COS 226-3 - The legacy of acid rain in the Adirondacks strongly affected leaf chemical composition but not the respiratory physiology of sugar maple (Acer saccharum) and American beech (Fagus grandifolia) leaves.

Background/Question/Methods

The legacy of acidic deposition in the U.S. Northeast has been well studied for its negative impacts on forest soils and plants. Acidic deposition is thought to detrimentally affect sugar maple (Acer saccharum) and has been posited as a main driver of sugar maple decline syndrome. The physiological mechanism by which acid deposition impairs sugar maple growth is not well understood, but previous studies have suggested that increased respiration rates may be involved. We directly assessed this hypothesis with measurements of two species with contrasting acid tolerances (sugar maple and American beech, Fagus grandifolia) across a naturally occurring base saturation (BS) gradient in the Adirondack Mountains of New York state. This gradient spans northern hardwood stands growing on highly buffered soils that were not strongly impacted by acidification (e.g., soils with limestone and BS values near 100%) to stands on granitic soils that were strong impacted by acidification (e.g., BS values near 5%). We measured foliar respiration rates, total foliar chemistry (N, P, Ca, Mg, Mn, F, Al) and the soluble concentration of select elements (Ca, Mg, Mn) to examine how a range of nutrient availability may affect plant respiration.

Results/Conclusions

Foliar chemistry changed across the soil base saturation gradient in an expected manner, with decreases in base cations (e.g., Ca and Mg) and increases in phytotoxic metals (e.g., Mn) in highly acidified soils. However, foliar respiration rates were not correlated with any measure of acidity (e.g., BS, foliar concentrations of Ca, Mg, Mn, Al content; all P > 0.1). Respiration rates did correlate with other leaf traits (N content, leaf mass per unit area) reflective of leaf morphological variation that was unrelated to acidity. We found that nearly all of the leaf Ca and Mg content was in the soluble pools (slopes between total and soluble were not statistically different than 1.0; P > 0.1) but that most of the leaf Mn content was not soluble (slope between soluble and total pools was 0.02 and statistically different than 1.0; P < 0.001). These results indicate that the detrimental effects of acidic deposition on trees is unlikely to be directly related to increases in foliar respiration as was previously thought. These results also reflect a commonality in foliar chemistry response to acidification between species that clearly differ in their ecological acid tolerance.