Effect of the grass Leersia hexandra on the dispersal,seed germination,and establishment of Pachira aquatica seedlings

1. In wetlands, hydrochory is one of the main mechanisms of seed dispersal and there is often synchrony between propagule production and the flood season. Different sources of disturbance can prevent seed dispersal to suitable sites, and if environmental conditions are not adequate for germination and seedling establishment, recruitment will be limited, affecting succession.
2. We worked in a disturbed tropical freshwater swamp where the native grass Leersia hexandra has dominated open areas, creating a grass matrix that surrounds patches of swamp forest. Leersia grows vigorously, forming cushions of dry matter that cover the soil, forming a potential obstacle to seed dispersal. We asked whether the vegetative growth of this grass prevents the entry and dispersal of seeds of the tropical swamp tree Pachira aquatica, thwarting seed germination and seedling establishment, and arresting succession. We set up transects in the grass matrix in two zones: close to the river (R) and bordering the tree patches or fragments (F). We quantified tree seed and seedling presence, survival and growth in situ and experimentally introduced seeds and seedlings in the field and monitored seed germination and the survival and growth of their seedlings, as well as that of transplanted seedlings.
3. There was a negative relationship between the number of seeds and established seedlings, and the distance to river or fragment (r = −0.86, p < 0.001 for zone R; and r = −0.77, p < 0.001 for zone F) and with the grass cushion (r = −0.68, p = 0.005 for zone R; and r = −0.66, p = 0.007 for zone F); the grass creating a barrier to dispersal. When seeds were sown after clearing the grass cushion, germination success was high, so this stage is not limited. The transplanted seedlings had better survival and a greater final height than the seedlings of the sown seeds. Grass cover had a negative effect on both types of seedlings. Seedling survival rates were inversely related to grass cover, showing that seedlings overgrown by grass had low survival rates. Flooding is a stress factor for seedlings and produced mortality, in addition to the effects of the grass.
4. Together, the field survey and the experiment show that succession is being arrested in two ways: (1) by limiting seed dispersal because the grass cushion slows the dispersal and penetration of seeds into the vegetation; and (2) by limiting seedling establishment because the grass competes for space and light. Our results show that even where the grass is native, slower growing, seed-dependent species may struggle to compete and establish. If grass cover is increasing, these swamps are very vulnerable to a decrease in area because it is very difficult for them to regenerate naturally.

     

河岸带生态系统退化机制及其恢复研究进展

恢复和重建自然和人为干扰导致的退化河岸带生态系统是目前恢复生态学、流域生态学等学科研究的重要内容之一．对河岸带生态系统的干扰表现在河流水文特征改变、河岸带直接干扰和流域尺度干扰3个方面,分别具有不同的影响机制．河流水文特征改变通过改变河岸土壤湿度、氧化还原电位、生物生存环境以及沉积物传输规律对河岸带生态系统产生影响;河岸带直接干扰通过人类活动及外来物种入侵而直接影响河岸带植被多样性;流域尺度干扰则主要表现在河道刷深、河道淤积、河岸带地下水位降低和河流冲刷过程改变等．河岸带生态恢复评价对象包括河岸带生态系统各要素,评价指标已从单一的生态指标转向综合性指标．河岸带生态恢复应在景观或者流域尺度上进行考虑,识别对其影响的生物和物理过程以及导致其退化的干扰因子,通过植被重建与水文调控来进行．扩展研究尺度和研究对象及采用多学科的研究方法将是今后相关研究中的重要问题．     

Effects of physical disturbance and granivory on establishment of native and alien riparian trees in Colorado, U.S.A.

Abstract.  In western North America, the alien Elaeagnus angustifolia L. invades riparian habitats usually dominated by pioneer woody species such as Populus deltoides Marshall ssp. monilifera (Aiton) Eckenwalder . We conducted manipulative field experiments to compare the importance of physical disturbance and granivory for seedling establishment of these two species. We planted seeds of both species in disturbed and undisturbed study plots, and used exclosures, seed dish trials and live-trapping to assess the role of granivory. Seedling establishment of both species was increased by physical disturbance and seeds of both species were subject to granivory. However, the relative importance of these two factors differed between species. For P. deltoides , lack of physical disturbance prevented seedling establishment in uncleared subplots, but granivory did not prevent seedling establishment outside of exclosures. For E. angustifolia , granivory prevented seedling establishment outside of exclosures, but lack of physical disturbance did not prevent establishment in uncleared subplots. The lesser dependence on disturbance may enable E. angustifolia to invade areas characterized by low levels of fluvial disturbance, such as floodplains along regulated rivers, where P. deltoides recruitment does not occur. Populations of granivorous rodents may affect the susceptibility of riparian ecosystems to invasion by E. angustifolia .     

