Riparian Forest Structure Research Articles

Riparian forest structure and succession in second-growth stands of the central Cascade Mountains, Washington, USA

We examined the relationship between landform types and riparian forest structure and succession in second-growth stands along mid order streams in the Cascade Mountains, Washington, USA. We sampled tree, sapling, seedling, and shrub characteristics across a range of fluvial geomorphic surfaces, which were classified into four landform classes, including low floodplain, high floodplain, terrace and hillslope. Landform classification was based on topographic characteristics, position relative to the stream channel, and estimated flood frequency. Statistical analyses using generalized estimating equations (GEE) showed that landform exerted a strong influence on the distribution and abundance of conifer and deciduous species and of different tree life stages. The floodplain landforms were characterized by initial disturbance from timber harvest, and ongoing fluvial disturbance, which favored the establishment of deciduous communities dominated by red alder ( Alnus rubra) and maintenance of early successional riparian stands. In contrast, the terrace and hillslope landforms were also subject to timber harvest as the stand initiating agent but were unaffected by fluvial disturbance. However, based on differences in species distribution, we infer that forest structure on these two landforms differed from one another as a result of differences in soil moisture levels. Terraces and hillslopes were found to have high conifer tree abundance, but frequency of younger conifers was higher on hillslopes. Deciduous tree reproduction was very low on terraces and hillslopes. Our results also suggest that conifer recruitment in these second-growth riparian forests may be more successful on soil substrates than on coarse woody debris. We propose that the interplay between the disturbance regime (including type, frequency and intensity) and soil moisture conditions played an important role in influencing the course of riparian succession, present stand structure, and future successional trajectories and these were the primary mechanisms driving vegetation differences among landforms.

Canopy Composition and Forest Structure Provide Restoration Targets for Low‐Order Riparian Ecosystems

Abstract Many programs are in place to protect and restore low‐order streams and riparian zones. However, information on riparian zone forests is sparse for many biogeographical regions, especially compositional and structural data that would provide useful targets for restoration. This study provides quantitative data on riparian zone composition and forest structure from three physiographic provinces of eastern United States. Data from 219 low‐order (first‐ to fourth‐order) forested reaches were arranged by three basal area (BA) categories meant to represent successional categories and variations in forest structure. Detrended correspondence analysis (DCA) was used to illustrate differences among successional categories and physiographic provinces. The DCA ordination separated stands into four physiographic subregions, based on the species composition of late‐successional stands. Many early to mid‐successional stands (<30 m2/ha) were similar in composition to late‐successional reference stands (BA ≥ 30 m2/ha) in the same physiographic subregion. In such sites, natural successional processes would likely be sufficient to restore the compositional and structural attributes inherent in late‐successional stands if provided long‐term protection. Other sites with dissimilar compositions may have been recovering from more intensive types of alterations, such as mechanized land clearing. In such sites, restoration to historic compositions could benefit functionally by planting oaks (Quercus spp. L.) and other heavy mast species.

Impacts of Riparian Forest Removal on Palauan Streams

ABSTRACTThe 2006 completion of the circum‐island Compact Road on the island of Babeldaob in the Republic of Palau resulted in several deforested stream reaches with modified stream channels. To determine the impacts of deforestation and road construction, various ecosystem parameters were compared between road‐impacted reaches, reforested savanna reaches, and forested reaches. Compared to adjacent forested reaches, road‐impacted reaches received significantly more light (0.4 ± 0.1 vs. 87.8 ± 4.1 % light transmittance, respectively), were significantly warmer (25.7 ± 0.1 vs. 26.1 ± 0.1°C, respectively), and received higher nutrient and sediment loads, all of which were attributed to the removal of riparian vegetation and increased surface runoff from the road. These differences were believed to have shifted the benthic algal community in road‐impacted reaches from diatoms to filamentous algae with significantly greater chl a biomass (10×) and benthic algal ash free dry mass AFDM (3×) compared to adjacent forested reaches. Savanna‐impacted and forested reaches had similar chl a, algal AFDM, and received similar amounts of light. Nutrient and sediment concentrations varied between the two reach types. Results from this study emphasize the need for the maintenance of riparian forests especially with predicted increases in population, development, and deforestation. Future studies are needed to determine effective riparian widths and riparian forest community structure to help resource managers and land owners protect and preserve the many ecosystem services that Palauan streams and watersheds provide.

