Anticipating whether or not species range limits will shift in response to climate change is a goal of global change research, as these changes could alter ecosystem structure and function. Shifts in tree species distribution are the consequence of changes in main demographic rates governing tree population dynamics (growth, regeneration, reproduction and mortality). Thus, assessing tree species demographic responses across wide climatic gradients allow identifying potential changes in distribution. The present study analyzes the main demographic rates of Pinus ponderosa along altitudinal gradients between the uppermost and lowermost limits of distribution. Regeneration capacity (assessed as the ratio between number of seedlings/saplings and adults), sapling height growth, adult radial growth, reproductive investment, adult tree mortality and herbivory damage by mammals were sampled in 12 plots (approx. 3 ha each) at each altitudinal belt (high, medium and low elevations) in Northern Arizona. While a retreating rear edge would be characterized by low regeneration capacity, diminished reproductive investment and growth and high mortality, an expanding leading edge would have high regeneration capacity and enhanced reproductive investment and growth with low mortality rates. Biotic agents, such herbivory, could impact species performance and modulate responses to climate.
Results/Conclusions
Populations at medium elevations had the highest growth, reproductive investment and regeneration capacity and low mortality rates, representing the optimal climatic conditions for P. ponderosa. When compared with the medium elevation, P. ponderosa at low elevation had significantly reduced adult growth and regeneration capacity with high adult mortality rates, suggesting a future decline for these P. ponderosa populations. However, once established, saplings at the low elevation had high height growth rates, similar to those at medium elevation. At high elevation, adult growth and reproductive investment were high and mortality rates were low, which indicates a potential for upward expansion. Nevertheless, regeneration capacities of co-occurring conifer species at the high elevation were higher than for P. ponderosa. Finally, herbivory damages were significantly higher at high than at medium and low elevations. On one hand, our results point to a future decline of P. ponderosa populations at the lowermost distribution limit through reduced performance, although high sapling growth rates may compensate and promote species persistence. On the other hand, high performance at high elevations could allow an upward expansion of the species, but competition with other species and herbivory may limit this expansion.