Monte Carlo model framework to simulate settlement dynamics

We developed an open source Monte Carlo based model to simulate Settlement Dynamics in Drylands (SeDD). The model assigns partial probabilities to each pixel within a grid region, based on several factors that can influence the establishment and subsistence of settlements: groundwater depth, vegetation type, proximity to rivers, paved roads, old river beds, and existing settlements. Partial probabilities are considered to be independent from each other, and therefore multiplied to calculate an overall probability for each pixel. Settlements are assigned by maximum probabilities or randomly, according to pre-established threshold probability values. We also modeled the gradual reduction of vegetation caused by a new settlement in neighboring pixels, decreasing the probabilities related to vegetation type. The final distribution of settlements is given by an average over multiple Monte Carlo simulations. The model is computationally efficient and could be used to rapidly explore different scenarios of settlement dynamics and vegetation degradation in arid environments, and other environmental factors that can be added to the framework without performing changes in the source code.     

Localized impact of livestock settlements on vegetation patterns in fir forests of Kashmir Himalaya

Among the many global drivers of ecosystem degradation, the long-term impact of livestock settlements on vegetation patterns and composition, and is one of the greatest hurdles to successful forest ecosystem restoration. In order to identify the ecological implications of human land use changes on vegetation patterns and composition, the current study was conducted for the first time in Fir Forests of Kashmir Himalaya, to illustrate how vegetation responds to livestock settlements. We took samples from forest types both near and far from livestock settlements (ALS and NLS, respectively). To collect data on forest types, we employed a systematic random sampling strategy. After evaluation of the important value index (henceforth IVI) for each plant species along with anthropogenic data of forests, we analyzed data using ordination and cluster analysis techniques. On comparative analysis, the lesser values for diversity indices have been found for at the forest types near livestock settlement. It was discovered that 36% species were unique to away from livestock settlements forest type and 18% species in near to livestock settlements forest type, but that maximum 46% of species were common to all forest types studied. Species composition was unevenly distributed along with various anthropogenic factors, according to the canonical correspondence analysis. Forests near livestock settlements were also discovered to affect species abundance and diversity by promoting the growth of weedy species. The abundant growth of weedy flora such as Cirsium falconeri, Polygonum aviculare, and Urtica dioica around livestock settlement, results in alterations of plant community composition. The findings revealed that cut trees contributed 16% of reported density at ALS forest type and considerably higher values 35% at NLS forest type, respectively. Findings of the study could be used to develop effective livelihood options and management policies for long-term forest harvest impacts in the Himalayas, resulting in the sustainable use of local forest reserves.     

额济纳天然绿洲景观演化驱动因子分析

额济纳天然绿洲位于我国西北干旱内陆河流域黑河流域下游 ,近年来 ,随着黑河中、上游地区下泄地表径流量的减少 ,额济纳天然绿洲景观退化严重。基于研究区近十几年来的景观演化总体特征 ,分别从气候因素、水文因素、人类干扰因素等几个主要的景观变化驱动因子入手 ,探讨其演化机制、揭示其演化规律。研究结果表明 :黑河流域中、上游下泄地表径流量的锐减而导致研究区地下水水位下降、地下水矿化度升高以及水质恶化是额济纳天然绿洲景观发生退化的根本原因 ,绿洲人类活动强度的增加、绿洲关键区域的超载和过牧以及蒸发度指数的升高、湿润系数的下降均加剧了研究区景观退化的程度。     

