Fish community variation along physical gradients in short streams of northern Spain and the disruptive effect of dams

Relationships between the fish community and selected habitat features were examined in a set of short temperate streams located at the northern end of the Iberian Peninsula. The fish fauna in these streams consists mostly of diadromous or estuarine species. Species richness and diversity increased with stream order, depth and width and decreased with elevation and distance from the sea. Stream order (positively) and elevation (negatively) were the two variables most highly correlated with species richness and diversity; higher order streams (order 3–4) showed greater values of species diversity than lower order ones (order 1) even when the elevation effect was removed. Addition of species in the downstream direction, but no replacement or loss was evidenced. We also compared the observed values of species diversity with those predicted from habitat features for a set of locations above unpassable dams. A great majority of the sites showed lower than predicted diversity values, which is an expected outcome for this mainly migratory fish fauna.     

Stream flow, salmon and beaver dams: roles in the structuring of stream fish communities within an anadromous salmon dominated stream

The current paradigm of fish community distribution is one of a downstream increase in species richness by addition, but this concept is based on a small number of streams from the mid-west and southern United States, which are dominated by cyprinids. Further, the measure of species richness traditionally used, without including evenness, may not be providing an accurate reflection of the fish community. We hypothesize that in streams dominated by anadromous salmonids, fish community diversity will be affected by the presence of the anadromous species, and therefore be influenced by those factors affecting the salmonid population. Catamaran Brook, New Brunswick, Canada, provides a long-term data set to evaluate fish community diversity upstream and downstream of an obstruction (North American beaver Castor canadensis dam complex), which affects distribution of Atlantic salmon Salmo salar. The Shannon Weiner diversity index and community evenness were calculated for sample sites distributed throughout the brook and over 15 years. Fish community diversity was greatest upstream of the beaver dams and in the absence of Atlantic salmon. The salmon appear to depress the evenness of the community but do not affect species richness. The community upstream of the beaver dams changes due to replacement of slimy sculpin Cottus cognatus by salmon, rather than addition, when access is provided. Within Catamaran Brook, location of beaver dams and autumn streamflow interact to govern adult Atlantic salmon spawner distribution, which then dictates juvenile production and effects on fish community. These communities in an anadromous Atlantic salmon dominated stream do not follow the species richness gradient pattern shown in cyprinid-dominated streams and an alternative model for stream fish community distribution in streams dominated by anadromous salmonids is presented. This alternative model suggests that community distribution may be a function of semipermeable obstructions, streamflow and the distribution of the anadromous species affecting resident stream fish species richness, evenness, biomass and production.     

Control of freshwater fish and crayfish community structure in Taranaki,New Zealand: dams,diadromy or habitat structure?

1. Freshwater fish and crayfish communities were surveyed along elevational gradients in streams radiating from Mount Taranaki, New Zealand. Six of the 38 streams surveyed had dams or weirs and 32 of the 85 sites were above these barriers. 2. Of the 15 fish and one crayfish species captured, 14 were native. The number of species declined with increasing elevation and distance from the sea. Species richness was also lower above dams even when effects of elevation and distance from the sea were accounted for. 3. Linear regression models using sites without dams were constructed to predict the effect of elevation on fish and crayfish community structure and to allow the effect of dams to be evaluated. The number of species predicted by these models was consistently higher than the number of species found at the above-dam sites. 4. Four fish community groups were classified using two-way indicator species analysis (TWINSPAN). One high elevation group of sites consisted of short steep streams on the west side of the mountain and a second contained longer, lower gradient streams on the east side. The other two groups (3 and 4) consisted of mid-elevation sites and low-elevation sites, respectively. 5. Discriminant analysis was used to predict biotic group membership using the environmental data. Overall, 80% of sites were classified correctly. Correlation of environmental variables with axis scores in the canonical variate analysis revealed that distance from the sea, site elevation and presence of dams were most strongly associated with fish distribution patterns.     

