The main objective of this investigation was to examine the relationships
between various site factors and the productivity of Sitka
spruce (Picea sitchensis Bong., Carr) plantations.
The natural distribution of the species is notable for its great latitudinal
range, although it is confined to a narrow belt along the
Pacific coast of North America. The climatic conditions found in
this region, whilst not precisely matched, are sufficiently similar
to those found in North West Europe to have encouraged its use as an
exotic species on a large scale. Much of the planting has been successful,
but little was known of the relations between productivity and
site factors for stands that were growing normally and could confidently
be expected to reach utilisable sizes. The study was therefore
confined to healthy stands over 30 years of age.
The distribution of the productivity classes in Scottish forests
appeared to show no very marked influence of gross climatic factors
so four forest areas were selected for detailed examination. Each
of these forests showed a wide range of productivity classes within a
small area. Because of the constraints placed on the choice of stands
to be sampled the sampling had to be systematic and involved visiting
almost every stand in each area. Productivity was measured on a
height:age basis to avoid treatment effects, although other productivity
parameters such as basal area and volume were recorded in the 95 plots
sampled. Variation in some site factors was examined initially before
settling on sampling procedures. For each plot a series of physiographic
and soil parameters were measured. All results were expressed so
to describe the site potential for tree growth in the most
meaningful way possible.
The relation of the site factors to productivity of Sitka spruce was
examined both by single factor and multivariate analyses. The
results obtained by using different techniques of analysis were
similar. The factors which emerged as having the strongest relationships
to growth were also similar for the areas studied although any
individual factor might not have the same importance in each. To
aid in the elucidation of the relationships a cross -stratification on
a physiographic basis was made. For the statistical analysis the
total number of plots from which data had been collected was reduced
to 77 because of doubts about the age of the remainder as recorded.
The data showed that local climatic effects expressed as 'elevation'
or relative elevation were the dominating factors affecting productivity
at each of the forests. Within this effect the phosphorus
status of the site proved the next most important individual variable.
Multivariate analysis indicated a physico-chemical 'factor' which
combined phosphorus status and soil structure, consistently important
to productivity in each forest. Base status of the site was the best
means of separating the sites but did not itself directly contribute
much to productivity except at one forest.
Neither the soil type nor ground vegetation were suitable for classifying
potential productivity but prediction equations that accounted
for over 70% of the variation in productivity were devised. The
usefulness of these to management was thought less than their
value for directing attention to the factors that limit production.
Component analysis was considered more potent as a means of detecting
combinations of factors that might be investigated further.
The limitations to economically useful site improvements for this
species were thought to be defined by the effect of elevation. This
effect and the phosphorus nutrition of the tree should be the main
directions for immediate further research.