京津风沙源区不同分区植被覆盖度变化及归因分析

以2000—2018年MODIS NDVI影像为数据源,基于像元二分模型估算京津风沙源区植被覆盖度(FVC),分析京津风沙源区FVC的时空变化特征,并结合自然因素和人类活动,运用地理探测器模型探究自然、人类活动对区域尺度FVC空间分布的影响。结果表明:2000—2018年,京津风沙源区FVC呈增加趋势,增长速率为0.013·(10 a)^-1,植被增加率为8.2%,FVC较高的区域在燕山丘陵山地水源保护区,其次是农牧交错带沙化土地治理区和浑善达克沙地治理区,FVC较低的区域为北部干旱草原沙地治理区。不同分区各驱动因子对FVC空间分布的解释力不同。自然因素中,年降水量是控制北部干旱草原治理区、浑善达克沙地治理区和燕山丘陵山地水源保护区FVC空间分布的主要驱动因子,坡度是控制农牧交错带沙化土地治理区的主要驱动因子;人类活动中,年末大牲畜头数是控制北部干旱草原治理区和农牧交错带沙化土地治理区FVC空间分布的主要驱动因子,人口密度是控制浑善达克沙地治理区和燕山丘陵山地水源保护区FVC空间分布的主要驱动因子;其他因子对FVC空间分布的影响则存在区域差异。交互探测器结果显示,双因子交互作用以双协同作用和非线性协同作用为主。人类活动与年降水量、坡度等自然因素的共同作用能够更充分地解释FVC空间分布。风险探测器识别的适宜植被生长的范围为年降水量316.4～486.0 mm、平均相对湿度48.4%～57.6%、年均温2.5～7.9 ℃的区域,其他驱动因子则在不同分区之间存在差异。     

基于地下水恢复的塔里木河下游生态需水量估算

为探明生态输水后地下水响应带范围及地下水恢复下生态需水量,以塔里木河下游大西海子水库至台特玛湖段为研究区,基于2000—2010年生态输水和地下水埋深分布特征,分析了塔里木河下游生态输水后两岸地下水位恢复状况,并借助遥感和地理信息系统技术对研究区生态需水量进行了研究。结果表明:塔河下游地下水位的抬升幅度与输水量的大小呈一定的正相关关系,并存在一定的时效性。2004—2010年地下水处于长期的负均衡状态,多年下降幅度明显。塔河下游英苏、喀尔达依、阿拉干和依干不及麻断面地下水响应幅度分别为1195、1050、2281 m和1000 m。历经11a输水后,塔里木河下游地下水总恢复需水量为7.06×108m3,其中,齐文阔尔河段为4.98×108m3,老塔里木河段为2.09×108m3,地下水恢复至生态水位4.5m需要5—8a的时间。保护塔里木河下游大西海子以下所有天然植被面积(96114.09 hm2)的生态需水量为0.587×108m3,保护下游地下水响应带天然植被面积(41439.85 hm2)的生态需水量为0.21×108m3。     

Variability in flood disturbance and the impact on riparian tree recruitment in two contrasting river systems

The vegetation within the riparian zone performs animportant ecological function for in-stream processes.In Australia, riparian zones are regarded as the mostdegraded natural resource zone due to disturbancessuch as river regulation and livestock grazing. Thisstudy looks at factors influencing vegetation dynamicsof riparian tree species on two contrasting riversystems in Western Australia. The Blackwood River insouth-western Australia is influenced by aMediterranean type climate with regular seasonalwinter flows. The Ord River in north-western Australiais characterized by low winter base flows andepisodic, extreme flows influenced by monsoon rains inthe summer. For both rivers, reproductive phenology ofstudied overstory species is timed to coincide withseasonal river hydrology and rainfall. An evendistribution of size classes of trees on the BlackwoodRiver indicated recruitment into the population iscontinual and related to the regular predictableseasonal river flows and rainfall. In contrast, on theOrd River tree size class distribution was clustered,indicating episodic recruitment. On both rivers treeestablishment is also influenced by elevation abovethe river, microtopography, moisture status and soiltype. In terms of vegetation dynamics riparianvegetation on the Ord River consists of long periodsof transition with short lived stable states incontrast to the Blackwood river where tree populationstructure is characterized by long periods of stablestates with short transitions.     