Stream fish assemblages in relation to environmental factors on a montane plateau (Nyika Plateau,Malawi)

Few studies have examined river fishes of Malawi. This study is one of the first to examine the stream fish assemblages on the Nyika Plateau in northern Malawi. Twenty four sites were sampled over three different periods in two river systems of the plateau. Eighteen species were collected and among these was Hippopotamyrus ansorgii, the first collection of this species in the Lake Malawi catchment. Three species, including a non-native trout, were common in the two systems studied. Correspondence analysis (CA) suggested gradients in species composition related to altitude and river type. Species succession, from a trout dominated upstream to a downstream dominated by indigenous species, was shown on the first CA axis. The second CA axis showed the assemblage of the plateau separated by river type. A direct gradient analysis method, canonical correspondence analysis (CCA), showed the importance of two stream position metrics (stream order and c-link), depth, water temperature and substrate type in determining species composition. According to variation partitioning in CCA, the spatial and temporal components respectively explained 46% and 3.6% variation in assemblage composition based on the all species data matrix, and 48.7% and 2.6% variation in assemblage composition based on the native species data matrix. The species collected were also discussed in relation to the morphological adaptations in their body forms to the environmental conditions of the streams studied.     

溪流大小及其空间位置对鱼类群落结构的影响

2008年10月和2009年10月分别对黄山地区阊江流域与青弋江流域1-3级浅水溪流中鱼类群落结构进行了研究,着重探讨了溪流大小及其空间位置对鱼类群落空间分布的影响。研究共捕获35种鱼类,阊江和青弋江分别为26和29种,其Jaccard相似性系数为57.1%;两流域间每样点的物种数差异显著,个体数差异极显著。在溪流级别梯度(1-3级)下,鱼类的物种数和个体数都呈上升的变动趋势,且物种数的变化相对个体数较显著。综合相关分析、逐步多元回归分析与典范相关分析的结果,阊江和青弋江中鱼类物种数的空间变化主要与下游量级相关,而个体数的空间变化主要与水宽相关。因此,阊江和青弋江1-3级浅水溪流中鱼类群落结构的空间分布格局是溪流大小和空间位置的共同作用结果,且溪流大小和空间位置分别决定鱼类的个体数与物种数的空间分布。       

The stream as a habitat templet for bryophytes: species' distributions along gradients in disturbance and substratum heterogeneity

1. The structure of bryophyte communities in streams in relation to habitat characteristics, especially disturbance, is described. Disturbance in rivers is quantified as movement of the stream bed, whereas in small streams water level fluctuation is used as an indicator of disturbance frequency. 2. Canonical correspondence analysis differentiated frequently disturbed sites from more stable ones. The existence of a disturbance gradient was confirmed in a subset of the study sites using long-term records of discharge variation. A parallel change was detected in the species composition of bryophyte communities with low-statured, potentially fast colonizers dominating the disturbed end and large perennial species the stable end of the gradient. 3. A consistent pattern of zonation of bryophyte species was found along the gradient from continually submersed to persistently dry conditions in small streams and lake outlets. An abrupt increase in species richness occurred at or just above the water line, where facultatively aquatic species tolerant of both conditions formed the bulk of the community. 4. The relationship between species richness and standing crop in stream bryophyte communities was consistent with the hump-backed model of Grime (1979), especially at the within-habitat scale. Quadrats of low and very high standing crop were characterized by low species richness, while peak richness was observed at intermediate standing crops. 5. A few perennial species (e.g. Fontinalis spp. and Rhynchostegium riparioides) capable of monopolizing space dominated the most stable habitats. Species composition in low biomass sites was more variable, yet only one basic growth-form (small-statured species with high allocation to spore production) seemed possible in these highly disturbed streams. In habitats of intermediate biomass, small-scale disturbances apparently allow the formation of a more varied bryophyte community. 6. A habitat templet for stream bryophyte life strategies and community structure is presented. Disturbance is proposed to be the factor filtering out traits unsuitable for a given environment. The potential of stream bryophytes for testing and developing general ecological theory is emphasized.     

Effects of coalbed natural gas development on fish assemblages in tributary streams of the Powder and Tongue rivers

