Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.     

Limited Effects of Large‐Scale Riparian Restoration on Seed Banks in Agriculture

I examined the effect of riparian forest restoration on plant abundance and diversity, including weed species, on agricultural lands along the Sacramento River in California (United States). Riparian forest restoration on the Sacramento River is occurring on a large‐scale, with a goal of restoring approximately 80,000 ha over 160 km of the river. In multiuse habitats, such as the Sacramento River, effects of adjoining habitat types and movement of species across these habitats can have important management implications in terms of landscape‐scale patterns of species distributions. Increased numbers of pest animals and weeds on agricultural lands associated with restored habitats could have negative economic impacts, and in turn affect support for restoration of natural areas. In order to determine the distribution and abundance of weeds associated with large‐scale restoration, I collected seed bank soil samples on orchards between 0 and 5.6 km from adjacent restored riparian, remnant riparian, and agricultural habitats. I determined the abundance, species richness, and dispersal mode of plant species in the seed bank and analyzed these variables in terms of adjacent habitat type and age of restored habitat. I found that agricultural weed species had higher densities at the edge of restored riparian habitat and that native plants had higher densities adjacent to remnant riparian habitat. Weed seed abundance increased significantly on walnut farms adjacent to restored habitat with time since restored. I supply strong empirical evidence that large areas of natural and restored habitats do not lead to a greater penetration of weed species into agricultural areas, but rather that weed penetration is both temporally and spatially limited.     

Waste Not,Want Not: The Need to Utilize Existing Artificial Structures for Habitat Improvement Along Urban Rivers

Urban rivers have often experienced substantial engineering modification and consequently are highly degraded aquatic ecosystems with minimal riparian habitat. Habitat restoration and improvement efforts are needed within urban rivers to support ecological communities and increase ecosystem integrity. Most river restoration techniques are not feasible within large urban rivers, and so there is a need to develop novel methodologies. Artificial structures such as river walls can function as habitat for plant and invertebrate species in urban rivers, and in some cases can be more diverse than remnant habitat. Along the River Thames through central London, plant species richness was found to be significantly higher on river walls than intertidal foreshore, which represents the only remnant habitat for riparian species. Both this survey and other studies have suggested that the physical and environmental characteristics of river walls are likely to influence their capacity to function as ecological habitat, for example, walls composed of more complex construction materials (brick and boulders) being more diverse than simpler structures (concrete and sheet piling). The opportunity exists to use river walls and other artificial structures (e.g., jetties) to improve habitat along urban rivers by installing walls which are designed to be more complex, or by adding modifications to existing walls. Some trial modifications, such as the addition of wall ledges and timber fenders to sheet piling, have been installed at Deptford Creek along the River Thames, and have so far greatly supported the colonization and development of plant communities. The restoration possibilities of such modifications should be considered, and further development and rigorous testing of installations is required in urban rivers to make sound restoration recommendations.     

Flood‐deposited wood creates regeneration niches for riparian vegetation on a semi‐arid South African river

Question : The formation of large woody debris (LWD) piles during floods has significant impacts on riparian succession through pioneering plants often establishing in association with wood. We assess the importance of LWD for seed regeneration of riparian plants after a century‐scale flood disturbance in a semi‐arid environment. Location : The Sabie River within Kruger National Park in the semi‐arid northeast of South Africa. Methods : Our approach was to quantify the riparian soil seed bank, to record the frequency of establishment of riparian plants in woody debris piles, and to conduct experimental out‐plantings of common riparian trees in plots with and without LWD. Results : We found the abundance and diversity of seedlings were higher in soils taken from wood piles than from open reference areas, and most seedlings were herbaceous species. Surveys indicated that numbers of seedlings recorded within woody debris were significantly greater than in open reference areas or within established vegetation. Seedling establishment in various cover‐types also varied for different riparian tree species. Experimental out‐planting of seedlings of two riparian tree species (Philenoptera violacea and Combretum erythrophyllum) revealed that, after 433 days, planted seedlings survived only in woody debris piles. Conclusion : LWD formed after a large flood creates heterogeneous patches that may influence post‐disturbance regeneration of riparian vegetation by providing a variety of environmental niches for seedlings establishment. We suspect that higher seedling survival in LWD is due to increased moisture (particularly in the dry season) and nutrients, and protection from seasonal flooding and herbivory.     