Predicting cover types in a mountain range with long evolutionary grazing history: a GIS approach

Aim To determine how the distribution and cover of different vegetation types are affected by physical factors and livestock in a mountain range with a long evolutionary history of grazing. Location Upper vegetation belt of the Córdoba mountains (1700–2800 m a.s.l., 31º34′ S, 64º50′ W) in central Argentina. Methods Using GIS, we analysed the relationships of plant cover types to physical features (physiography and topography) and indicators of accumulated livestock pressure (distance to human settlements and roads) through multinomial logistic regression. We predicted a present vegetation map which was validated with a real map. We then constructed two maps simulating minimum and maximum values of accumulated livestock pressure for the whole area. Map comparisons allowed evaluation of the possible influence of livestock, both in extension and intensity. Results Both physical features and livestock pressure influenced the occurrence of vegetation units. The overall accuracy of the predicted map at the pixel level was low (26%) indicating low habitat specificity of the vegetation units. We suggest that some part of the unaccounted for variance was due to livestock pressure patterns that were not fully captured by our indicators. Our models proved adequate for predicting the total percentages of vegetation units at coarser scales. The extrapolations showed that under a history of low livestock pressure, such as in sites far away from human settlements and roads, the area would be dominated by woodlands, tussock grasslands and natural rock outcrops. Under a history of heavy livestock pressure, in turn, rock exposed by erosion, tussock grasslands and natural rock outcrops would dominate. Main conclusions Vegetation units showed low habitat specificity, and were associated with accumulated livestock pressure, indicating that livestock and its associated activities are important factors structuring the landscape and have important consequences for the integrity of the ecosystem. Results suggest that although this system evolved with large herbivores, it has experienced irreversible degradation processes, and intensification of current domestic livestock pressure is likely to lead to even more land degradation.     

Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi‐arid coastal ecosystem

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi‐arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water‐use functional types? We examined a suite of physiological parameters and water‐uptake depths of the dominant, functionally distinct woody vegetation along a water‐table depth gradient in a Mediterranean semi‐arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water‐source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi‐arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi‐arid coastal ecosystems to groundwater drawdown.     

可调式沉水植被网床对泥沙型富营养化河道生态修复工程研究

分别采用水花生(Alternanthera philoxeroides (Mart.) Griseb)和狐尾藻(Myriophyllum verticillatum)2种可调式沉水植被网床对临港新城环湖流动泥沙型河道进行修复, 通过4个月连续跟踪测定修复河道总氮(TN)、硝态氮(NO₃-N)、亚硝态氮(NO₂-N)、总磷(TP)、活性磷酸盐(PO₄-P)的含量变化以及水体透明度(SD)变化以评价可调式沉水植物网床生态修复效果。结果显示, 沉水植物网床对流动泥沙型河道透明度具有明显改善作用。其中, 水花生网床和狐尾藻网床对TN、NO₃-N、NO₂-N、TP、PO₄-P的削减率分别为69.58%、67.91%、84.48%、62.26%、82.61%和45.48%、58.32%、76.56%、43.12%、73.68%, 且分别对泥沙型河道透明度提高了18%和26%。可见, 水花生网床对营养盐的削减效果优于狐尾藻网床, 而狐尾藻网床对透明度改善效果优于水花生网床。因此把这2种可调式沉水植物网床结合起来, 对流动泥沙型富营养化河道具有更好去除富营养物及泥沙的生态修复效果。     

Pastoralist's livestock and settlements influence game bird diversity and abundance in a savanna ecosystem of southern Kenya

