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Measurements of canopy opening, understory light, throughfall precipitation, and understory herb response in leaf surface area per square meter in pine forests on light—textured soils in northern Wisconsin show that the understory herb layer is more responsive to differences in throughfall precipitation determined by canopy opening than to difference in light. Canopy opening controls both the level of light reaching the herb layer and the proportion of total precipitation reaching the litter layer, but the light levels are apparently considerably above the minimum thresholds for these species. On the light—textured soil—moisture recharge in the surface layers, as influenced by canopy opening, exerts the major control over the total cover of herbaceous species. Moisture content of the lower litter layers and upper soil layer, where most of the northern understory herbs are rooted, is greatly influenced by variations in throughfall determined by canopy opening.
Anderson et al. (Sat,) studied this question.