Vegetation

Ca deficiency thresholds for trees in native forests exist for some species with established nutritional vulnerabilities. For example, based on greenhouse (Swan 1971) and field studies (Borer and others 2004, DeHayes and others 1999), minimal sufficiency and deficiency Ca thresholds have been determined for red spruce foliage (1200 and 800 µg g^-1 , respectively). The deficiency threshold is associated with reduced cold tolerance and growth and increased winter injury of trees (Borer and others 2004, DeHayes and others 1999, Swan 1971). Similarly, based on surveys of forest-grown trees, a foliar Ca deficiency threshold of about 5000 µg g^-1 has been established for sugar maple, (e.g., see Kolb and McCormick 1993). Foliar Ca concentrations below this threshold have been associated with increases in crown dieback and reduced growth of trees (Schaberg and others 2006), reduced growth following overstory release and impaired stem wound closure (Huggett and others 2007). However, an analysis of these thresholds reveals that threshold concentrations are not uniform among species and can vary greatly (here more than fivefold). Obviously, not all tree species access or require Ca in equal amounts. Depending on tree rooting habits, (e.g., depth of roots, possible mycorrhizal association, etc.), access to soil Ca can differ greatly. Furthermore, perhaps because Ca cycling within forests has historically been adequate to amply supply this critical nutrient, specific thresholds of Ca depletion have been developed for only a few key species (such as red spruce and sugar maple). Even for well-studied species, internal chemical sequestration of Ca, (e.g., the precipitation of Ca as insoluble oxalate crystals within cell walls; Fink 1991), may mask biological Ca deficiencies and complicate the establishment of universally relevant deficiency thresholds. Indeed, discovery of the mechanism through which acid deposition reduces the cold tolerance of red spruce foliage relied on the differentiation of biologically labile Ca from total foliar pools in order to remove the confounding influence of foliar Ca sequestration (DeHayes and others 1999, Schaberg and others 2000). Thus, due to great species-to-species variation in Ca nutrition and use, combined with questions of tissue sequestration and biological availability, foliar assessments of Ca are not universally valuable in assessing Ca deficiency. However, despite limitations, measurements of foliar Ca concentrations may have value if conducted as part of monitoring efforts to gauge spatial and temporal changes in Ca nutrition, thereby assessing trends in Ca accrual or depletion.