Soil seed banks of two montane riparian areas: implications for restoration

Understanding the role of seed banks can be important for designing restoration projects. Using the seedling emergence method, we investigated the soil seed banks of two montane, deciduous riparian forest ecosystems of southeastern Arizona. We contrasted the seed banks and extant vegetation of Ramsey Canyon, which is the site of riparian restoration activities, with that of Garden Canyon, which has been less affected by human land uses. Fewer plant species were found at Ramsey Canyon than Garden Canyon, for both the seed bank and extant vegetation, and the vegetation at Ramsey Canyon (seed bank and extant) had consistently drier wetland indicator scores. As well, vegetation patterns within sampling zones (channel margins and adjacent riparian forests) differed between canyons. At Garden Canyon channel margins, the seed bank and extant vegetation had relatively high similarity, with herbaceous wetland perennial species dominating. Extant vegetation in the floodplain riparian forest zone at Garden Canyon had a drier wetland indicator score than the seed bank, suggesting that the floodplains are storing seeds dispersed from wetter fluvial surfaces. Vegetation patterns for Ramsey Canyon channel margins were similar to those for Garden Canyon floodplains. Vegetation patterns in the Ramsey Canyon riparian forest zone were indicative of non-flooded conditions with an abundance of upland species in the soil seed bank and extant vegetation. Channel geomorphology measurements indicated that much of the riparian forest zone at Ramsey Canyon is functionally a terrace, a condition that may be a legacy of channel erosion from historic land uses. Steep, erodible channel slopes may contribute to the low seed bank germinant density at Ramsey Canyon channel margins, and narrower flood-prone area may explain the greater terrestrialization of the vegetation in both sampling zones. We recommend testing the use of donor soils from more diverse stream reaches to restore biodiversity levels at Ramsey Canyon, following restoration activities such as channel-widening. Seed banks from Garden Canyon, for example, although predominantly consisting of herbaceous perennials, would supply species with a range of moisture tolerances, life spans, and growth forms. We also recommend that restorationists take care not to harm seed banks exposed during removal of introduced species; at Ramsey Canyon, soil seed banks were equally diverse in areas with high and low cover of the introduced Vinca major (a legacy of Ramsey Canyon land use).     

Seed bank,seed dispersal and vegetation cover: Colonization along a newly‐created river channel

Question : What is the relative importance of the initial seed bank and subsequent seed dispersal for floristic composition of bank vegetation two years after creation of a newly‐cut reach of a river channel? Location : River Cole, West Midlands, United Kingdom. Methods : We took bank and bed sediment samples from a 0.5‐km reach of a new river channel cut into intact flood‐plain. After river diversion, seed samples deposited on artificial turf mats placed on the river banks and flood‐plain edge were taken in summer and winter 2002 and 2003. Seed rain samples from funnel traps were taken during summer 2002 and 2003. We undertook greenhouse germination trials to assess viable seed species within these samples. In summer 2004, we surveyed river bank vegetation. Agglomerative cluster analysis was used to investigate floristic similarity between seed bank, seed rain, seed deposition samples and final bank vegetation cover. DCA was used to explore contrasts between the samples and to assess whether these reflected interpretable environmental gradients. Results : Seed rain samples contained a small subset of species in the summer depositional samples. 38 species were found within the final vegetation, the seed bank, and at least one of the four sets of depositional samples; a further 30 species not present in the seed‐bank samples were present in at least one of the four sets of depositional samples and the final vegetation. Floristic composition of the vegetation was most similar to the depositional samples from winter 2002 and 2003 and summer 2003. DCA axis 1 reflected a time sequence from seed‐bank samples through depositional samples to the final vegetation. Conclusions : Newly cut river banks were colonized rapidly. Seed remobilization and hydrochorous transport from the upstream catchment are important for colonization. Species richness was highest in samples deposited during winter when high river flows can remobilize and transport viable seeds from upstream. This process would also have enhanced the species richness of seed production along the banks during the second summer (2003).     

