The Role of Understory Vegetation in the Nutrient Cycle of Forested Ecosystems in the Mountain Hemlock Biogeoclimatic Zone

The biogeochemical role of understory vegetation was investigated in three sites representing a typical topographic sequence of xeric, mesic, and hygric site types characteristics of the Mountain Hemlock Biogeoclimatic Zone. Overstory biomass on the three sites was estimated to be 60.9, 55.7, and 34.5 kg/m² for the hygric, mesic, and xeric site types, respectively. Average values for understory biomass were: 44.1, 66.1, and 309 g/m² for the hygric, mesic, and xeric site types. Understory aboveground production (UAP) values of 26.0, 14.2, and 63.1 g°m— ²°yr— ¹ for the hygric, mesic, and xeric site types were equivalent to 11.3, 6.1, and 48.6% of the estimated overstory aboveground production. The understory was found to cycle a much greater proportion of its total nutrient (N, P, Ca, Mg, K, Zn, Cu) standing crop annually compared to overstory. Approximately 80% of the macronutrients present in the understory standing crop are found in the understory annual production on the hygric site type. Estimates of 17.6, 8.3, and 20.6 g°m— ²°yr— ² of total understory aboveground litterfall (exclusive of the moss layer) were obtained for the hygric, mesic, and xeric sites, respectively. Understory was shown to return a significant proportion of the litterfall nutrients on a yearly basis, the bulk of which was retuned as a single pulse during the first autumn snowfall. Understory vegetation above the moss layer was shoen to have a significant effect on the quantity of nutrients present in throughfall precipitation. The effect was seasonal with PO₄—P, NO₃—N, and NH₄—N being removed in the spring and Ca, Mg, and K being added to overstory throughfall in the autumn. Modifications of water chemistry previously attributed to the forest floor may in some cases reflect unmeasured influences of understory vegetation.