I examined factors regulating decomposition rates of red alder (Alnus rubra)) and Douglas-fir (Pseudotsuga menziesii) leaf litter in Coast Range riparian areas in western Oregon. Overall, this study was designed to examine the influence that leaf litter quality characteristics and decomposition site treatment have on decomposition rates, to provide a better understanding of how vegetation management can impact nutritional subsidies and nutrient cycles within these riparian systems. I employed the litterbag method to compare decomposition rates of litter with different initial chemistry in sites of different N availability. Specifically, this study investigates the role of litter source, riparian decomposition site, and how differences in N (both endogenous and exogenous) may influence the decomposition dynamics of red alder and Douglas-fir leaf litter. I addressed the following research questions: 1) How do the decomposition rates of red alder and Douglas-fir differ? 2) Do differences in chemical measures of initial litter quality (eg. N, Ca, lignin, cellulose, C:N) correlate with different rates of decomposition in Douglas-fir (8 different sources of Douglas-fir litter)? 3) Does dominance of a site by either red alder or Douglas-fir overstory) influence decomposition rates? 4) Does N fertilization increase the rate of litter decomposition under Douglas-fir overstories? Results suggest that red alder litter decomposes more rapidly than Douglas-fir litter under either canopy, but the difference in decomposition rates is greater under a red alder overstory than under a Douglas-fir overstory. N mineralization began immediately following placement of the red alder litter bags and more N was mineralized in red alder litter decomposing under red alder overstories than under Douglas-fir overstories. Compared to red alder, Douglas-fir litter decomposition did not vary by overstory treatment. Generally, Douglas-fir litter went through an immobilization period, with only high N litter mineralizing N under unfertilized Douglas-fir overstories. Both low- and high-N Douglas-fir litter immobilized more N under red alder overstories, and under fertilized Douglas-fir conditions. In fertilized plots under Douglas-fir overstories, high-N litter was still immobilizing N after two years. In contrast, low-N Douglas-fir litter immobilized N throughout the 2 year period under all treatments. This study indicates strong species-specific effect of overstory composition on riparian ecosystem processes. These effects can influence energy and nutrient budgets of riparian food webs, and suggest a need for broader consideration of potential impacts resulting from conversion of red alder to Douglas-fir dominated riparian area. Surprisingly, rates of Douglas-fir litter decomposition were negatively related to initial litter nitrogen concentrations across the range 0.7 - 1.4% N, contrary to patterns observed across species in other ecosystems. N fertilization exerted a minor influence on decomposition rates of Douglas-fir, with decomposition rates slower in fertilized Douglas-fir plots. These results highlight the complicated relationship between decomposition of high lignin litter and N availability and suggest that under such conditions decomposition can be dramatically reduced.

Riparian zone vegetation can influence terrestrial and aquatic food webs through variations in the amounts, timing, and nutritional content of leaf and other litter inputs. Differences in vegetation composition and density, as well as riparian topography, may modulate the strength and quality of these inputs. Changes in inputs to small order streams affect the processes and condition of adjacent and downstream reaches based on the amount of particulate organic matter that is intercepted, retained, or exported. The central Oregon Coast Range provides an ideal opportunity to study how deciduous dominated and coniferous dominated riparian forests influence small streams within a matrix of managed riparian forests. In coastal Oregon riparian forests, we investigated lateral and vertical litter inputs to sixteen streams throughout a year and assessed how these inputs were influenced by density of deciduous dominated (mainly red alder (Alnus rubra)) or coniferous dominated (mainly Douglas-fir (Pseudotsuga menziesii)) overstory, understory, and lateral slope. Deciduous site vertical litter inputs (504 g m-2 yr-1 (95% CI: 447-562)) were estimated to exceed those from coniferous sites (394 g m-2 yr-1 (336-452)) by 110 g m-2 (29-192) over the full year. Annual lateral inputs (per meter of stream bank on one side) at deciduous sites (109 g m-1 yr-1 (76-143)) were estimated to be 47 g m-1 (1-95) more than coniferous sites (63 g m-1 yr-1 (29-97)). Annual inputs at coniferous sites were dominated by deciduous leaves, coniferous needles, and twig litter types. Deciduous leaves, deciduous-other, and small unidentifiable litter types dominated the annual inputs at deciduous sites. When evaluated temporally, November was the most pivotal month differentiating coniferous and deciduous site litter inputs. At deciduous sites, lateral litter movement increased with slope, but we did not see the same relationship for coniferous sites except in spring/summer months. Lateral inputs were quantitatively greatest in autumn months for both overstories, but were proportionately greater in winter. Regardless of slope, there was no indication that understory plants were obstructing annual lateral litter inputs or that annual lateral litter inputs were moving more than 5 m down slope. The percent nitrogen of annual total vertical litter was estimated to be 1.9% N (1.5-2.4) at deciduous sites and 1.2% N (0.8-1.7) at coniferous sites. Average % nitrogen of individual litter types were either greater in deciduous sites or not different among overstories, indicating that one can generally expect coniferous sites to have lower % N litter inputs overall. The annual nitrogen flux entering each meter-length (from above and both sides) of standard 4 m-wide streams at a deciduous sites (42 g N m-1 of stream) was twice that of coniferous sites (21 g N m-1 of stream). Annual total litter carbon flux into each meter-length of 4 m-wide streams was estimated to be 1154 g C m-1 of stream at deciduous sites and 880 g C m-1 of stream at coniferous sites. On average, autumn months (October-December) accounted for 46-59% of annual vertical C flux and 56-70% of annual vertical N flux at coniferous and deciduous sites. Our results suggest that red alder dominated riparian zones of the central Oregon Coast Range have significantly different quantity, timing, and quality of leaf litter inputs to streams than conifer dominated forests. Varied topography adjacent to streams with red alder dominated overstory has greater impact on the quantity, quality, and timing of total inputs than at coniferous sites. The cumulative effects from many small red alder dominated streams exporting to downstream reaches include more pronounced seasonality of litter delivery, with greater carbon and nitrogen loading annually, than expected from conifer dominated streams. Differences in overstory and topography in Oregon Coast Range riparian forests directly impact the delivery of nutrients and can affect the structure and composition of food webs in these ecosystems.

Five randomly assigned treatments were used to quantify effects of adding varying numbers of red alder (Alnus rubra Bong.) or nitrogen (N) fertilization on growth of a 10-year-old conifer plantation at a medium quality site in the western Washington Cascade Range. Zero, 20, 40, and 80 alder trees per acre were retained along with about 300 conifers per acre. Nearly all conifers were coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco). A fifth treatment substituted N fertilizer for N-fixing alder. Changes in average tree height, and in numbers of trees, basal area, and volume per acre between plantation ages of 10 and 27 are compared. In pure conifer plots, gross volume growth averaged 26 percent greater on fertilized than nonfertilized plots, indicating measurable benefits of additional N. On both fertilized and nonfertilized plots, an average of 13 percent of the original conifers died. Retaining 20, 40, or 80 alder per acre (7, 13, and 27 percent of the associated conifer trees per acre, respectively) was associated with reduced numbers of Douglas-fir by about 19, 5, and 17 percent, respectively, in the next 17 years. Mortality and growth of Douglas-fir were not related to alder density, but losses of Douglas-fir were especially large on plots where relatively large red alder (20 per acre) were retained. Neither total stand nor conifer yields were changed by retaining alder. Additional comparisons are needed at other locations, especially those with known N deficiency.

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.