Effect of thinning on production and mortality of fine roots in a Pinusradiata plantation on a fertile site in New Zealand

Abstract

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.