From a Tree’s Perspective: Climate Drivers of Extreme Growth

Recent research supported by the Southwest CASC points to major risks to the health and survival of western forests in a future arid climate. In a new publication, researchers used tree-ring records from seven tree species across 604 sites in the western U.S. to better understand which climate factors drive both extreme low- and high-growth and whether those drivers differ among species and across sites. 

Trees can document both historic and current climate events in their annual tree-rings, which widen or shrink depending on growing conditions. By studying these natural records, scientists can uncover how climate has affected growth in the past and learn what that might mean for the future of forests.

Across multiple species, the researchers found that winter precipitation stood out as the strongest driver of extreme tree growth. Nearly all years with low-growth showed below average current winter precipitation, hinting that poor growth years are typically driven by especially dry winters. The results also showed that these years were more likely to occur across a variety of sites than years of high growth, especially during regional droughts. 

The story around climate and strong growth was more complicated. High growth years were more varied and often required a combination of favorable climate conditions, such as multiple years of wet winters and summer rainfall. Interestingly, temperature and summer climate were generally found to be less significant as drivers of both kinds of extreme growth, especially when compared to precipitation and atmospheric moisture demand. 

Low-growth years, driven by dry winters, will likely become more frequent as the climate changes, increasing the chances of forest mortality. Additionally, a combination of favorable conditions, which favor high growth, will be less likely in the future. These combined factors imply that the populations of some tree species in the West may shrink, which is consistent with widespread mortality seen recently in the region. The findings from this research contribute to our understanding of the impacts of extreme climate, all from a tree’s perspective. 

Read the full publication in Global Change Biology.