1. Extraction of coalbed natural gas (CBNG) often results in disposal of large quantities of CBNG product water, which may affect aquatic ecosystems. We evaluated the effects of CBNG development on fish assemblages in tributary streams of the Powder and Tongue rivers. We used treatment and control, impact versus reference sites comparisons, surveys of CBNG product‐water streams and in situ fish survival approaches to determine if CBNG development affected fish assemblages. 2. Several of our results suggested that CBNG development did not affect fish assemblages. Species richness and index of biotic integrity (IBI) scores were similar in streams with and streams without CBNG development, and overall biotic integrity was not related to the number or density of CBNG wells. Fish occurred in one stream that was composed largely or entirely of CBNG product water. Sentinel fish survived in cages at treatment sites where no or few fish were captured, suggesting that factors such as lack of stream connectivity rather than water quality limited fish abundance at these sites. Fish species richness did not differ significantly from 1994 to 2006 in comparisons of CBNG‐developed and undeveloped streams. Biotic integrity declined from 1994 to 2006; however, declines occurred at both impact and reference sites, possibly because of long‐term drought. 3. Some evidence suggested that CBNG development negatively affected fish assemblages, or may do so over time. Specific conductivity was on average higher in treatment streams and was negatively related to biotic integrity. Four IBI species richness metrics were negatively correlated with the number or density of CBNG wells in the catchment above sampling sites. Bicarbonate, one of the primary ions in product water, was significantly higher in developed streams and may have limited abundance of longnose dace (Rhinichthys cataractae). Total dissolved solids, alkalinity, magnesium and sulphate were significantly higher in developed streams. 4. Biological monitoring conducted before the development of CBNG, and continuing through the life of development and reclamation, together with data on the quantity, quality and fate of CBNG product water will allow robust assessment of potential effects of future CBNG development worldwide.     

Patterns in diversity of anurans along an elevational gradient in the Western Ghats, South India

Aim To examine patterns in anuran species richness along an elevation gradient and identify factors that govern anuran species richness on a tropical elevational gradient. Location Sampling for anurans was carried out in Kalakad Mundanthurai Tiger Reserve (KMTR) in the southern Western Ghats, India. Methods Night‐time sampling for anuran species richness was carried out from 20 November 2004 to 20 April 2005, during the north‐east monsoon and dry seasons, using transects (50 × 2 m) and visual encounter surveys along the streams. The entire gradient was classified into thirteen 100‐m elevation zones. Sampling at the alpha (single drainage basin) level was carried out in the Chinnapul River drainage basin (40–1260 m a.s.l.) and at the gamma (landscape) level in four drainage basins. Additionally, published records were used to arrive at an empirical species richness (S) for the entire landscape. Mid‐Domain Null software was used to test for the possible influence of geometric constraints on anuran species at both the alpha and gamma levels. The influence of area under each elevation zone on empirical S was tested. The pattern in anuran species richness along the elevational gradient was investigated using: (1) species boundaries in each elevation zone and their habitat correlates, (2) abiotic factors as predictor variables, (3) mean snout vent lengths of anurans, and (4) correlation between the matrices of distance in the elevation zones based on microhabitat parameters and species composition. Cluster analysis on species presence–absence in the elevation zones was used to categorize the entire gradient into high, middle and low elevations. In these three elevation categories, pattern in composition of species was examined for endemism in Western Ghats–Sri Lanka biodiversity hotspot, uniqueness to an elevation zone, adaptations of adults and modes of breeding. Results Species richness at the alpha level increased linearly with elevation, while at the gamma level there were three peaks. Maximum species richness was observed at the highest elevation (1200 m) at both the alpha and the gamma levels. The observed patterns differed significantly from mid‐domain null predictions. The multi‐modal pattern in species richness was a consequence of overlapping species range boundaries. Soil temperature was the best single measure in explaining the majority of variation in species richness at the alpha level (r² = 0.846, P < 0.01). However, soil moisture was the best predictor when both the alpha and the gamma sites were pooled (r² = 0.774, P < 0.01). Anuran body size decreased with an increase in elevation. The highest proportions of endemic and unique species were found at high elevations (> 700 m). The proportion of arboreal anurans increased from low to high elevation. Anurans exhibiting direct development were predominantly found at high elevations. Main conclusions Geometric constraints did not influence anuran species richness along the elevational gradient. Overlapping range boundaries influenced species richness at the gamma level. Abiotic factors such as soil temperature and moisture influenced anuran species richness in the mountain range. The ‘Massenerhebung effect’ could be responsible for range restriction and endemism of anurans, differences in guilds and mode of reproduction. These findings highlight the importance of cloud forests for endemic anurans.     

