|Abstract or Summary
- Red alder (Alnus rubra Bong.) is a common associate and a potentially severe competitor of Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco) in the Oregon Coast Range. However, because it fixes nitrogen and increases rates of soil nutrient cycling, red alder has the potential to benefit Douglas-fir. The objective of this study was to quantify and attempt to explain differences in growth of young Douglas-fir and red alder, growing in mixed and pure stands, by analyzing six-year trends in survival, growth, animal damage of saplings, cover of understory vegetation, and foliar and soil nutrients. In March 1986, a replacement series study was established by planting Douglas-fir and red alder at 3-m spacing in 0.073-ha plots with the following species proportions (Douglas-fir/red alder): 1/0, 0.9/0.1, 0.7/0.3, 0.5/0.5, 0.25/0.75, 0/1. Each proportion was replicated three times in a randomized complete block design. Six annual measurements (fall 1986-1991) of survival, growth, and animal damage and one measurement of stand yield (1991 basal area, volume, and predicted biomass) were used in response surface analyses to test for significant polynomial relationships (a=0.05) between tree response and species proportion. Relationships of Douglas-fir height increment to red alder proportion varied from positive linear (year 2) to no relationship (years 3-4) to negative linear (years 5-6). Relationships of Douglas-fir diameter increment to red alder proportion varied from no relationship (years 2-3) to negative linear (year 4) to negative quadratic (years 5-6). In the sixth year, competition from red alder caused relative yield (relative to monoculture yield) of Douglasfir volume index and predicted biomass to be significantly less than the yield that would be expected in the absence of intraspecific competition (expected yield). Increasing proportion of Douglas-fir decreased intraspecific competition of red alder, and as a result, relative yield of basal area and predicted biomass for red alder was significantly greater than its expected yield. Two easurements (March 1986 and 1991) of mineralizable nitrogen (NH4 and NO3); total N, P, and C; and pH from 3 soil depths (0-15, 15-30, and 60-90 cm) were used in analysis of variance to test for significant differences (a=O.05) in sixth-year amount and five-year change in these variables among three species proportions (Douglas-fir/red alder): 1/0, 0.5/0.5, and 0/i. There was no net N accretion; however, there was higher mineralizable N as NH4 in pure Douglas-fir stands than in mixed stands. One explanation for these results is that possibly the more open conditions of Douglas-fir stands increased rates of N mineralization. Douglas-fir foliar N and P also were significantly higher in pure stands than in mixed stands, probably because of the increased ability of large, vigorous saplings growing in absence of red alder to absorb soil nutrients rather than because of greater supplies of soil nutrients. The effect of increased Douglas-fir vigor and size in enhancing nutrient uptake was more prominent for P than for N. These data indicate that the effects of red alder on Douglas-fir can range from positive to neutral to negative. In general, competition was the most important factor limiting survival and growth of Douglas-fir; animal damage and understory vegetation under red alder canopies caused further limitations in Douglas-fir response. In addition, results of this research suggest that red alder competition can be tolerated with little loss of Douglas-fir survival and growth through an age of 4 years.