Composition of riparian litter input regulates organic matter decomposition: Implications for headwater stream functioning in a managed forest landscape

Although the importance of stream condition for leaf litter decomposition has been extensively studied, little is known about how processing rates change in response to altered riparian vegetation community composition. We investigated patterns of plant litter input and decomposition across 20 boreal headwater streams that varied in proportions of riparian deciduous and coniferous trees. We measured a suite of in‐stream physical and chemical characteristics, as well as the amount and type of litter inputs from riparian vegetation, and related these to decomposition rates of native (alder, birch, and spruce) and introduced (lodgepole pine) litter species incubated in coarse‐ and fine‐mesh bags. Total litter inputs ranged more than fivefold among sites and increased with the proportion of deciduous vegetation in the riparian zone. In line with differences in initial litter quality, mean decomposition rate was highest for alder, followed by birch, spruce, and lodgepole pine (12, 55, and 68% lower rates, respectively). Further, these rates were greater in coarse‐mesh bags that allow colonization by macroinvertebrates. Variance in decomposition rate among sites for different species was best explained by different sets of environmental conditions, but litter‐input composition (i.e., quality) was overall highly important. On average, native litter decomposed faster in sites with higher‐quality litter input and (with the exception of spruce) higher concentrations of dissolved nutrients and open canopies. By contrast, lodgepole pine decomposed more rapidly in sites receiving lower‐quality litter inputs. Birch litter decomposition rate in coarse‐mesh bags was best predicted by the same environmental variables as in fine‐mesh bags, with additional positive influences of macroinvertebrate species richness. Hence, to facilitate energy turnover in boreal headwaters, forest management with focus on conifer production should aim at increasing the presence of native deciduous trees along streams, as they promote conditions that favor higher decomposition rates of terrestrial plant litter.     

The Effect of Nutrient Enrichment of Either the Bank or the Surface Water on Shoreline Vegetation and Decomposition

Riparian ecosystems can harbor great diversity and provide important ecological functions such as improving water quality. The impact of eutrophication on riparian ecosystems, however, is unclear. We conducted a mesocosm experiment to study the effects of nutrient loading on riparian ecosystems. We specifically asked whether the source of nutrients in the riparian zone affects the complex interactions that occur between surface water and adjacent wetlands. We also studied litter decomposition in the wetland component of the mesocosms, because litter accumulation in fens is assumed to control succession toward floating mats. Each mesocosm consisted of an upland component, referred to as the bank, and a water compartment. The bank and water compartments were planted with typical riparian zone and open water fen species prior to the addition of nitrogen (N) and phosphorus (P) in different combinations to either the bank or the surface water. Nutrient addition (mainly P) resulted in increased plant production and higher expansion rates of plants on the bank and in the water. There were also clear interactions in plant responses between the bank and water. Only eutrophic species increased shoot densities after fertilization. Nutrient addition further resulted in higher litter production, especially on the banks, and stimulated decomposition. Both the plant responses and the litter experiment indicated that eutrophication would accelerate succession to floating mats. Such floating fen mats are not likely to have the typical species-rich combination of desirable species; however, as our results suggest that they would be dominated by a few eutrophic species.     

Pattern of tree species diversity in riparian forest fragments of different widths (SE Brazil)

Width is an essential element of the spatial configuration of riparian forests and may be fundamental in determining their corridor function. In the present study we tested the effect of forest width on floristic structure (tree species composition and diversity) in 15 fragments of riparian forest in an agricultural fragmented landscape of SE Brazil. All these fragments were chosen in a geomorphological homogeneous river reach under similar soil, topographic and human disturbance conditions in order to minimize the influence of these factors. The forest widths considered ranged from 30 to 650 m. The results showed that total species richness and climax species richness were significantly greater when we consider larger fragments, as has been observed in other studies. Nevertheless, species diversity and evenness were not significantly correlated with forest width. The analysis of species composition showed that the narrowest fragments were characterized by species well adapted to temporary flood conditions, while medium and wide fragments showed a composition typical of drier upland areas. Therefore, the effect of forest width on floristic structure appears to be more strongly linked to the effect of river floods in the case of the fragments studied. The existence in riparian corridors of a drier forest, in general richer and more diversified than the annually flooded forest, seems to favor the maintenance of regional species diversity in fragmented landscapes.     

