Frequencies of Multivariate Air Masses Drive Tree Growth

Abstract

Midlatitude surface meteorological conditions are embedded within—and affected by—synoptic-scale systems, including the movement and persistence of air masses (AMs). Changes in AM frequencies (number of daily occurrences) over the past several decades could have large effects on ecosystems: each organism is exposed to the synergistic effects of the entire suite of atmospheric variables acting upon it—an inherently multivariate environment—which is best captured using AMs. Utilizing a global-scale AM classification system and a large network of tree-ring chronologies, we investigate how variation in AM frequency impacts tree growth at over 900 locations. We find that AM frequencies are well-correlated with tree growth, especially in the 12-month period from July in the year prior to growth through June in the year of growth. The most impactful AMs are Dry-Warm and Humid-Cool AMs, which exhibit average correlations of ρ = −0.4 and ρ = +0.4 with tree growth, respectively, for certain tree species, with correlations at some sites exceeding ρ = ±0.8 in some seasons. Compared to empirical models based solely on temperature and precipitation, modeling using only AM frequencies proved superior at nearly 60% of the sites and for over 80% of the well-sampled (n ≥ 10) species. These results should provide a foundation for using AMs to improve forecasts of tree growth, tree stress and wildfire potential. Long-term reconstructions of AM frequencies back several centuries may also be feasible using tree-ring data, which will help contextualize and temporally extend multivariate perspectives of climate change that utilize such air masses.