Sugar maple (Acer saccharum) is a major component of the northern hardwood forest ecosystem in North America. Sugar maple has experienced a well documented regional decline in the northeastern region of North America since the 1950s. While initially attributed to several different factors including, drought stress and insect defoliation, recent studies have strongly implicated the legacy of chronic acidic deposition as a driving force of declining sugar maple health, recruitment, and growth. The causal explanation has been that sugar maple is a highly calcium-dependent species, while acid deposition causes the depletion of base cations, particularly calcium, in forest soils resulting in nutrient deficits. Here, we test two hypotheses: (1) depletion of soil calcium results increased respiration and therefore decreases growth (2) depletion of base cations in the soil directly limits growth by reducing calcium available for synthesis of cell walls. We collected upper canopy leaves from mature sugar maple and American beech (Fagus grandifolia), a sympatric species in northern hardwood forests. We chose sites that span a well-studied base saturation gradient across the Adirondack region. Foliar samples were used to measure a suite of leaf traits, including chlorophyll content (greenness), dark respiration rate, soluble leaf chemistry and total leaf chemistry.
Results/Conclusions
Sugar maple and American beech were the two most dominant overstory species at all study sites. Estimates of foliar chlorophyll were not correlated with base saturation for either sugar maple or American beech (p > 0.1). We expected dark respiration rate to increase with decreasing soil base saturation since this would indicate a greater metabolic cost with increasing stress related to nutrient deficits. However, we found instead that respiration rate and soil base saturation had no correlation (p= 0.401). These results were not consistent with the predictions from hypothesis (1), nor are they consistent with previous suggestions in the literature linking soil acidification and foliar respiration. Ongoing work to analyze variation in soluble and total leaf chemistry as a function of soil chemistry will address hypothesis (2). Overall, This study provides new insights on the mechanisms underlying the past and potential future impacts of acid rain, which has declined in recent years. However, calcium depletion in forest soils, and the potentially related decline of sugar maple, may represent a longer term legacy of acid pollution.