Warming water and leaf litter quality but not plant origin drive decomposition and fungal diversity in an experiment

The fungi associated with leaf litter play a key role in decomposition and can be affected both by the warming water and the invasion of non-native species in riparian vegetation. Warming water and invasion of non-native riparian species on stream fungal communities have been studied mainly in temperate ecosystems. We tested the effects of warming water and non-native plant Psidium guajava on leaf litter decomposition, conidia density, species richness and beta diversity of tropical stream fungi. Thus, we carried out an experiment using the current mean temperature of streams from northwestern Paraná in South Brazil (22 °C) and two temperatures above the current mean temperature (26 °C and 29 °C). We also used the leaves of a non-native plant (P. guajava), and two native plants (one of similar nutritional quality, and the other of higher nutritional quality than the non-native species) occurring in Neotropical streams riparian vegetation. Warming water accelerated leaf litter decomposition and reduced conidia density and fungal richness in native and non-native plants. However, species composition and beta diversity were not affected by water temperature. Our study showed that warming affects the fungi of streams, the main microorganisms responsible for decomposition and that the nutritional quality of the leaves may be more important than the origin of riparian plant species. Despite this, further investigations should be conducted on the interaction of P. guajava with the flow of nutrients in these environments and how it can affect other ecosystem processes and the food chain. Efforts to study the effects of water warming and biological invasion on the attributes and distribution of fungi in streams are vital, making them a tool for the conservation of riparian ecosystems.     

Ecosystem response to removal of exotic riparian shrubs and a transition to upland vegetation

Understanding plant community change over time is essential for managing important ecosystems such as riparian areas. This study analyzed historic vegetation using soil seed banks and the effects of riparian shrub removal treatments and channel incision on ecosystem and plant community dynamics in Canyon de Chelly National Monument, Arizona. We focused on how seeds, nutrients, and ground water influence the floristic composition of post-treatment vegetation and addressed three questions: (1) How does pre-treatment soil seed bank composition reflect post-treatment vegetation composition? (2) How does shrub removal affect post-treatment riparian vegetation composition, seed rain inputs, and ground water dynamics? and (3) Is available soil nitrogen increased near dead Russian olive plants following removal and does this influence post-treatment vegetation? We analyzed seed bank composition across the study area, analyzed differences in vegetation, ground water levels, and seed rain between control, cut-stump and whole-plant removal areas, and compared soil nitrogen and vegetation near removed Russian olive to areas lacking Russian olive. The soil seed bank contained more riparian plants, more native and fewer exotic plants than the extant vegetation. Both shrub removal methods decreased exotic plant cover, decreased tamarisk and Russian olive seed inputs, and increased native plant cover after 2 years. Neither method increased ground water levels. Soil near dead Russian olive trees indicated a short-term increase in soil nitrogen following plant removal but did not influence vegetation composition compared to areas without Russian olive. Following tamarisk and Russian olive removal, our study sites were colonized by upland plant species. Many western North American rivers have tamarisk and Russian olive on floodplains abandoned by channel incision, river regulation or both. Our results are widely applicable to sites where drying has occurred and vegetation establishment following shrub removal is likely to be by upland species.     

The role of vegetation and litter in the nitrogen dynamics of riparian buffer zones in Europe

Plant uptake and denitrification are considered to be the most important processes responsible for N retention and mitigation in riparian buffers. In many riparian buffers, however, nutrients taken up by plants remain in the system only temporarily and may be gradually released by mineralization later. Still, plants increase the residence time of nutrients considerably by reducing their mobility. We investigated the importance of plant N uptake and N immobilization in litter for N retention in riparian buffers. Nitrogen uptake in vegetation and N dynamics in litter were measured over a two-year period in a range of forested and herbaceous riparian buffers along a climatic gradient in Europe, receiving different loadings of N-enriched groundwater. Plant production, nitrogen uptake, and N retention were significantly higher in the forested buffer sites compared to the herbaceous buffer sites. However, in herbaceous buffers, periodic harvesting of herbaceous biomass contributed considerably to the N retention. No relationship between lateral N loading and plant productivity or N uptake was observed; this indicated that plant growth was not N-limited. In the winter period, decaying leaf litter had a small but significant role in N retention in a majority of the riparian ecosystems studied. Moreover, no responses to the climatic gradient were found. Generally, we can state that annual N retention in the vegetation and litter compartment is substantial, making up 13–99% of the total N mitigation.     

