Climate is often assumed to control range limits, with species limited by their physiological tolerances to both high and low values of temperature, precipitation, and/or snowfall. However, biotic interactions, like competition, may also prevent species from expanding beyond their range margins. Understanding the relative importance of abiotic and biotic factors in determining species distributions is particularly relevant for forecasting range shifts in response to climate change. Here, we quantify 100 year growth chronologies and contemporary competitive neighborhoods across the altitudinal ranges of three Pacific Northwestern tree species (Abies amabilis, Callitropis nootkatensis, Tsuga mertensiana) in Mount Rainier National Park, Washington (USA). We use these data to ask how climatic factors and/or competitive neighborhoods together limit performance across species ranges.

Results/Conclusions

We found that growth of all three species increased with warm years and longer growing seasons at upper altitudinal range limits, suggesting that climatic constraints prevent these species from expanding upwards. However, annual growth of trees at lower range limits was insensitive to annual variability in temperature, precipitation and snowfall. All species showed reduced growth in dense neighborhoods, implying that competition with neighbors also limits population growth rates. However, these negative impacts were not greater at lower altitudinal range margins, suggesting that competition is also not responsible for preventing population spread below lower altitudinal range limits. We speculate that biotic drivers on growth other than competition; or biotic and abiotic processes at other life history stages (e.g. adult mortality, juvenile recruitment) are responsible for lower altitudinal range limits. In summary, results suggest that climate may not always directly determine range limits, and thus, that short-term range shifts in response to global warming may be difficult to predict.