Climate isn't everything: Biotic interactions, life stage, and seed origin will also affect range shifts in a warming world

Background/Question/Methods

It is often assumed that temperature drives species’ distributions, leading to forecasts of widespread range shifts with climate warming. However, the extent to which climate determines species’ ranges is uncertain. Inconsistencies in the direction and rate of range shifts are prevalent, and may be attributable to biotic interactions, such as competition, as well as intraspecific variation in responses to climate and competition across life stages and/or populations, all of which can alter range boundaries. However, the potential effects of biotic interactions and intraspecific variation on climate change-induced range shifts are unknown as few empirical studies have explicitly examined competitive interactions across species' ranges, and potential variation by life stage and origin population is often ignored. We tested the importance of competition in limiting growth of three conifer species on Mt. Rainier, through observational studies of two life stages (adults and saplings) growing at different elevations and in varying competitive environments. We examined the importance of origin population and competitive interactions, by transplanting seedlings from different origins into varying competitive environments within and beyond their elevational ranges.

Results/Conclusions

We found that climate limits growth and survival at high elevations, whereas competition limits growth and survival at low elevations. These results support a classic, but little tested, hypothesis: biotic factors are more important at lower range limits, whereas abiotic factors control upper range limits. Furthermore, focal species differed in sensitivity to climate and competition by life stage and seed origin. Sapling growth was generally less sensitive to climate than adult growth, and was more sensitive to competition. Survival and growth of seedlings differed by origin, as well, but effects were weaker than those of competitive environment. Taken together, our findings suggest that climate change will lead to increased tree growth and upward expansion of Mt. Rainier’s forests, beyond current high elevation treeline. Climate change will minimally affect low elevations in this system in the short-term, however, as the competitive interactions that limit growth and survival primarily at early life-history stages will require long time-scales to cause shifts in geographic range boundaries. These biotically mediated impacts of climate change to species’ ranges are less predictable, especially if disturbance rates such as fire and disease outbreaks are altered in a future warmer world.