Abstract:
Competitive interactions between Douglas-fir seedlings (Pseudotsuga menziesii), and seedlings of a nitrogen-fixing hardwood
associate, red alder (Alnus rubra), were investigated in an addition series experiment. The primary objective of the research was to identify and quantify key processes underlying intra- and interspecific competitive interactions of Douglas-fir and red alder seedlings. Seedlings of both species were planted into five monoculture densities of 1, 2, 4, 8, and 16 trees m⁻², and into mixtures of all possible pairwise combinations of these densities. Stem diameter and height were measured monthly, and were used to calculate relative growth rates. Soil moisture depletion was monitored using the neutron scattering method. An index of light
availability was measured through fisheye photography and computer digitization. Leaf water potential served as a physiological
indicator of water stress physiology. Predictive equations were developed for estimating the affect of species densities on tree yield and growth as they changed with time. Patterns in resource use and
physiology were also modeled as functions of competitive regime. Yield, absolute growth, and relative growth rates based on an index
of stem volume of both species declined quantitatively in response to intra- and interspecific competition. The competitive effects of red alder intensified with time, as the dominant overstory species grew
taller and shaded the understory Douglas-fir. Resource availability and physiological performance were reduced quantitatively in response to the density of each species. Manipulation of both species
densities influenced productivity through the modification of canopy structure, light pentration, and soil moisture availability. This study demonstrated that species density, resources, and physiology interact
in a complex fashion to produce patterns of yield in stands of competing trees.
