Nutrient fluctuations in Sitka spruce (Picea sitchensis) plantations: the implications for future forest management practice

The nutrient supply of forest soils is strongly influenced and changed by a number of forest management practices such as the choice of tree species ('biological acidification') and the biomass export (thinning, harvesting). This study investigates the soil chemical conditions of Sitka spruce plantations of different age classes regarding the fluctuations in nutrient supply within a rotation period on nutrient-poor soils, and discusses implications for future forest management. The study is based on composite samples of the topsoil (Ah-horizon) that were collected from 30 plots in Sitka spruce plantations of different age classes and semi-natural oak woodlands in the Grizedale Forest area, Cumbria, UK. A number of soil chemical analyses was performed. Across the age classes, the soil chemical variables were found to fluctuate rather than linearly increase or decrease, showing distinct resilient behaviour. Furthermore, both the analysis of the fluctuation of the soil chemical variables, and the sample plot ordination assessing the importance of soil chemical variables at the plots, strongly emphasize three major groups of interacting soil chemical variables: (1) soil bases (including Ca, Mg, base saturation, cation exchange capacity, cation molar ratios); (2) soil acids (including Al, H, pH); and (3) soil organics (including organic matter content, P). A key ion was found to be reactive Al, which strongly affects the soil nutrient balance and supply to the site, as well as being toxic to the fine root system of Sitka spruce. The research findings show that unfavourable environmental factors such as acid deposition, when combined with current forest management practices are responsible for the reduction in the long-term stability of the nutrient supply. This in turn causes a diminution in site productivity and potential limitation to the subsequent establishment of native deciduous tree species. In order to maintain long-term stability of the nutrient supply and to preserve the feasibility of subsequent broadleaf restocking, some changes to current forest management practices of Sitka spruce plantations are proposed.