1A: Using tree rings to understand Utah’s long-term trends for drought and forest health

The drought that’s currently impacting the entire southwestern U.S. has recently been rated as the worst in the last 1,200 years^[1]. But what about Utah? Are local conditions different from these dire regional averages? To determine this, we examined tree-ring data from bristlecone pine trees, snags, and logs in Cedar Breaks National Monument and tracked conditions in southern Utah over more than a millennium. We specifically wanted to know if this historical record shows that there have been more extreme droughts compared to what we are now experiencing.

In the analysis we could see evidence of the current drought. We found that even resilient bristlecone pines showed a decrease in growth in 2020-2021, consistent with the regional trends. But we also found that trees in southern Utah experienced prolonged periods of low growth even more severe than what we are experiencing today. During the DustBowl (mid-1930s), the early 1400s, and several periods before 1000 A.D., tree growth occurred much more slowly than it does now—most likely because of intense drought. Some years even showed missing rings, indicating conditions so detrimental to tree growth that no growth was even added. Other factors may have also played a part in trends, including long, cold winters, tree crowding, outbreaks of disease/insects and fire, but the most likely long-term cause for these trends overtime is drought.

These remarkably dry periods sometimes lasted many decades—much longer than 20 years. What this implies is that if such scenarios happened in the past, they could happen again. As bad as the current drought is, we may not have yet reached a worst-case scenario, based on the natural record over the past millennium. Tree-ring patterns show that such droughts can occur and can last longer than two decades.