Altitudinal gradients in stream fish diversity and the prevalence of diadromy in the Sixaola River basin,Costa Rica

Landscape-scale patterns of freshwater fish diversity and assemblage structure remain poorly documented in many areas of Central America, while aquatic ecosystems throughout the region have been impacted by habitat degradation and hydrologic alterations. Diadromous fishes may be especially vulnerable to these changes, but there is currently very little information available regarding their distribution and abundance in Central American river systems. We sampled small streams at 20 sites in the Sixaola River basin in southeastern Costa Rica to examine altitudinal variation in the diversity and species composition of stream fish assemblages, with a particular focus on diadromous species. A set of environmental variables was also measured in the study sites to evaluate how changes in fish assemblage structure were related to gradients in stream habitat. Overall, fish diversity and abundance declined steeply with increasing elevation, with very limited species turnover. The contribution of diadromous fishes to local species richness and abundance increased significantly with elevation, and diadromous species dominated assemblages at the highest elevation sites. Ordination of the sampling sites based on fish species composition generally arranged sites by elevation, but also showed some clustering based on geographic proximity. The dominant gradient in fish community structure was strongly correlated with an altitudinal habitat gradient identified through ordination of the environmental variables. The variation we observed in stream fish assemblages over relatively small spatial scales has significant conservation implications and highlights the ecological importance of longitudinal connectivity in Central American river systems.     

Spatial variation in composition and richness of fish communities in a southwestern Australian river system

Thirteen species of fish were recorded from the non-tidal reaches of the Murray River system, southwestern Australia. Of these, nine were indigenous species. Although this system occurs in a zone of moderate to high rainfall, the species richness of this system is comparable to that of much harsher environments, such as the Canadian boreal zone or the Nevada desert. Species richness tends to increase in a downstream direction, and most changes in species composition are due to addition, rather than replacement of species. Stream order was strongly and significantly positively correlated with species richness (P < 0.001), but was in effect functioning as a composite variable. Stepwise multiple regression analysis showed that stream width, pH range, distance from the main stream, distance from the estuary, barriers and stream gradient together explained more than 80% of the variation in species richness. The variables, barriers, distance from the main stream and distance from the estuary, support island biogeography type explanations for variations in species richness, while the variable, stream width complies with the river continuum concept. A predictive model for species richness of any given reach of a lower west coastal stream in Australia was tried using variables that can be gathered almost entirely from detailed topographic maps and aerial photographs.     

Modelling Stream-Fish Functional Traits in Reference Conditions: Regional and Local Environmental Correlates

Identifying the environmental gradients that control the functional structure of biological assemblages in reference conditions is fundamental to help river management and predict the consequences of anthropogenic stressors. Fish metrics (density of ecological guilds, and species richness) from 117 least disturbed stream reaches in several western Iberia river basins were modelled with generalized linear models in order to investigate the importance of regional- and local-scale abiotic gradients to variation in functional structure of fish assemblages. Functional patterns were primarily associated with regional features, such as catchment elevation and slope, rainfall, and drainage area. Spatial variations of fish guilds were thus associated with broad geographic gradients, showing (1) pronounced latitudinal patterns, affected mainly by climatic factors and topography, or (2) at the basin level, strong upstream-downstream patterns related to stream position in the longitudinal gradient. Maximum native species richness was observed in midsize streams in accordance with the river continuum concept. The findings of our study emphasized the need to use a multi-scale approach in order to fully assess the factors that govern the functional organization of biotic assemblages in ‘natural’ streams, as well as to improve biomonitoring and restoration of fluvial ecosystems.     

Species richness and assemblage structure of Trichoptera in Danish streams

• 1 Species richness and assemblages of Trichoptera from 157 ‘pristine’ Danish lowland stream sites were analyzed, for dependence on geographical position of the sites and simple physical variables, using two way indicator species analysis and detrended correspondence analysis.
• 2 A total of 106 species were recorded, representing ≈ 90% of the species pool known from Danish streams. Only seven species occurred at more than half the sites, whereas an additional 11 species were found at more than a quarter of the sites.
• 3 Although sites showed significant regional differences in environmental variables (stream order, width, slope and presence/absence of riparian forest), species richness and assemblages were primarily correlated with stream order, width and slope. Maximum richness was found at the largest (5th order) stream sites.
• 4 Regional differences in species assemblages were found, with several species being absent from the islands Funen and Bornholm. Species assemblages also differed between forested and non‐forested stream sites.
• 5 We found evidence that stream temperature may be of only minor importance in determining Trichoptera species richness and assemblage composition in Danish streams compared to other size‐related physical factors.

