Nursing effects of birch on Sitka spruce grown on an industrial cutaway peatland.

Abstract

The suitability of major conifer species for afforestation of industrial cutaway peatlands is limited to a narrow range of site types. There is some evidence that establishment of Sitka spruce (Picea sitchensis) or Norway spruce (Picea abies) with birch (Betula spp.), as a mixed stand or by under planting spruce in an established birch canopy can improve the productivity of the conifer crop. Management of mixed Norway spruce and birch crops is now a well-established management model used in southern Sweden (Kronoberg approach). In this study, a mixed spruce-birch trial, established in 2000 under the previous BOGFOR programme, was re-evaluated to determine if there was any evidence of a nurse effect of birch on Sitka spruce. Analysis of various planting configurations showed that planting the two species at the same time in alternate rows produced the best results, in terms of total basal area, top height, mean DBH and height of Sitka spruce. When compared to pure Sitka spruce stands, the productivity was c. 38% higher, for trees planted at the same time in alternate rows with birch. Although the definitive physiological factors contributing to the nursing effect of birch on Sitka spruce is still unclear, these results and others suggest the nursing effect is probably due to enhanced foliage nutrition, possibly associated with increased nutrient availability due to decomposition of birch litter or increased root-zone aeration. There was no evidence of a reduction in exposure or in frost stress in mixed species treatments. The implications of these findings are that the potential area suitable for Sitka spruce on cutaway industrial peatland sites can be expanded when planted in combination with birch. Moreover, the potential utilisation of birch thinnings for biomass and the final Sitka spruce crop for timber may be a particularly suitable option for Bord na Móna, since it may potentially fulfil both bioenergy and timber production objectives. Further research is, however, required to assess whether the nursing effect will continue and to evaluate the viability of the proposed silvicultural system on cutaway peats. The timing of silvicultural interventions is particularly important to ensure that a Sitka spruce crop in not suppressed whilst still preserving the birch nursing effect.