Effects of ice and floods on vegetation in streams in cold regions: implications for climate change

Riparian zones support some of the most dynamic and species‐rich plant communities in cold regions. A common conception among plant ecologists is that flooding during the season when plants are dormant generally has little effect on the survival and production of riparian vegetation. We show that winter floods may also be of fundamental importance for the composition of riverine vegetation. We investigated the effects of ice formation on riparian and in‐stream vegetation in northern Sweden using a combination of experiments and observations in 25 reaches, spanning a gradient from ice‐free to ice‐rich reaches. The ice‐rich reaches were characterized by high production of frazil and anchor ice. In a couple of experiments, we exposed riparian vegetation to experimentally induced winter flooding, which reduced the dominant dwarf‐shrub cover and led to colonization of a species‐rich forb‐dominated vegetation. In another experiment, natural winter floods caused by anchor‐ice formation removed plant mimics both in the in‐stream and in the riparian zone, further supporting the result that anchor ice maintains dynamic plant communities. With a warmer winter climate, ice‐induced winter floods may first increase in frequency because of more frequent shifts between freezing and thawing during winter, but further warming and shortening of the winter might make them less common than today. If ice‐induced winter floods become reduced in number because of a warming climate, an important disturbance agent for riparian and in‐stream vegetation will be removed, leading to reduced species richness in streams and rivers in cold regions. Given that such regions are expected to have more plant species in the future because of immigration from the south, the distribution of species richness among habitats can be expected to show novel patterns.     

Effects of climate‐induced increases in summer drought on riparian plant species: a meta‐analysis

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Hydrologic effects on riparian vegetation in a boreal river: an experiment testing climate change predictions

Climate change is expected to alter the magnitude and variation of flow in streams and rivers, hence providing new conditions for riverine communities. We evaluated plant ecological responses to climate change by transplanting turfs of riparian vegetation to new elevations in the riparian zone, thus simulating expected changes in water‐level variation, and monitored the results over 6 years. Turfs moved to higher elevations decreased in biomass and increased in species richness, whereas turfs transplanted to lower elevations gained biomass but lost species. Transplanted plant communities responded slowly to the new hydrologic conditions. After 6 years, biomass of transplanted turfs was statistically indistinguishable from target level controls, but species richness and species composition of transplants were intermediate between original and target levels. By using projections of future stream flow according to IPCC climate change scenarios, we predict likely changes to riparian vegetation in boreal rivers. Climate‐driven hydrologic changes are predicted to result in narrower riparian zones along the studied Vindel River in northern Sweden towards the end of the 21st century. Present riparian plant communities are projected to be replaced by terrestrial communities at high elevations as a result of lower‐magnitude spring floods, and by amphibious or aquatic communities at low elevations as a result of higher autumn and winter flows. Changes to riparian vegetation may be larger in other boreal climate regions: snow melt fed spring floods are predicted to disappear in southern parts of the boreal zone, which would result in considerable loss of riparian habitat. Our study emphasizes the importance of long‐term ecological field experiments given that plant communities often respond slowly and in a nonlinear fashion to external pressures.     

Riparian plant community responses to increased flooding: a meta‐analysis

A future higher risk of severe flooding of streams and rivers has been projected to change riparian plant community composition and species richness, but the extent and direction of the expected change remain uncertain. We conducted a meta‐analysis to synthesize globally available experimental evidence and assess the effects of increased flooding on (1) riparian adult plant and seedling survival, (2) riparian plant biomass and (3) riparian plant species composition and richness. We evaluated which plant traits are of key importance for the response of riparian plant species to flooding. We identified and analysed 53 papers from ISI Web of Knowledge which presented quantitative experimental results on flooding treatments and corresponding control situations. Our meta‐analysis demonstrated how longer duration of flooding, greater depth of flooding and, particularly, their combination reduce seedling survival of most riparian species. Plant height above water level, ability to elongate shoots and plasticity in root porosity were decisive for adult plant survival and growth during longer periods of flooding. Both ‘quiescence’ and ‘escape’ proved to be successful strategies promoting riparian plant survival, which was reflected in the wide variation in survival (full range between 0 and 100%) under fully submerged conditions, while plants that protrude above the water level (>20 cm) almost all survive. Our survey confirmed that the projected increase in the duration and depth of flooding periods is sufficient to result in species shifts. These shifts may lead to increased or decreased riparian species richness depending on the nutrient, climatic and hydrological status of the catchment. Species richness was generally reduced at flooded sites in nutrient‐rich catchments and sites that previously experienced relatively stable hydrographs (e.g. rain‐fed lowland streams). Species richness usually increased at sites in desert and semi‐arid climate regions (e.g. intermittent streams).     

