Earth’s hydroclimatic variability is increasing, with changes in the frequency of extreme events that may negatively affect forest ecosystem processes. If plant growth and primary production respond asymmetrically (i.e., nonlinearly) to precipitation, then an increase in precipitation variability can lead to long-term growth trends even with negligible changes in mean precipitation. If dry extremes reduce growth more than wet extremes increase growth, then an increase in precipitation variability will result in reduced growth during dry years that is not offset by increased growth during wet years, leading to both increased interannual variability and long-term growth declines. Here, we examine asymmetries in the relationship between tree growth and precipitation using more than 100 years of tree-ring widths from across the conterminous United States in combination with historical, high-resolution precipitation estimates. Further, we examine how these asymmetries vary as a function of: i) species, ii) seasonality of precipitation, and iii) regional aridity. Finally, we examine the implications of asymmetric precipitation–growth relationships in the context of a more variable hydroclimate.
Results/Conclusions
We find that precipitation variability has increased across much of the U.S., and many tree species – particularly in semiarid regions – respond asymmetrically to this variability, such that reductions of tree growth during dry years are greater than, and not fully compensated by, increases of growth during wet years. The U.S. Southwest in particular shows both a large increase in cool-season precipitation variability coupled with asymmetric responses of growth to cool-season precipitation. This asymmetry was particularly pronounced for piñon pine (Pinus edulis), Douglas-fir (Pseduotsuga menziesii), ponderosa pine (Pinus ponderosa), and blue oak (Quercus douglasii). Simulations suggest a two-fold increase in the probability of strong negative regional growth anomalies in the Southwest resulting from increases in precipitation variability alone. Eastern U.S. tree species were generally less likely to be moisture limited, and those that were moisture limited were responded more symmetrically to precipitation than western U.S. species. However, both bur oak (Quercus macrocarpa) and post oak (Quercus stellata) responded asymmetrically to precipitation at a large number of sites in at least one season. Climate models project continued increases in precipitation variability, portending significant future reductions of tree growth across much of the U.S., particularly in vulnerable semiarid forest ecosystems of the western U.S.