Spatial and temporal patterns of species richness in a riparian landscape

Aim To test for control of vascular plant species richness in the riparian corridor by exploring three contrasting (although not mutually exclusive) hypotheses: (1) longitudinal patterns in riparian plant species richness are governed by local, river‐related processes independent of the regional species richness, (2) riparian plant species richness is controlled by dispersal along the river (longitudinal control), and (3) the variation in riparian plant species richness mirrors variation in regional richness (lateral control). Location The riparian zones of the free‐flowing Vindel River and its surrounding river valley, northern Sweden. Methods We used data from three surveys, undertaken at 10‐year intervals, of riparian reaches (200‐m stretches of riverbank) spanning the entire river. In addition, we surveyed species richness of vascular plants in the uplands adjacent to the river in 3.75‐km² large plots along the same regional gradient. We explored the relationship between riparian and upland flora, and various environmental variables. We also evaluated temporal variation in downstream patterns of the riparian flora. Results Our results suggest that local species richness in boreal rivers is mainly a result of local, river‐related processes and dispersal along the corridor. The strongest correlation between species richness and the environment was a negative one between species number and soil pH, but pH varied within a narrow range. We did not find evidence for a correlation between species richness on regional and local scales. We found that the local patterns of species richness for naturally occurring vascular plants were temporally variable, probably in response to large‐scale disturbance caused by extreme floods. Most previous studies have found a unimodal pattern of species richness with peaks in the middle reaches of a river. In contrast, on two of three occasions corresponding to major flooding events, we found that the distribution of species richness of naturally occurring vascular plants resembled that of regional diversity: a monotonic decrease from headwater to coast. We also found high floristic similarity between the riparian corridor and the surrounding landscape. Main conclusions These results suggest that local processes control patterns of riparian species richness, but that species composition is also highly dependent on the regional species pool. We argue that inter‐annual variation in flood disturbance is probably the most important factor producing temporal variability of longitudinal species richness patterns.     

Riparian leaf litter decomposition on pond bottom after a retention on floating vegetation

Allochthonous (e.g., riparian) plant litter is among the organic matter resources that are important for wetland ecosystems. A compact canopy of free‐floating vegetation on the water surface may allow for riparian litter to remain on it for a period of time before sinking to the bottom. Thus, we hypothesized that canopy of free‐floating vegetation may slow decomposition processes in wetlands. To test the hypothesis that the retention of riparian leaf litter on the free‐floating vegetation in wetlands affects their subsequent decomposition on the bottom of wetlands, a 50‐day in situ decomposition experiment was performed in a wetland pond in subtropical China, in which litter bags of single species with fine (0.5 mm) or coarse (2.0 mm) mesh sizes were placed on free‐floating vegetation (dominated by Eichhornia crassipes, Lemna minor, and Salvinia molesta) for 25 days and then moved to the pond bottom for another 25 days or remained on the pond bottom for 50 days. The leaf litter was collected from three riparian species, that is, Cinnamomum camphora, Diospyros kaki, and Phyllostachys propinqua. The retention of riparian leaf litter on free‐floating vegetation had significant negative effect on the carbon loss, marginal negative effects on the mass loss, and no effect on the nitrogen loss from leaf litter, partially supporting the hypothesis. Similarly, the mass and carbon losses from leaf litter decomposing on the pond bottom for the first 25 days of the experiment were greater than those from the litter decomposing on free‐floating vegetation. Our results highlight that in wetlands, free‐floating vegetation could play a vital role in litter decomposition, which is linked to the regulation of nutrient cycling in ecosystems.     

Changes in vegetation and soil properties following 6 years of enclosure in riparian corridors

河岸带是维持生物多样性的重要生态系统之一。然而,由于过度放牧引起的植被消耗和过度开垦等人类活动的干扰,河岸带植被多样性和植被盖度受到严重的破坏,甚至威胁了河道的稳定性。围栏封育在退化草地生态系统修复中被广泛应用,但对退化河岸带植被群落和土壤性质的影响尚不明确。本研究的目的旨在明确围封的实施是否会促进河岸带植被群落的物种组成、物种丰富度和物种多样性恢复,土壤氧分如何随围封年限的增加而变化。辽河干流自2012年起被围栏封育管理,本研究在辽河干流河岸带沿岸设置了20个草本群落长期观测样地,记录了2012–2017年样地中植被高度、盖度和个体数量等参数用于物种丰富度和物种多样性的统计分析。同时,分别测定了2012年和2017年植被群落土壤氧分含量,验证了植被群落和土壤氧分对围封的反馈,研究了2012–2017年辽河干流河岸带的围栏封育对物种多样性和土壤氧分的影响。结果表明,随着围封年限的增加,辽河干流河岸带草本群落植被丰富度和多样性显著增加。物种组成方面,菊科植物的优势度显著增加,禾本科植物优势度显著下降。围封后植被群落的恢复和禁止耕作,加速了土壤中磷和钾的消耗,表现为显著降低,土壤有机质含量对围封的响应表现的相对滞后,并没有显著变化。综上所述,本研究为河岸带植被群落物种多样性、物种组成对围封的响应提供了新的见解。     

