The resilience of young trees is vital for forest recovery after die-off events caused by drought, heat, wildfire, insects, or pathogens. Researchers tend to make predictions using knowledge about how adult trees respond to stress – but do younger trees respond to harsh conditions the way that older trees do? By better understanding how younger generations respond to environmental stress resource managers can better manage the trees that will create the forests of the future. 

To study how extreme conditions could impact forests’ “recruitment failure”, the inability of younger generations to survive, Southwest CASC-supported researchers studied five tree species from the Jemez Mountains in New Mexico (ponderosa pine, piñon pine, Englemann spruce, Douglas fir, and limber pine). While controlling for other conditions, researchers first exposed four-year-old plants to normal temperatures but gave them no water. The trees dealt with the drought through week eight but progressively died through week 36. Ponderosa pine died first, followed by piñon pine, Englemann spruce, Douglas Fir, then finally limber pine. This result was counter-intuitive because the lower elevation trees that experience warmer temperatures in the field were the first to die. Then, by exposing trees to 10-degree higher temperatures than normal for one week, the researchers mimicked a heat wave around the trees. The species died in the same sequence as in the drought conditions, just a little bit faster.  

Results from this Southwest CASC-supported study were published in the journal Frontiers in Forest and Global Change. This research can help forest managers understand how landscapes may change following extreme events and guide species recovery plans by identifying which species are the most vulnerable (ponderosa pine) and most resilient (limber pine) to heat and drought. 

This work is supported by the Southwest CASC Project, “Science to Help Move From Mortality to Recovery in Western Forests and Woodlands.”