Spatial and temporal patterns of sulfur and nitrogen in wood of trees across the north central United States

Abstract

Tree wood tissue was sampled in 169 forest stands, which represented five forest types, across the states of Minnesota, Wisconsin, and Michigan. The molar ratio of elemental sulfur to nitrogen for 3 decades of wood growth (1956 to 1985) was related to (i) location, as defined by five sampling zones, (ii) the molar ratio of elemental sulfur to nitrogen in soils, and (iii) wet sulfate deposition. There were also differences in molar ratios of sulfur to nitrogen among tree species and no significant interactions between species and zone. The sulfur to nitrogen ratios of both soil and wood increased from west to east across the sulfate deposition gradient. This trend is consistent with growing evidence that sulfate and other sulfur ions deposited through air pollution are accumulating in various compartments of forest ecosystems in the north central United States. These results also support studies that indicate that more sulfur is being taken up by trees than needed for nutrition.