- Knowledge of stand structure, stand dynamics, and production ecology of
species mixtures lags well behind that of single-species, even-aged stands. Two
mixed-species spacing trials in central Oregon allowed investigation of mixed-species
dynamics in a controlled experimental setting. The first site, Pringle Butte, is a
mixture of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and lodgepole pine
(Pinus contorta Dougl. ex Loud.). The second site, Lookout Mountain, is a mixture of
ponderosa pine and grand fir (Abies grandis (Dougl. ex D. Don) Lindl.). Both studies
were planted in the early 1970's and established as replacement series under a split-plot
design with spacing as the whole plot factor and species composition as the split-plot
factor. Plot data have been collected since 1975. In the summer of 2001, 95 trees
outside the plots were destructively sampled and 114 plots were intensively sampled.
From these data the following were developed and assessed: (i) volume growth
dynamics; (ii) models predicting individual tree vertical foliage distributions; (iii)
models describing the profile of maximum branch diameter within a tree; and (iv)
volume growth efficiency. In both studies, the least shade tolerant species had the fastest early growth
rates. Over time, volume development depended on both spacing and composition;
P. ponderosa was able to catch up with P. contorta within the mixtures and between
the pure plots at Pringle Butte, whereas A. grandis still lags behind P. ponderosa,
although its relative contribution increases with increasing spacing at Lookout
Mountain. Relative yields of mixtures were greater than one, but significantly so only
in the A. grandis - P. ponderosa mixtures.
Branch leaf area equations indicate that, given branch diameter, position in the
crown is an important factor in estimating leaf area. Tree leaf area was best predicted
by the product of tree basal area and the ratio of crown length above breast height, a
surrogate for sapwood area at crown base. Branch- and tree-level predictions differed
significantly between sites for P. ponderosa. Relative vertical foliage distribution on
individual trees of both A. grandis and P. ponderosa shifted up with an decrease in
relative height, while increased spacing resulted in a downward shift in relative foliage
distribution on P. ponderosa at Lookout Mountain. Spacing and competing species
also affected absolute foliage distributions in a manner consistent with expected
influence on crown length.
For all species and spacings, profiles of maximum branch diameter were
curvilinear, decreasing near the crown base. Tree variables such as diameter, height,
and crown length were able to account for spacing. The effect of species composition
on branch profiles was more difficult to assess. Profiles of maximum branch diameter increase with increasing spacing and tree relative height, but the effects of species
composition depended on spacing in all species. More pronounced increases in
maximum branch diameter profiles with increasing relative height within the crown
were found in the subordinate species in mixtures than in adjacent pure plots and in its
overtopping competitor. In contrast, the overtopping species had a larger spacing
response in the pure plots than in mixed plots.
The ratio of leaf biomass to crown biomass decreased with increasing spacing,
but was also influenced by species composition. Growth efficiency decreased with
increasing spacing, except in Abies grandis, which peaked at the intermediate spacing.
Results suggest that plot growth efficiency peaks at intermediate densities depending
on composition. At wider spacings, growth efficiency appears to be limited by greater
allocation of carbon to branches for both construction and maintenance respiration. At
tighter spacings, growth efficiency appears to be limited by competition among
individuals, reducing resources per individual and promoting differentiation. In dense,
differentiated stands, the poorest individuals contribute leaf area but little growth,
reducing stand growth efficiency.
Spacing and species composition play an important role in stand development
and resulting crown structure. Mixtures can produce a more diverse array of stand
structures and yield similar if not more volume than pure stands of the higher yielding
species. Management of mixed-species stands must take into account the interactions between spacing and species' growth dynamics, but this same interaction opens
possibilities for a wide variety of stand structures for a given species composition.