We investigated the vegetation structure and density of game birds along a successional gradient created by varying intensity of human settlement in a pastoral community in Shompole group ranch in southern Kenya. We examined four habitat types including heavily-grazed grass in currently occupied settlements, short grass in seasonal settlements, patches of bushed woodland in settlement sites that had been abandoned up to 30 years ago, and tall grass in a wildlife sanctuary. Vegetation biomass was the highest in the sanctuary where livestock are excluded and the lowest in the vicinity of currently occupied settlements. Abundance of doves ( Oena spp . and Streptopelia spp. ) was best associated with moderate grazing and per cent green grass, which provide good foraging opportunities. Francolin and spurfowl ( Francolinus spp .) were positively associated with vegetation biomass. Quail ( Coturnix spp .) were positively associated with tree cover. These data reflect the importance of maintaining a balance of wildlife and livestock grazing with patches of ungrazed grasslands and bushed woodlands in providing a mosaic of habitats that support a diverse population of game birds. The results have wide application for an integrated management approach to livestock, wildlife and game bird management in the savannas.     

Multi-Scale Controls on Water Quality Effects of Submerged Aquatic Vegetation in the Tidal Freshwater Hudson River

Patches of submerged vegetation can be important sites of primary production and habitat for organisms in many aquatic ecosystems. In the tidal freshwater Hudson River they make up about 6% of the river bottom area. Direct sampling of water masses passing through patches of vegetation and week-long continuous monitoring of water characteristics were used to determine plant effects on dissolved oxygen and suspended sediments. Vegetated areas could have dissolved oxygen concentrations substantially higher than in the main channel and suspended sediments and turbidity were frequently higher in vegetated areas. Patches of Vallisneria americana had variable capacity to maintain super-saturated oxygen concentrations; patch size accounted for some of the variability whereas larger-scale differences in main-channel influent water also contributed. Differences in turbidity among sites were harder to account for; width of plant beds and abundance of neighboring vegetated areas contributed weakly to predictions of local turbidity. Functional heterogeneity within ecosystems is common and attempts to explain variability may require understanding different controlling factors for different functions and appreciating that factors operate at multiple scales.     

Hyporheic rehabilitation in rivers: restoring vertical connectivity

1. The hyporheic zone below the channel and banks of many rivers where surface water and ground water exchanges plays a crucial functional role in the biogeochemical transformation of water, mediated by active microbial biofilms. This zone also harbours assemblages of invertebrates that graze biofilms, contribute to secondary production, and can alter the porosity of the hyporheic zone through their movement or burrowing activities. 2. Many human activities cause interstitial sedimentation or disrupt surface–groundwater hydrological linkages, impacting upon ecological processes in the hyporheic zone. However, strategies for river rehabilitation seldom explicitly consider the hyporheic zone or seek to restore lost vertical linkages with groundwater. Instead, restoration goals target surface, riparian or floodplain features even though current river ecosystem theory emphasises the three dimensions of hydrological connectivity. To guide effective, holistic river restoration, scientists and managers therefore need information on the mechanisms by which energy and material are transferred in the hyporheic zone and which ecosystem services are thus provided. 3. Other gaps in our understanding of hyporheic zone rehabilitation include recruitment processes of the hyporheos and the relative importance of groups of hyporheic invertebrates in rivers differing in substratum size, disturbance frequency and groundwater linkages. Carefully designed experiments that assess responses to hyporheic rehabilitation strategies will provide valuable data at varying scales (e.g. distribution of hyporheic habitat types at the reach scale) for management as well as providing insights into the mechanisms controlling hyporheic invertebrate assemblages and ecological processes. Fully successful river rehabilitation must include restoration of vertical linkages between the river and its shallow groundwater aquifers.     

