Frequencies of Multivariate Air Masses Drive Tree Growth

Cameron C. Lee; Matthew P. Dannenberg

doi:10.1029/2022JG007064

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. Plain Language Summary Tree‐ring widths depend on the weather a particular tree experiences and are a direct indicator of tree growth and tree health. Often, the relationship between tree rings and weather are described using temperature, precipitation, or drought indices. However, a tree is exposed not just to temperature and/or precipitation alone, but to all weather elements acting together, which can be defined as an air mass. In this research, we explore how air masses impact tree growth at over 900 locations across the Northern Hemisphere. We find that tree‐ring widths are significantly related to the frequency of certain air masses, especially a Dry and Warm air mass, and a Humid and Cool air mass. Further, we find that air masses often affect tree growth more strongly than temperature and precipitation alone, up to a year prior to when the tree ring grew. Air mass information could therefore help in forecasting tree/forest health. Key Points Tree‐ring widths are significantly related to variability in air mass frequency (number of daily occurrences) in the year preceding growth The air masses that impact tree‐ring widths the most are dry, dry‐warm, and humid‐cool air masses, especially in relatively hot locations Air mass models explain more variability in tree‐ring widths than models based on temperature and precipitation for most sites and species

Frequencies of Multivariate Air Masses Drive Tree Growth

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