Theoretical Calculations of the Influence of Spacing on Stand Stability

Abstract

Theoretical stability calculations for unthinned plantations of Sitka spruce ( Picea sitchensis ) at spacings of 0.9, 1.4, 1.8, 2.0, 2.4 and 3.0 m showed that for both static and dynamic loading the increase in the resistance to uprooting or stem breakage as a result of increasing the mean tree size outweighed the greater drag force on the crown. Thus, increasing the spacing beyond the currently accepted norm of 2 m would appear to improve stability. The results are sensitive to assumptions about the effect of spacing on the relationship between the maximum turning moment and tree size, and on the effect of spacing on the wind-loading of the crowns. Spaced-furrow ploughing is likely to have a more adverse effect on the root anchorage of widely spaced stands than on that of closely spaced stands and further work is required before it can be confidently asserted that wider spacings will improve tree stability on ploughed, poorly drained soils.