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Vertical distribution and production of fine roots in an old-growth forest, Japan

Fine roots (≤ 2 mm in diameter) account for up to 50% of total net primary production in forests, representing a major flow of both carbon and nutrients into the soil. We investigated the vertical distribution and production of fine roots in a warm temperate old-growth evergreen broadleaved forest in southwestern Japan. We used a continuous inflow method that considered different rates of diameter-dependent root mortality, decomposition, and thickening. Fine roots were classified into two classes (≤ 1 mm and 1–2 mm diameter). The experiment was conducted over a 1-year period to collect data on the mass of live fine roots and mass of dead fine roots in January, May, November and the following January. Decomposition ratios were assessed for three intervals (January to May, May to November, and November to January). More than 70% of fine roots occurred in the 0–20 cm soil layer, and less than 4% were found in the 50–80 cm soil layer. Decomposition ratios varied seasonally in both root size classes, peaking in summer and reaching a minimum in winter. The same pattern was found for production, mortality, and decomposition. The peak rate of production was 1.62 g·m–2·day–1 in ≤ 1 mm and 0.63 g·m-2·day–1 in 1–2 mm fine roots. The lowest production was 0.62 g·m–2·day–1 in ≤ 1 mm and 0.38 g·m–2·day–1 in 1–2 mm fine roots. Total fine root production over a 1-year period was 6.61 t·ha–1. A mass of 2.70 t·ha–1yr–1 of dead fine roots was decomposed to return nutrients to the soil. It is concluded that a warm temperate old-growth evergreen broadleaved forest in southwestern Japan plays an important role in carbon cycle and nutrient return through a high amount of production and decomposition.