Interactions between tree species and their effects on nitrogen and phosphorus transformations in the forest floor

Abstract

Natural woodlands and forests contain a wide diversity of plant species and the potential for interactions between the different components is great. Mixed conifer plantations are less complex, but provide an opportunity to study interactions that may occur between tree species. Plantations of conifers in Britain have been mainly of single species unless there have been good reasons for planting mixtures. An example of this includes the increasing practice of planting mixed coniferous species on nutrient poor soils. Species mixtures have been planted on podzolized heathland soils and deep peats to establish crops of Sitka spruce (Picea sitchensis (Bong.) Carr.) during the past 60 years or more. On heathlands dominated by Calluna,the objective was to suppress the growth of heather by planting pine, usually Scots pine (Pinus sylvestris L.), in mixture with the spruce (Weatherall, 1957). On the west coast of Scotland, mixed species have been used to establish self-thinning crops on deep peats to surmount the problem of windthrow and to avoid uneconomic thinning. In this situation, larch (Larix sp.) was also used as the nurse species (O’Carroll, 1978). The practice of planting mixed species gathered momentum when it was realised that spruce planted in mixture could be established on nutrient poor soils without repeated additions of nitrogen fertilizers and herbicides (Mackintosh, 1983). In a report on the nutritional requirements of Sitka spruce on different site types in Britain, Taylor and Tabbush (1990) suggested that the mixtures effect involved more than just suppression of the vegetation. The overall effect appears as an increased supply of nitrogen to the spruce planted in mixture which it does not obtain in pure stands. Similar conclusions had been reached by Richards (1962) in a study of the effects of different Pinus species on Hoop pine (Araucaria cunninghamii Aik) and by Fisher and Stone (1969) investigating the effects of larch and pine on the growth of herbaceous vegetation.