Some considerations on the functioning of tropical riparian ecotones

The fundamental function of a riparian wetland is the prevention against eutrophication and various types of pollution of a water body. This is done through the typical and dominant vegetation of the African tropical ecotone wetland, the papyrus which is very productive and has considerable adsorbing root surfaces as it spreads out over the water and at the river and lake edges. Because of the twelve-hour light day and high temperatures, the production is high. In this way, nutrients (nitrogen, phosphorus and others) are concentrated and accumulated in different parts of the plant from water flowing through the ecotone wetland. The amounts of, and the rate of flow of the material substances are regulated and the interface zone acts as a sink to most of those anthropogenic substances. It also functions as a refuge for waterfowl and other wildlife, and contributes to human food needs and well-being.     

Aspen and Willow Restoration Using Beaver on the Northern Yellowstone Winter Range

Aspen (Populus tremuloides) on the northern Yellowstone winter range has declined over the last half‐century. Beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991 in an attempt to reverse this trend. In 2005, we assessed the efficacy of this project by quantifying the long‐term effects of beaver on aspen stands and the riparian area in this drainage. Between 1990 and 2005, the canopy cover of mature aspen decreased more than 62%, whereas immature aspen cover more than tripled, resulting in a total aspen canopy cover decrease (p < 0.05) from 43 to 25% (a loss of 7.25 ha). Willow canopy cover increased from 10 to 14% during the same period. The impacts of beaver on aspen stands were estimated by comparing vegetative changes among control sites with less than 10% beaver use (n = 5), active beaver sites (n = 6), sites inactive for 1–3 years (n = 7), sites inactive for 4–6 years (n = 4), and sites inactive for 7–11 years (n = 5). Aspen sprout and sapling densities were greater (p = 0.01) on sites which were active and inactive for 1–3 years compared to the other sites. Aspen ramets were not able to grow taller than 2 m on sites without beaver activity for 4–11 years due to ungulate herbivory. Although beaver stimulated the growth of aspen sprouts and saplings, ungulate herbivory prevented successful aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range 14 years after beaver reintroduction.     

Geomorphic predictors of riparian vegetation in small mountain watersheds

Aims Hydrogeomorphic processes operating at watershed, process zone and site scales influence the distribution of riparian vegetation. However, most studies examining the relationships between hydrogeomorphic processes and riparian vegetation are conducted at site scales. We quantified the relative importance of watershed, process zone and site geomorphic characteristics for predicting riparian plant community types and plant species abundances in four small mountain watersheds in central Nevada, USA.Methods We mapped riparian vegetation types and identified process zones (based on dominant geomorphic process and valley fill material) within the watersheds. We sampled sites in each combination of vegetation type and process zone (n = 184 sites) and collected data on watershed scale factors, valley and stream geomorphic characteristics and on plant cover of each geomorphic surface. Plant community types were defined by cluster and indicator species analyses of plant cover data, and related to geomorphic variables using ordination analysis (nonmetric multidimensional scaling). Linear mixed effects models were used to predict abundances of indicator species.Important findings Variables describing position in the watershed (elevation, contributing area) that are related to gradients of temperature, moisture and stream discharge were of primary importance in predicting plant community types. Variables describing local geomorphic setting (valley width, stream gradient, channel sediments, geomorphic surface height) were of secondary importance, but accurately described the geomorphic setting of indicator species. The process zone classification did not include position in the watershed or channel characteristics and only predicted plant community types with unique geomorphic settings. In small mountain watersheds, predicting riparian vegetation distribution requires explicit consideration of scale and geomorphic context within and among watersheds in addition to site variables.     

Abundance Patterns of Landbirds in Restored and Remnant Riparian Forests on the Sacramento River, California, U.S.A.

Riparian vegetation along the Sacramento River—California's largest river—has been almost entirely lost, and several wildlife species have been extirpated or have declined as a result. Large-scale restoration efforts are focusing on revegetating the land with native plants. To evaluate restoration success, we conducted surveys of landbirds on revegetated and remnant riparian plots from 1993 to 2003. Our objectives were to estimate population trends of landbirds, compare abundance patterns over time between revegetated and remnant riparian forests, and evaluate abundance in relation to restoration age. Of the 20 species examined, 11 were increasing, 1 was decreasing (Lazuli Bunting [ Passerina amoena ]), and 8 showed no trend. The negative trend for Lazuli Bunting is consistent with information on poor reproductive success and with Breeding Bird Survey results. There was no apparent guild association common to species with increasing trends. Nine species were increasing on revegetated and remnant plots, four were increasing on revegetated plots only, three were increasing on remnant plots only, the Lazuli Bunting was decreasing on both, and three species were stable on both. Although many species were increasing at a faster rate on revegetated plots, their abundance did not reach that of the remnant plots. For revegetated plots, "year since planting" was a strong predictor of abundance trends for 13 species: positive for 12, negative for 1. Our study shows that restoration activities along the Sacramento River are successfully providing habitat for a diverse community of landbirds and that results from bird monitoring provide a meaningful way to evaluate restoration success.     