Structure, reproduction and flood-induced dynamics of riparian Tugai forests at the Tarim River in Xinjiang, NW China

Tugai forests are the riparian forests along the rivers in the continental desert regions of Central Asia, i.e. the Tarim River, Amu Darya and Syr Darya. They mainly consist of Populus euphratica Oliv., Populus pruinosa Schrenk. and Elaeagnus oxycarpa Schltdl. As a consequence of land opening campaigns, large areas of Tugai forests were destroyed after the 1950s. Due to excessive use of water for irrigation, the remaining Tugai forests are under severe threat. Near natural Tugai forests still exist along the Tarim middle reaches in the Tarim Huyanglin Nature Reserve, Xinjiang, NW China. There is a gap in understanding, how the seedlings of P. euphratica establish as trees which continuously connect to the groundwater. Therefore, the set of conditions which must be met for germination and successful establishment, i.e. formation of Tugai forests, was investigated along a representative transect still under natural conditions. P. euphratica seedlings germinate in belts during the retreat of the summer flood on freshly deposited sites bare of other vegetation. Such germination sites are formed by river dynamics. While germination takes place regularly in the study area, successful establishment is restricted to few germination events. Seedlings face dropping groundwater levels during spring and early summer of the second year after germination. Therefore, for successful establishment, it is essential that the flood of the second year starts in time and is high enough, in order to replenish the groundwater. Furthermore, clayey soil layers in the subsoil may play a role for successful establishment, too, as they store water better than sandy soil layers.

Influence of urbanization on riparian forest diversity and structure in the Georgia Piedmont, US

Riparian forests are increasingly threatened by urban expansion and land use change worldwide. This study examined the relationships between landscape characteristics and woody plant diversity, structure, and composition of small order riparian corridors along an urban-rural land use gradient in the Georgia Piedmont, US. Riparian plant diversity, structure, and composition were related to landscape metrics and land use. Species richness was negatively associated with impervious surfaces and landscape diversity, and positively associated with forest cover and largest forest patch index. Shannon species diversity was strongly related to the biomass of non-native species, especially for the regeneration layer. Urban sites were characterized by high richness of non-native and pioneer species. Developing sites were dominated by the non-native shrub, Ligustrum sinense Lour., and several native overstory trees, mainly Acer negundo L. While agricultural and managed forest sites were composed of ubiquitous species, unmanaged forest sites had a structurally distinct midstory indicative of reduced disturbance. Urban and agricultural land uses showed decreased native stem densities and signs of overstory tree regeneration failure. Results from this study highlight the impact of the surrounding landscape matrix upon riparian forest plant diversity and structure.

MATURE AND OLD-GROWTH RIPARIAN FORESTS: STRUCTURE, DYNAMICS, AND EFFECTS ON ADIRONDACK STREAM HABITATS

Riparian forests regulate linkages between terrestrial and aquatic ecosystems, yet relationships among riparian forest development, stand structure, and stream habitats are poorly understood in many temperate deciduous forest systems. Our research has (1) described structural attributes associated with old-growth riparian forests and (2) assessed linkages between these characteristics and in-stream habitat structure. The 19 study sites were located along predominantly first- and second-order streams in northern hardwood-conifer forests in the Adirondack Mountains of New York (U.S.A.). Sites were classified as mature forest (6 sites), mature with remnant old-growth trees (3 sites), and old-growth (10 sites). Forest-structure attributes were measured over stream channels and at varying distances from each bank. In-stream habitat features such as large woody debris (LWD), pools, and boulders were measured in each stream reach. Forest structure was examined in relation to stand age using multivariate techniques, ANOVA, and linear regression. We investigated linkages between forest structure and stream characteristics using similar methods, preceded by information-theoretic modeling (AIC). Old-growth riparian forest structure is more complex than that found in mature forests and exhibits significantly greater accumulations of aboveground tree biomass, both living and dead. In-stream LWD volumes were significantly (alpha = 0.05) greater at old-growth sites (200 m3/ha) compared to mature sites (34 m3/ha) and were strongly related to the basal area of adjacent forests. In-stream large-log densities correlated strongly with debris-dam densities. AIC models that included large-log density, debris-dam density, boulder density, and bankfull width had the most support for predicting pool density. There were higher proportions of LWD-formed pools relative to boulder-formed pools at old-growth sites as compared to mature sites. Old-growth riparian forests provide in-stream habitat features that have not been widely recognized in eastern North America, representing a potential benefit from late-successional riparian forest management and conservation. Riparian management practices (including buffer delineation and restorative silvicultural approaches) that emphasize development and maintenance of late-successional characteristics are recommended where the associated in-stream effects are desired.

