Climate Factors Leading to Asymmetric Extreme Capture in the Tree-Ring Record

Paleoclimate data play a critical role in contextualizing recent hydroclimate extremes, but asymmetries in tree-ring responses to extreme climate conditions pose challenges for reconstruction and interpretation of past climate. Here we establish the extent to which existing tree-ring records capture precipitation extremes in western North America and evaluate climate factors hypothesized to lead to asymmetric extreme capture, including timing of precipitation, seasonal temperatures, snowpack, and atmospheric river events. We find that while there is dry-biased asymmetry in one third of western North American tree-ring records, 45% of sites capture wet extremes as well as or better than dry extremes. Summer extremes are rarely captured at any sites. Latitude and elevation affect site-level extreme responses, as do seasonal climate conditions, particularly in the autumn and spring. Directly addressing asymmetric extreme value capture in tree-ring records can aid our interpretation of past climate and help identify alternative avenues for future reconstructions. Plain Language Summary Climate extremes, which include droughts, floods, and heat waves, are of great concern due to their impacts on people and commerce. Human-caused climate change may already be increasing the frequency and severity of climate extremes. To evaluate whether changes are occurring, recent climate extremes must be compared to climate extremes hundreds of years ago. Trees, which record past climate in their rings, are often used for this purpose. However, trees are thought to record these extremes unevenly; for example, droughts more than floods. We used tree-ring and climate data from western North America and found that while many trees do record dry extremes more often than wet extremes, this is not universal and varies by location, season, and year. Other climate factors, such as how warm or cloudy it is in different seasons, or when the rain and snow fell, consistently influence whether trees record the extreme year. These results can aid our interpretation of past climate and suggest ways to improve records of past climate extremes, which in turn can help us better understand our current climate and how it might change in the future.