Plant species richness in ephemeral and perennial reaches of a dryland river

Ephemeral reaches are common along desert rivers but are less well studied than those with perennial stream flow. This study contrasted riparian plant species richness and composition (extant vegetation and soil seed bank) between stream reaches with different low-flow conditions (perennial vs. ephemeral flow) but similar flood patterns and similar watershed-derived species pools. Data were collected at Cienega Creek (Arizona, USA) over a 2 year period spanning drought conditions and wetter conditions. Consistent with expectations relating to water limitation effects on diversity, species richness in the riparian zone was lower at ephemeral-flow sites during a season with minimal precipitation and no overbank flooding; under these conditions, the more permanent water sources of the perennial-flow sites sustain the larger number of species. During seasons with greater precipitation and elevated stream flows, in contrast, species richness at ephemeral-flow sites increased to levels at or slightly above those of perennial-flow sites. For values pooled across two wet seasons of a calendar year, year-round richness was greater at the two ephemeral-flow sites (total of 92 vascular plant species) than at the two perennial-flow sites (68 species). This greater year-round richness was a combination of multiple factors: greater light, space, and bare ground, a diverse soil seed bank (with the seed banks equally species-rich among hydrologic types), and moderately abundant precipitation and flooding sufficient to stimulate establishment of opportunistic species (mainly annuals) during the bimodal wet seasons. These results indicate that long-term patterns of site water availability, by influencing woody plant cover, mediate the diversity response to episodic water pulses in dryland rivers. The results also have implications for riparian conservation efforts, which to date have focused primarily on perennial stream reaches: ephemeral reaches of spatially intermittent rivers harbor many riparian plant species, and warrant conservation efforts, as well.     

Soil seed bank in a patchy vegetation of coastal sandy plains in southeastern Brazil

Large seed banks have been found in tropical dry forests and also in habitats with high seasonality in rainfall. However, patchily structured vegetation could induce great spatial variation in the seed bank. We characterized the seed bank in a patchy vegetation of restinga, a common type of coastal vegetation found in the Atlantic forest biome. We also evaluated whether there is any spatial variation between the litter and soil layer, bare sand, and the edge and center of vegetation patches with distinct species dominance. We found 104 seeds/m² in the seed bank using a 5‐cm‐depth sampling. Seven out of 16 species found in the restinga seed bank germinated; two of these were found in the early stages of vegetation patches. We found a higher number of seeds at the edge than in the center of vegetation patches. However, there were no significant differences in the number of seeds in the seed bank between the litter and soil layer, and between vegetation patches with distinct species dominance. Bare sandy soils had lower seed bank densities than vegetation patches. A small seed bank size might be explained by the low proportion of seeds from herbaceous and woody species, which are pioneers in the Atlantic forest. However, seed bank might play an important role in the early stages of the successional process, due to the occurrence of the few species that are able to colonize new young vegetation patches.     

Does the seed bank contribute to the restoration of species‐rich vegetation in wet dune slacks?

Questions: What is the contribution of the seed bank to restoration of species‐rich vegetation in oligotrophic wet dune slacks? Does the restoration management affect the seed bank? Location: Calcareous coastal dune slacks at the west coast of The Netherlands. Methods: Species composition of the seed bank and the above‐ground vegetation was sampled in dune slacks that had a variable extent of groundwater level rise in combination with either topsoil removal or mowing. Results: The seed bank had a high potential for restoration of species‐rich vegetation: 60 species were found in the seed bank of which 14 were characteristic of oligotrophic, wet dune vegetation. While topsoil removal almost completely removed the seed bank, groundwater level rise did not permanently submerge the seed bank of species of oligotrophic, wet conditions. Changes in abundance in the established vegetation were unrelated to species abundance in the seed bank. Of all new species establishments in the vegetation relevés, 76% occurred where the species was not found in the seed bank. The chance that presence of a species in the seed bank led to establishment the following year was only 11%. Conclusion: The seed bank was not the dominant source for newly establishing species following the large disturbance that was induced by restoration management. Changes in species abundance after the restoration impact were not related to species abundance in the seed bank, but to ongoing succession and current year dispersal. To attain a high number of new establishments, restoration projects should preferably be planned in the proximity of refuge populations, rather than relying on the seed bank alone.     

