Abstract:
Bioassays using red alder and snowbrush plants grown in soils collected
from a clearcut, a young Douglas-fir plantation, and an old-growth stand were
conducted. Sites are located at the Andrews Experimental Forest, Oregon. In
the first bioassays, more alder than snowbrush plants survived and nodulated.
Of the plants that survived, more red alder plants nodulated when grown in
clearcut soils than in other soils, and more snowbrush nodulated when grown in
soils from the young stand. With the exception of acetylene reduction per plant,
response variables differed among the three sites, however soil samples within
sites were also a significant source of variation. Red alder biomass and nodule
weight were highest when plants were grown in clearcut soils. Snowbrush
biomass and nodule weight were highest when grown in soils from the young
stand. The biomass of snowbrush plants grown in clearcut soils averaged higher
in bottom slope soils than in soils from any other position.
Two additional bioassays using red alder and snowbrush plants
consisted of adding sequentially to clearcut soils Frankia plus macronutrients,
micronutrients, mycorrhizal fungi, and Pseudomonas fluorescens. There was no
interaction between treatment and location for either species. There were no
significant treatment effects for snowbrush, but there were significant treatment
effects for red alder. Red alder seedlings given Frankia and macronutrients had
greater biomass and reduced more acetylene than seedlings grown without
additions. Adding Alpova diplophloeus increased acetylene reduction by 33%
over that attained with Frankia and macronutrients alone. The combined effect
of Frankia, macronutrients and the mycorrhizal fungus was to increase acetylene
reduction by 136% over controls. Adding micronutrients to Frankia and
macronutrients reduced acetylene reduction by nearly one-half, completely
negating the positive effect of the Frankia and macronutrients. The presence of
the mycorrhizal fungus appeared to buffer the negative effects of micronutrients.
Red alder seedlings grown in upper slope soil had greater biomass and reduced
more acetylene than seedlings grown in down slope soil. In contrast, snowbrush
plants grown in bottom slope soil had greater biomass, nodule weight, and
reduced more acetylene than seedlings grown in any of the other slope
positions. Because slope positions were not replicated, any conclusions we
draw apply only to the four locations we measured on the single slope.
