Increasing mortality and declining recruitment driven by warm, dry conditions leads to decline of western US woodlands

Background/Question/Methods

Dry forests inhabit areas with climatic conditions near the physiological limits for trees and may be especially vulnerable to climate change. Despite evidence of increasing mortality in response to drought and warming temperatures, population trends of dry forests are largely unknown because we lack comparable tree recruitment data at range-wide scales. We fit range-wide demographic models for five widespread pinyon and juniper species using forest inventory and analysis from western US and estimated current population trends, climate vulnerability, and the relative role of both morality and recruitment in driving population trends using integral projection model.

Results/Conclusions

We estimate that four of the five species pinyon-juniper species are declining in parts of their range. Population vulnerability increases with aridity and temperature, with up to ~50% of populations declining in the warmest and driest conditions. Mortality and recruitment were both essential to explaining where populations are declining, with few populations declining with elevated mortality alone. Our results suggest that increasing temperatures and ongoing drought may threaten the long-term viability of many dry forest populations in their current range, and that understanding patterns and drivers of tree recruitment will be essential to anticipating climate driven range contractions. These models also provide an approach to anticipate coming shifts in forest distributions based on underlying demography.