Soil seed bank in different vegetation types in the Loess Plateau region and its role in vegetation restoration

In the Loess Plateau region, soil erosion is a serious problem. Vegetation restoration is an effective approach to control soil erosion and improve ecosystems. The soil seed bank generally plays an important role in vegetation restoration after disturbance. Thus, we reviewed soil seed bank studies to reveal the soil seed bank characteristics and its role in vegetation restoration in three vegetation types (forest, forest‐steppe, and steppe). We selected 38 seed bank studies and analyzed several seed bank characteristics, such as seed density, species composition, and the relationship between seed size and seed bank. We also assessed the role of the soil seed bank in vegetation restoration. The soil seed bank density ranged from 2,331 ± 1,993 to 6,985 ± 4,047 seeds/m² among the different vegetation types. In the soil seed bank, perennial herbs and grasses accounted for 51.5% of the total species. Native species that were dominant or common in the standing vegetation usually had relatively high seed bank densities. Moreover, species with smaller seeds generally had higher soil seed bank densities. The present study indicates that the soil seed bank plays a significant role in spontaneous vegetation restoration, especially during the early successional stages in abandoned slope farmlands and grazing‐excluded grasslands. However, species with large seeds or transient soil seed banks should be reintroduced through seeding to accelerate target species restoration. More studies on soil seed banks need to be conducted to comprehensively reveal their characteristics.     

Comparison of Designed Channel Restoration and Riparian Buffer Restoration Effects on Riparian Soils

Stream restoration is often employed in efforts to stabilize eroding channel banks. Banks are stabilized through a designed channel approach, which involves grading and armoring of stream banks using heavy machinery, or alternatively through planting of seedlings and saplings to establish forested riparian buffers. We hypothesized that designed channel restoration would have detrimental impacts on riparian soils but that soils would recover over time, and we hypothesized that riparian buffer restoration would not impact riparian soils. We tested these hypotheses by comparing soil attributes (bulk density, soil organic matter, and root biomass) at reaches that had undergone designed channel and riparian buffer restoration in different years (project ages ranged from 2 to 16 years) to paired urban (unrestored) control reaches. Soil properties in sub‐surface soil layers (10–20 and 20–30 cm depth) at both recent (<10 years old) and older (>10 years old) designed channel reaches differed significantly from paired urban control soils; bulk density was higher and root biomass lower in manipulated reaches compared to urban control reaches. At many designed channel reaches, bulk density exceeded values known to restrict root growth. These results indicate that compaction and disturbance of riparian soils may be a significant unintended consequence of designed channel restoration and can persist for at least a decade. In contrast, we found no significant differences in soil properties between riparian buffer restoration reaches and urban control reaches. Thus, the results indicate that riparian buffer restoration is a more ecologically favorable method than designed channel restoration for bank stabilization.     

三峡库区消涨带植被重建

重建三峡水库消涨带植被对于恢复消涨带功能、维持三峡工程安全和修复长江流域退化生态系统具有十分重要的意义.消涨带可分为自然消涨带和人工消涨带,自然消涨带及其植被是流域生态系统的组成部分,具有重要的生态、社会和经济价值.人为控制水位涨落而形成的人工消涨带很少有植被覆盖,属于退化的生态系统.三峡消涨带包括三峡自然消涨带和三峡水库消涨带,三峡工程的建设将淹没三峡自然消涨带及其植被并产生没有植被覆盖的三峡水库消涨带.作者认为开发利用三峡水库消涨带土地资源,发展库区经济将导致库区生态环境恶化、危及三峡工程安全且不利于长江流域生态系统的健康.解决三峡水库消涨带问题的关键是重建消涨带植被,并恢复其功能.该文从水利建设与环境保护的关系、三峡水库管理与库区景观建设的需要、消涨带功能恢复与流域生态系统康复和促进水库消涨带研究的深入阐述了开展三峡水库消涨带植被重建的必要性.     

消涨带及三峡水库消涨带植被重建

重建三峡水库消涨带植被对于恢复消涨带功能、维持三峡工程安全和修复长江流域退化生态系统具有十分重要的意义。消涨带可分为自然消涨带和人工消涨带,自然消涨带及其植被是流域生态系统的组成部分,具有重要的生态、社会和经济价值。人为控制水位涨落而形成的人工消涨带很少有植被覆盖,属于退化的生态系统。三峡消涨带包括三峡自然消涨带和三峡水库消涨带,三峡工程的建设将淹没三峡自然消涨带及其植被并产生没有植被覆盖的三峡水库消涨带。作者认为开发利用三峡水库消涨带土地资源,发展库区经济将导致库区生态环境恶化、危及三峡工程安全且不利于长江流域生态系统的健康。解决三峡水库消涨带问题的关键是重建消涨带植被,并恢复其功能。该文从水利建设与环境保护的关系、三峡水库管理与库区景观建设的需要、消涨带功能恢复与流域生态系统康复和促进水库消涨带研究的深入阐述了开展三峡水库消涨带植被重建的必要性。     

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.     

