SummaryStanding crops of live and dead fine (< I mm diameter) and small (1 to 5 mm diameter) roots

Production and replacement of fine roots (diam. < 1 mm) takes 8–67% of net primary production in forests. Most of this production is lost through mortality; little appears as an increment. Traditional biomass methods underestimate fine-root production because estimating production or mortality from changes in standing crop alone does not adequately account for simultaneous and compensating processes of growth, death, and replacement which occur continuously. We propose a compartment–flow model to solve this problem and estimate fine-root production and mortality at a monthly resolution for a pine plantation in New Zealand. The main component of the model is fine-root decomposition, an exponential decay function driven by soil temperature. The model "produces" and "turns over" enough fine roots to maintain observed standing crops of live and dead fine roots given losses through decomposition each month. We have formulated the model as differential and difference equations. Monthly estimates from the model indicated smooth modal patterns. Production and mortality peaked in early spring (September) at about 600 kg•ha−1•month−1 and fell to near zero in summer (January–February). The periodicity of these two processes was out of phase with soil temperature at 10 cm. Decomposition occurred continuously; it peaked in early summer (December) and declined to low levels during winter and was in phase with soil temperature. In a validation of the decomposition portion of the model with an independent set of decomposition data, measured standing crops of dead fine root were not statistically different from predicted values.

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The effects of heavy thinning (60% reduction in basal area) on fine (< 1 mm diam.) and small roots (1–5 mm diam.) were evaluated during the 2nd year following treatment by periodic soil core sampling in a 12-year-old plantation of Pinusradiata D. Don. Data from these samples enabled us to estimate monthly standing crops of live and dead fine roots and seasonal rates of fine-root decomposition. We used a compartment-flow model to estimate production and mortality of fine roots with monthly resolution from these data. The general pattern of production and mortality was modal and out of phase with soil temperature. On an area basis, thinning reduced the overall standing crop of live fine roots from 1.38 to 0.55 Mg/ha; the standing crop of dead fine roots remained unchanged at 4.37 Mg/ha. The standing crop of live small roots declined from 1.03 to 0.54 Mg/ha. Annual production of fine roots was estimated at 2.2 and 1.9 Mg•ha−1•year−1 in the control and thinned treatment, respectively, and mortality was estimated at 2.1 and 2.0 Mg•ha−1•ear−1 in the control and thinned treatment, respectively. Thinning shortened mean fine-root longevity from 6.2 to 2.5 months. With respect to total dry matter production, fine-root production remained a minor component following a heavy thinning. It accounted for only 4.6 and 6.1% of the stand total in the control and thinned treatments, respectively. These results indicate that on a fertile site with a mild climate the opportunity to shift production from fine roots to another component, such as stem wood, is likely to be small.

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