Abstract:
This thesis examines the 8-year growth trend of
Douglas-fir (Pseudotsuga menziesii (mirb.)Franco), ponderosa
pine (Pinus ponderosa Dougl. ex laws.), and whiteleaf
manzanita (Arctostaphylos viscida Parry). Manzanita stands
ranging from 1700/ha to 27000/ha density were grown among
mixed conifers of the same age, which were planted in 1981
at 250x250 cm spacing as 2-0 bareroot seedlings. Four years
of herb control was provided by herbicides or paper mulch in
all spacings. At a density of 13500/ha of manzanita, plots
were grown with no control of herbs, and others were grown
with no conifers. Mixed conifers also were grown with no
competitors, and some plots, initially free of all
vegetation, later supported only annual herbs. Three sites
were used in a complete randomized block design with nine
treatments as above. All were located on south slopes at low
elevation in southwest Oregon, representing the warmest and
driest area of the Douglas-fir region. Conifer stem volume and biomass yield were inhibited
quantitatively by manzanita competition. As manzanita
density increased the yield of conifers decreased in an
increasing trend through time. Significant biomass yield
decrease (14 percent) was associated with 17 percent brush
cover in ponderosa pine. Douglas-fir had high mortality in
manzanita at less than 27000/ha density, while ponderosa
pine mortality was minor. Herb competition severely
inhibited survival and growth of both manzanita and
conifers. Shrub canopy cover reached a maximum by age seven
at 27000 manzanita/ha density, but continued to expand at
lower densities. Shrub yield was not affected by conifer
growth, but intraspecific competition at the high densities
of manzanita decreased yield and caused high mortality by
the seventh year.
Soil moisture was depleted most rapidly in the top 30
cm of soil. When manzanita competition occurred, soil water
was depleted more than in pure conifer or pure herb stands.
Manzanita used most of the available soil water to below 60
cm early in the season (May-June). Trees in pure conifer
plots grew at the fastest rate, and ended the summer with
the highest soil moisture content.
Conifers can survive and grow satisfactorily under
harsh environments if brush cover is reduced to low
densities, and herbs are sprayed in the first two to three
years. Ponderosa pine is better suited for reforestation of
hot south aspects in the foothill region. Douglas-fir needs higher levels of brush and herb control, and remain
vulnerable to intense heat and drought through age eight.