Germinable soil seed banks in abandoned grasslands in central and western Norway and their significance for restoration

Questions: Could the seed bank increase biodiversity during restoration of abandoned, species‐poor, formerly cultivated vegetation? Is it possible to identify how climate, soil and former and present management and vegetation affected the seed bank? Location: The study sites were eight abandoned grasslands, four in Orkdal, central Norway and four in Gaular, western Norway. Methods: 144 seed bank samples were collected from three depths. Each sample was sown and placed in a greenhouse. After three months, the trays were dried and stored at 4°C in a dry place for two months. This was repeated twice. Results: There was a separation of the two regions along the first DCA axis in both the seed bank and in the vegetation analysis and also a clear separation of the seed bank from the vegetation along the second axis. These results are caused by differences in former management as well as temperature, precipitation and soil type between Gaular and Orkdal. We found more annuals, short‐lived species and species demanding light open conditions in the seed bank than in the vegetation probably because these species have the capacity for producing persistent seeds. Most of the species found only in the seed bank were found in very few samples and with few individuals. Conclusion: These results suggest that it may be difficult to increase vegetation biodiversity through restoration of grasslands such as those investigated if the natural soil seed bank is the main seed source.     

Spatial relationships between the standing vegetation and the soil seed bank in a fire-prone encroached dehesa in Central Spain

Postfire vegetation regeneration in many fire-prone ecosystems is soil seed bank dependent. Although vegetation and seed bank may be spatially structured, the role of prefire vegetation patterns and fire in determining postfire vegetation patterns is poorly known. Here, we investigated the spatial patterning of species abundance and richness in the vegetation and seed bank of a Mediterranean encroached dehesa in Central Spain. The seed bank was studied with and without a heat shock simulating a spatially homogeneous fire. Semivariograms and cross-semivariograms showed that species richness in the vegetation was aggregated in patches, mainly of herbs, with highest values corresponding to high herb cover and low tree cover. Species richness in the seed bank was also structured in patches, but the spatial pattern was weak. Seedling density of germinates in the seed bank also showed weak spatial pattern. Heating increased overall germination and species richness, and the intensity of the spatial pattern of species richness, particularly of herbaceous species. However, seed bank density patterns disappeared after heat shock because of increased germination of shrubs without spatial pattern. Our results document that the spatial structure of plant richness in the vegetation may persist after fire due to the spatial patterns of herbaceous species in the seed bank, and that postfire species richness patterns can arise independently of fire intensity patterns. However, the spatial structure of the vegetation after fire can be altered by the feedback between shrub encroachment and an eventual fire because of the ubiquitous germination of shrubs.     

Tracing carbon sources in small urbanising streams: catchment‐scale stormwater drainage overwhelms the effects of reach‐scale riparian vegetation

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城市化地区河岸带植被特征及其与河岸硬度的关系——以晋江市为例

确立了滨河河道硬度与滨河土地硬度两个指标,用以研究城市化地区河岸带的植物构成、多样性与均匀度、优势植物等特征与河岸硬度的关系。选定晋江市都市区12条主要河流作为研究对象,通过样地调查收集数据,并采用相应指数进行数据处理分析,结果表明:(1)调查河流的河岸带植被物种共计70科143属159种,其中乔木20科30属41种,灌木15科15属20种,草本35科98属98种,草本居主导地位,河流间种类分布不均衡;(2)河岸带植物乔灌草区系均以广义热带性成分居多,其次为世界性成分、温带性成分,无中国特有分布类型,乔木中广义热带成分占绝对主导,直观反映地带性特征,灌木与草本中的温带成分多于乔木中的温带成分,类型趋于多元,种类更丰富;(3)从河岸植被优势植物构成来看,主要优势乔木与灌木基本为本土植物,而主要草本中外来植物入侵严重,对地带性植物景观的指示显然不及乔木与灌木;(4)从河岸植被的人工美学属性来看,灌木优势植物中园林观赏植物的种类数显著多于乔木、草本优势植物,反映出晋江人工审美主导的滨河景观空间主要改变与塑造了灌木层植被景观;(5)从河岸植物的人居需求属性来看,龙眼、杨桃等多见于庭院林、水岸林的水果树种也在河岸植被中频繁出现,反映出人口密集区由于对植物生产用途的重视而对河岸植被产生的影响;(6)滨河土地硬度与河道硬度对河岸带灌木植物的影响最强烈,二者均对河岸带植物多样性造成威胁,且前者具有更大的影响力,而后者直接影响了河岸植物的分布形态,出于生产与审美目的的人为干扰对河岸带植物优势种的影响力随滨河硬度的增加而加强,低滨河硬度有助留存原生植被群落;(7)天然弯曲的河流形态对河流植被特征具有积极影响,有助于保留更多本土植被类型,并能在某种程度上丰富乔灌草的植物种类,但这种缓和作用无法根本扭转河岸硬化对植物多样性的胁迫影响。研究表明,城市化地区河流的岸带植被特征与沿线人为干扰类型和强度密切相关,恢复与塑造河岸生态景观要以乡土植被为主,通过乔木景观塑造地带性景观,并以灌木与草本丰富植被景观。     