Structural and functional responses of plant communities to climate change‐mediated alterations in the hydrology of riparian areas in temperate Europe

The hydrology of riparian areas changes rapidly these years because of climate change‐mediated alterations in precipitation patterns. In this study, we used a large‐scale in situ experimental approach to explore effects of drought and flooding on plant taxonomic diversity and functional trait composition in riparian areas in temperate Europe. We found significant effects of flooding and drought in all study areas, the effects being most pronounced under flooded conditions. In near‐stream areas, taxonomic diversity initially declined in response to both drought and flooding (although not significantly so in all years) and remained stable under drought conditions, whereas the decline continued under flooded conditions. For most traits, we found clear indications that the functional diversity also declined under flooded conditions, particularly in near‐stream areas, indicating that fewer strategies succeeded under flooded conditions. Consistent changes in community mean trait values were also identified, but fewer than expected. This can have several, not mutually exclusive, explanations. First, different adaptive strategies may coexist in a community. Second, intraspecific variability was not considered for any of the traits. For example, many species can elongate shoots and petioles that enable them to survive shallow, prolonged flooding but such abilities will not be captured when applying mean trait values. Third, we only followed the communities for 3 years. Flooding excludes species intolerant of the altered hydrology, whereas the establishment of new species relies on time‐dependent processes, for instance the dispersal and establishment of species within the areas. We expect that altered precipitation patterns will have profound consequences for riparian vegetation in temperate Europe. Riparian areas will experience loss of taxonomic and functional diversity and, over time, possibly also alterations in community trait responses that may have cascading effects on ecosystem functioning.     

Effects of river ice on riparian vegetation

1. Many rivers and streams experience pronounced ice dynamics caused by the formation of anchor and frazil ice, leading to flooding and disturbance of riparian and aquatic communities. However, the effects of dynamic ice conditions on riverine biota are little known. 2. We studied the formation of anchor ice in natural streams over 2 years and assessed the effects of anchor ice on riparian vegetation by comparing sites with frequent or abundant and little or no anchor ice formation. We also studied the direct impact of ice on riparian plants by experimentally creating ice in the riparian zone over three winters and by exposing plants of different life forms to ?18 °C cold ice in the laboratory. 3. Riparian species richness per 1‐m² plot was higher at sites affected by anchor ice than at sites where anchor ice was absent or rare, whereas dominance was lower, suggesting that disturbance by ice enhances species richness. Species composition was more homogenous among plots at anchor ice sites. By experimentally creating riparian ice, we corroborated the comparative results, with species richness increasing in ice‐treated plots compared to controls, irrespective of whether the sites showed natural anchor ice. 4. Because of human alterations of running waters, the natural effects of river ice on stream hydrology, geomorphology and ecology are little known. Global warming in northern streams is expected to lead to more dynamic ice conditions, offering new challenges for aquatic organisms and river management. Our results should stimulate new research, contributing to a better understanding of ecosystem function during winter.     

Flow regulation affects temporal patterns of riverine plant seed dispersal: potential implications for plant recruitment

1. Changes to the natural flow regime of a river caused by flow regulation may affect waterborne seed dispersal (hydrochory), and this may be an important mechanism by which regulation affects riverine plant communities. We assessed the effect of altered timing of seasonal flow peaks on hydrochory and considered the potential implications for plant recruitment. 2. We sampled hydrochory within five lowland rivers of temperate Australia, three of which are regulated by large dams. These dams are operated to store winter and spring rains and release water in summer and autumn for agriculture. At three sites on each river, hydrochory was sampled monthly for 12 months using passive drift nets. The contents of the drift samples were determined using the seedling‐emergence method. 3. More than 33 000 seedlings from 142 taxa germinated from the samples. In general, more seeds and taxa were observed in the drift at higher flows. By altering the period of peak flows from winter–spring to summer–autumn, flow regulation similarly affected the period of peak seed dispersal. The effect of regulation on seed dispersal varied between taxa depending on their timing of seed release and whether or not they maintain a persistent soil seed bank. 4. Hydrochory in rivers is a product of flow regime and the life history of plants. By altering natural flow regimes and thus hydrochorous dispersal patterns, flow regulation is likely to affect adversely the recruitment of native plant species with dispersal phenologies adapted to natural flow regimes (such as many riparian trees and shrubs) and encourage the spread of non‐native (exotic) species. 5. Changes to hydrochorous dispersal patterns are an important mechanism by which altered flow timing affects riverine plant communities. Natural seasonal flow peaks (in this case spring) are likely to be important for the recruitment of many native riparian woody taxa.     

