Riparian Plant Diversity Research Articles

Passive Reestablishment of Riparian Vegetation Following Removal of Invasive Knotweed (Polygonum)

AbstractJapanese knotweed and congeners are invasive to North America and Europe and spread aggressively along rivers establishing dense monotypic stands, thereby reducing native riparian plant diversity, structure, and function. Noxious weed control programs attempt to eradicate the knotweed with repeated herbicide applications under the assumption that the system will recover to a native assemblage which will inhibit future invasions. However, eradication efficacy studies typically only measure the amount of knotweed reduced, not the reestablished species diversity or plant origins. For a community scale efficacy study, we measured vascular plant species diversity and cover in riparian areas along five rivers in Washington State, 3 to 6 years after Bohemian knotweed was initially treated with herbicide. Plant species composition was compared between riparian sites treated to remove knotweed and reference sites where knotweed was absent. Sites where knotweed had been removed had significantly more exotic species and vegetative cover than reference sites; however, native species richness and cover were greater in reference sites and areas with more overstory vegetation. The native plants observed were primarily shade tolerant and perennial, as opposed to many of the exotics, which were shade-intolerant annuals. In general, reestablishment of native and exotic vegetation was not related to pretreatment knotweed stem count, size of the invaded area, or timing of herbicide application. However, residual native tree cover was negatively correlated with initial knotweed stem count. Monitoring the success of restoration objectives (e.g., native plant reestablishment or increased species diversity) and characterizing associated habitat features following knotweed eradication will help in the development of site-specific protocols for successful plant community scale restoration.

Vertical distance from drainage drives floristic composition changes in an Amazonian rainforest

Background: Plant composition changes with topography and edaphic gradients that correlate with soil-water and nutrient availability. Data on soil water for the Amazon Basin are scarce, limiting the possibility of distinguishing between soil and soil-water influences on plant composition. Aim: We tested a new proxy for water table depth, the terrain height above nearest drainage (HAND), as a predictor of composition in trees, lianas, palms, shrubs, and herbs and compared HAND to conventional measures of height above sea level (HASL) and horizontal distances from nearest drainage (HDND). Methods: Plant-species composition in 72 plots distributed across 64 km2 of lowland evergreen terra firme forest was summarised using non-metric multidimensional scaling (NMDS). NMDS scores were regressed against estimates of HAND, HASL and HDND. Results: Plant composition was highly correlated with the vertical distance from water table, capturing up to 82% of variation. All life forms showed highest turnover rates in the zone with seasonally water-saturated soils, which can extend 350 m from stream margins. Conclusions: Floristic composition is closely related to water table depth, and HAND appears to be the most robust available topographical metric of soil-water gradients. Brazilian conservation laws protecting 30-m-wide riparian buffers are likely to be too narrow to encompass the full zone of highest floristic turnover and may be ineffective in safeguarding riparian plant diversity.

Higher temperature reduces the effects of litter quality on decomposition by aquatic fungi

Summary1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss.2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms.3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant.4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species.5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves.6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams.7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams.

Factors affecting mammalian carnivore use of riparian ecosystems in Mediterranean climates

Riparian ecosystems serve as movement and dispersal corridors; however, the factors that determine their use by multiple species of carnivores remain unknown. Two hypotheses can explain carnivore presence in riparian ecosystems. Higher riparian plant richness, diversity, and structure provide the resources used by carnivores (resting sites, cover, and food). Alternatively, areas with higher water availability allow species to withstand water loss and thermal gradients in the high summer temperatures. In southern Portugal we surveyed 70 transects 2 km long along riparian ecosystems during the wet winter months and again in the dry summer months, recording signs of carnivore species and the environmental context in which they occurred (vegetation descriptors, surrounding landscape, and waterway type and condition). Five carnivore species used riparian ecosystems (stone marten [Martes foina], Eurasian badger [Meles meles], common genet [Genetta genetta], Egyptian mongoose [Herpestes ichneumon], and red fox [Vulpes vulpes]). Riparian ecosystems had a diverse and heterogeneous plant community with a mix of obligate riparian, Mediterranean sclerophyllous, and exotic species. Most carnivore species responded to water channel type and standing water availability in both seasons, except for the stone marten in the wet season, which uses areas with rich riparian vegetation. Our results suggest that the use of riparian ecosystems is linked to water availability that provides water, prey, and external temperature regulation.

Multiscale Patterns of Riparian Plant Diversity and Implications for Restoration

Planning riparian restoration to resemble historic reference conditions requires an understanding of both local and regional patterns of plant species diversity. Thus, understanding species distributions at multiple spatial scales is essential to improve restoration planting success, to enhance long‐term ecosystem functioning, and to match restoration planting designs with historic biogeographic distributions. To inform restoration planning, we examined the biogeographic patterns of riparian plant diversity at local and regional scales within a major western U.S.A. drainage, California's Sacramento—San Joaquin Valley. We analyzed patterns of species richness and complementarity (β‐diversity) across two scales: the watershed scale and the floodplain scale. At the watershed scale, spatial patterns of native riparian richness were driven by herbaceous species, whereas woody species were largely cosmopolitan across the nearly 38,000 km2 study area. At the floodplain scale, riparian floras reflected species richness and dissimilarity patterns related to hydrological and disturbance‐driven successional sequences. These findings reinforce the importance of concurrently evaluating both local and regional processes that promote species diversity and distribution of native riparian flora. Furthermore, as restoration activities become more prevalent across the landscape, strategies for restoration outcomes should emulate the patterns of species diversity and biogeographic distributions found at regional scales.

