We examined the nutrition and Ca-related physiology of red spruce (Picea rubens Sarg.) trees from two watersheds at the Hubbard Brook Experimental Forest (Thornton, NH): a reference where pollution-induced Ca depletion is well documented and a Ca-addition watershed, to evaluate the bases of recent (2003) differences in winter injury there.  Current-year foliage from red spruce in both watersheds were collected in November (2005) and February (2006) and compared for their cation nutrition, soluble sugar concentrations, ascorbate peroxidase (APX; a key antioxidant enzyme) activity, and cold tolerance.  Ca content and total sugar concentration were significantly greater in foliage of trees from the Ca-addition watershed during both fall and winter (P = 0.037, 0.035 for November; P = 0.055, 0.036 for February, respectively).  Individual sugar concentrations of fructose and glucose in November (P = 0.013, 0.007, respectively), and sucrose in winter (P = 0.005), were also found to be significantly greater in foliage from the Ca-addition watershed.  APX activity was similar in trees from both watersheds during fall (P = 0.278), but greater in the Ca addition watershed during winter (P = 0.063).  Cold tolerance of foliage from the Ca-addition watershed was also significantly greater than that from the reference watershed (P < 0.001).  The suppression of various Ca-dependent processes in red spruce from the reference watershed likely predisposed them to greater winter injury in 2003.  Because the reference watershed reflects ambient forest conditions in the region, the consequences of impaired physiological function due to Ca-depletion may have more widespread implications for forest health.