Influence of floods on natural riparian forests along the Ergis River, west China

The riparian forests along the Ergis River, west China, composed mainly of Salicaceae species, play an important role in eco-environment protection and sustainable development of local agriculture, stockbreeding, and social economy of the northern desert region of Xinjiang Uygur Autonomous Region. The study of the influence of floods on the natural riparian forests is imperative for the understanding of the successional process and the acceleration of conservation and restoration of forests. By investigating the relationship between floods and dispersal of seeds, sprouting, natural regeneration, the structure of the forests, and their current distribution, we conclude that: 1) the ripening and dispersal periods of Salicaceae species seeds overlap largely with flood occurrence periods, and the sprouting and natural regeneration of seeds depend greatly on flood events; 2) floods supply soil water and increase groundwater level of riparian land through flood irrigation and horizontal infiltration to maintain the normal growth of the riparian forests; 3) floods have a decisive influence on the structure, composition, and distribution pattern of riparian forests, and any disturbance in the water flow has a profound effect on these characteristics. Given these facts, some management measures for conservation and restoration of the riparian forests are proposed, including the establishment of riparian forest buffer belt, bank stabilization measures, and maintenance of flood protection.

Classifying fine-scale spatial structure of riparian forests using hyperspectral high-resolution remotely sensed imagery at the Cedar River municipal watershed in western Washington, USA

Logging in the Pacific Northwest has often removed conifer trees from riparian forests, which are then colonized and dominated by red alder (Alnus rubra) and deciduous shrubs. As a result, deciduous-dominated sites are over-represented in streamside areas compared to historic conditions, causing degradation of stream habitat. A popular restoration strategy in the region is the reintroduction of conifer trees into riparian areas. An accurate characterization of streamside forest conditions at large spatial scales is required to identify disturbed areas. Traditional remote sensing imagery has not been capable of capturing the fine-scale spatial heterogeneity within riparian zones. We analyzed a MODIS–ASTER (MASTER) image collected at the Cedar River municipal watershed in western Washington to determine if this high-resolution (5 m) hyperspectral imagery (50 bands) would be better able to characterize the high level of heterogeneity common in this environment. A supervised classification procedure using a maximum likelihood algorithm was conducted delineating nine of the dominant land cover types in the study. Subsequent field observations and validation procedures indicated that the classification had an overall accuracy of roughly 80%. The results show that this technique provided a cost-effective means of mapping the fine-scale spatial heterogeneity typical of streamside forest stands in the region.

Urbanization alters the functional composition, but not taxonomic diversity, of the soil nematode community

We evaluated the response of riparian forest soil nematode community structure to the physico-chemical environment associated with urban land use. Soils were sampled seasonally between December 2000 and October 2002 along an urban—rural transect in Asheville, North Carolina. We characterized the taxonomic (to genus) and functional composition (trophic groups) of the nematode community of forest soils, as well as several nematode ecological indicators (maturity index, channel index, weighted faunal index). The diversity of nematode genera was not affected by urban land use. However, there tended to be functional differences in the nematode communities along the land use gradient. The urban soils tended to have lower abundances of predatory and omnivorous nematodes. Differences in channel index scores indicated that there was less fungal dominance in the soil food webs of the urban soils. Our results indicate that the functional composition of the soil food web is an important component of soil biodiversity that can be affected by land use practices. This study was conducted in a relatively small city; hence the influence of pollutants on the soil environment was not as great as in larger cities. Correspondingly, the impact on the soil nematode community was not very severe. The utilization of the nematode community assemblage as an indicator of soil conditions should be further explored in urban places of differing magnitudes of environmental effects.

