Riparian Forest Structure Research Articles

Structure and Composition of Old-Growth and Unmanaged Second-Growth Riparian Forests at Redwood National Park, USA

Restoration of second-growth riparian stands has become an important issue for managers of redwood (Sequoia sempervirens [D. Don] Endl.) forest reserves. Identifying differences between old-growth and second-growth forest vegetation is a necessary step in evaluating restoration needs and targets. The objective of this study was to characterize and contrast vegetation structure and composition in old-growth and unmanaged second-growth riparian forests in adjacent, geomorphologically similar watersheds at Redwood National Park. In the old-growth, redwood was the dominant overstory species in terms of stem density, basal area, and importance values. Second-growth was dominated by red alder (Alnus rubra Bong.), Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco), and redwood. Understory species were similar in both forests, with several key differences: Oxalis oregana Nutt. and Trillium ovatum Pursh had greater importance values in the old-growth, and Vaccinium parvifolium Sm., Dryopteris spp. and sedges Carex spp. had greater importance values in the second-growth. Notable differences in structure and composition suggest that restoration practices such as thinning could expedite the acquisition of old-growth characteristics in second-growth riparian forests.

Comparing streambed light availability and canopy cover in streams with old-growth versus early-mature riparian forests in western Oregon

Light availability strongly influences stream primary production, water temperatures and resource availability at the base of stream food webs. In headwater streams, light is regulated primarily by the riparian forest, but few studies have evaluated the influence of riparian forest stand age and associated structural differences on light availability. In this study, we evaluated canopy cover and streambed light exposure in four second-order streams within paired reaches of primary old-growth versus second- growth mature riparian forests. Stand age class was used as a proxy here for canopy complexity. We estimated stream canopy cover using a spherical densiometer. Local streambed light exposure was quantified and compared within and between reaches using fluorescein dye photo- degradation. Reaches with complex old-growth riparian forests had frequent canopy gaps which lead to greater stream light availability compared to adjacent reaches with simpler second-growth riparian forests. We quantified light exposure at relatively high resolution (every 5 m) and also found greater variability in stream light along the reaches with old-growth riparian forests in three of the four streams. Canopy gaps were particularly important in cre- ating variable light within and between reaches. This work demonstrates the importance of the age, developmental stage, and structure of riparian forests in controlling stream light. The highly variable nature of light on the stream benthos also highlights the value of multiple measurements of light or canopy structure when quantifying stream light.

Influence of riparian vegetation and forest structure on the water quality of rural low-order streams in SE Brazil

Land use changes have resulted in large deforestation of rural landscapes, thus influencing transport of water and materials along the watersheds. Riparian zones have strong effects on stream water quality, but most studies evaluated the effects of riparian vegetation (forested vs. deforested), although riparian forests may greatly differ in structure. Here we evaluated the effects of riparian vegetation characteristics (RV) and riparian forest structure (RFS) on stream water quality in a tropical rural landscape in SE Brazil. We sampled 15 low-order streams along a gradient in riparian degradation, from completely deforested streams to those with well-developed riparian forests. In each stream we established a 100m reach and evaluated RV (trees, grasses, vines, bamboo, canopy closure, and riparian forest width), RFS (tree density and height, vertical canopy structure, mean basal area and diameter at breast height), and stream habitat and water quality (mean water depth, fine sediment cover (FSC), electric conductivity (EC), dissolved oxygen (DO), ammonium, nitrate, total N, dissolved, particulate, and total P). We used Principal Components Analyses to reduce dimensionality of RV and RFS variables, and evaluated the separate effects of RV and RFS on water quality variables using conditional autoregressive models. We found effects of both RV and RFS on FSC, EC, DO and ammonium concentrations, and effects of only RFS on total and dissolved P concentrations. These results suggest that although RV variables are good predictors of the buffering role of riparian zones, the structure of the riparian forest can influence stream water quality variables. Thus, heterogeneity in riparian forest structure due to forest degradation or restoration should be considered when evaluating buffering effects of riparian zones.