     

Fish assemblages across a complex,tropical freshwater/marine ecotone

Synopsis Riverine fish assemblages in the temperate zone generally show strong longitudinal patterns of faunal turnover and increases in species richness with increasing stream order. We examined the composition and structure of tropical fish assemblages across a complex freshwater/marine ecotone in Tortuguero National Park on the Caribbean coast of Central America. Species turnover was high between four characteristic habitats that largely corresponded with a longitudinal gradient of stream order over distances of less than 30 km. Suites of common fish species characterized each habitat: creeks, rivers, lagoons, and the sea. In addition to the habitat endemics, several species spanned two habitat types, but only three species were collected in more than two habitats. Multivariate gradient analysis of fish assemblages reflected a gradient of habitats that to some extent corresponded to fluvial distances. Due to the unusual configuration of coastal lagoons lying parallel to the coast, the ordination gradient showed little correlation with linear distance to the coast. Environmental variables related to habitat size and salinity showed greatest correspondence with the fish assemblage ordination gradient. Invertebrate-feeding fishes were the predominant trophic group in 15 of 16 fish assemblages, and inland creek sites contained a greater proportion of herbivores and omnivores than other sites. The relative fraction of herbivorous and detritivorous fishes showed a monotonic decline along the longitudinal habitat gradient from inland to coast. Patterns of species composition and richness at Tortuguero Park appeared to agree well with earlier models of factors influencing temperate zone stream fishes. Headwaters have low aquatic primary productivity and contain small colonizing fish species subject to large fluctuations in local densities and intermittent competition. Lagoons contain both large and small species, the latter being restricted largely to shallow edge habitats by predation. Lagoons exhibit more lentic environmental conditions, experience relatively fewer periodic disturbances than headwaters, and their assemblages are inferred to be under relatively greater influence of biotic factors. Fish assemblages of rivers and caños (swampy side channels and braids) appear to be under less abiotic control than headwaters and influenced less by biotic factors than lagoons.     

Population Genetic Structure of Astyanax scabripinnis (Teleostei, Characidae) from an Urban Stream

Despite the great anthropogenic interference on urban streams, information is still scarce about the genetic variability and structure of native fish populations inhabiting such streams. In the present study, random amplified polymorphic DNA (RAPD) markers were used to analyze genetic variability and structure of populations assigned to the Neotropical fish species Astyanax scabripinnis from an urban stream located in Londrina, Paraná State, southern Brazil. Thirty individuals of this species were collected from three sites throughout the upper Cambé stream. A total of 10 primers amplified 159 loci, of which 128 (80.5%) were polymorphic. Each of the three populations showed very similar proportions of polymorphic loci, which ranged from 63.5 to 64.8%. Unbiased genetic distances varied from 0.0612 to 0.0646. Theta_p-test values indicated moderate to high genetic differentiation among individuals from different localities. The number of migrants varied from 1.34 to 1.46, suggesting a low gene flow between populations. The genetic similarity among all individuals studied ranged from 0.424 to 0.848. The results suggest that populations of A. scabripinnis in Cambé stream are undergoing genetic differentiation.     

Diatom Assemblages and their Associations with Environmental Variables in Oregon Coast Range Streams, USA

The Oregon Coast Range, rich in natural resources, is under increasing pressure from rapid development. The purpose of this study was to examine diatom species patterns in relation to environmental variables in streams of this region. Diatoms, water quality, physical habitat and watershed characteristics were assessed for 33 randomly selected stream sites. Watershed size, elevation, geology, vegetation and stream morphology varied substantially among sites. Streams were characterized by dilute water chemistry and a low percent of fine substrate. A total of 80 diatom taxa were identified. Taxa richness was low throughout the region (median 15, range 10–26). Assemblages were dominated by two adnate species, Achnanthidium minutissimum and Achnanthes pyrenaicum. Diatoms sensitive to organic pollution dominated the assemblages at all sites (median 85%). Non-metric multidimensional scaling (NMDS) and correlational analysis showed quantitative relationships between diatom assemblages and environmental variables. NMDS axes were significantly correlated with watershed area, watershed geology, conductivity, total nitrogen, total solids and stream width. Diatom-based site classification (Two-way Indicators Species Analysis, (TWINSPAN)) yielded 4 discrete groups that displayed weak correlations with environmental variables. When stream sites were classified by dominant watershed geology, overall diatom assemblages between groups were significantly different (Analysis of Similarity (ANOSIM) global R = 0.19, p < 0.05). Our results suggest that streams in the coastal region are in relatively good condition. High natural variability in stream conditions in the Oregon Coast Range ecoregion may obscure quantitative relationships between environmental variables and diatom assemblages. A bioassessment protocol that classifies sites by major landscape variables and selects streams along the major human disturbance gradient might allow for detection of early signs of human disturbance in environmentally heterogeneous regions, such as the Pacific Northwest.     

