- 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.