Evolution of the Riparian Forest Corridor in a Large Mediterranean River System

The central reach of the Ebro River, one of the largest rivers in the Mediterranean region, is characterised by a meandering channel that develops into a wide floodplain. In the present study, we analysed the evolution of the landscape structure and function of riparian forests in the Middle Ebro River (~ 250 km long) in 1927, 1957 and 2003 in order to evaluate the forest dynamics in this river corridor.

Historical Condition and Change in Riparian Vegetation, Hood Canal and Eastern Strait of Juan de Fuca, Washington

Pacific Northwest riparian forests occupy dynamic environments, host high biodiversity, and support important ecological services like nutrient storage/transformation, flood attenuation, and water quality protection. Lack of information on their historical condition and changes hinder effective ecosystem management. We combined archival records and contemporary field surveys to reconstruct historical riparian conditions and change in Hood Canal and the eastern Strait of Juan de Fuca, Washington. Historical riparian forest composition and structure varied by shoreline type and landform. Conifer species dominated all shoreline types, with hardwoods relatively abundant in estuaries and stream bottomlands. Older, more structurally diverse forests clustered in stream ravines, with the size of stream-riparian fir and redcedar dominants measuring 2.5 and 8 times larger than in upland stands, respectively. Over half (57.9%) of stream-riparian redcedar-spruce forest sites shifted to hardwood/mixed forest over the historical period, versus fir-hemlock, alder, and hardwood/mixed forest types which all showed ≤35.3% change. Classification and regression tree models highlighted historical forest type, elevation, and proximity to bottomlands as key predictors of vegetation change, with historical conifer sites in/near bottomlands two times more likely to undergo change than upland stands. Our findings provide spatially explicit historical context for ecosystem managers and modelers, and suggest new questions pertinent to riparian forest management.

Reoccupation of floodplains by rivers and its relation to the age structure of floodplain vegetation

River channel dynamics over many decades provide a physical control on the age structure of floodplain vegetation as a river occupies and abandons locations. Floodplain reoccupation by a river, in particular, determines the interval of time during which vegetation can establish and mature. A general framework for analyzing floodplain reoccupation and a time series model are developed and applied to five alluvial rivers in the United States. Channel dynamics in these rivers demonstrate time‐scale dependence with short‐term oscillations in active channel area in response to floods and subsequent vegetation growth and progressive lateral movement that accounts for much of the cumulative area occupied by the rivers over decades. Rivers preferentially reoccupy locations recently abandoned causing a decreasing probability of reoccupation with time since abandonment. For a typical case, a river is 10 times more likely to reoccupy an area it abandoned in the past decade than it is to reoccupy an area it abandoned 30 yrs ago. The decreasing probability of reoccupation over time is consistent with observations of persistent stands of late seral stage floodplain forest. A power function provides a robust approach for estimating the cumulative area occupied by a river and the age structure of riparian forests resulting from a specific historical sequence of streamflow in comparison to either linear or exponential alternatives.

Structure and composition of riparian forests in an old-growth northern hardwood–hemlock watershed