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.     

The role of ice dynamics in shaping vegetation in flowing waters

Ice dynamics is an important factor affecting vegetation in high‐altitude and high‐latitude streams and rivers. During the last few decades, knowledge about ice in streams and rivers has increased significantly and a respectable body of literature is now available. Here we review the literature on how ice dynamics influence riparian and aquatic vegetation. Traditionally, plant ecologists have focused their studies on the summer period, largely ignoring the fact that processes during winter also impact vegetation dynamics. For example, the freeze‐up period in early winter may result in extensive formation of underwater ice that can restructure the channel, obstruct flow, and cause flooding and thus formation of more ice. In midwinter, slow‐flowing reaches develop a surface‐ice cover that accumulates snow, protecting habitats under the ice from formation of underwater ice but also reducing underwater light, thus suppressing photosynthesis. Towards the end of winter, ice breaks up and moves downstream. During this transport, ice floes can jam up and cause floods and major erosion. The magnitudes of the floods and their erosive power mainly depend on the size of the watercourse, also resulting in different degrees of disturbance to the vegetation. Vegetation responds both physically and physiologically to ice dynamics. Physical action involves the erosive force of moving ice and damage caused by ground frost, whereas physiological effects – mostly cell damage – happen as a result of plants freezing into the ice. On a community level, large magnitudes of ice dynamics seem to favour species richness, but can be detrimental for individual plants. Human impacts, such as flow regulation, channelisation, agriculturalisation and water pollution have modified ice dynamics; further changes are expected as a result of current and predicted future climate change. Human impacts and climate change can both favour and disfavour riverine vegetation dynamics. Restoration of streams and rivers may mitigate some effects of anticipated climate change on ice and vegetation dynamics by, for example, slowing down flows and increasing water depth, thus reducing the potential for massive formation of underwater ice.     

The role of hydrochory in structuring riparian and wetland vegetation

Hydrochory, or the passive dispersal of organisms by water, is an important means of propagule transport, especially for plants. During recent years, knowledge about hydrochory and its ecological consequences has increased considerably and a substantial body of literature has been produced. Here, we review this literature and define the state of the art of the discipline. A substantial proportion of species growing in or near water have propagules (fruits, seeds or vegetative units) able to disperse by water, either floating, submerged in flowing water, or with the help of floating vessels. Hydrochory can enable plants to colonize sites out of reach with other dispersal vectors, but the timing of dispersal and mechanisms of establishment are important for successful establishment. At the population level, hydrochory may increase the effective size and longevity of populations, and control their spatial configuration. Hydrochory is also an important source of species colonizing recruitment‐limited riparian and wetland communities, contributing to maintenance of community species richness. Dispersal by water may even influence community composition in different landscape elements, resulting in landscape‐level patterns. Genetically, hydrochory may reduce spatial aggregation of genetically related individuals, lead to high gene flow among populations, and increase genetic diversity in populations receiving many propagules. Humans have impacted hydrochory in many ways. For example, dams affect hydrochory by reducing peak flows and hence dispersal capacity, altering the timing of dispersal, and by presenting physical barriers to dispersal, with consequences for riverine plant communities. Hydrochory has been inferred to be an important vector for the spread of many invasive species, but there is also the potential for enhancing ecosystem restoration by improving or restoring water dispersal pathways. Climate change may alter the role of hydrochory by modifying the hydrology of water‐bodies as well as conditions for propagule release and plant colonization.     

