Climate change impacts on population growth across a species’ range differ due to nonlinear responses of populations to climate and variation in rates of climate change

Here, we use an uncommon boreal plant species, Douglasia alaskana (Primulaceae), to test how the pattern of response to changes in climate variables and different magnitudes of cli- mate change combine to influence population growth rate across much of a species’ range. Douglasia alaskana has a moderately-sized geographic range (southcentral Alaska), but has high habitat specificity (alpine scree fields) and small population sizes, satisfying two of the three components of rarity (habitat restriction and small local populations) described by ref. [15]. Rates of climate change are likely to be large but quite variable among populations, because the species occurs in one of the fastest warming regions in the world (the boreal zone), but some populations are coastal and others inland. We construct climate-driven integral pro- jection models [16] for five populations using observed climate responses coupled with realis- tic projections of future climate. We then use these models to determine whether impacts of climate change on population growth rate are driven by the magnitude of climate change in each population v. climate responsiveness, and how nonlinearities in population responses to multiple components of climate (e.g., summer v. winter temperatures) contribute to variation in climate change effects across the range. Methods Douglasia alaskana is a perennial alpine plant with 1–3 rosettes and fruits presented on short (<15 cm) peduncles. It occurs in alpine scree on ridgelines and mountaintops in southcentral

Alaska, USA (Fig 2). Populations are small, ranging from 4–300 individuals in all populations we have found. The species is uncommon, with a very small fraction of seemingly suitable hab- itat occupied. Seeds are dropped within 15 cm of the parent plant into unvegetated scree fields (A. Louthan, pers. obs.). The species is partially semelparous, with most individuals (775 of Fig 1. Non-linear effects of climate on population growth rate. Population growth rate is in general a non-linear function of climate (solid black lines), either: A. accelerating; or B. decelerating. λC and λF are the current and future growth rates at the current and future values of a climate variable; CC represents current and CF represents future climate. Blue lines show the slope of the population growth rate vs. climate evaluated at the current climate (i.e., “climate sensitivity”). The slope of the red line (“climate responsiveness”) is the ratio of the actual change in population growth Δλ to the actual change in climate ΔC (CF -CC). λL is the linear projection of the future population growth rate using the sensitivity, which underestimates λF when population growth is an accelerating function of climate but overestimates λF when population growth is a decelerating function. https://doi.org/10.1371/journal.pone.0247290.g001 PLOS ONE Nonlinear responses of population growth rate to climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0247290 March 3, 2021 3 / 20