Post‐Impoundment Biomass and Composition of Phytoplankton in the Yangtze River

Damming, and thus alteration of stream flow, promotes higher phytoplankton populations and encourages algal blooms (density >10⁶ cells L^–1) in the Three Gorges Reservoir (TGR). Phytoplankton composition and biomass were studied in the Yangtze River from March 2004 to May 2005. 107 taxa were identified. Diatoms were the dominant group, followed by Chlorophyta and Cyanobacteria. In the Yangtze River, algal abundance varied from 3.13 × 10³ to 3.83 × 10⁶ cells L^–1, and algal biomass was in the range of 0.06 to 659 mg C m^–3. Levels of nitrogen, phosphorus and silica did not show consistent longitudinal changes along the river and were not correlated with phytoplankton parameters. Phytoplankton abundance was negatively correlated with main channel discharge (Spearman r = –1.000, P < 0.01). Phytoplankton abundance and biomass in the Yangtze River are mainly determined by the hydrological conditions rather than by nutrient concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Post‐storm sediment burial and herbivory of Vallisneria americana in the Hudson River estuary: mechanisms of loss and implications for restoration

This article investigates the mechanics of loss of Hudson River Vallisneria americana after the high volume storms at the end of the 2011 growing season, when two severe weather events—Tropical Storm Irene and the remnants of Tropical Storm Lee—struck the Hudson River watershed. In 2012, the distribution of the most common species of submerged aquatic vegetation (SAV), Vallisneria americana (wild celery, water celery, or tape grass), in the Hudson River estuary declined by more than 90%, with no appreciable recovery in 2013 and 2014. Because of its important habitat value for aquatic life and for increasing dissolved oxygen, managers and scientists have begun discussing the reasons for the loss, as well as how to assist its recovery through assisted restoration efforts in the estuary. Supported by in situ and in vitro experiments, the article posits the hypothesis that sediment, washed into the river by the storm, buried overwintering tubers of the plant, thus reducing sprouting success. Sprouting was as low as 50% with sediment depth between 2 and 5 cm; sprouting did not occur with sediment depth greater than 10 cm. Field experiments found no support for the hypothesis that herbivory inhibited regrowth of the plant after the storm events. These results suggest that future assisted restoration of Vallisneria americana and SAV in general may require attention to system‐specific factors.     

Relationship between vegetation,hydrology and fluvial landforms on an unregulated sand‐bed stream in the Hunter Valley,Australia

A survey of fluvial landforms was conducted at Widden Brook, an unregulated sand‐bed stream in the Hunter Valley, New South Wales (NSW), Australia, to investigate the physical factors associated with vegetation pattern in Riverine Oak Forest. Groundwater depth and chemistry (pH, dissolved oxygen and electrical conductivity) were measured using piezometers and submersible data loggers on three fluvial landforms (i.e. toe of bank, top of bank and floodplain) along five transects. Floristic composition, canopy cover, bare ground and leaf litter were assessed within 45 quadrats on the three landforms along the five transects. Elevation above the bed and flood return period were determined by cross‐sectional survey and flood frequency analysis, while flow duration was determined from the gauge record. Canonical correspondence analysis demonstrated that vegetation composition was associated with average watertable depth and flood variables to a similar extent. The relative importance of these factors would be expected to vary with flood‐ and drought‐dominated climatic periods on a scale of several decades. Floristic composition was moderately associated with the canopy cover of the dominant woody species, Casuarina cunninghamiana (Miq.), but weakly correlated with bare ground and groundwater chemistry. Suites of species were associated with particular fluvial landforms and their corresponding flood and watertable conditions. The reach examined has characteristics similar to both the semi‐arid and mesic riparian ecosystems of the USA. The coarse sediments, high flood variability, short flood duration and dominance by a pioneer tree that relies on groundwater are similar to riparian ecosystems in the western USA, while the relatively broad floodplain and the development of a forest canopy that is associated with the distribution of understorey plants are similar to the mesic riparian systems in the eastern USA.     

