Background/Question/Methods A number of anthropogenic factors (most notably pollution-induced acidic loading) can deplete beneficial base cations (e.g., calcium – Ca) and increase the availability of potentially toxic elements (e.g., aluminum - Al) in forest soils.  This dynamic of depletion and availability is complicated because 1) Al can competitively inhibit Ca uptake, and 2) once within plants these cations can either promote plant health and support stress response physiology (Ca) or act as a direct stress agent (Al).  The combination of reduced stress response as Ca becomes deficient, with increased direct toxicity as Al availability increases, could be especially problematic for sensitive and economically important species like sugar maple (Acer saccharum Marsh.).  Because alterations to Ca and Al nutrition may disproportionately influence plant energy relations, perturbations of these cations may particularly affect forest carbon sequestration.  To evaluate these possibilities, we measured the nutrition and physiology of native sugar maple trees in a long-term, replicated Ca manipulation study at the Hubbard Brook Experimental Forest in New Hampshire, USA. Plots received applications of Ca (to boost Ca availability above depleted ambient levels) or Al (to compete with Ca uptake and test for a low-level toxicity response).

Results/Conclusions First we assessed intermediate and co-dominant trees in 2004.  We found that trees on Ca-addition plots had significantly greater foliar Ca concentrations, reduced crown dieback and greater basal area growth than trees on control (ambient Ca depleted) and Al-addition plots.  An associated test showed that trees from Ca-addition plots experienced about twice as much wound closure in one year than trees from control and Al-addition plots.  Subsequent measurements with dominant trees in 2006 confirmed that soil Ca addition increased foliar Ca levels above deficiency thresholds.  However, following two additional Al applications since 2004, trees on Al-addition plots had elevated activities of two antioxidant enzymes (glutathione reductase and ascorbate peroxidase) in their leaves compared to trees from control and Ca-addition plots.  Stems of trees from Al-addition plots also stored significantly less soluble sugars (fructose, sucrose, glucose and total) than trees from control and Ca-addition plots.  Al treatment increased antioxidant enzyme activity and reduced stem sugar reserves even though no differences in foliar Al concentrations attributable to treatment were detected.  Overall results suggest that mature sugar maples are vulnerable to Ca deficiency and Al toxicity, and that reduced carbon sequestration (growth and/or stored sugars) is one likely consequence of nutritional disruption.