Litter dynamics in two Sierran mixed conifer forests. I. Litterfall and decomposition rates

Abstract

Litterfall was measured for 4 years and leaf litter decomposition rates were studied for 3.6 years in two mixed conifer forests (giant sequoia–fir and fir–pine) in the southern Sierra Nevada of California. The giant sequoia–fir forest (GS site) was dominated by giant sequoia (Sequoiadendrongiganteum (Lindl.) Buchh.), white fir (Abiesconcolor Lindl. & Gord.), and sugar pine (Pinuslambertiana Dougl.). The fir–pine forest (FP site) was dominated by white fir, sugar pine, and incense cedar (Calocedrusdecurrens (Torr.) Florin). Litterfall, including large woody debris <15.2 cm in diameter, at the GS site averaged 6364 kg•ha−1•year−1 compared with 4355 kg•ha−1•year−1 at the FP site. Compared with other temperate coniferous forests, annual variability in litterfall (as computed by the ratio of the annual maximum/minimum litterfall) was extremely high for the GS site (5.8:1) and moderately high for the FP site (3.4:1). In the GS site, leaf litter decomposition after 3.6 years was slowest for giant sequoia (28.2% mass loss), followed by sugar pine (34.3%) and white fir (45.1%). In the FP site, mass loss was slowest for sugar pine (40.0%), followed by white fir (45.1%), while incense cedar showed the greatest mass loss (56.9%) after 3.6 years. High litterfall rates of large woody debris (i.e., 2.5–15.2 cm diameter) and slow rates of leaf litter decomposition in the giant sequoia–fir forest type may result in higher litter accumulation rates than in the fir–pine type. Leaf litter times to 95% decay for the GS and FP sites were 30 and 27 years, respectively, if the initial 0.7-year period (a short period of rapid mass decay) was ignored in the calculation. A mass balance approach for total litterfall (<15.2 cm diameter) decomposition yielded lower decay constants than did the litterbag study and therefore longer times to 95% decay (57 years for the GS site and 62 years for the FP site).