Influence of stand age and spatial location on throughfall chemistry beneath black spruce

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Total summer precipitation and throughfall chemistry are investigated beneath black spruce (Piceamariana (Mill.) B.S.P.) in central Canada under the null hypothesis that no variation occurs with postfire stand age nor from one location on the feather moss dominated forest floor to another. Data from collectors at inner, mid, and edge radial positions beneath individual tree crowns and between neighbouring trees (gaps), within each of a 61-, 90-, and 120-year-old stand, were summed to yield a growing season total volume and the deposition of NO3−-N, NH4+-N, PO43−-P, K+, Ca2+, and Mg2+. Relative to incident precipitation, NO3−-N and NH4+-N showed highly significant net uptake by the canopy foliage. In contrast, K+, Ca2+, and Mg2+ showed net losses from the canopy throughfall. No appreciable difference in net throughfall (i.e., throughfall minus precipitation) volume and K+ deposition appeared among the stands. However, NH4+-N, NO3−-N, PO43−-P, Ca2+, and Mg2+ showed significant differences. NH4+-N and NO3−-N were less readily retained by foliage of the old stand than by that of the young stand, whereas Ca2+ and Mg2+ leached more readily from trees in the 120-year-old stand than in the other two. PO43−-P showed no net throughfall in the 61- and 120-year-old stands, whereas there were net losses from the canopy of the 90-year-old stand. This may reflect soil differences between the 90-year-old site and the other two. Significant differences in spatial location emerged for all variables measured. Net throughfall deposition of PO43−-P, K+, Ca2+, and Mg2+ declined from the inner to gap positions, whereas volume, NO3−-N, and NH4+-N increased. These effects were proportional to the canopy profile depth above each collector. The heterogeneity observed in nutrient deposition on the forest floor has important implications for the distribution and growth of forest floor plants, such as mosses and lichens, which seem to depend on precipitation and throughfall for their nutrient supply.