White ash (Fraxinus americana) decline and mortality: The role of site nutrition and stress history

Over the past century, white ash (Fraxinus americana) populations throughout its range have deteriorated as a result of declining tree health and increased mortality rates. Although co-occurring factors including site nutritional deficiencies and punctuated stress events (e.g., defoliations, drought) are hypothesized to trigger white ash decline, there are no empirical assessments of these factors at regional scales. In this study, we evaluated ash crown dieback, crown health condition, and mortality on 190 plots paired along a topographic gradient known to differ in site nutrition across a 3000 km2 area of northwestern Pennsylvania, USA. Additionally, we assessed white ash foliar nutrient content and additional factors including defoliation history as potential explanatory variables at all sites. White ash populations on upper slopes consistently had significantly greater dieback, poorer crown condition, and greater mortality than populations in paired plots on lower slopes. Despite nearly two decades since the last major elm spanworm defoliation, this stressor further amplified the differences in health and mortality seen between slope positions. On the relatively cation deficient upper slope positions, crown dieback and crown condition improved with increasing foliar cation (Ca2+, Mg2+) concentration. Our results indicate that white ash health is strongly influenced by site nutrition, and defoliation can trigger declines in cation-deficient sites. Knowledge of how landscape position and nutrition influence white ash health may alter management responses to insect outbreak events.

► We examine current white ash health and decline as a function of site variables.
► Base cation concentrations were lower on upper slope sites than lower slope sites.
► Ash on cation deficient sites experienced greater crown dieback and tree mortality.
► Past defoliation stress exacerbated differences in crown health and tree mortality.