Effects of wind‐driven spatial structure and environmental heterogeneity on high‐altitude wetland macroinvertebrate assemblages with contrasting dispersal modes

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Post‐clearcut dynamics of carbon,water and energy exchanges in a midlatitude temperate,deciduous broadleaf forest environment

Clearcutting and other forest disturbances perturb carbon, water, and energy balances in significant ways, with corresponding influences on Earth's climate system through biogeochemical and biogeophysical effects. Observations are needed to quantify the precise changes in these balances as they vary across diverse disturbances of different types, severities, and in various climate and ecosystem type settings. This study combines eddy covariance and micrometeorological measurements of surface‐atmosphere exchanges with vegetation inventories and chamber‐based estimates of soil respiration to quantify how carbon, water, and energy fluxes changed during the first 3 years following forest clearing in a temperate forest environment of the northeastern US. We observed rapid recovery with sustained increases in gross ecosystem productivity (GEP) over the first three growing seasons post‐clearing, coincident with large and relatively stable net emission of CO₂ because of overwhelmingly large ecosystem respiration. The rise in GEP was attributed to vegetation changes not environmental conditions (e.g., weather), but attribution to the expansion of leaf area vs. changes in vegetation composition remains unclear. Soil respiration was estimated to contribute 44% of total ecosystem respiration during summer months and coarse woody debris accounted for another 18%. Evapotranspiration also recovered rapidly and continued to rise across years with a corresponding decrease in sensible heat flux. Gross short‐wave and long‐wave radiative fluxes were stable across years except for strong wintertime dependence on snow covered conditions and corresponding variation in albedo. Overall, these findings underscore the highly dynamic nature of carbon and water exchanges and vegetation composition during the regrowth following a severe forest disturbance, and sheds light on both the magnitude of such changes and the underlying mechanisms with a unique example from a temperate, deciduous broadleaf forest.     

Soil moisture dynamics and restoration of self‐sustaining native vegetation ecosystem on an open‐cut coal mine

Post‐mining landscape reconstruction on open‐cut coal mines aims to support restoration of self‐sustaining native vegetation ecosystems that in perpetuity require no extra inputs relative to unmined analogs. Little is known about the soil moisture retention capacity of the limited layer of topsoil replaced (often <30 cm deep), impacts of deep ripping of the profile, and the combined impacts of these on plant available water during the mine restoration process. We examined changes in soil moisture parameters (soil water potential, Ψ, and soil water content, Θ) daily using automated soil sensors installed at 30 and 45–65 cm depths on mine restoration sites aged between 3 and 22 years and on adjacent remnant vegetation sites following heavy rainfall events at Meandu mine, southeast Queensland, Australia. Consistent patterns in soil moisture attributes were observed among rehabilitated sites with generally marked differences from remnant sites. Remnant site soil profiles had generally higher Θ after drying than rehabilitated sites and maintained high Ψ for extended periods after rain events. There was a relatively rapid decline of Ψ on reconstructed soil profiles compared with remnant sites although the times of decline onset varied. This response indicated that vegetation restoration sites released soil moisture more rapidly than remnant sites but the rate of drying decreased with increasing rehabilitation age and increased with increasing tree stem density. The rapid drying of mine rehabilitated sites may threaten the survival of some remnant forest species, limit tree growth, and delay restoration of self‐sustaining native ecosystem.     

The effects of disturbance on the population structure and regeneration potential of five dominant woody species – in Hugumburda‐Gratkhassu National Forest Priority Area,North‐eastern Ethiopia

Vast areas of forests in North‐eastern Ethiopia have been replaced by cropland, shrub land or grazing areas. Thus, information about how vegetation composition and structure varies with disturbance is fundamental to conservation of such areas. This study aimed to investigate the effects of disturbance on the population structure and regeneration potential of five dominant woody species within forest where local communities harvest wood and graze livestock. Vegetation structure and environmental variables were assessed in 50 quadrats (20 m × 20 m). In most of both disturbed and undisturbed treatments, Juniperus procera was the highest contributor to the basal area of the forest, while that of Olinia rochetiana was the lowest. Analysis of population structure showed high density at lower Diameter at Breast Height (DBH) and low density at higher DBH classes. Undisturbed forest treatments had 84% canopy cover, 22 m mean vegetation height and a density of 1320 trees of dominant species and 1024 seedlings/saplings ha^?1. In disturbed habitats, canopy cover (73%), mean vegetation height (18 m) and density of dominant trees and saplings were significantly lower than in undisturbed habitats. Thus, to ensure species, survival and maintain species diversity managed use of the protected area is essential.     

