One of the greatest challenges ecologists face is forecasting how species distributions will respond to climate change. In general, species distributions have moved polewards and upslope with recent climate change (i.e. range shifts), supporting the assumption that range limits are climatically determined. However, studies also document a surprising number of species whose distributions have remained unchanged, despite significant warming where they occur. This apparent lack of response to warming can arise for species whose range limits are determined by factors other than climate (e.g. species interactions, edaphic factors) OR for species whose range shifts are unable to keep pace with climate change (e.g. due to dispersal limitation). Unfortunately, while these two possibilities are often difficult to distinguish, they have very different implications for the long-term viability of the species in question. Here, we use extensive data derived from field observations, manipulative experiments, and modeling for four long-lived and slow-growing conifers, collected across the wide range of climates provided by Mount Rainier (WA, USA) to explore A) evidence for climatically determined range limits and B) the rate at which altitudinal distributions could shift in response to climate change in the region.
We found strong evidence that climate constrains performance at upper treeline. Specifically, heavy snowpack and short growing season length constrains recruitment (i.e. seed germination and seedling survival) and adult tree growth of four treeline species, suggesting that upper range limits are determined by climate. Moreover, dispersal rates of wind-dispersed focal trees are high, implying that range expansion is likely with climate change. However, current recruitment rates of and growth rates of focal trees are extremely low at upper range limits, which will constrain the rates of range expansion. In all, this suggests that the rate of climate change induced range expansion in these systems depends on the extent to which a warming climate increases extremely low population growth rates of long-lived trees. More broadly, our results suggest that the absence of a discernible ecological response to climate change (i.e. range shifts) should not necessarily be interpreted as a lack of climate sensitivity or ecological resilience.