Summary1. Riparian trees regulate aquatic ecosystem processes, such as inputs of light, organic matter and nutrients, that can be altered dramatically when these trees are harvested. Riparian buffers (uncut strips of vegetation) are widely used to mitigate the impact of clear-cut logging on aquatic ecosystems but there have been few experimental assessments of their effectiveness. 2. Forests along 13 headwater stream reaches in south-western British Columbia, Canada, were clear-cut in 1998, creating three riparian buffer treatments (30-m buffer, 10-m buffer and clear-cut to the stream edge), or left as uncut controls, each treatment having three or four replicates. 3. We predicted that periphyton biomass and insect consumers would increase as buffer width decreased, because of increased solar flux. We used two complementary studies to test this prediction. 4. In one study, we compared benthic communities before and after logging in all 13 streams; a second study focused on periphyton and insect colonization dynamics over 6-week periods in each of four seasons in four streams, one in each treatment. 5. Photosynthetically active radiation, and mean and maximum water temperature, increased as buffer width narrowed. 6. Periphyton biomass, periphyton inorganic mass and Chironomidae abundance also increased as buffer width narrowed, with the largest differences occurring in the clearcut and 10-m buffer treatments. 7. Photosynthetically active radiation, water temperature, periphyton biomass and periphyton inorganic mass were significantly greater in the 30-m buffer treatment than in controls during some seasons. 8. Synthesis and applications. We have shown that a gradient of riparian buffer widths created a gradient in light and temperature that led to non-linear increases in periphyton biomass and insect abundance. For example, Chironomidae abundance was generally greater in the 10-m and 30-m buffer treatments than in controls, whereas this was not always the case in the clear-cut treatment. This pattern may be due to the high sediment content of the periphyton mat in the clear-cut treatment, which potentially limited the response of some insects to increased food resources. Overall, our results indicate that uncut riparian buffers of 30-m or more on both sides of the stream were needed to limit biotic and abiotic changes associated with clear-cut logging in headwater, forested watersheds.

An important question for salmon restoration efforts in the western USA is 'How should habitat restoration plans be altered to accommodate climate change effects on stream flow and temperature?' We developed a decision support process for adapting salmon recovery plans that incorporates (1) local habitat factors limiting salmon recovery, (2) scenarios of climate change effects on stream flow and temperature, (3) the ability of restoration actions to ameliorate climate change effects, and (4) the ability of restoration actions to increase habitat diversity and salmon population resilience. To facilitate the use of this decision support framework, we mapped scenarios of future stream flow and temperature in the Pacific Northwest region and reviewed literature on habitat restoration actions to determine whether they ameliorate a climate change effect or increase life history diversity and salmon resilience. Under the climate change scenarios considered here, summer low flows decrease by 35-75% west of the Cascade Mountains, maximum monthly flows increase by 10-60% across most of the region, and stream temperatures increase between 2 and 6 C by 2070-2099. On the basis of our literature review, we found that restoring floodplain connectivity, restoring stream flow regimes, and re-aggrading incised channels are most likely to ameliorate stream flow and temperature changes and increase habitat diversity and population resilience. By contrast, most restoration actions focused on in-stream rehabilitation are unlikely to ameliorate climate change effects. Finally, we illustrate how the decision support process can be used to evaluate whether climate change should alter the types or priority of restoration actions in a salmon habitat restoration plan.

Investigations utilizing a one-dimensional sediment routing model demonstrate that moderate inputs of water and sediment at tributary junctions greatly increase physical heterogeneity in the recipient channel. Simulated physical heterogeneity is most sensitive to the ratios of tributary to mainstream bed load flux and bed load grain size and is less sensitive to relative discharge. Within the model, aggradation drives the processes that augment habitat variability, and in general, any aggradational confluence will be associated with elevated physical diversity. Model output reveals elevated physical diversity at two scales: between distinctive upstream and downstream zones separated by a confluence step and within each zone as a function of local environmental gradients. Total diversity increases as tributary sediment load and caliber increase relative to the mainstream. The ecological implications of the patterns and magnitude of tributary-induced physical heterogeneity are considered, and testable hypotheses are presented. Results highlight the need to accurately characterise patterns of sediment production, delivery, and routing in order to predict local tributary impacts and thereby understand patterns of habitat diversity and biodiversity at network scales. Résumé :Des recherches utilisant un modèle unidimensionnel de transport des sédiments montrent que des apports modérés d'eau et de sédiments aux points de jonction des tributaires augmentent considérablement l'hétérogénéité du chenal récepteur. L'hétérogénéité physique simulée est très sensible au rapport des flux des charges de fond du tributaire sur ceux du cours principal ainsi qu'au rapport des tailles des particules; il est moins sensible aux débits relatifs. Dans le modèle, l'alluvionnement entraîne les processus qui augmentent la variabilité de l'habitat et, en général, toute confluence d'alluvionnement est associée à une augmentation de la diversité physique. Les résultats du modèle montrent des augmentations de la diversité physique à deux échelles, soit entre des zones distinctes d'amont et d'aval séparées par un seuil de confluence, soit dans un même zone en fonction des gradients environnementaux locaux. La diversité totale augmente en fonction de l'accroissement de la charge et du calibre des sédiments du tributaire par rapport à ceux du cours principal. Nous examinons les conséquences écologiques des patterns et de l'amplitude de l'hétérogénéité physique générés par les tributaires et nous présentons des hypothèses testables. Nos résultats soulignent la nécessité de caractériser avec précision les patrons de production, de livraison et de cheminement des sédiments afin de pouvoir prédire les impacts locaux des tributaires et ainsi comprendre les patrons de diversité d'habitat et de biodiversité à l'échelle des réseaux.[Traduit par la Rédaction] Rice et al. 2566