Impact of nutrient enrichment and alkalinization on periphyton communities in the New Jersey Pine Barrens

The impact of residential and agricultural development on stream periphyton communities in the New Jersey Pine Barrens was examined by comparison with communities in undeveloped areas. Watershed disturbance resulted in stream water primarily characterized by greatly elevated pH levels and nitrate concentrations. A total of 53 periphyton species were encountered in bimonthly samples over a one year period in the three disturbed and three undisturbed study streams. Species richness was significantly greater in the disturbed streams based on three criteria: the average number of species per stream on each sampling occasion (disturbed = 6.3; undisturbed = 4.9), the average number of species per stream for the entire year (disturbed = 19.3; undisturbed = 16.0), and the total number of species found in streams within a type (disturbed = 40; undisturbed = 31). Species composition also changed significantly as the result of disturbance. There appeared to be replacement of species characteristic of undisturbed Pine Barrens streams with species peripheral to the region. The expected effects of both elevated pH and nitrate were consistent with these results.     

Variation in freshwater fish assemblages along a regional elevation gradient in the northern Andes,Colombia

Studies on elevation diversity gradients have covered a large number of taxa and regions throughout the world; however, studies of freshwater fish are scarce and restricted to examining their changes along a specific gradient. These studies have reported a monotonic decrease in species richness with increasing elevation, but ignore the high taxonomic differentiation of each headwater assemblage that may generate high β‐diversity among them. Here, we analyzed how fish assemblages vary with elevation among regional elevation bands, and how these changes are related to four environmental clines and to changes in the distribution, habitat use, and the morphology of fish species. Using a standardized field sampling technique, we assessed three different diversity and two structural assemblage measures across six regional elevation bands located in the northern Andes (Colombia). Each species was assigned to a functional group based on its body shape, habitat use, morphological, and/or behavioral adaptations. Additionally, at each sampling site, we measured four environmental variables. Our analyses showed: (1) After a monotonic decrease in species richness, we detected an increase in richness in the upper part of the gradient; (2) diversity patterns vary depending on the diversity measure used; (3) diversity patterns can be attributed to changes in species distribution and in the richness and proportions of functional groups along the regional elevation gradient; and (4) diversity patterns and changes in functional groups are highly correlated with variations in environmental variables, which also vary with elevation. These results suggest a novel pattern of variation in species richness with elevation: Species richness increases at the headwaters of the northern Andes owing to the cumulative number of endemic species there. This highlights the need for large‐scale studies and has important implications for the aquatic conservation of the region.     

Plant species richness and diversity along an altitudinal gradient in the Sierra Nevada, Mexico

This article presents an analysis of plant species richness and diversity and its association with climatic and soil variables along a 1300‐m elevation gradient on the Cerro Tláloc Mountain in the northern Sierra Nevada in Mexico. Two 1000‐m² tree sampling plots were created at each of 21 selected sampling sites, as well as two 250‐m² plots for shrubs and six 9‐m² plots for herbaceous plants. Species richness and diversity were estimated for each plant life form, and beta diversity between sites was estimated along the gradient. The relationship between species richness and diversity and environmental variables was modelled using simple linear correlation and regression trees. Species richness and diversity showed a unimodal pattern with a bias towards high values in the lower half of the elevation gradient under study. This response was consistent for all three life forms. Beta diversity increased steadily along the elevation gradient, being lower between contiguous sites at intermediate elevations and high – the species replacement rate was nearly 100%– between sites at the extremes of the gradient. Few species were adapted to the full spectrum of environmental variation along the elevation gradient studied. The regression tree suggests that differences in species richness are mainly influenced by elevation (temperature and humidity) and soil variables, namely A₂ permanent wilting point, organic matter and horizon field capacity and A₁ horizon Mg²⁺.     

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.