We examined riparian areas in an old-growth northern hardwood watershed of the Porcupine Mountains, Upper Michigan, focusing on how stream valley geomorphology and associated factors influence the structure and composition of both the overstory and ground-flora vegetation. We collected data from 92 transects stratified among three different geomorphic settings characteristic of portions of the northern Lake States, including a low-gradient, bedrock controlled geomorphic type, a high-gradient bedrock controlled geomorphic type, and a clay lake plain geomorphic type. Individual transects extended from the stream channel to at least 40m into the adjacent uplands, with transects arrayed roughly perpendicular to streamflow. Along each transect we collected: (1) overstory basal area, species composition, and canopy closure data on a series of 100m2 plots; (2) ground-flora species composition on a series of 1m2 plots; and (3) physiographic data (e.g., distance from and elevation above the stream, landform) collected across each transect. Overall, the structure and composition of overstory and ground-flora vegetation is ordered along a complex environmental gradient running from the stream channel into the adjacent uplands. Additionally, our multivariate analyses suggest that the structure and composition of the overstory and ground-flora is related to specific valley floor landforms, as well as distance from and elevation above the bankfull stream channel. Conifer basal area increases across the valley floor landforms (floodplains, terraces, and hillslopes or valley walls) in all three geomorphic settings, while hardwood basal area remains relatively constant across these same landforms. Hardwood basal area, however, dominates the adjacent upland forest ecosystems in all but the clay lake plain geomorphic type which is characterized by fine-textured soils. In all three geomorphic settings, the near stream environment often lacks a significant overstory component, and these fluvial surfaces are often dominated by a mixture of woody shrub and herbaceous species. These results suggest that changes in vegetation are associated with the proximity of the stream, and these patterns are influenced strongly by large-scale landscape factors such as stream valley geomorphology and associated disturbances. Additionally, our results provide much-needed baseline information on the vegetation patterns and dynamics of riparian forests in the northern Lake States, and suggest that riparian areas may require different management strategies than the adjacent upland forest ecosystems.

Estructura y composición florística del bosque ribereño subandino de la subcuenca de Yumbillo, Yumbo (Valle del Cauca)

Se analizaron la estructura y la composición florística del bosque ripario subandino en la subcuenca del río Yumbillo, Municipio de Yumbo (Valle del Cauca), vertiente oriental de la Cordillera Occidental de los Andes, Colombia. Mediante 20 parcelas de 100 m2, en el rango altitudinal 1500-2235 msnm se registraron datos de individuos con diámetros a la altura del pecho (DAP) 10 cm, composición florística y estructura vertical y horizontal, mediante cálculo de abundancia, frecuencia, dominancia e índice de valor de importancia (IVI). La composición florística se constituyó de 825 individuos distribuidos en 93 especies, 71 géneros y 50 familias botánicas; las familias dominantes fueron Myrtaceae, Lauraceae, Melastomataceae, Moraceae, Euphorbiaceae, Fagaceae, Acanthaceae y Tiliaceae. La especie de mayor peso ecológico fue el Truco (Hyeronimasp) y las de mayor frecuencia fueron el Truco (Hyeronimascabrida), el Roble (Quercus humboldtii) y el Higuerón de nacimiento (Ficus apollinaris). Los mayores valores de importancia correspondieron a las especies Truco (Hyeronima scabrida), Arrayán (Myrcia sp), Nacedero (Trichanthera gigantea), Otobo (Dialyanthera lehemannii) y Naranjuelo (Lacistema aggregatum). Se categorizó la estructura vertical en 3 estratos, 508 individuos en el I (3 y 16 m), 270 en el II (17 y 23 m) y 47 en el emergente (alturas superiores a 24 m). Se concluye que el bosque ripario posee un alto grado de especies pioneras y típicas de bosques en recuperación, donde se destaca el Medio comino (Ocotea sp), especie maderable amenazada que puede hacer parte de las acciones de restauración ecológica y enriquecimiento florístico en áreas de bosques riparios y en la protección de cuencas.

The Ebro River in the 20th century or the ecomorphological transformation of a large and dynamic Mediterranean channel

ABSTRACTIn the first decades of the 20th century, the Ebro River was the Iberian channel with the most active fluvial dynamics and the most remarkable spatial‐temporal evolution. Its meandering typology, the dimensions of its floodplain, and the singularities of its flow regime produced an especially interesting set of river functions.The largest dynamics of the Ebro River are concentrated along the meandering profile of the central sector. During the 20th century, this sector experienced a large alteration of its geomorphological structure. We present here an analysis of this evolution through the cartographic study of a long segment of the river (~250 km) in 1927, 1956 and 2003. The results show a large reduction in bank sinuosity, a progressive loss of fluvial territory, and a large decrease in channel width. These changes are especially clear in the areas previously most ecologically connected with the active channel. The fluvial territory of the river in 2003 was approximately half that found during the first decades of the 20th century. Forest plantations, which were non‐existent in 1927, occupied more than 1500 ha of the study area in the last decade.This intense geomorphological transformation becomes ecologically visible in (i) a 35% reduction of the area occupied by riparian vegetation; (ii) a loss of the heterogeneity of riparian forest spots, which were formerly structured in an irregular mosaic far from the river thalweg; and (iii) a modification of the riparian forest structure, which is currently linear, uniform, thin and very close to the river axis. The ecomorphological alteration was intensified by the remarkable reduction in bank length (13%) and the reduced dynamism of the present river system, indicated by an increase in the percentage of fluvial territory occupied by riparian forests and a reduction in the area occupied by the active channel. Copyright © 2002 John Wiley & Sons, Ltd.

