To persist, species are expected to shift their geographical ranges poleward or to higher elevations as Earth’s climate warms. However, while many species’ ranges have shifted in historical times, many others have not, or have shifted only at the high-latitude or high-elevation limits, leading to possible range expansions.  Given these idiosyncratic responses to climate warming, the critical task for identifying vulnerable species is to understand: 1) why some species have not shifted their ranges, particularly at the equatorial or low-elevation limits; and 2) whether such resilience will last as warming continues. We report on a comprehensive, long-term demographic study of two species of tundra plants throughout their latitudinal ranges in western North America (including two LTER sites).  

Results/Conclusions

We show that compensatory changes in different demographic rates are buffering southern populations of both plants against the negative effects of climate change, slowing their northward range shifts, but that this buffering may not continue indefinitely. Southern populations of both species showed lower survival and recruitment but higher growth of individual plants, possibly due to longer, warmer growing seasons. Because of these compensatory changes, the population growth rates of southern populations are not lower than those of northern ones. However, continued warming may yet prove detrimental, as even those demographic rates that improved with moderate warming declined in the warmest years throughout the ranges. Our results emphasize the need for long-term, range-wide  measurement of all population processes to detect demographic compensation and to identify non-linear responses that may lead to sudden range shifts as critical temperature or precipitation thresholds are exceeded.