Range Edge Red Spruce in New Jersey, U.S.A.: Bog Versus Upland Population Structure and Climate Responses

Red spruce (Picea rubens Sarg.) was investigated at its lower elevational range limit in northwestern New Jersey, USA, where small disjunct populations of scattered young trees occur in bogs and on adjacent rocky upland slopes. The bog habitat, unusual for the species elsewhere in its range, supported slower tree growth but more robust populations, with older trees, more cone production, more trees of canopy status, and higher seedling densities. Surrounding upland slopes supported short-lived, increasingly drought-sensitive populations with somewhat higher incidence of nonlethal adelgid galls and old leader damage. Tree ring chronologies showed a decrease in growth rate within four of five populations beginning around 1975, 15 years after such a decrease in mature red spruce elsewhere in its range, with recovery from decline seen after 1983 for younger but not for older trees. Existing trees exhibited low levels of crown discoloration but fairly extensive (24% of trees) defoliation due to insect leader damage. Roots were heavily colonized by ectomycorrhizae, through with less diversity in wetlands and with the stress-indicator Cenococcum geophilum dominanting in all populations. Inexplicably dead trees were found in high numbers (22%) in both bog and upland populations at one of three localities. Tree ring index chronologies reveal a lack of climate sensitivity in bog populations and one upland population, where no monthly temperature or precipitation variable explains growth decline. The other two upland populations recorded consistent climatic signals, with better growth during years with a wet June, wet March, or warm January; these two upland populations also showed increasing drought sensitivity over time, indicating an uncoupling of climate/growth relationships like that attributed elsewhere to regional air pollution.