Impact of anthropogenic disturbances on riparian forest ecology and ecosystem services in Southern India

This article highlights the impact of anthropogenic disturbance on forest structure and plant diversity in the riparian forest in the Cauvery Wildlife Sanctuary, Karnataka, Southern India. We clustered 11 transects into groups with low and high anthropogenic disturbances. In total, 73 tree species were recorded on 11 transects, of which 57 species were found in less-disturbed sites and 39 species in high-disturbed sites. Shannon–Wiener diversity confirmed higher values for less-disturbed sites (3.2 ± 0.7) compared with high-disturbed sites (2.7 ± 0.5). Evenness index suggested no complete evenness in the two sites. Mean species richness and number of individuals in the majority girth classes were found to be higher in less-disturbed sites than in high-disturbed sites. Non-native plants dominated the high-disturbed sites. The occurrence of native and non-native species was correlated with the level of disturbance. Current disturbance intensities may have led to loss of native species such as Ixora bracheata, Madhuca latifolia, Syzygium cumini and Terminalia arjuna in riparian forests. This study concludes that species-rich areas in the riparian forests are under threat. Protection of these areas should be prioritized in policy, because anthropogenic disturbance has led to decreasing riparian forest species diversity and structure.

Species composition and minimum sampling area of a riparian mixed broadleaved-Korean pine forest in Changbai Mountain Nature Reserve

Riparian areas are unique although often small component of the overall watershed landscape. The structure of riparian forests along Erdaobai River on the north slope of Changbai Mountain were investigated by using field data collected from eight sampling transects perpendicular to the Erdaobai River channel. Two kinds of species-area saturation curves were used to examine the relationship between species number and minimum sampling area. The results showed that riparian gymnosperms accounted for a high proportion of all gymnosperms in the Changbai Mountain Nature Reserve while riparian ferns and angiosperms accounted for a relatively low proportion. The average minimum sampling areas of riparian forest that included 60%, 80%, and 90% of the community species pool were about 85, 185, and 328 m 2 , respectively; while those for nonriparian forest were about 275, 390, and 514 m 2 , correspondingly.

Woody species composition, diversity and structure of riparian forests of four watercourses types in Burkina Faso

Riparian forests are classified as endangered ecosystems in general, particularly in sahelian countries like Burkina Faso because of human-induced alterations and civil engineering works. The modification of this important habitat is continuing, with little attention being paid to the ecological or human consequences of these changes. The objective of this study is to describe the variation of woody species diversity and dynamic in riparian forests on different type of watercourse banks along phytogeographical gradient in Burkina Faso. All woody species were systematically measured in 90 sample plots with sides of 50 m × 20 m. Density, dominance, frequency and species and family importance values were computed to characterize the species composition. Different diversity indices were calculated to examine the heterogeneity of riparian forests. A total of 196 species representing 139 genera and 51 families were recorded in the overall riparian forests. The species richness of individuals with dbh ≥ 5cm increased significantly from the North to the South along the phytogeographical gradient and varied significantly between the different types of riparian forests. Similarity in tree species composition between riparian forests was low, which indicates high beta diversity and reflects differences in habitat conditions and topography. The structural characteristics varied significantly along the phytogeographical gradient and between the different types of riparian forests. The diameter class distribution of trees in all riparian forests showed a reverse “J” shaped curve except riparian forest of stream indicating vegetation dominated by juvenile individuals. Considering the ecological importance of riparian forest, there is a need to delineate and classify them along watercourses throughout the country.

