Atmospheric deposition is an important pathway for moisture, nutrient, and pollutant exchange among the atmosphere, forest, and soils. Previous work has shown the importance of proximity to the forest edge to chemical fluxes in throughfall, but far less research has considered stemflow. This study examined the difference in acid neutralization capacity (ANC) of stemflow of nineteen Liriodendron tulipifera L. (yellow poplar) trees between the forest edge and interior in a rural area of northeastern Maryland. We measured ANC directly via potentiometric titration. Stemflow from trees at the forest edge was found to have significantly higher and more variable pH and ANC than in the forest interior (p < 0.01). No mathematical trend between ANC and distance to the forest edge was observed, indicating the importance of individual tree characteristics in stemflow production and chemistry. These results reaffirm the importance of stemflow for acid neutralization by deciduous tree species.

Forest ecosystems in the Eastern USA are threatened by acid deposition rates that have increased dramatically since industrialization. We utilized two watersheds at the Fernow Experimental Forest in West Virginia to examine long-term effects of acidification on ecological processes. One watershed has been treated with ammonium sulfate (approximately twice the ambient deposition rate) since 1989 to simulate elevated acidic deposition, while the other served as a control. Prior to treatment, both watersheds were similar in age and species composition. Ten dominant overstory Prunus serotina and Liriodendron tulipifera trees were selected and cored from each watershed to measure bolewood concentrations of essential elements through time. In addition, changes in tree species basal area were analyzed utilizing 50 long-term growth plots. Results of this experiment show lower calcium and magnesium concentration and increased acidic cation concentration for both species in the treated watershed, indicating a negative treatment effect. Growth response, measured through relative growth rates of cored trees and changes in basal area from growth plots, was not as conclusive and appeared to differ by species. The resulting difference in species response indicates that acidification sensitivity is something that land managers should consider when managing forests affected by acidification.

Liriodendron tulipifera (known as the tulip tree) is a woody species that has been previously classified as sensitive to ozone (O₃) in terms of visible leaf injuries and photosynthetic primary reactions. The objective of this work is to give a thorough description of the detoxification mechanisms that are at the basis of O₃ sensitivity. Biochemical and molecular markers were used to characterize the response of 1-year-old saplings exposed to O₃ (120 ppb, 5 h day⁻¹, for 45 consecutive days) under controlled conditions. O₃ effects resulted in a less efficient metabolism of Halliwell-Asada cycle as confirmed by the diminished capacity to convert the oxidized forms of ascorbate and glutathione in the reduced ones (AsA and GSH, respectively). The reduced activity of AsA and GSH regenerating enzymes indicates that de novo AsA biosynthesis occurred. This compound could be a cofactor of several plant-specific enzymes that are involved in the early part of the phenylpropanoid and flavonoid biosynthesis pathway, as confirmed by the significant rise of PAL activity (+75%). The induction of the defence-related secondary metabolites (in particular, rutin and caffeic acid were about threefold higher) and the concomitant increase in transcript levels of PAL and CHS genes (+120 and 30%, respectively) suggest that L. tulipifera utilized this route in order to partially counteract the O₃-induced oxidative damage.