Nursing mixtures can enhance long-term productivity of Sitka spruce (Picea sitchensis (Bong.) Carr.) stands on nutrient-poor soils

The growth of Sitka spruce (Picea sitchensis (Bong.) Carr.) on nutrient-poor sites in the British Isles is often improved during the late establishment phase when grown in intimate ('nursing') mixture with pioneer species such as pines and larches. However, there is very little information on the longer term effects of such mixtures on stand development and productivity. Therefore, we analysed results from four experiments established in northern Scotland in the 1960s which included nursing mixtures of Sitka spruce with Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl.) or larches (Larix spp.), as well as plots of pure Sitka spruce given different establishment regimes, sometimes including nitrogen fertilization. At three nutrient-poor sites, spruce in nursing mixture was significantly taller after 15 years than pure spruce without nitrogen. Analysis of foliar nitrogen status showed that pure spruce without nitrogen became deficient by similar to 8-10 years with no recovery for at least 20 years. Spruce grown in mixture also showed an initial nitrogen deficiency, but with recovery by 10-15 years to optimal or marginal levels. After more than 40 years growth, on the nutrient-poor sites the basal areas in the nursing mixtures were significantly higher than pure spruce without remedial nitrogen and comparable to those of pure spruce given several applications of nitrogen, However, these differences were not evident at the more fertile site. Nearly all mixed plots self-thinned towards dominance by Sitka spruce. The magnitude of this nursing effect is striking and harnessing the mechanisms underpinning this effect could be very important for sustaining productivity of forests on nutrient-poor soils in upland Britain.