Composition and Structure of Riparian Forest Dominated by Tsuga canadensis: Contrasts Between Old-Growth and Second-Growth Stands in Northwestern Pennsylvania

Riparian forests of northwestern Pennsylvania's Allegheny High Plateau are often dominated by eastern hemlock, Tsuga canadensis, a tree species under threat from the introduced hemlock woolly adelgid (Adelges tsugae). It is vital to document the structure and composition of these forests before adelgid invasion to understand how they may respond to this disturbance and to provide information useful for future restoration and management efforts. To meet these goals, we compared the composition and structure of old-growth and second-growth stands in a T. canadensis-dominated riparian forest along Henry Run, Cook Forest State Park, Pennsylvania. Tsuga canadensis strongly dominated the large tree stratum of the old-growth stand, followed by Betula lenta and Fagus grandifolia. Tsuga canadensis and Pinus strobus were codominant in the second-growth stand with B. lenta, Fraxinus americana, and Acer rubrum as secondary species. Seedlings and saplings of dominant tree species in both old-growth and second-growth stands were under represented relative to small and large trees, indicating regeneration difficulties. The alien shrub Berberis thunbergii was the only sapling species to occur in the second-growth stand, perhaps a land-use legacy, but was absent from the seedling stratum. We predict that the potential loss of T. canadensis from the canopy at Henry Run may cause a shift in composition to B. lenta and/or P. strobus as these species capture gaps created by dead and dying T. canadensis. However, browsing pressure by white-tailed deer (Odocoileus virginianus) and competition with B. thunbergii may affect regeneration success of specific tree species and ultimately, future forest canopy composition.

Importance des sites sacrés pour la conservation des forêts-galeries au Centre-Bénin

Les mares, bassins ou sites sacrés de forêtsgaleries le long des cours d'eau, encore appelés Íbú ódó en langue Tchabè, sont respectés par les communautés Tchabè le long des fleuves Ouémé et Okpara au Centre-Bénin (Afrique de l'Ouest). Le caractère sacré de ces sites et points d'eau a un impact évident sur la conservation de la biodiversité, à travers la prohibition de l'utilisation des poisons pour la pêche, de la pollution des eaux, l'interdiction de l'élevage, les défrichements dans leur voisinage immédiat, etc. La structure et la diversité des forêts-galeries et Íbú ódó ont été examinées dans trois sites différents (Djabata, Idadjo et Monka) de forêts-galeries adjacents à des sites et points d'eau sacrés. En outre, les caractéristiques des forêts-galeries ont été analysées du point de vue des différents modes d'utilisation et de gestion des terres, en zones agricole, domaines utilisés par les villages et sites sacrés des religions traditionnelles. La diversité des ligneux (dbh 10 cm) est plus élevée dans les sites des forêts sacrées, tandis que la surface terrière est plus grande dans les sites à forte emprise humaine ou adjacents aux Íbú ódó. Sur le plan floristique, les forêts-galeries les plus éloignées des villages sont les plus diversifiées, avec les plus grandes surfaces terrières. L'enquête ethnobotanique auprès des anciens et des chefs chasseurs a révélé l'importance culturelle et la diversité des ressources des sites de forêtsgaleries sacrés. Les Íbú ódó sont respectés par tous les villageois y compris les immigrants et colons agricoles non Tchabè. Les fonctions des écosystèmes riverains (telles que brisevent, fertilité des sols, sites privilégiés de chasse) sont reconnues et valorisées par les populations riveraines. La présente étude recommande d'inclure les sites sacrés de forêts-galeries dans une stratégie de gestion durable du bassin du fleuve Ouémé, afin d'inverser la destruction de la végétation et de préserver la diversité des forêts-galeries. C'est une nécessité pour un contrôle efficace des inondations et la conservation de la biodiversité au Centre-Bénin. (Résumé d'auteur)

Late-successional riparian forest structure results in heterogeneous periphyton distributions in low-order streams