Grazing without grasses: Effects of introduced livestock on plant community composition in an arid environment in northern Patagonia

Question: How does grazing intensity affect plant density, cover and species richness in an Patagonian arid ecosystem? Location: Monte steppe ecoregion, SW Argentina. Methods: I analysed the effect of grazing on plant density, cover and species richness using a stocking rate gradient within the same habitat. Six paddocks were used with stocking rates ranging between 0.002 – 0.038 livestock/ha. Plant density, species richness, plant cover and percentage of grazed branches were determined by sampling plots within each paddock. The percentage of grazed branches was used as an independent measurement of grazing intensity. Results: Higher stocking rates were related to lower plant density, species richness and plant cover. The paddock with the lowest grazing intensity had 86% more plants per unit area, 63% more plant cover and 48% higher species richness. The percentage of grazed branches and the quantity of dung increased with stocking rate. Conclusions: Introduced livestock seriously affect native vegetation in the Patagonian Monte. The damage observed in this xerophytic plant community suggests that plant adaptations to aridity do not provide an advantage to tolerate or avoid grazing by vertebrate herbivores in this region. Plant degradation in this arid environment is comparable to the degradation found in more humid ecosystems.     

The eco-hydrology of partly cleared,native ecosystems in southern Australia: a review

Water use by the native vegetation that existed in southern Australia prior to European settlement was largely in balance with rainfall. European settlers altered the landscape by clearing land to grow agricultural crops and pastures, and with the introduction of livestock to graze the partly cleared, native ecosystems. The aim of this review is to contrast the hydrology of grazed, partly cleared ecosystems, intact indigenous ecosystems, and entirely cleared agricultural systems in the intensive land-use zone (350–1000 mm annual rainfall zone) of southern Australia. Since European settlement, the areas of forests and woodlands in the Murray–Darling Basin have declined by approximately 64% to make way for agricultural enterprises. Modern-day vegetation surveys estimate between 52 and 58% of the intensive land-use zone of the Murray–Darling Basin has been entirely cleared, while about 40% is in the partly cleared state (disturbed ecosystems with canopy cover exceeding 5%). The replacement of native vegetation by agricultural crops and pastures has disturbed the water cycle that existed prior to European settlement, and has markedly elevated the amount of water leaking beyond the root zone of introduced species, and contributing to groundwater systems. Estimates of annual leakage beneath the root zone of annual crops range from 0 to 63 mm per annum; however, no estimates of leakage for partly cleared woodlands exist. Yet, because the groundwater beneath partly cleared woodlands rises considerably more slowly than under entirely cleared landscapes, it is likely that less water leaks beneath the roots of grazed woody ecosystem. However, aging of these systems by livestock grazing will reduce the numbers of woody individuals and will impact on groundwater recharge.     

River Otter Latrine Site Selection in Arid Habitats of Western Colorado,USA

ABSTRACT River otters (Lontra canadensis) select specific habitat features when establishing latrines, but no studies have described latrine features in arid and semiarid environments. We developed a model describing those habitat features that influence otter latrine site selection on rivers in arid and semiarid watersheds of western Colorado, USA. River otters selected latrine sites with the presence of beaver (Castor canadensis) activity, large prominent rocks, adjacent to deeper water, with shading over the site, and rock or cliff overstory. Our model provides a robust predictive tool for identifying river otter latrine sites in arid environments of southwestern North America.     

Review of distribution of nitrogen and phosphorus in riparian zones of Chinese inland water bodies

This review tries to explain the effects of river infiltration and the interaction between groundwater and surface water on nitrogen (N) and phosphorus (P) distribution in riparian zones. On the basis of previous studies, this review summarized the distribution characteristics of N and P of 7 different riparian zones in China, compared the water quality of water bodies adjoining these riparian zones, proposed the distribution regularity of N and P in diverse riparian zones. The concentrations of N and P in the riparian zones with better water quality have increasing trend from riverside to upland. And those rivers with poorer water quality have decreasing trend. If the infiltrated concentration of runoff pollutant is more than river pollutant infiltration, the upland soil may obtain higher contaminants. On the contrary, if the concentration of river infiltration is more than that of the runoff, the riverside soil will show higher concentration. Similarly, if those two effects are equivalent, the concentration of N and P in the riparian zone will be low on riverside and upland, and somewhere in the middle will be higher than elsewhere. Clean rivers and polluted rivers taken with different quantity of contaminants will infiltrate diverse concentrations of N and P. It is expected that this review can provide a new theoretical basis for revealing the environmental effects of riparian zones and managing, protecting and restoring the riparian ecosystems.     