Influences of logging history and riparian forest characteristics on macroinvertebrates and brook trout (Salvelinus fontinalis) in headwater streams (New Hampshire, U.S.A.)

Summary1. Logging can strongly affect stream macroinvertebrate communities, but the direction and magnitude of these effects and their implications for trout abundance are frequently region‐specific and difficult to predict.2. In first‐order streams in northern New England (U.S.A.) representing a chronosequence of logging history (<2 to >80 years since logging), we measured riparian forest conditions, stream macroinvertebrate community characteristics and brook trout (Salvelinus fontinalis) abundance. Principal component analysis was used to collapse forest data into two independent variables representing variation in logging history, riparian forest structure and canopy cover. We used these data to test whether logging history and associated forest conditions were significant predictors of macroinvertebrate abundance and functional feeding group composition, and whether brook trout abundance was related to logging‐associated variation in invertebrate communities.3. Catchments with high PC1 scores (recently logged, high‐density stands with low mean tree diameter) and low PC2 scores (low canopy cover) had significantly higher total macroinvertebrate abundance, particularly with respect to chironomid larvae (low PC2 scores) and invertebrates in the grazer functional feeding group (high PC1 scores). In contrast, proportional representation of macroinvertebrates in the shredder functional feeding group increased with time since logging and canopy cover (high PC2 scores). Brook trout density and biomass was significantly greater in young, recently logged stands (high PC1 scores) and was positively related to overall macroinvertebrate abundance. In addition, three variables – trout density, invertebrate abundance and shredder abundance – successfully discriminated between streams that were less‐impacted versus more‐impacted by forestry.4. These results indicate that timber harvest in northern New England headwater streams may shift shredder‐dominated macroinvertebrate communities supporting low trout abundance to a grazer/chironomid‐dominated macroinvertebrate community supporting higher trout abundance. However, while local effects on brook trout abundance may be positive, these benefits may be outweighed by negative effects of brook trout on co‐occurring species, as well as impairment of habitat quality downstream. Research testing the generality of these patterns will improve understanding of how aquatic ecosystems respond to anthropogenic and natural trajectories of forest change.

Relationship between riparian forest structure and deposition of CWD caused by the 2003 flooding in the Shukushubetsu River, Hokkaido

Relationship between riparian forest structure and deposition of CWD caused by the 2003 flooding in the Shukushubetsu River, Hokkaido

Floristic composition and stand structure of riparian forests in the Tadami River basin and the ecological distribution of an endangered tree, Salix hukaoana

Floristic composition and stand structure of riparian forests in the Tadami River basin and the ecological distribution of an endangered tree, Salix hukaoana

Effects of fluvial geomorphology on riparian tree species in Rekifune River, northern Japan

Classification of riverbed geomorphic surfaces based on flooding frequency was conducted and the relationship between their distribution and river morphology was analyzed, to provide an understanding of the structure and species composition of riparian forests dominated by Chosenia arbutifolia. The channel floors of two contrasting river morphologies (bar-braided and incised meandering channels), were divided into five geomorphic surfaces (gravel bar, lower and upper floodplains, secondary channel, and terrace) based on the water level of a 2-yr and a 20-yr recurrence interval. The environmental variables of the same geomorphic surfaces showed similar trends regardless of braided and meandering channel morphology, but differed significantly among the five geomorphic surfaces, which influenced the dominance of tree species. The geomorphic surface map based on recurrence interval of flood and physiognomical vegetation map based on aerial photos appeared almost identical. Geomorphic surface distribution, determined by river channel dynamics and the sediment transport processes occurring at a larger scale and a longer time frame, played an important role in shaping the structure and composition of the riparian forests. C. arbutifolia dominated gravel bar, and the upper and lower floodplains, because these geomorphic surfaces were characterized by gravelly soils which have lower soil moisture availability than soils of other geomorphic surfaces. Thus, an extensive distribution of C. arbutifolia in the braided channel section can be attributed to the frequent lateral migrations of river channels, which resulted in a high ratio of gravel bars, and lower and upper floodplains. In order to preserve indigenous plant communities in riparian zone, dynamic nature and processes of braided rivers should be maintained.