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts

Nematomorph parasites manipulate crickets to enter streams where the parasites reproduce. These manipulated crickets become a substantial food subsidy for stream fishes. We used a field experiment to investigate how this subsidy affects the stream community and ecosystem function. When crickets were available, predatory fish ate fewer benthic invertebrates. The resulting release of the benthic invertebrate community from fish predation indirectly decreased the biomass of benthic algae and slightly increased leaf break-down rate. This is the first experimental demonstration that host manipulation by a parasite can reorganise a community and alter ecosystem function. Nematomorphs are common, and many other parasites have dramatic effects on host phenotypes, suggesting that similar effects of parasites on ecosystems might be widespread.     

Arthropods within the woody element of hedgerows and their distribution pattern

1 The diversity and abundance of arthropods within hedgerows was investigated using insecticide fogging. In total, 13 390 arthropods were collected from 181 m³ of hedge (2% of the total volume). The taxonomic diversity of the total sample included 51 families in 13 orders, all within the phylum Arthropoda. Five orders accounted for 90% of all arthropods: Araneae, Coleoptera, Diptera, Hemiptera and Hymenoptera. The predators were the dominant functional group accounting for 40% of the total sample. 2 The linear distribution of hedgerow arthropod assemblages was investigated by sampling arthropods in 13 hedges at seven equidistant points along each hedge. Abundance of most arthropod taxa and the four functional groups (predators, parasitoids, herbivores and scavengers) consistently showed a clumped distribution along hedges, with high numbers at both ends and in the middle section. Hedge ends were defined either as a node, where the hedge intersected with another hedge, or as a gateway prior to an adjacent landscape feature such as a wood. Aggregation of arthropods at the nodes may reflect preferable microclimatic conditions at hedgerow junctions, whereas aggregation at a gap suggests the gap acted as a barrier to movement. The aggregation at the centre remains unexplained but may be a manifestation of the movement of individual arthropods along the hedgerow. 3 The great abundance and diversity of arthropods found in the present study emphasizes the status of hedges as one of the most important noncrop habitats on farmland. The arthropods that they contain may act as food for other farmland species, aid pest control and contribute to crop pollination.     

Litter input from riparian vegetation to streams: a case study of the Njoro River,Kenya

Allochthonous coarse particulate organic matter (CPOM) input into the Njoro River was measured between January and June 1998 at two contrasting sites: open-canopy and closed-canopy sites. Bank runoff and aerial drift traps were used for collecting CPOM inputs over periods of two weeks. Collected litter was sorted into four categories: leaves, fruits, wood and plant fragments. Monthly input ranged from 77 to 228 g ash free dry weight m^–1 for bank runoff input and from 64 to 129 g ash free dry weight m^–2 for aerial input. The highest input of 228 g ash free dry weight m^–1 was recorded in May at the closed-canopy site. Wood, fruits and plant fragments of particle size >100 m contributed a mean ± SE of 60±9% of the total inputs with the rest from leaf litter. The closed-canopy site had higher inputs (P<0.05) of bank and aerial input than the open canopy site. There was no relationship between total bank runoff input and rainfall (r _s = 0.08), however, total aerial input increased with decrease in rainfall (r _s = – 0.59). There were differences between inputs from different plant species (P<0.05) that ranked in the following order: Syzygium cordatum > Rhus natalensis > Pittosporum viridiflorum > Vangueria madagascariensis. Removal of riparian vegetation from the banks of the Njoro River would alter the quantity and quality of the litter and reduce CPOM inputs to the river and to a downstream lake with attendant consequences to the energy budget of biocoenoses in the two ecosystems.     

Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones

1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs. 2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift‐feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed‐canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes. 3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25–100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low‐level flux from autumn to spring in ice‐free streams. This flux varies with in‐stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short‐term behaviour, growth, and abundance of terrestrial consumers. 4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short‐term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs. 5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream–riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. Study of these reciprocal connections is helping to define a more holistic perspective of catchments, and has the potential to shape new directions for ecology in general.     

根区不同灌溉方式对苹果幼苗水流阻力的影响

研究了滴灌条件下根区不同灌溉方式（交替滴灌ADI、固定滴灌FDI和常规滴灌CDI）和灌水量对苹果幼苗及各组成部分水流阻力、气孔导度和叶面积的影响.结果表明：根区灌溉方式和灌水量对苹果幼苗水流阻力（R）的影响显著; 在相同的根区灌溉方式下,苹果幼苗根系阻力（R_r）随着灌水量的减少而增大,冠层阻力（R_s）随着灌水量的减少而减小.在相同灌水量下,与常规滴灌相比,交替滴灌和固定滴灌均提高了苹果幼苗叶片和叶柄阻力（R_l+p）,降低了幼苗全株阻力（R_t）、R_r、R_s以及侧枝和主杆阻力（R_lb+mr）.在20 mm和30 mm灌水定额下,交替滴灌的R_l+p分别比常规滴灌高1.06%和0.63%.在平均节水达33%的前提下,交替滴灌和固定滴灌的平均R_l+p分别比常规滴灌高19.65%和24.34%,但交替滴灌和固定滴灌的平均R_lb+mr分别降低了4.83%和14.97%.交替滴灌和固定滴灌等局部根区不同灌溉方式通过有效减小苹果气孔导度和叶面积,提高了R_l+p,从而减少了叶片的蒸腾失水,提高了苹果幼苗的水分利用效率,通过降低R_r和R_lb+mr提高了苹果幼苗调控水分的能力和抗干旱能力.