the two species planted
beyond their northern range limits. Most species failed to establish a
reproductive population at one or more sites within their current ranges,
due to extremely low germination and seedling survival. We found little
evidence of demographic compensation buffering populations to the climate
treatments. Synthesis: These results support predictions across a suite of
species that ranges will need to shift with climate change as populations
within current ranges become increasingly vulnerable to decline. Species
capable of dispersing beyond their leading edges may be more likely to
persist, as our evidence suggests that projected changes in climate may
benefit such populations. If species are unable to disperse to new habitat
on their own, assisted migration may need to be considered to prevent the
widespread loss of vulnerable species.