Responses of riparian plants to accumulation of silt and plant litter: the importance of plant traits

Abstract. A 2‐yr field experiment was used to determine the response of riparian plants to accumulation of litter or silt in a river flood‐plain meadow in northern Sweden. Such disturbances occur regularly in free‐flowing rivers but are likely to change as a result of global changes in land use or climate. We anticipated that plants with different traits would differ in their response to litter and silt accumulation. We quantified plant response as relative change in above‐ground biomass, and regressed it on either litter mass or silt depth, and on plant traits such as lateral spread, plant height, relative growth rate, seed mass and seed persistence in soil. The relative changes in riparian plant biomass following litter or silt accumulation were negatively related to litter mass and silt depth, and positively related to most examined plant traits such as seed mass, seed persistence and lateral spread. The vegetation recovery in the second season was largely determined by plant traits; litter or silt accumulation had no significant effect. Litter accumulation selected for large‐seeded species, but silt accumulation selected for species with strong ability of lateral spread. Seed persistence was a useful variable in predicting species recovery from both litter and silt accumulation. Plant height was negatively related to plant recovery, but relative growth rate was not significantly related to relative change in plant biomass after silt or litter accumulation. Our results imply that plant traits are important variables to consider for predicting the responses of riparian vegetation to deposition of organic and inorganic matter.     

Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.     

The effect of flooding and flood timing on leaf litter breakdown rates and nutrient dynamics in a river red gum (Eucalyptus camaldulensis) forest

Comparisons of litter standing-stocks in low-lying and higher areas of the floodplain and the effects of controlled flooding events on leaf litter decomposition and leaf litter nutrients were examined during autumn and winter in a southeastern Australian river red gum (Eucalyptus camaldulensis) floodplain forest. The mean mass of total litter and some litter components was significantly greater in autumn than in winter but there were few differences in litter mass between low-lying flood runners and higher sites (1.5 m) on the floodplain, regardless of season. Leaf decomposition was more rapid in flooded areas than in non-flooded areas and was significantly faster in autumn than in winter. In flooded leaves, concentrations of phosphorus and nitrogen dropped rapidly during the first 3 days of each experiment, increased to near original after 7–10 weeks and then decreased again. After 112 days of decomposition the C:N:P ratios of leaf litter increased, but this effect was most marked for flooded leaves. Simple models of leaf litter dynamics indicated that leaf litter standing-stocks in low-lying flood runners would be reduced by flooding, particularly during autumn. In contrast, models predicted a net gain in standing-stocks of leaf litter to be higher on the floodplain, particularly in autumn. Alteration to the seasonal timing of floods by river regulation has probably decreased litter standing-stocks and nutrients available in low-lying areas of the floodplain to support the production of macrophytes and biofilms during winter and spring floods.     

Resource Pulses in Desert River Habitats: Productivity-Biodiversity Hotspots,or Mirages?

Resource pulses in the world''s hot deserts are driven largely by rainfall and are highly variable in both time and space. However, run-on areas and drainage lines in arid regions receive more water more often than adjacent habitats, and frequently sustain relatively high levels of primary productivity. These landscape features therefore may support higher biotic diversity than other habitats, and potentially act as refuges for desert vertebrates and other biota during droughts. We used the ephemeral Field River in the Simpson Desert, central Australia, as a case study to quantify how resources and habitat characteristics vary spatially and temporally along the riparian corridor. Levels of moisture and nutrients were greater in the clay-dominated soils of the riverine corridor than in the surrounding sand dunes, as were cover values of trees, annual grasses, other annual plants and litter; these resources and habitat features were also greater near the main catchment area than in the distal reaches where the river channel runs out into extensive dune fields. These observations confirm that the riverine corridor is more productive than the surrounding desert, and support the idea that it may act as a refuge or as a channel for the ingress of peri-desert species. However, the work also demonstrates that species diversity of invertebrates and plants is not higher within the river corridor; rather, it is driven by rainfall and the accompanying increase in annual plants following a rain event. Further research is required to identify the biota that depend upon these resource pulses.     