Late-successional riparian forests often regulate autotrophic microhabitats in low-order streams through shading provided by canopies. However, few studies have linked forest structure with periphyton microhabitat in adjoining streams. Our hypotheses were that (1) the heterogeneous horizontal structure in old-growth forests creates more spatially variable below-canopy light environments compared with mature forests and (2) site-specific light availability over streams correlates with spatial distributions of periphyton microhabitat. We surveyed 15 low-order stream reaches in late-successional northern hardwood–hemlock forests in the Adirondack Mountains of New York, USA. We measured forest structure and the below-canopy light environment at all reaches and the periphyton chlorophyll a concentration on artificial substrates in eight reaches. While stand age was not statistically significant, multivariate models of horizontal forest structure (e.g., gap distribution) and topography showed strong relationships (R2 > 0.70) with the below-canopy light environment across all late-successional forests. Furthermore, metrics of below-canopy light availability explained a small but statistically significant proportion of the variation in chlorophyll a concentration. This variation in chlorophyll a indicates that complex late-successional riparian forests, both mature and old-growth, create a mosaic of heterotrophic (shaded) and autotrophic (lighted) microhabitats along low-order streams. These results reveal important and previously unrecognized links between stream habitat heterogeneity and the horizontal heterogeneous late-successional forest structure.

Impacts of Wildfire on the Composition and Structure of Riparian Forests in Southern California

In southern California, wildfire is a ubiquitous agent shaping plant communities. Although fire impacts have been widely studied in chaparral-covered uplands, few data are available regarding fire and riparian vegetation. This study provides an example of the impact of a severe fire on riparian habitat. Plant species found in southern California gallery forests are typically adapted to maintaining populations following flood disturbances; we seek to determine whether structural and compositional changes following fire here demonstrate a similar quasi-equilibrium response. We sampled 65 quadrats on 11 transects along two streams in the Los Padres National Forest to characterize tree species size–class distributions before and after the 2002 Wolf Fire. We tested whether species exhibited differential patterns of survivorship and regeneration following the fire, and also tested for spatial variability in mortality within the floodplain. Alnus rhombifolia dominated the pre-fire forest, but experienced severe mortality in the fire and showed very limited resprouting after 3 years. Other prominent taxa (Populus, Salix, Quercus spp.) also lost considerable standing basal area, but had substantially greater rates of resprouting, resulting in a dramatically altered post-fire vegetation composition and structure. Fire impacts did not vary with landform position, leading to a distinctive homogenizing disturbance that contrasts with the spatially zoned and relatively stabilizing compositional influence that flood events have in this same riparian setting.

On the retreat of forested, cohesive riverbanks

Data from the Brandywine Creek in Pennsylvania and the South River in Virginia document spatial and temporal patterns describing the retreat of gently curving bends with forested riparian zones containing a mix of very large and smaller trees, cohesive banks, and slow rates of migration (∼ 0.1 m/yr). Field data include 2.5 years of bank profile surveys, measurements of tree morphology, and ground-based laser scanner (LiDAR) surveys repeated biannually. These banks retreat through a cycle of erosion on decadal timescales. Initially, small volumes (mean = 0.173 m 3 per bank erosion event) of soil are removed between large trees growing on the bank. Near trees, the rate of bank retreat is initially negligible, creating a scalloped bank morphology buttressed by large trees. As the trees are increasingly exposed to the flow, they become slowly undercut, gradually leaning into the channel and sliding down to the toe of the bank, eventually toppling into the river after a few years, which restarts the cycle. A magnitude–frequency analysis of monitoring data from the Brandywine Creek indicates that the effective bank erosion event only 0.11 m 3; thus, forested bank retreat is dominated by small soil failures. Large erosion events, many associated with trees toppling into the river, greatly increase the variance of time-averaged rates of bank erosion, such that almost 4 years of monitoring are needed to measure time-averaged rates of erosion to within an accuracy of 10%. Because of the limited spatial extent of our survey data and the absence of large discharges, these results are provisional. Improved process-based models to predict the retreat of forested riverbanks should account for the spatial structure of riparian forests and the interactions between the flow, the gradual removal of bank soils, and the ongoing efforts of the trees to remain attached to the bank.