Annual survival and breeding dispersal of a migratory passerine,the Scissor‐tailed Flycatcher

Knowledge of survival rates is critical for advancing our understanding of the dynamics of populations and here we report apparent annual survival and breeding dispersal of Scissor‐tailed Flycatchers (Tyrannus forficatus) breeding at two sites in southwest Oklahoma (Ft. Sill and Wichita Mountain Wildlife Refuge [WMWR]). Our Cormack‐Jolly‐Seber estimate of apparent adult survival for the period from 2008 to 2105 was relatively low (0.514) compared to estimates for 36 other migratory and socially monogamous passerines breeding in North America, and was independent of sex (males: N = 151; females: N = 119), breeding status (territory holder or floater), body mass, site, year, and precipitation during the year prior to breeding. Although apparent survival did not differ between sites, dispersal (N = 66 individuals) was more common and breeding dispersal distance (BDD) was greater for Scissor‐tailed Flycatchers at Ft. Sill where anthropogenic disturbance was more frequent. BDD also increased with body mass at Ft. Sill (but not at WMWR) and, after accounting for it, BDD at Ft. Sill tended to be greater for birds that failed to breed successfully in the past year. Older birds and males had the longest BDDs at WMWR, and males exhibited a similar tendency at Ft. Sill. We contend that our estimate of apparent survival is low, not because of inherently low survivorship, but, instead, as a consequence of frequent permanent emigration from our population. We also suggest that the greater BDD of older birds (WMWR) and males (both sites) reflects a history of selection for dispersal in response to frequent habitat disturbance. Frequent habitat disturbance, in addition to the opportunity to prospect for territories both before and after breeding, probably enable the earliest spring arrivals (typically older birds and males) to often relocate between years.     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Determinants of successful establishment and post‐translocation dispersal of a new population of the critically endangered St. Croix ground lizard (Ameiva polops)

Translocation to areas free of exotic predators, habitat degradation, or disease may be the most viable restoration option for many endangered species. We report on a successful translocation of the critically endangered St. Croix ground lizard, Ameiva polops, extirpated from St. Croix, U.S. Virgin Islands, Caribbean, by predation from introduced mongooses (Herpestes auropunctatus). We translocated 57 adult A. polops from Green Cay to Buck Island in May 2008. We placed 4 females and 3 males each in eight, 100 m², enclosures on Buck Island for 71 days, then the enclosures were opened. During the enclosure period, 20 individuals were identified and 32 others were seen. The average number sighted per survey was only 5.28 (range = 2–10). One hatchling was sighted in an enclosure, indicating a translocated female successfully nested. Body condition of the translocated individuals increased significantly by the end of the enclosure period. Population monitoring surveys at 61 sites across Buck Island showed that 5 years after the initial translocation in June 2013, the new population had grown to an estimated 1,473 individuals and occupied 58.9% of the island. We attribute eradication of mongoose, life history of the species, large propagule size, condition of habitat, soft‐release, use of adults, interagency collaboration, and systematic assessment as primary factors that facilitated this successful translocation. Our findings provide meaningful insights on factors that enhance the potential for successful translocations, and point to new strategies aimed at restoring populations of endangered reptiles in their native ranges.     

Effect of land‐use and land‐cover change on mangrove blue carbon: A systematic review

Mangroves shift from carbon sinks to sources when affected by anthropogenic land‐use and land‐cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer‐reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re‐establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre‐disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.     

An assessment of riparian restoration outcomes in two rural catchments in south‐western Victoria: Focusing on tree and shrub species richness,structure and recruitment characteristics