Spatial Variability of Plant Litter Decomposition in Stream Networks: from Litter Bags to Watersheds

The decomposition of plant litter plays a fundamental role in the cycling of carbon and nutrients and is driven by complex interactions of biological and physical controls, yet little is known about its variability and controls across spatial scales. Here we address the indirect effects of riparian canopy cover on litter decomposition and decomposers and their variability within a set of hierarchical scales (watershed, stream segments and reaches) controlling for confounding factors that could co-vary with canopy cover (for example, temperature and nutrients), in high-altitude subtropical streams. Total, microbial and invertebrate-driven decomposition rates were approximately 1.4–6.6 times higher in closed-canopy than in open-canopy watersheds. Riparian canopy cover accounted for 62–69% of total variability of decomposition rates and indirectly (via light availability and litter inputs) promoted fungal facilitation of shredders through leaf litter conditioning. In contrast to what we expected, much of the spatial variability in the decomposition occurred at smaller scale (4–20% of total variability among reaches versus <1% among watersheds) and coincided with the greatest variability in shredder abundance and fungal biomass (70 and 17% among reaches, respectively). We conclude that riparian canopy cover may be an important control of natural variability of litter decomposition at the watershed scale through its effects on fungal decomposers and shredder consumption. We also provide evidence of higher reach and minor watershed variability of litter decomposition in stream networks. Our results point to the importance of identifying the sources of natural variability of decomposition and how they interact within and among spatial scales.     

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.     

The response of riparian vegetation to flood‐maintained habitat heterogeneity†

Abstract Riparian environments are subject to the scouring and depositional effects of floods. Riparian vegetation and substrates are scoured during high flows, while litter and sediment is deposited downstream. In the Prosser and Little Swanport River catchments in south‐east Tasmania, vascular plant species were surveyed in large riparian relevés. Within these relevés, 1 × 1 m subplots were placed in both flood‐scoured and depositional environments. Species composition was compared between these three datasets, to investigate the importance of floods in determining species richness and species composition of riparian vegetation. Species richness and diversity were highest in areas experiencing flood scour. Herbs appear particularly reliant on the creation of gaps for colonization, and some major riparian shrub species may also require disturbance to maintain their abundance. The depositional environment tended to favour shrubs and graminoids. Given that differences in species composition are related to flood‐induced features of the riparian environment, the regulation of these rivers might reduce the diversity of the riparian vegetation downstream of dams.     

Do riparian plant community characteristics differ between <Emphasis Type="Italic">Tamarix</Emphasis> (L.) invaded and non-invaded sites on the upper Verde River,Arizona?

Invasion by Tamarix (L.) can severely alter riparian areas of the western U.S., which are globally rare ecosystems. The upper Verde River, Arizona, is a relatively free-flowing river and has abundant native riparian vegetation. Tamarix is present on the upper Verde but is a minor component of the vegetation (8% of stems). This study sought to determine whether riparian vegetation characteristics differed between sites where Tamarix was present and sites where Tamarix was absent during the invasion of the upper Verde. We hypothesized that herbaceous understory and woody plant communities would differ between Tamarix present and absent sites. Our hypothesis was generally confirmed, the two types of sites were different. Tamarix present sites had greater abundance of all vegetation, native understory species, graminoids, and native trees, and a positive association with perennial native wetland plant species. Tamarix absent sites had greater abundance of exotic plants and upland adapted plants and an association with greater abiotic cover and litter. These results are contrary to other reports of Tamarix association with depauperate riparian plant communities, and suggest that Tamarix invasion of a watershed with a relatively natural flow regime and a robust native plant community follows similar establishment patterns as the native riparian plant community.     

Soil seed bank dynamics in response to an extreme flood event in a riparian habitat

A significantly increased water regime can lead to inundation of rivers, creeks and surrounding floodplains- and thus impact on the temporal dynamics of both the extant vegetation and the dormant, but viable soil-seed bank of riparian corridors. The study documented changes in the soil seed-bank along riparian corridors before and after a major flood event in January 2011 in southeast Queensland, Australia. The study site was a major river (the Mooleyember creek) near Roma, Central Queensland impacted by the extreme flood event and where baseline ecological data on riparian seed-bank populations have previously been collected in 2007, 2008 and 2009. After the major flood event, we collected further soil samples from the same locations in spring/summer (November–December 2011) and in early autumn (March 2012). Thereafter, the soils were exposed to adequate warmth and moisture under glasshouse conditions, and emerged seedlings identified taxonomically. Flooding increased seed-bank abundance but decreased its species richness and diversity. However, flood impact was less than that of yearly effect but greater than that of seasonal variation. Seeds of trees and shrubs were few in the soil, and were negatively affected by the flood; those of herbaceous and graminoids were numerous and proliferate after the flood. Seed-banks of weedy and/or exotic species were no more affected by the flood than those of native and/or non-invasive species. Overall, the studied riparian zone showed evidence of a quick recovery of its seed-bank over time, and can be considered to be resilient to an extreme flood event.     