Impacts of Salmon on Riparian Plant Diversity

The study of natural gradients in nutrient subsidies between ecosystems allows for predictions of how changes in one system can affect biodiversity in another. We performed a large-scale empirical test of the role of Pacific salmon (Oncorhynchus spp.) in structuring riparian plant communities. A comparison of 50 watersheds in the remote Great Bear Rainforest of British Columbia's central coast in Canada shows that salmon influence nutrient loading to plants,shifting plant communities toward nutrient-rich species, which in turn decreases plant diversity.These effects are mediated by interactions between salmon density and the physical characteristics of watersheds. Predicting how salmon affect terrestrial ecosystems is central to conservation plans that aim to better integrate ecosystem values into resource management.

Ecosystem engineers maintain a rare species of butterfly and increase plant diversity

We evaluated whether ecosystem engineers can accomplish two conservation goals simultaneously: (1) indirectly maintain populations of an endangered animal through habitat modification and (2) increase riparian plant diversity. We tested for effects of a prominent ecosystem engineer, the beaver Castor canadensis, on populations of St. Francis' satyr butterfly Neonympha mitchellii francisci and plant species richness and composition. We performed our test by surveying riparian vegetation communities in all stages of beaver-influenced wetland succession. We found that beavers created wetland habitats that supported plant species not found elsewhere in riparian zones and increased plant species diversity across the landscape by creating a novel combination of patch types. Our results confirmed what others have found about engineering effects on plant diversity, but these results further demonstrated a case where ecosystem engineers indirectly maintain populations of rare animals by modifying the composition and diversity of plant communities within wetlands. Our research demonstrates how an ecosystem engineer can influence habitat availability and composition of plant communities important for an endangered insect, and maintain overall plant species diversity by increasing habitat heterogeneity.

Fluvially-deposited large wood and riparian plant diversity

Out-of-channel wood jams centred by fluvially-deposited large wood represent an important habitat for riparian plant species along large rivers dominated by exposed riverine sediments. Such wood jams often display distinct physical features associated with the jams, such as areas of scour immediately upstream, plumes of fine sediment downstream and abundant organic material deposited throughout the jam. This paper examines the relationship between physical characteristics of young wood jams (1–3 years since deposition) and riparian plant diversity along the braided River Tagliamento in Northeast Italy. Species richness of riparian plants was significantly positively correlated to the depth of scour features and amount of fine sediment around the wood jams, which facilitate improved access to the water table and increase available water in the sediments, respectively. Species diversity was significantly positively correlated to scour pool depth only. Scour depth was in turn influenced by wood jam size, indicating that larger jams may indirectly increase riparian plant diversity. Management and restoration of riparian zones containing exposed riverine sediments should allow for the formation of wood jams and their associated features where possible, in order to potentially increase localised biodiversity.

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.

MULTISCALE CONTROLS ON WOODY PLANT DIVERSITY IN WESTERN OREGON RIPARIAN FORESTS

Riparian forests are known to be floristically diverse and influenced by multiscale phenomena, yet few studies have explicitly compared how these factors contribute to various aspects of riparian plant diversity. We analyzed woody riparian plant species and environmental data from four western Oregon watersheds distributed across a wide climate‐driven productivity gradient at three scales (40‐m2 sample plots [alpha diversity], 1‐ha plots, and species pools from 16 1‐ha plots in each watershed) to compare three hypotheses of control on species diversity: (1) local control, (2) direct climatic control, and (3) indirect climatic control. We used a process model (3‐PG) to model gross primary productivity (GPP) as a functional climate index across our study area. We performed multiple linear regression to determine the best predictors of alpha (sample‐plot scale) diversity, compositional change within riparian forests (beta diversity), and hectare‐scale diversity and used path analysis to explore hypothesized causal linkages between climate and other factors and species diversity. We also analyzed a companion data set of gap and forest environments from a subset of the same sites to determine the influence of disturbance on species diversity across the gradient.We found evidence for strong spatial patterning in woody plant richness consistent with indirect climatic control on woody plant richness. Climate (GPP) showed negative relationships with alpha diversity and hectare richness of trees, shrubs, and woody plant species and was the most commonly selected explanatory variable in regression analyses. GPP and Rubus spectabilis cover increased from the least to most productive climates while understory light and moisture heterogeneity across the riparian area decreased. These environmental changes coincided with declines in alpha, beta, and hectare‐scale diversity. Disturbance gaps yielded higher richness at most sites, but differences in species richness between gap and forest sample plots did not increase at high levels of GPP, as hypothesized.This study points toward an integrated conceptual model whereby regional and landscape scale controls such as climate and watershed position complement and interact with local controls (i.e., vegetation structure, environmental gradients) to jointly govern woody plant diversity in riparian forests.