Context-dependent defence in terrestrial plants: the effects of light and nutrient availability on plant resistance against herbivory

The context‐dependent defence (CDD) hypothesis predicts that defence levels of plant species against herbivory are not fixed but vary with environmental conditions, in a way that is specific for plant species that share evolutionary adaptations to resource conditions exemplified by similar maximum relative growth rates. More specifically, we expected plants from resource‐poor environments to display high defence levels but not when grown under resource‐rich conditions, whereas the reverse – plants from resource‐rich conditions displaying low defence levels but not when grown under resource‐poor conditions – is not necessarily the case. In this study, we used multiple‐choice bioassays in which leaf discs were fed to larvae of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) as an efficient and effective way of indicating plant defence levels. This generalist herbivore was capable of detecting both inter‐ and intraspecific differences in defence among plant species. The CDD was tested by exploring the effects of various experimental resource conditions (light, nutrients) upon the herbivore preferences and by comparing these preferences with the maximum relative growth rate of plant species. The experimental results provide general support for the CDD hypothesis with respect to nutrient‐level variation but the effects were not related to the origin of the plant species tested. Variation in light conditions did not result in consistent effects upon herbivore preferences. The CDD therefore can be formulated more precisely as: defence levels of plant species vary under different environmental conditions but in a way that is specific for plant species that share evolutionary adaptations to similar nutrient conditions. This more precise CDD hypothesis is a useful addition to existing optimal‐defence theory because of its focus on the possible plastic effects of resource conditions upon plant defence levels. This is relevant when designing experimental plant–herbivore studies.     

伊洛河河岸带生态系统草本植物功能群划分

植物功能群是对环境有相同响应和对主要生态系统过程有相似作用的组合。伊洛河是黄河中游南岸的一条重要水系,其河岸带生态系统不同生境类型中草本植物优势种变化明显,能较好地反映出植被与环境的动态关系。根据调查结果,结合出现次数和重要值,选取27种优势种进行种间联结及相关性分析。以$2检验为基础,结合联结系数AC和共同出现百分率PC来测定草本层优势种间的联结性,根据优势种间的联结性及其在不同生境中的变化异同,以优势种为主体划分伊洛河河岸带生态系统中草本植物功能群。结果表明,对27种草本植物优势种共划分了5组功能群:"广布型"、"湿生型"、"中旱生型"、"农田逃逸型"和"入侵型"。表明以优势种为主体对伊洛河河岸带生态系统草本植物进行功能群划分可行性高,有较强的代表性。同一功能群物种间表现出显著正联结性,一起出现在同一生境下的几率较大,在长期的生长演化过程中,能够适应相似的资源环境且对干扰有相似的响应。伊洛河河岸带生态系统由于长期的自然和人为干扰,加上外来物种的入侵,河岸带生态系统的生物多样性和生态安全面临着严峻的挑战。     

三峡水库奉节以东秭归和巫山段消落带植物群落动态特征

基于2008—2012年对三峡水库奉节以东秭归和巫山段消落带固定样地不同海拔区段植物群落的5a定位监测,研究消落带植物群落的物种组成、优势植物、植物生活型和物种多样性的动态变化,结果表明:1)截止2012年,消落带海拔156—172 m区段共经历了4次水库水位涨落。经历首次后(2009年),消落带原生植物由55科147种减少到18科33种,经历4次后(2012年),减少到14科39种。与经历水库水位涨落前(2008年)比较,经历首次后的科数减少了67.3%,种数减少了77.6%;经历4次后的科数减少了74.5%,种数减少了73.5%。在消落带原生植物减少的同时,出现了许多"新"植物。经历首次后出现了49种,经历4次后出现了23种,分别占调查当年样地植物种类总数的59.8%和32.9%。海拔172—175 m区段共经历了2次水库水位涨落,消落带原生植物由40科91种(2008年)减少到了13科20种。与经历水库水位涨落前比较,科数减少了67.5%,种数减少了78.0%。出现"新"植物21种,约占调查当年样地植物种类总数的44.7%。通过对历次调查中消落带植物"消失"和"出现"的数量比较表明,消落带植物对经历首次水库水位涨落的反应最为敏感,此后,虽又经历过几次水库水位涨落,但其变化速率趋于减小。2)不同海拔区段、不同生态适应型植物的"消长"动态和优势种组成不完全相同。海拔156—172 m区段,经历4次水库水位涨落后,在消落带植物群落中占优势的草本植物种为菊科(Compositae)的鬼针草(Bidens pilosa)、禾本科(Gramineae)的狗牙根(Cynodon dactylon)、毛马唐(Digitaria chrysoblephara)、狗尾草(Setaria viridis)、莎草科(Cyperaceae)的碎米莎草(Cyperus iria),占优势的灌木树种为漆树科(Anacardiaceae)的盐肤木(Rhus chinensis)和大戟科(Euphorbiaceae)的算盘子(Glochidion puberum);在海拔172—175 m区段,除鬼针草、毛马唐仍为优势种外,还增加了大戟科的湖北算盘子(Glochidion wilsonii),马鞭草科(Verbenaceae)的黄荆(Vitex negundo),葡萄科(Vitaceae)的五叶地锦(Parthenocissus quniquefolia)等树种。3)消落带植物群落的优势生活型为一年生和多年生草本;物种多样性随着水库水位涨落次数的增加总体变化呈减少趋势。4)三峡水库水位周期性涨落导致消落带发生水陆环境交替变化,不同生态适应型植物对变化生境的适应能力有所不同,是消落带植物群落发生变化的主要驱动因素。     