雅鲁藏布江中游河岸带几种主要沙生植物种群点格局分析

以雅鲁藏布江中游河岸带几种主要沙生植物为研究对象, 采用相邻格子法分别在不同生境条件下的藏沙蒿(Artemisia wellbyi)、藏白蒿(Artemisia younghusbandii)和砂生槐(Sophora moorcroftiana)种群设立4个20 m × 30 m的样方, 运用点格局分析方法, 研究了这几种沙生植物的种群结构、空间分布、空间关联及其影响因素, 以期为西藏高寒河谷风沙化土地植被恢复提供依据。结果表明: 1)藏沙蒿种群结构呈增长型, 藏白蒿和砂生槐种群结构呈衰退型。藏沙蒿和藏白蒿种群在所有尺度上均呈集群分布, 砂生槐种群随着尺度增大表现为集群→随机→集群→随机分布。沙生植物种群分布格局主要是由较小的大小级决定的, 而较大的大小级是造成种群分布格局波动的主要因素。2)这几种沙生植物种间正关联往往存在于一定的尺度范围内, 种间关联时常出现不同关联方式交替出现和摆动的现象。半裸露砂砾地和半固定沙地藏沙蒿-砂生槐种间关联均表现为无关联→正关联→无关联→正关联, 半裸露砂砾地藏沙蒿-毛瓣棘豆( Oxytropis sericopetala)种间关联表现为正关联→无关联→正关联→无关联→负关联, 藏白蒿-砂生槐种间关联表现为正关联→无关联。3)这几种沙生植物种群均随植株形体增大, 聚集强度减弱, 大小级较小时表现为集群分布, 随着大小级增大, 表现为随机分布或随机与集群分布交错出现。不同大小级的空间正关联随着植株形体大小差异的增大而减弱, 甚至会转变为负关联, 各大小级在较小的空间尺度上易表现为正或无空间关联。     

Responses to Riparian Restoration in the Spring Creek Watershed, Central Pennsylvania

Riparian treatments, consisting of 3‐ to 4‐m buffer strips, stream bank stabilization, and rock‐lined stream crossings, were installed in two streams with livestock grazing to reduce sediment loading and stream bank erosion. Cedar Run and Slab Cabin Run, the treatment streams, and Spring Creek, an adjacent reference stream without riparian grazing, were monitored prior to (1991–1992) and 3–5 years after (2001–2003) riparian buffer installation to assess channel morphology, stream substrate composition, suspended sediments, and macroinvertebrate communities. Few changes were found in channel widths and depths, but channel‐structuring flow events were rare in the drought period after restoration. Stream bank vegetation increased from 50% or less to 100% in nearly all formerly grazed riparian buffers. The proportion of fine sediments in stream substrates decreased in Cedar Run but not in Slab Cabin Run. After riparian treatments, suspended sediments during base flow and storm flow decreased 47–87% in both streams. Macroinvertebrate diversity did not improve after restoration in either treated stream. Relative to Spring Creek, macroinvertebrate densities increased in both treated streams by the end of the posttreatment sampling period. Despite drought conditions that may have altered physical and biological effects of riparian treatments, goals of the riparian restoration to minimize erosion and sedimentation were met. A relatively narrow grass buffer along 2.4 km of each stream was effective in improving water quality, stream substrates, and some biological metrics.