Interacting effects of stand density, site factors, and nutrients on productivity and productive efficiency of Douglas-fir plantations in the Oregon Cascades

The objectives of this research were to study: a) the
effect of thinning and treatments consisting of pruning
and inultinutrient fertilization on aboveground biolnass
increment, growth efficiency (GE), and foliar nutrients;
b) the influence of topoedaphic variables (soil
nutrients, slope, aspect, and rock content) and foliar
nutrients on both leaf area increment and GE; and c) the
influence of topoedaphic variables on the rate of
iiJneralizable N. Studies were conducted in young Douglasfir
plantations in the western Cascades of Oregon.
Net aboveground biomass increment over a 6-year
period averaged 14.5, 7.8, and 5.5 Mgha.yr for
unthinned, noderately thinned, and heavily thinned stands, respectively. Aboveground biornass increment and
GE were analyzed in three 2-year periods. Density
affected aboveground biomass increment in all periods,
and there was an increasingly significant treatment
effect in each period, but no significant interaction
between stocking density and treatment. Stand density had
the major effect on GE, but there was also a significant
interaction between stocking density and treatment during
the 1985-'87 period. Foliar analysis indicated that
thinning improved N, K, and Mg nutrition, and resulted in
increased translocation of K from one-year old to current
year foliage.
Nultivariate and regression analyses suggest that
relative leaf area increment is correlated most closely
with one or another measure of Mg, K, and N availability,
whereas GE correlates most strongly with leaf area index,
mineralizable N, and foliar Mg content.
Mineralizable N in two soil depths did not vary
significantly by stocking density, treatment, or densitytreatment
interaction. The rate of mineralizable nitrogen
expressed as concentration basis averaged 49 per cent
lower at the 20-40 cm depth than at the 0-20 cm depth.
Mineralizable N expressed on an area basis correlated
positively with total soil N, exchangeable Ca, and
adjusted aspect, and negatively with rock content and
slope steepness.