Riparian Trees and Flow Paths between the Hyporheic Zone and Groundwater in the Oberer Seebach,Austria

We investigated lateral subsurface water exchange in a 2^nd order mountain stream with a piezometer method. At both banks the stream hyporheic zone lost water to the riparian groundwater zone. Independently, the hydraulic heads at three sites in the streambed and in the riparian zone exhibited periodic, diurnal fluctuations. We attributed them to water consumption by the riparian trees, as solar radiation explained part of this additional variation. Our results demonstrate that subsurface water exchanges take place between the hyporheic zone and lateral riparian groundwater in spatially defined small‐scale flow paths. These small‐scale interactions occur within the context of large‐scale patterns of loss and gain of channel water.     

Using water plant functional groups to investigate environmental water requirements

1. Analysis of the distribution and abundance of water plants can be a useful tool for determining the ecological water requirements of sites in a catchment. 2. Seed‐bank and vegetation surveys of wetland and riparian sites were undertaken in the Angas River catchment in South Australia to determine the distribution and abundance of plants associated with riparian habitats. Plant species were allocated to water plant functional groups (WPFGs sensu Brock and Casanova, Frontiers in Ecology; Building the Links, 1997, Elsevier Science). In addition to the seven functional groups already recognised, three new groups containing submerged and woody growth forms were included in this study. 3. Cluster analysis of sites on the basis of species presence/absence was compared with site clustering obtained from analysis of representation of WPFGs. Functional group analysis provided a similar segregation of species‐poor sites to that resulting from analysis of species presence/absence, but provided better resolution of clusters for species‐rich sites. Three clusters of species‐rich sites were delineated: riparian sites that require year‐round permanent water but have fluctuating water levels, spatially and temporally variable riparian sites with shrubs and trees and temporary wetlands that dry annually. 4. Segregation of sites on the basis of functional group representation can provide information to managers about the water requirements of suites of species in different parts of the catchment. Knowledge of the environmental water requirements of sites within a catchment can help managers to prioritise water management options and delivery within that catchment.     

Responses of riparian trees and shrubs to flow regulation along a boreal stream in northern Sweden

1. Flow dynamics is a major determinant of riparian plant communities. Therefore, flow regulation may heavily affect riparian ecosystems. Despite the large number of dams worldwide, little specific information is available on the longitudinal impacts of dams on vegetation, for example how far downstream and at what degree of regulation a dam on a river can influence riparian woodlands. 2. We quantified the long‐term responses of riparian trees and shrubs to flow regulation by identifying their lateral distribution and habitat conditions along a boreal river in northern Sweden that has been regulated by a single dam since 1948. The regulation has reduced annual flow fluctuations, this effect being largest at the dam, downstream from which it progressively decreases following the entrance of free‐flowing tributaries. 3. We related changes in the distribution patterns, composition, abundance and richness of tree and shrub species to the degree of regulation along the river downstream from the dam. Regulation has triggered establishment of trees and shrubs closer to the channel, making it possible to measure ecological impacts of flow regulation as differences in vegetation attributes relative to the positions of tree and shrub communities established before and after regulation. 4. Trees and shrubs had migrated towards the mid‐channel along the entire study reach, but the changes were largest immediately downstream of the dam. Shrubs were most impacted by flow regulation in terms of lateral movement, but the effect on trees extended furthest downstream. 5. The species composition of trees progressively returned to its pre‐regulation state with distance downstream, but entrance of free‐flowing tributaries and variation in channel morphology and substratum caused local deviations. Species richness after regulation increased for trees but decreased for shrubs. The changes in species composition and richness of trees and shrubs showed no clear downstream patterns, suggesting that other factors than the degree of regulation were more important in governing life form.     

