Shade and hemlock woolly adelgid infestation increase eastern hemlock foliar nutrient concentration
Tsuga canadensis (L.) Carr. (eastern hemlock) is dying across much of eastern North America from the invasive hemlock woolly adelgid (HWA, Adelges tsugae Annand). Survey studies show that eastern hemlock populations with high foliar nutrient concentrations are associated with low infestation rates, and also suggest that deeply shaded trees may be more susceptible to infestation. Here we examined (1) how foliar nutrient concentration of eastern hemlock changes with varying shade levels; and (2) how nutrient concentration might further change with sustained shade and subsequent HWA infestation. Foliar samples from three years—pretreatment, post-shade, and post-shade and infestation—were collected and analyzed for [N], [P], and [K]. Pretreatment, all seedlings had similar foliar nutrient concentrations. After nine months in the shade tents, seedlings under higher levels of shade exhibited increased foliar [N]. For each 10% increase in shade, foliar [N] increased 35.09 μg/mg over baseline levels. The combined effects of prolonged shade with HWA infestation increased foliar [N], [P], and [K]. The mechanism for increasing foliar nutrients is unknown, but may be due to reduced growth causing a concentration effect, or nutrients mobilized by the plant in response to infestation.