黑河下游河岸绿洲区包气带土壤水分与植被生长状况的研究

黑河下游荒漠平原区近年来由于河水流入量的持续减小,致使依赖河水补给来源的地下水呈减少趋势,造成区域性地下水位下降,从而引起一系列生态环境问题。通过研究黑河下游河岸绿洲区非饱和带的含水量、地下潜水埋深、土壤类型结构等与河岸绿洲植被生长状况之间的关系,发现土壤结构、地下水位埋深和土壤盐分是决定研究区植被生长状况的3个主要因素,而土壤结构是不可改变的,土壤盐分又与地下水关系密切,因此,地下水是极端干旱区植被生长状况的决定因素。     

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.     

Response of biota to land use changes and water quality degradation in two medium-sized river basins in southwestern Greece

To assess biotic responses to anthropogenic pressures, a seasonal monitoring of benthic macroinvertebrates, physicochemical and hydromorphological parameters has been applied in 11 sites along two highly impacted river basins in southwestern Greece, mainly during the year 2006. In the first river basin, the upper reaches were covered by thick riparian forest, replaced by intensive agriculture and livestock in the mid-reaches, while the lower parts were characterized mainly by agroindustrial activities. The second river basin was impacted by the discharge of raw cheese whey effluents in the mid-reaches, while the lower parts were protected by thick riparian vegetation. The mid- and lower parts of the first river basin (Peiros–Parapeiros rivers) were impaired by dam constructions, agricultural intensity and urbanization. In the second river basin (Vouraikos river), the 3/5 of the river length, located in the mid-reaches, was degraded due to the discharge of untreated cheese whey effluents. Water quality of the lower reaches was highly recovered due to the presence of thick riparian vegetation. Macroinvertebrate diversity was strongly correlated to pollution, as sites with major physicochemical and hydromorphological degradation presented low taxonomic richness and diversity values, while non-impacted sites were characterized by a considerable presence of E.P.T. taxa and higher diversity. The current study indicates the significant impact of land use changes to the water quality, which consequently influence the distribution of biota, pointing out the critical role of the riparian vegetation to the detoxification of surface waters.     

Altered stream-flow regimes and invasive plant species: the Tamarix case

Aim  To test the hypothesis that anthropogenic alteration of stream-flow regimes is a key driver of compositional shifts from native to introduced riparian plant species.
Location  The arid south-western United States; 24 river reaches in the Gila and Lower Colorado drainage basins of Arizona.
Methods  We compared the abundance of three dominant woody riparian taxa (native Populus fremontii and Salix gooddingii , and introduced Tamarix ) between river reaches that varied in stream-flow permanence (perennial vs. intermittent), presence or absence of an upstream flow-regulating dam, and presence or absence of municipal effluent as a stream water source.
Results  Populus and Salix were the dominant pioneer trees along the reaches with perennial flow and a natural flood regime. In contrast, Tamarix had high abundance (patch area and basal area) along reaches with intermittent stream flows (caused by natural and cultural factors), as well as those with dam-regulated flows.
Main conclusions  Stream-flow regimes are strong determinants of riparian vegetation structure, and hydrological alterations can drive dominance shifts to introduced species that have an adaptive suite of traits. Deep alluvial groundwater on intermittent rivers favours the deep-rooted, stress-adapted Tamarix over the shallower-rooted and more competitive Populus and Salix . On flow-regulated rivers, shifts in flood timing favour the reproductively opportunistic Tamarix over Populus and Salix , both of which have narrow germination windows . The prevailing hydrological conditions thus favour a new dominant pioneer species in the riparian corridors of the American Southwest. These results reaffirm the importance of reinstating stream-flow regimes (inclusive of groundwater flows) for re-establishing the native pioneer trees as the dominant forest type.