Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr.)

This study investigated the effects of Alaskan lodgepole pine (Pinus contorta Loud.) and hybrid larch (Larix x eurolepis Henry) nurses, planted in triplet mixture with Sitka spruce (Picea sitchensis (Bong.) Carr.), on growth rate, stem form, branching habit and wood properties of the spruce. These mixtures were compared with pure stands of Sitka spruce which had been regularly and periodically fertilised with nitrogen. Hybrid larch promoted diameter increments in Sitka spruce greater than those achieved by the other treatments over the last 12 years of the experiment (current age 30 years). Growth of regularly fertilised pure Sitka spruce was not significantly greater than that of periodically fertilised pure spruce, suggesting that regular applications of nitrogen fertiliser did not result in increased stem growth. The greatest increase in growth of the larch-nursed spruce occurred during the period immediately following canopy closure, thus demonstrating the failure of the larch to compete with the spruce crowns. This period of high growth was associated with a high branch, cross-sectional area and deep knots within the wood on the lower part of the spruce stems in comparison with other treatments. While each treatment was associated with a similar number of branches, lodgepole pine-nursed spruce had more small branches (0-10 mm diameter) and fewer big branches (>20 mm diameter) compared with other treatments, highlighting the capacity of lodgepole pine to control branch development of the spruce. Spruce trees nursed by larch had deeper and more imbalanced living crowns with longer lived branches in comparison with lodgepole pine-nursed spruce, and both the pure spruce treatments, suggesting that more juvenile wood may have formed within the stem. Larch-nursed spruce also had the highest stem taper and lodgepole pine-nursed spruce the lowest. Basic wood density was not influenced by treatments.

Overall, the evidence from this study suggests that the use of larch as a nursing species on deep peats is inadvisable and that Alaskan lodgepole pine is better able to control the branching habit of Sitka spruce with the prospect of better quality sawlogs in the future, albeit with a lower average tree size. Periodically fertilised pure spruce appears to maintain a growth rate consistently above that of the lodgepole pine-nursed spruce but without many of the disadvantages linked with the regularly fertilised pure spruce, particularly the development of large branches and an irregular pattern of annual rings within the wood. (C) 1999 Elsevier Science B.V. All rights reserved.