Increased Site Fertility and Litter Decomposition Rate in High-Pollution Sites in the San Bernardino Mountains

Abstract

Abstract Some possible factors causing enhanced litter decomposition in high-pollution sites in the San Bernardino Mountains of southern California were investigated. Nitrogen concentration of soil, as well as foliage and litter of ponderosa pine (Pinus ponderosa Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.) were greater in high-pollution plots than in moderate- or low-pollution plots. Nitrogen concentration of soil, foliage, and litter of ponderosa pine (ozone-sensitive), and of the ozone-tolerant spedes, sugar pine (Pinus lambertiana Dougl.) and incense cedar (Calocedrus decurrens [Torr.] Florin.), were all higher at a higher pollution site than at a moderate-pollution site. The rate of litter decomposition for all three species was also greater at the high-pollution site. Results suggest that the primary factor causing enhanced decomposition of L-layer litter in high-pollution plots is greater site fertility, leading to the production of foliage and litter that is higher in N than litter from moderate- or low-pollution plots. Although foliage of ponderosa pine trees exhibiting symptoms of severe ozone damage in high-pollution plots abscissed 2-4 years earlier than foliage of trees in low-pollution plots, retranslocation of N and P prior to needle abscission was not inhibited in high-pollution plots. Higher N levels in litter in the high-pollution plots does not appear to be due to reduced retranslocation of N from senescing foliage. Litter from ponderosa pine trees with severe symptoms of ozone damage (chlorotic mottle) decomposed at the same rate as litter from adjacent trees with no visible chlorotic mottle. Treating ponderosa and Jeffrey pine litter from four plots with 2, 20, or 200 mM NH4NO3 significantly increased the rate of litter decomposition at one or more N concentrations in three of the four plots. Total CO2 generated by the decomposing litter during a 15 or 17 day period tended to increase with N amendment, but was significantly increased only with litter from the dogwood plot treated with 200 mM NH4NO3. In some areas in the San Bernardino Mountains, chronic N deposition may stimulate litter decomposition by: (1) increasing site fertility, leading to higher concentrations of N in foliage and litter, and (2) accumulation of N onto litter and plant canopies, which stimulates litter decomposition upon wetting. For. Sci. 37(4):1163-1181.