TRACERS OF CALCIUM IN ECTOMYCORRHIZAL TREES: CA/SR AND δ⁴⁴CA

Northeastern forests have been prominent in research related to anthropogenic effects on ecosystem health. Calcium depletion resulting from acid deposition is one of the main topics discussed in the literature. Shortle and Smith (1988) were some of the first to find and suggest that the declining health of red spruce stands was at least partially caused by Ca depletion. More recently, Juice et al. (2006) working with sugar maples at Hubbard Brook Experimental Forest, NH, USA, discovered that the decline of these trees throughout the Northeast is caused in part by Ca deficiencies, which had been, before this study, a favored assumption. Others have reported that in certain areas, soil calcium depletion is not as damaging due to the apparent ability of some trees to access calcium in apatite (Blum et al. 2002) and therefore thrive in areas with depleted soil pools of exchangeable Ca (Yanai et al. 2005). A consensus cannot be reached on the state of the Ca budget and its relation to ecosystem health until an accurate accounting of Ca in the ecosystem is completed. All of the sources and fluxes of Ca have yet to be fully identified and quantified in part due to the lack of an appropriate tracer of calcium. In an effort to improve our knowledge of terrestrial Ca cycling, we've examined two tracers of calcium, Ca/Sr values and Ca isotope ratios (expressed as δ⁴⁴Ca) for their ability to provide information about tree calcium sources. Using seedling Scotch pines (Pinus sylvestris) with and without mycorrhizal fungi (Suillus bovinus or Thelephora terrestris) grown at two rates of nutrient application we conclude that both tracers undergo fractionation within individual plants and throughout the substrate-fungi-root-needle system. Despite this, the two tracers combined are a useful and more accurate tool than existing tracers for clarifying the sources and movement of calcium in a forest ecosystem.

Blum, J.D., et al. 2002. Nature 417: 729-731. Juice, S.M., et al. 2006. Ecology 87(5):1267-1280. Shortle, W. and K.T. Smith. 1988. Science 240: 1017-1018. Yanai, R.D., et al. 2005. Journal of Forestry 103:14-20.