Flow variability maintains the structure and composition of in‐channel riparian vegetation

1. Naturally variable river flows are considered to be important for structuring riparian vegetation. However, while the importance of floods for the ecology of riparian vegetation is well recognised, much less is known about the importance of small fluctuations in river flows. 2. We investigated the effect of water supply diversion weirs on the riparian vegetation of upland streams. These weirs remove within‐channel fluctuations in flow but do not prevent large floods downstream. We surveyed the in‐channel and banktop vegetation of five streams, three of which were regulated by weirs and two of which acted as controls. 3. Unexpectedly, we observed greater species richness within the channel downstream of the weirs. This was because of increased numbers of exotic and terrestrial (‘dry’) plant species. Grass cover was also greater downstream of the weirs. There were no significant differences in the banktop vegetation between the upstream and downstream sites of the regulated streams. 4. Our results highlight the role of within‐channel flow variability in maintaining the composition of vegetation within the stream channel. We suggest that greater species richness does not necessarily indicate a less‐disturbed environment. Rather, a greater number of ‘dry’ species is indicative of the impacts of flow regulation. 5. Small fluctuations in river flows are probably necessary to protect the ecosystem structure and function of regulated streams. It is recommended that variable within‐channel flows be provided in regulated streams.     

Impact of the arundo wasp,Tetramesa romana (Hymenoptera: Eurytomidae), on biomass of the invasive weed,Arundo donax (Poaceae: Arundinoideae), and on revegetation of riparian habitat along the Rio Grande in Texas

An invasive grass, Arundo donax, occupies thousands of hectares of arid riparian habitat along the Rio Grande in Texas and Mexico, and has negative impacts on national security, water resources, and riparian ecosystems. The shoot-tip-galling wasp Tetramesa romana was released in 2009 between Brownsville and Del Rio, Texas, and has dispersed over 800?km along the river channel. Plots along the river were surveyed for shoot counts of arundo and all other plant species in 2016 at seven sites in regions in which prior studies had documented a 22% decline in arundo biomass (estimated from live shoot length) from 2007 to 2014. Estimated live biomass declined a further 32% between 2014 and 2016. Native plants accounted for 86% of the 44 species encountered in plots. Individual plots averaged five plant species, and arundo was most abundant in only 9 of 21 plots. Arundo live biomass and shoot density were negatively associated with plant diversity, indicating that live arundo interferes with germination and/or survival of other plant species. The proportion of dead shoots in plots, proportion of wasp-galled shoots, and density of exit holes made by emerging adult wasps per metre live main shoot length were positively associated with plant diversity in a combined model. Regressions indicated that the effects of wasp damage measures on diversity were mediated through their effects on main shoot mortality. By reducing live arundo biomass, the arundo wasp is fostering recovery of native plant communities at riparian sites along the Rio Grande.     

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition. 2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River. 3. Patterns of species richness varied with temporal scale of analysis, that is between single‐year and multi‐year time frames. At the annual timescale, quadrat species richness (m^?2) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single‐year pattern, the highest long‐term richness occurred at intermittent sites. 4. Quadrat species richness, total species richness at a site (per 18 1‐m² plots) and cover were more variable year to year at non‐perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter‐annual compositional variance, while the perennial sites had the lowest. 5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types. 6. High long‐term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non‐perennial sites.     

Butterflies as an indicator group of riparian ecosystem assessment

Riparian ecosystems play an important role in modulating a range of ecosystem processes that affect aquatic and terrestrial organisms. Butterflies are a major herbivore in terrestrial ecosystems and are also common in riparian ecosystems. Since butterflies use plants for larval food and adult nectar sources in riparian ecosystems, butterfly diversity can be utilized to evaluate riparian ecosystems. We compiled butterfly data from 33 sites in three riparian ecosystem types across the country and compared butterfly diversity in terms of number of species and quality index in relation to riparian environmental variables. Number of butterfly and plant species was not different among three riparian habitat types. Additionally, there was no significant ecological variable to distinguish the butterfly communities on three riparian habitats. Non-metric multi-dimensional scaling ordination showed that butterfly communities in three riparian ecosystem types differed from each other, and butterfly riparian quality index was the main variable for butterfly assemblages. Five indicator species for moor and another five species for riverine riparian ecosystems were identified. Three and one indicator species for moor and riparian ecosystems, respectively, were plant specialists, while 44 butterflies were general feeders, feeding on a wide range of hostplants in several habitats. These results suggest that butterfly species use actively riparian habitats for nectar and larval food, and the butterfly riparian quality index can be employed to track faunal change in riparian habitats, which are frequently threatened by disturbances such as water level and climate changes, and invasive species.