Riparian ecosystems are among the most degraded systems in the landscape, and there has been substantial investment in their restoration. Consequently, monitoring restoration interventions offers opportunities to further develop the science of riparian restoration, particularly how to move from small‐scale implementation to a broader landscape scale. Here, we report on a broad range of riparian revegetation projects in two regions of south‐western Victoria, the Corangamite and Glenelg‐Hopkins Catchment Management Areas. The objectives of restoration interventions in these regions have been stated quite broadly, for example, to reinstate terrestrial habitat and biodiversity, control erosion and improve water quality. This study reports on tree and shrub composition, structure and recruitment after restoration works compared with remnant vegetation found regionally. Within each catchment, a total of 57 sites from six subcatchments were identified, representing three age‐classes: <4, 4–8 and >8–12 years after treatment, as well as untreated (control) sites. Treatments comprised fencing to exclude stock, spraying or slashing to reduce weed cover, followed by planting with tube stock. Across the six subcatchments, 12 reference (remnant) sites were used to provide a benchmark for species richness, structural and recruitment characteristics and to aid interpretation of the effects of the restoration intervention. Vegetation structure was well developed in the treated sites by 4–8 years after treatment. However, structural complexity was higher at remnant sites than at treated or untreated sites due to a higher richness of small shrubs. Tree and shrub recruitment occurred in all remnant sites and at 64% of sites treated >4 years ago. Most seedling recruitment at treatment sites was by Acacia spp. This assessment provides data on species richness, structure and recruitment characteristics following restoration interventions. Data from this study will contribute to longitudinal studies of vegetation processes in riparian landscapes of south‐western Victoria.     

Paddock‐scale patterns of seed production and dispersal in the invasive shrub Acacia nilotica (Mimosaceae) in northern Australian rangelands

Paddock‐scale Acacia nilotica L. Willd. ex Del. (prickly acacia) spatial distribution, seed production and dispersal patterns were investigated in the Astrebla (Mitchell) grasslands of northern Australia as a step toward predicting future patterns of invasion. A number of hypotheses were tested based on what we know of this species in both Australia and regions where it is native, for example South Africa. It was hypothesized that most A. nilotica seeds would be produced by trees in riparian habitats with access to permanent water. In addition, we predicted that seeds would be dispersed throughout the Astrebla grassland landscape by cattle, following observations that cattle readily ingest and pass seeds and that cattle have access to all areas within paddock boundary fences. Tree density, seed production and seed dispersal by cattle were measured along a series of transects from permanent watering points to paddock boundary fences. Trees associated with permanent watering points produced more seeds per unit area and occurred at higher density than their non‐riparian counterparts. The importance of riparian trees decreased in years with high rainfall and in paddocks with only small areas of riparian habitat. Cattle spread dung and seeds throughout paddocks, with peaks of deposition adjacent to permanent watering points. These results suggest that invasion patterns are likely to be uneven across the landscape and may be reactive to climate. High seedling recruitment and possible thicket formation is expected adjacent to permanent watering points and wherever cattle congregate. Patterns of recruitment in non‐riparian areas are likely to be relatively sparse. The importance of post‐dispersal factors in determining recruitment patterns is discussed.     

三峡库区9种植物种子萌发特性及其在植被恢复中的意义

三峡大坝蓄水后形成的库区消涨带面临植被消亡、生态退化等问题。为了筛选出适用于库区消涨带植被恢复的植物, 将9种1年生植物种子放置在库区消涨带不同海拔进行水淹(W 165-8 m, 121 d;W 155-18 m, 230 d;W 147-26 m, 271 d), 然后在实验室条件下进行萌发, 研究在消涨带淹水胁迫下这些种子的萌发特性。结果表明: (1) 除马唐(Digitaria sanguinalis)、小蓬草(Conyza canadensis)、金色狗尾草(Setaria glauca)种子在各条件下萌发率都较低外, 不同水淹条件对萌发率影响不同: 与新鲜种子相比, W 165水淹后, 旱稗(Echinochloa hispidula)和婆婆针(Bidens bipinnata)种子萌发率显著上升, 其余种子萌发率均显著下降; W 155水淹后, 所有种子的萌发率都显著下降且只有鱧肠(Eclipta prostrate)、黄花蒿(Artemisia annua)、合萌(Aeschynomene indica)3个物种有萌发, 萌发率分别为11.0%、7.3%和2.7%; W 147水淹后, 旱稗和婆婆针种子萌发率显著上升, 鱧肠种子无显著差异, 其余种子萌发率显著下降。(2) 鱧肠、黄花蒿、婆婆针和旱稗种子比其它物种更耐水淹。W 165水淹后, 鱧肠、黄花蒿、婆婆针、旱稗种子萌发率分别为44.7%、42%、20.7%和4.3%, W 147水淹后分别为76.3%、23%、15%和26.3%, 高于其他物种。(3) 水淹后种子萌发时间格局也受到影响, 大部分种子起始萌发时间推迟、萌发速度变慢。鱧肠、黄花蒿、婆婆针和旱稗的种子对三峡库区消涨带的水淹胁迫具有一定的适应能力, 可根据它们对水淹条件适应能力的差异在消涨带不同海拔高度进行植被恢复。