Forty Years of Tropical Forest Recovery from Agriculture: Structure and Floristics of Secondary and Old-growth Riparian Forests in the Dominican Republic1

Interest in tropical secondary forests has grown as large areas of agriculture have been abandoned in recent decades; yet, there are few long-term studies of post-agriculture vegetation recovery in the tropics. In this study, we compared the vegetation structure and floristic composition of old-growth and 40-year-old secondary riparian forests in the Cordillera Central, Dominican Republic. Canopy height and stem density of woody plants were similar between forest types, but basal area of trees was 27 percent lower in secondary forests. Introduced tree species comprised 20 percent of the basal area and dominated the understory of secondary forests. Life-form diversity was higher in old-growth forests as arborescent ferns, the palm species, and epiphytic bromeliads, orchids, and bryophytes were much more abundant. The number of species of epiphytic orchids and bromeliads, ground ferns, and herbaceous plants was also significantly higher in old-growth forests. The species density of woody plants and vines, however, was comparable between forest types, and vine abundance was significantly higher in secondary forests. The high importance of introduced tree species and the delayed recovery of several plant life-forms have important implications for the conservation of plant diversity in secondary forests in the tropics. The robust regeneration of woody structure despite the long land tenure (ca 60 yr) by farmers is probably due to the nutrient-rich alluvial soils and low-intensity agriculture. This study revealed the potential for the rapid recovery of woody plant diversity and structure in fertile secondary forests adjacent to mature forest seed sources and the more delayed recovery of nonwoody plant diversity and abundance.

INFLUENCES OF ENVIRONMENT AND DISTURBANCE ON FOREST PATTERNS IN COASTAL OREGON WATERSHEDS

Modern ecology often emphasizes the distinction between traditional theories of stable, environmentally structured communities and a new paradigm of disturbance-driven, nonequilibrium dynamics. However, multiple hypotheses for observed vegetation patterns have seldom been explicitly tested. We used multivariate statistics and variation partitioning methods to assess the relative influences of environmental factors and disturbance history on riparian and hillslope forests. Our study area was the Cummins Creek Wilderness, located in the Oregon Coast Range. Most of the wilderness burned at least once between the mid-19th and early 20th centuries, creating a mosaic of younger forests with a few old-growth patches. Species composition on hillslopes varied primarily along a climatic gradient from moist maritime environments to drier inland climates but was relatively insensitive to forest age structure. The abundance of Tsuga heterophylla, a fire-sensitive, late-successional tree species, decreased with distance from old-growth patches, suggesting possible seed-source limitations following the historical fires. In contrast to species composition, hillslope forest structure was primarily related to fire history but was largely independent of environmental gradients. Old-growth structure characteristics such as large dominant trees, large snags, high down-wood volumes, and high tree size diversity increased with stand age and with the presence of remnant trees that survived the fires. Riparian forests had high shrub cover, abundant hardwoods, and high down-wood volumes, while the conifer-dominated hillslopes had high overstory density and basal area. Maritime climates and their associated plant species extended further inland in riparian areas than on hillslopes. Advance regeneration densities were higher in riparian forests within 5 km of the coast than in any other portion of the study area. Riparian forest structure and composition were related to both environmental and disturbance variables, with stream gradient and size integrating much of the fine-scale variability in disturbance regimes. No single theoretical framework was sufficient to explain the vegetation patterns observed in these forested watersheds. Our findings suggest a conceptual model of forest landscapes in which the relative influences of environment and disturbance on vegetation patterns are contingent on the facet of vegetation considered (composition vs. structure) and the portion of the landscape examined (hillslope vs. riparian).

