The nursing of Sika spruce by Douglas fir

摘要

Nursing of Sitka spruce (Picea sitchansis) by Douglas fir (Pseudotsuga menziesii) arose unexpectedly in a field experiment established at Ballyhoura Forest in 1976. The original aim of the experiment was to examine the potential of intensive soil reclamation to improve the establishment of Sitka spruce and Douglas fir on poor Old Red Sandstone soils. The soil at the site was a modified podzol, changed from its original state by past management practices. It was nonetheless marginal for Sitka spruce,requiring frequent nitrogen fertilisation in the absence of nurse species. The experiment layout placed plots of the two species immediately adjacent to one another. The nursing effect was first noted eight years after the establishment in plots of Sitka spruce adjacent to Douglas fir. Height, leader length and diameter of the Sitka spruce were measured in lines parallel and adjacent to the Douglas fir to determine the spatial progress of nursing. Foliage analysis indicated that the effect was nutritional, rather than due to shelter. The nursing effect occurred in a less than ideal spatial arrangement of the two species; it is contended that a more favourable result for the Sitka spruce could have been achieved with an intimate mixture. Douglas fir/Sitka spruce mixtures should be considered on similar soils, the main benefits being the extended species choice, increased diversity, and reduced need for fertilisation.