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The role of understory vegetation in the nutrient cycle of forested ecosystems in the mountain hemlock biogeoclimatic zone Yarie, John

Abstract

A study was carried out to ascertain the biogeochemical role of understory vegetation in three representative sites characteristic of the Mountain Hemlock Biogeoclimatic Zone. The three sites were selected to represent a typical topographic sequence of plant associations and were classified as members of the Vaccinio (membranacei) -Tsugetum mertensianae, Abieto (amabilis) - Tsugetum mertensianae and Streptopo (rosei) - Abietetum amabilis plant associations (xeric, mesic, and hygric site types, respectively). The overstory layer was found to be typical of old growth, high elevation forests of southwestern coastal British Columbia. Overstory biomass on the three sites was estimated to be 60.88, 55.68, and 34.05 kg•m⁻² for the hygric, mesic, and xeric site types, respectively. Understory aboveground biomass was found to be less than one percent of the aboveground overstory biomass. Average values for the three sites were: 44.1, 66.1, and 399.3 g•m⁻² for the hygric, mesic, and xeric site types, respectively. Understory aboveground production (UAP) was found to represent a greater proportion of overstory aboveground production, as indicated by the mean annual increment (MAI), than the biomass figures might suggest. UAP values of 25.95, 14.19, and 63.12 g•m⁻²•yr⁻¹ for the hygric, mesic, and xeric site types, respectively, were equivalent to 11.28 percent, 6.06 percent, and 48.55 percent of the estimated aboveground overstory production. Only a small percentage of the total aboveground nutrient standing crop was found in the understory. This is in agreement with comparable published values for old growth forest ecosystems. However, the understory was found to cycle a much greater proportion of its total standing crop annually compared to overstory. Approximately 80 percent of the macronutrients present in the understory standing crop are found in the understory annual production on the Streptopo - Abietetum amabilis site (hygric site type). Estimates of 17.6, 8.3, and 20.6 g•m⁻²•yr⁻¹ of understory aboveground litterfall (exclusive of the moss layer) were obtained for the hygric, mesic, and xeric sites, respectively. These values are substantially less than for overstory litterfall, but the biomass of different litterfall components (e.g. understory or overstory) was shown to be a poor indicator of the proportional contribution of the components to the quantity of nutrients in aboveground litterfall. Understory was shown to return a significant proportion of the litterfall nutrients on a yearly basis, the bulk of which was returned as a single pulse during the first autumn snowfall. Understory vegetation above the moss layer was shown to have a significant effect on the quantity of nutrients present in throughfall precipitation reaching the ground. The effect was seasonal in nature with PO₄-P, N0₃-N, and NH₄-N being removed in the spring and Ca, Mg, and K being added to overstory throughfall in the autumn. It was concluded that modifications of water chemistry previously attributed to the forest floor may in some cases reflect unmeasured influences of understory vegetation. The understory aboveground nutrient cycles follow two basic patterns. The first pattern, a conservative cycle, is exemplified by nitrogen and phosphorus and has the following characteristics: (1) removal of nitrogen and phosphorus from overstory throughfall by the non-bryophyte understory, (2) estimated annual nitrogen and phosphorus uptake up bryophyte production in excess of the remaining throughfall nitrogen and phosphorus content and (3) a large proportion of the annual requirement was accounted for by internal redistribution within the understory plants. The second cycling pattern, an open cycle, is characteristic of calcium and magnesium and displays characteristics opposite to those of the "conservative cycle". The potassium, manganese, zinc, and copper cycles are intermediate between the "conservative" and "open" nutrient cycles. The results are discussed with respect to a proposed model of ecosystem function and it is hypothesized that understory plays a major role in maintaining ecosystem stability by promoting nutrient cycling.

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