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 contor ta Loud.) and hybrid larch (Larix x eurolepis Henry) nurses, planted in tri plet mixture with Sitka spruce (Picea sitchensis (Bong.) Carr.), on growth rate, stem form, branching habit and wood properties of the spruce. These m ixtures were compared with pure stands of Sitka spruce which had been regul arly and periodically fertilised with nitrogen. Hybrid larch promoted diame ter increments in Sitka spruce greater than those achieved by the other tre atments over the last 12 years of the experiment (current age 30 years). Gr owth of regularly fertilised pure Sitka spruce was not significantly greate r than that of periodically fertilised pure spruce, suggesting that regular applications of nitrogen fertiliser did not result in increased stem growt h. The greatest increase in growth of the larch-nursed spruce occurred duri ng 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 kn ots within the wood on the lower part of the spruce stems in comparison wit h other treatments. While each treatment was associated with a similar numb er of branches, lodgepole pine-nursed spruce had more small branches (0-10 mm diameter) and fewer big branches (>20 mm diameter) compared with other t reatments, highlighting the capacity of lodgepole pine to control branch de velopment of the spruce. Spruce trees nursed by larch had deeper and more i mbalanced living crowns with longer lived branches in comparison with lodge pole pine-nursed spruce, and both the pure spruce treatments, suggesting th at 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 n ursing species on deep peats is inadvisable and that Alaskan lodgepole pine is better able to control the branching habit of Sitka spruce with the pro spect 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 witho ut many of the disadvantages linked with the regularly fertilised pure spru ce, 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.