Emerging Examples in the United States and Europe

In addition to the well-established connections between Ca depletion and tree health outlined above, new associations between the Ca status and health of trees periodically emerge—particularly in regions that experience continued pollution-induced Ca leaching. Differences in the susceptibility of eastern hemlock trees to damage by the hemlock woolly adelgid (HWA; Adelges tsugae Annand) in the Northeastern United States may provide an example of this. The HWA is a small, aphid-like insect that was likely introduced to the mid-Atlantic States from Asia in the 1950s and has since expanded its range and influence, devastating hemlock forests over an ever-widening portion of eastern hemlock’s native range (Orwig and Foster 1998). Although it was first believed that little or no variation in susceptibility to HWA damage existed among hemlock trees (McClure 1995), recent work has shown that differences in site conditions and the presence of other stressors are associated with differential damage and decline (Orwig and Foster 1998, Sivaramakrishnan and Berlyn 1999). In particular, Pontius and others (2006) recently concluded that foliar chemistry was linked to the infestation and susceptibility of eastern hemlock to HWA. Among other evidence, results of a regional monitoring effort showed that concentrations of Ca, K, N, and P were strongly correlated with HWA densities (a driving factor in hemlock decline). From this and other findings, they hypothesized that, whereas foliar N and K concentrations may influence hemlock decline through an alteration in insect behavior due to palatability issues, Ca and P concentrations may deter severe HWA damage through an alteration in tree physiology (Pontius and others 2006). Experimental tests are needed to assess whether these changes in physiology involve Ca-induced alterations in plant stress response systems.

Evidence of acid deposition damage to forest health and productivity in Europe is anything but new or emerging. Indeed, media attention and resulting public concern about the possible connection between pollution exposures and forest death (Waldsterben) helped spur initial efforts to control acidifying pollution additions (Kakebeeke and others 2004). Research eventually identified pollution-induced cation depletion (particularly Mg and Ca) as an instigating component of forest decline (Schulze 1989). Although the mechanism through which acid deposition influences the health of European forests has not been explicitly defined, it is generally agreed that it acts as a predisposing agent, weakening forests and making them more susceptible to damage by other stresses including insect attack, extreme climate events, or storm damage (Materna and Lomský 2002, UNECE 2004). This scenario is strikingly similar to the Ca depletion and stress response suppression hypothesis developed primarily using evidence from declines in the United States (Schaberg and others 2001). In fact, a recent example of forest damage in Europe that is thought to be predisposed by pollution exposure involves the reddening and abscission of foliage of Norway spruce in the late winter, presumably due to freezing injury (Lomský and Šrámek 2002, Materna 2002). Patterns of injury (preferentially impacting the youngest foliage with the intensity and extent of damage increasing with elevation, Lomský and Šrámek 2002, Materna 2002) are identical to those documented for red spruce winter injury (DeHayes 1992, Lazarus and others 2004), which has been mechanistically linked to Ca depletion (see previous section).