Projected changes in plant species richness and extent of riparian vegetation belts as a result of climate‐driven hydrological change along the Vindel River in Sweden

1. Riparian plant communities are primarily structured by the hydrological regime of the stream. Models of climate change predict increased temperatures and changed patterns of precipitation that will alter the flow of rivers and streams with consequences for riparian communities. In boreal regions of Europe, stream flows will exhibit earlier spring‐flood peaks of lower magnitude, lower summer flows and higher flows in autumn and winter. We quantified the effects of predicted hydrological change on riparian plant species richness, using four different scenarios for the free‐flowing Vindel River in northern Sweden. 2. We calculated the hydrological niche of vegetation belts by relating the occurrence of species and vegetation belts to data on flood duration for 10 years preceding the vegetation survey. We then used the flood duration predicted for 2071–2100 to estimate expected changes in the extent of each vegetation belt. Using species accumulation curves, we then predicted changes in plant species richness as a result of changes in extent. 3. The two most species‐rich vegetation belts, riparian forest and willow shrub, were predicted to decrease most in elevational extent, up to 39 and 32%, respectively. The graminoid belt below the shrub belt will mainly shift upwards in elevation while the amphibious vegetation belt at the bottom of the riparian zone increases in size. 4. In the Vindel River, the riparian forest and willow shrub zone will lose most species, with reductions of 5–12% and 1–13% per site, respectively, depending on the scenario. The predicted loss from the entire riparian zone is lower, 1–9%, since many species occur in more than one vegetation belt. More extensive species losses are expected in the southern boreal zone for which much larger spring‐flood reductions are projected. 5. With an expected reduction in area of the most species‐rich belts, it becomes increasingly important to manage and protect riparian zones to alleviate other threats, thus minimising the risk of species losses. Restoring river and stream reaches degraded by other impacts to gain riparian habitat is another option to avoid species losses.     

Nutrient Vectors and Riparian Processing: A Review with Special Reference to African Semiarid Savanna Ecosystems

Abstract This review article describes vectors for nitrogen and phosphorus delivery to riparian zones in semiarid African savannas, the processing of nutrients in the riparian zone and the effect of disturbance on these processes. Semiarid savannas exhibit sharp seasonality, complex hillslope hydrology and high spatial heterogeneity, all of which ultimately impact nutrient fluxes between riparian, upland and aquatic environments. Our review shows that strong environmental drivers such as fire and herbivory enhance nitrogen, phosphorus and sediment transport to lower slope positions by shaping vegetative patterns. These vectors differ significantly from other arid and semiarid ecosystems, and from mesic ecosystems where the impact of fire and herbivory are less pronounced and less predictable. Also unique is the presence of sodic soils in certain hillslopes, which substantially alters hydrological flowpaths and may act as a trap where nitrogen is immobilized while sediment and phosphorus transport is enhanced. Nutrients and sediments are also deposited in the riparian zone during seasonal, intermittent floods while, during the dry season, subsurface movement of water from the stream into riparian soils and vegetation further enrich riparian zones with nutrients. As is found in mesic ecosystems, nutrients are immobilized in semiarid riparian corridors through microbial and plant uptake, whereas dissimilatory processes such as denitrification may be important where labile nitrogen and carbon are in adequate supply and physical conditions are suitable—such as in seeps, wallows created by animals, ephemeral wetlands and stream edges. Interaction between temporal hydrologic connectivity and spatial heterogeneity are disrupted by disturbances such as large floods and extended droughts, which may convert certain riparian patches from sinks to sources for nitrogen and phosphorus. In the face of increasing anthropogenic pressure, the scientific challenges are to provide a basic understanding of riparian biogeochemistry in semiarid African savannas to adequately address the temporal and spatial impact of disturbances, and to apply this knowledge to better regional land and water management. An integrated, multidisciplinary approach applied in protected as well as human-disturbed ecosystems in southern Africa is essential for underpinning a strong environmental basis for sustainable human-related expansion.