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The influence of red alder in adjacent conifer stands : nutrient cycling and light transmission Lavery, John Meredith

Abstract

Red alder (Alnus rubra(Bong.)) is an important successional deciduous species in British Columbia with expanding commercial markets. Alder is a pioneer species which grows rapidly on disturbed sites and in riparian areas of coastal British Columbia. Alder's impressive early height growth and ability to increase the rate of cycling of nitrogen and other nutrients lead to a challenge in managing alder with conifers. Managing alder in mixedwood stands requires finding an appropriate balance between competition for light and the improvement in N status of a site, which would lead to improved conifer growth over time. Light transmission to the understory of adjacent alder and conifer stands was measured along a transect between the two stands using photodiode and quantum sensors, as well as hemispherical photography. Light varied with age and site, but alder stands generally had more light in the understory than the adjacent conifer stands. This difference was more marked in younger stands. Modeling using SLIM and LITE correlated well with the sensor information, and further modelling of the alder stands suggests that the amount of light available to conifers adjacent to an alder stand is of adequate quantity to sustain conifer growth within 5m of the alder/conifer stand boundary. The assessment of nutrient cycling in alder stands involved independent studies of litterfall, soil qualities, and a bioassay of the soils using Douglas-fir seedlings. Higher concentrations of N and several other nutrients were found in alder litter, but the conifer litter had very high levels of several micronutrients. The alder litterfall made significant contributions to nutrient inputs up to 15m into the conifer stands. The soil showed elevated N, and Total C, C:N and pH were influenced by alder presence in the oldest stands. This series of experiments also showed the influence of alder on the concentration of N in the shoots of the seedlings, as well as an influence on boron. The influence of alder litterfall, soil and bioassay N at points along the transects was correlated at the oldest and middle aged sites, suggesting that red alder produces a significant change in the nutrient cycling in adjacent conifer stands. Implications for managers and suggestions for management are discussed.

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