Lateral channel migration effects on riparian forest structure and composition, Congaree River, South Carolina, USA

Lateral channel migration initiates complex and dynamic biogeomorphic responses that are fundamental to the creation and maintenance of riparian habitats along low-gradient, coastal plain rivers. This study examines the effects of lateral migration rates on the structure and composition of riparian forests along the Congaree River, Congaree National Park, in the Atlantic Coastal Plain, South Carolina. Lateral channel migration rates were measured in a GIS using aerial photos from 1938–2006. Forest variables were measured from a stratified-random sample of 25 paired edge-interior plots, and analyzed using Mann-Whitney tests, Spearman’s correlations, multi-response permutation procedures (MRPP), and non-metric multi-dimensional scaling (NMS) ordinations. Lateral channel migration produced a significant directional control on riparian forests. Pointbar forests exhibited classic forward succession dependent on spatial and temporal controls related to elevation, flood frequency, and lateral migration rates. Cutbank forests responded positively to exposure along the edge and increased in structural complexity with increasing proximity to the river; density, basal area, and richness varied inversely with lateral migration rates. Cutbank edges with low lateral migration allowed longer time for trees to colonize and they contained greater density, basal area, and richness. Cutbanks with high lateral migration contained lower tree density, basal area, and richness.

Community and ecosystem consequences of giant knotweed ( Polygonum sachalinense) invasion into riparian forests of western Washington, USA

The invasive, non-native herb, giant knotweed ( Polygonum sachalinense), is becoming increasingly common in riparian corridors throughout North America and Europe. Despite its prevalence, there has been limited study of its ecological impacts. We investigated the effects of knotweed invasion on the abundance and diversity of forest understory plants, and the quantity and nutrient quality of leaf-litter inputs, in riparian forests in western Washington, USA. Among 39 sampling locations, knotweed stem density ranged from 0 to 8.8 m −2. Richness and abundance (cover or density) of native herbs, shrubs, and juvenile trees (⩽3 m tall) were negatively correlated with knotweed density. Where knotweed was present (>5.3 stems m −2), litter mass of native species was reduced by 70%. Carbon:nitrogen ratio of knotweed litter was 52:1, a value 38–58% higher than that of native woody species (red alder [ Alnus rubra] and willow [ Salix spp.]). Resorption of foliar N prior to leaf drop was 76% in knotweed but only 5–33% among native woody species. By displacing native species and reducing nutrient quality of litter inputs, knotweed invasion has the potential to cause long-term changes in the structure and functioning of riparian forests and adjacent aquatic habitats.

The influence of timber harvest on the structure and composition of riparian forests in the Coastal Redwood region

The composition and structure of riparian forests in the coast redwood region were analyzed in relation to the length of time since timber harvest, and the width of riparian buffer zone. Ten sites were sampled in the central range of the coast redwood forest type within a variety of post-harvest age groups and riparian buffer zone widths. Data was collected using randomly selected sample plots adjacent to perennial coastal steams. Correlation Fisher's r to z tests and two-tailed t-test were used to examine the relationship between the sample variables and the experimental parameters “years since harvest”, and “width of buffer.” Results indicate that canopy cover was negatively correlated to “years since harvest” with the highest level of canopy cover found on the youngest sites and the lowest level found on the old-growth sites. The hardwood to conifer dominance ratio and the basal area of Alnus rubra (red alder) were correlated negatively to both “years since harvest” and “buffer width” indicating that timber harvest favored hardwood species. Late seral associates such as Oxalis oregana (redwood sorrel), Anthyrium filix- femina (lady fern), and Vaccinium parviflorum (billberry) were found preferentially in older forests and sites with wider buffer zones, while non-native species such as Hedera helix (English ivy), Pampas cortedaria (pampas grass), and Myosotis latifolia (forget-me-not) were found preferentially in younger forests and areas with smaller buffer zones.