Structure and Composition of Riparian Boreal Forest: New Methods for Analyzing Edge Influence

Structure and Composition of Riparian Boreal Forest: New Methods for Analyzing Edge Influence

Stream channel configuration, landform, and riparian forest structure in the Cascade Mountains, Washington

The hierarchical relationship of five key elements, valley constraint, riparian landform, riparian plant community, channel type, and channel configuration, are described for 21 sites in mature to old-growth riparian forests of the western Cascades Mountains, Washington, U.S.A. Channel type (bedrock, plane-bed, and forced pool-riffle) was closely related to channel configuration (especially large woody debris (LWD) volume, density, and LWD-formed pools) at the smallest spatial scale and valley constraint at the largest. Valley constraint significantly influenced off-channel habitat (r2 = 0.71) and LWD volume within forced pool-riffle channels (r2 = 0.58). Riparian plant community composition was differentiated by four landform classes: three alluvial landforms based on height above the channel and one based on hillslope. Just above the active channel, floodplain landforms contained more deciduous stems than conifer and greater conifer basal area than deciduous. Conifers dominated other landforms. The diameter of in-channel LWD increased with the age of the riparian forest (r2 = 0.34). In old-growth forests, LWD diameter was equivalent to or greater than the average riparian tree diameter for all sites. In younger forests, the mixed relationship between LWD and riparian tree diameter may reflect a combination of LWD input from the previous old-growth stand and LWD input from the existing stand.

Stream channel configuration, landform, and riparian forest structure in the Cascade Mountains, Washington

The hierarchical relationship of five key elements, valley constraint, riparian landform, riparian plant community, channel type, and channel configuration, are described for 21 sites in mature to old-growth riparian forests of the western Cascades Mountains, Washington, U.S.A. Channel type (bedrock, plane-bed, and forced pool-riffle) was closely related to channel configuration (especially large woody debris (LWD) volume, density, and LWD-formed pools) at the smallest spatial scale and valley constraint at the largest. Valley constraint significantly influenced off-channel habitat (r2 = 0.71) and LWD volume within forced pool-riffle channels (r2 = 0.58). Riparian plant community composition was differentiated by four landform classes: three alluvial landforms based on height above the channel and one based on hillslope. Just above the active channel, floodplain landforms contained more deciduous stems than conifer and greater conifer basal area than deciduous. Conifers dominated other landforms. The diam...

Structure and composition of unmanaged riparian forests in the coastal mountains of Oregon, U.S.A.

We characterized the structure and composition of unmanaged riparian forests in three river basins in Oregon's coastal mountains. Our objective was to evaluate stand attributes at three spatial scales: streamside (site), drainage network (stream order), and basin (subregion). Data on basal area, species composition, snag density, canopy cover, and tree regeneration were collected along transects at 124 sites. Conifer basal area increased with distance from stream, a trend similar among subregions, and was highest at sites along first-order streams. Hardwood basal area was relatively constant with distance from stream and was proportionally higher at sites along second- and third-order streams than at sites along first-order streams. Conifer and hardwood tree regeneration occurred infrequently and varied by topographic position, stream order, and subregion. Conifer regeneration was associated with basal area of shade-tolerant conifers and appeared to be limited by shrub competition. The unmanaged forests we studied were characterized by a patchy mosaic of structure and composition. Hardwoods and shrubs were major components of the near-stream environment in these forests, whereas dominance of conifers was limited to hillslopes. It appears that fine-scale patterns associated with proximity to the stream are influenced by coarser scale factors such as valley-floor width and climate.

Comparisons of riparian vs. over stream trap location in the estimation of vertical litterfall inputs

Vertical inputs of allochthonous organic matter to streams can be estimated directly with traps; traps may be placed over the stream, in the riparian forest or at other locations in the catchment. In this study, we compared litter inputs estimated from traps located over the stream with those obtained from traps placed on the stream bank in two stretches (orders 2 and 3) of the Mera stream, with similar tree species composition and riparian forest structure. The amount of annual litter input measured with the traps on the stream was similar in the two stretches, 748 and 730 g m-2 yr-1, for orders 2 and 3, respectively. However, there appeared to be differences in the composition of litter; a lower percentage of leaves and a higher percentage of wood was found in the stretch of order 3 than in the stretch of order 2. In both sites, the mean input measured under the riparian forest was lower than the input recorded with the traps placed over the stream by approximately 10%. In relation to the traps located over the stream, the traps located on the bank overestimated the input from adjacent terrestrial tree species and, therefore, underestimated the input from riparian species.