Sugar Maple Decline

Sugar maple (Acer saccharum Marsh.) decline has been documented throughout parts of the Northeastern United States and Quebec over many recent decades (Allen and others 1992a, Kelley 1988, Mader and Thompson 1969, Wilmot and others 1995). These declines have been characterized using various measures, including crown deterioration, increased leaf chlorosis, and reduced growth. Stress factors such as drought (Payette and others 1996), freezing (Robitaille and others 1995), and insect defoliation (Allen and others 1992b) have been implicated with the decline and mortality of sugar maple. Regardless of stressor, decline has also been associated with deficiencies or imbalances of various elements including N, phosphorous (P), K, Mg, manganese (Mn), or Ca (Bernier and Brazeau 1988, Horsley and others 2000, Mader and Thompson 1969, Ouimet and Fortin 1992, Paré and Bernier 1989, Wilmot and others 1995). Although the specific elements associated with decline can vary among sites, deficiencies in Ca have been highlighted as a potential contributor to sugar maple decline in recent studies throughout the region, (e.g., Ellsworth and Liu 1994, Ouimet and Camiré 1995, Wilmot and others 1996), in part because experimental additions of Ca or lime or both have been shown to reduce decline symptoms (Long and others 1997, Moore and others 2000, Wilmot and others 1995).

Proposed mechanism

Consistent with these observations, Schaberg and others (2001) hypothesized that sugar maple decline may be another example of Ca depletion’s influence on tree stress response systems and health. Variations in maple decline symptoms (crown condition and basal area growth) coinciding with differences in soil Ca status across a range of sites are consistent with this hypothesis (Schaberg and others 2006). However, as with red spruce, controlled additions of Ca at the Hubbard Brook Experimental Forest have provided a more specific test of the influence of Ca depletion on sugar maple health. For example, Juice and others (2006) compared the nutrition, reproductive success, and physiology of sugar maple seedlings on the reference and Ca-addition watersheds there. They found that seedlings on the Ca-treated watershed had higher root and foliar Ca concentrations, experienced greater survivorship, existed in greater densities, and had higher foliar chlorophyll concentrations than seedlings on the reference watershed (Juice and others 2006). Mycorrhizal colonization of seedlings was also greater in the treated than the reference watersheds (Juice and others 2006). In another study, Huggett and others (in press) surveyed and wounded forest-grown sugar maple trees in a replicated Ca manipulation study at Hubbard Brook. Similar to past studies, they found that Ca addition increased foliar Ca levels and resulted in improved crown vigor, reduced branch dieback, and greater basal area growth among trees (Huggett and others, in press). However, new were findings that Ca addition particularly improved the growth release of trees following a severe ice storm and significantly increased stem wound closure— a capacity particularly important to a species that is regularly wounded as part of maple sugar production (Huggett and others, in press). These new findings are particularly noteworthy because they more specifically test the influence of Ca nutrition on the ability of sugar maple trees to respond to environmental change (release from competition) and stress (wounding).