DECOMPOSITION OF DIVERSE LITTER MIXTURES IN STREAMS

In view of growing interest in understanding how biodiversity affects ecosystem functioning, we investigated effects of riparian plant diversity on litter decomposition in forest streams. Leaf litter from 10 deciduous tree species was collected during natural leaf fall at two locations (Massif Central in France and Carpathians in Romania) and exposed in the field in litter bags. There were 35 species combinations, with species richness ranging 1-10. Nonadditive effects on the decomposition of mixed-species litter were minor, although a small synergistic effect was observed in the Massif Central stream where observed litter mass remaining was significantly lower overall than expected from data on single-species litter. In addition, variability in litter mass remaining decreased with litter diversity at both locations. Mean nitrogen concentration of single- and mixed-species litters (0.68-4.47% of litter ash-free dry mass) accounted for a large part of the variation in litter mass loss across species combinations. For a given species or mixture, litter mass loss was also consistently faster in the Massif Central than in the Carpathians, and the similarity in general stream characteristics, other than temperature, suggests that this effect was largely due to differences in thermal regimes. These results support the notion that decomposition of litter mixtures is primarily driven by litter quality and environmental factors, rather than by species richness per se. However, the observed consistent decrease in variability of decomposition rate with increasing plant species richness indicates that conservation of riparian tree diversity is important even when decomposition rates are not greatly influenced by litter mixing.

Connecting variation in vegetation and stream flow: the role of geomorphic context in vegetation response to large floods along boreal rivers

Summary Flooding governs riparian plant diversity along boreal rivers but the ecological role of extreme floods is only partly understood. We studied the dynamics of riparian plant composition and richness in the free‐flowing Vindel River in northern Sweden, and the importance of reach type in sustaining high species richness. We conducted three surveys of riparian plant species richness over a period of two decades. The first and last of these surveys were conducted 1–3 years after significant flooding and the second was carried out after a period of more moderate flooding. Our results suggest that extreme floods reduce riparian plant species richness in tranquil (slow‐flowing) reaches but that a subsequent period of less extreme flood events facilitates recovery. Tranquil river reaches were also more prone to invasion by ruderal species following major floods. Species richness in turbulent reaches (rapids and runs) remained constant during all surveys. One possible explanation for this pattern is that tranquil reaches become more anoxic during floods because they have more fine‐grade soils with lower hydraulic conductivity than turbulent reaches. Anoxic conditions may cause stress and plant death, opening up space for colonization. Turbulent reaches maintain a better oxygenation in the root zone of plants through high groundwater turnover, reducing negative effects of prolonged floods. The fact that turbulent reaches preserved species richness regardless of flood magnitude suggests that they are important for the resistance of riparian ecosystems to prolonged inundation. In contrast, tranquil reaches, with a higher water‐holding capacity, might instead maintain their species richness during drought periods. Synthesis and applications. Our findings highlight the importance of spatial and temporal variation in riverine plant species richness and composition. To conserve these habitats at a landscape scale, a full range of reach types is necessary to allow for recovery in reaches where species richness has declined. To maintain healthy riparian zones, river managers should focus restoration efforts on interactions between hydrology, geomorphology and biota.

Riverscape-level patterns of riparian plant diversity along a successional gradient, Willamette river, Oregon

Riparian forest communities dominated by Populus balsamifera ssp. trichocarpa L. (Torr. and Gray ex Hook.) Brayshaw are important contributors to biodiversity in terrestrial and aquatic ecosystems of the Western United States. Species composition along a successional gradient from stand initiation to late-succession of P. balsamifera-dominated riparian forests was investigated along 145 km of the Willamette River, Oregon. There were 151 total species encountered across 28 stands and a mean species richness of 33.3 species per stand. Young stands were dominated by P. balsamifera and Salix tree spp. and opportunistic herbaceous species. Understory trees, shrubs, and herbaceous species as well as late-successional tree species established 12–15 years after stand initiation. Fraxinus latifolia Benth. was the dominant late-successional tree species. Vertical structural diversity, P. balsamifera mean diameter at breast height, large tree biomass, and stand age were strongly correlated with understory species presence and abundance based on non-metric multidimensional scaling (NMS) ordination. There were no young stands on mid and high terraces and this was reflected in geomorphic position being strongly correlated with the stand age gradient. Abundance of Phalaris arundinacea L. an invasive grass species, was also significantly correlated with plant species composition and abundance. This study indicates that Willamette River riparian forests are diverse and therefore important to the biodiversity of the Willamette River valley and that their presence as a mosaic of communities of different successional stages may be threatened by human interventions, including influences exerted by introduced plant species.

Effects of Leaf-Litter Accumulation on Riparian Plant Diversity across Europe

Effects of Leaf-Litter Accumulation on Riparian Plant Diversity across Europe