Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.     

How do grazers affect periphyton heterogeneity in streams?

The effects of grazing by stream invertebrates on algal biomass and spatial heterogeneity were tested experimentally in flow-through microcosms with natural substrates (rocks). One experiment tested the effects of fixed densities of three species of grazers (the caddisfly Allomyia sp. and two mayflies, Epeorus deceptivus and Baetis bicaudatus) on periphyton. Baetis was tested with and without chemical cues from fish predators, which reduced grazer foraging activity to levels similar to the less mobile mayfly (Epeorus). Mean algal biomass (chlorophyll a; chl a) was reduced in grazer treatments compared to ungrazed controls, but there were no differences among grazer treatments. Algal heterogeneity (Morisita index) increased with grazer mobility, with the highest heterogeneity occurring in the Baetis-no fish treatment (most mobile grazer) and the lowest in the caddisfly treatment (most sedentary grazer). A second experiment used a three factorial design, and tested whether initial resource distribution (homogeneous vs. heterogeneous), Baetis density (high vs. low) and fish odor (present vs. absent) affected grazer impact on algal resources. Abundances of Baetis and chl a on individual rocks were recorded to explore the mechanisms responsible for the observed distributions of algae. Initial resource heterogeneity was maintained despite being subjected to grazing. Mean chl a was highest in controls, as in experiment I, and effects of Baetis on algal biomass increased with grazer density. There were no fish effects on algal biomass and no effects of grazer density or fish on algal heterogeneity. At the scale of individual rocks Baetis was unselective when food was homogeneously distributed, but chose high-food rocks when it was heterogeneously distributed. Results of these mechanistic experiments showed that Baetis can track resources at the scale of single rocks; and at moderate densities mobile grazers could potentially maintain periphyton distributions observed in natural streams.     

Does leaf quality mediate the stimulation of leaf breakdown by phosphorus in Neotropical streams?

1. Lowland tropical streams have a chemically diverse detrital resource base, where leaf quality could potentially alter the effect of high nutrient concentrations on leaf breakdown. This has important implications given the extent and magnitude of anthropogenic nutrient loading to the environment. 2. Here, we examine if leaf quality (as determined by concentrations of cellulose, lignin and tannins) mediates the effects of high ambient phosphorus (P) concentration on leaf breakdown in streams of lowland Costa Rica. We hypothesised that P would have a stronger effect on microbial and insect processing of high‐ than of low‐quality leaves. 3. We selected three species that represented extremes of quality as measured in leaves of eight common riparian species. Species selected were, from high‐ to low‐quality: Trema integerrima > Castilla elastica > Zygia longifolia. We incubated single‐species leaf packs in five streams that had natural differences in ambient P concentration (10–140 μg soluble reactive phosphorus (SRP) L^?1), because of variable inputs of solute‐rich groundwater and also in a stream that was experimentally enriched with P (approximately 200 μg SRP L^?1). 4. The breakdown rate of all three species varied among the six streams: T. integerrima (k‐values range: 0.0451–0.129 day^?1); C. elastica (k‐values range: 0.0064–0.021 day^?1); and Z. longifolia (k‐values range: 0.002–0.008 day^?1). Both ambient P concentration and flow velocity had significant effects on the breakdown rate of the three species. 5. Results supported our initial hypothesis that litter quality mediates the effect of high ambient P concentration on leaf processing by microbes and insects. The response of microbial respiration, fungal biomass and invertebrate density to high ambient P concentration was greater in Trema (high quality) than in Castilla or Zygia (low quality). Variation in flow velocity, however, confounded our ability to determine the magnitude of stimulation of breakdown rate by P. 6. Cellulose and lignin appeared to be the most important factors in determining the magnitude of P‐stimulation. Surprisingly, leaf secondary compounds did not have an effect. This contradicts predictions made by other researchers, regarding the key role of plant secondary compounds in affecting leaf breakdown in tropical streams.     

Do soil organisms affect aboveground litter decomposition in the semiarid Patagonian steppe, Argentina?

Surface litter decomposition in arid and semiarid ecosystems is often faster than predicted by climatic parameters such as annual precipitation or evapotranspiration, or based on standard indices of litter quality such as lignin or nitrogen concentrations. Abiotic photodegradation has been demonstrated to be an important factor controlling aboveground litter decomposition in aridland ecosystems, but soil fauna, particularly macrofauna such as termites and ants, have also been identified as key players affecting litter mass loss in warm deserts. Our objective was to quantify the importance of soil organisms on surface litter decomposition in the Patagonian steppe in the absence of photodegradative effects, to establish the relative importance of soil organisms on rates of mass loss and nitrogen release. We estimated the relative contribution of soil fauna and microbes to litter decomposition of a dominant grass using litterboxes with variable mesh sizes that excluded groups of soil fauna based on size class (10, 2, and 0.01 mm), which were placed beneath shrub canopies. We also employed chemical repellents (naphthalene and fungicide). The exclusion of macro- and mesofauna had no effect on litter mass loss over 3 years (P = 0.36), as litter decomposition was similar in all soil fauna exclusions and naphthalene-treated litter. In contrast, reduction of fungal activity significantly inhibited litter decomposition (P < 0.001). Although soil fauna have been mentioned as a key control of litter decomposition in warm deserts, biogeographic legacies and temperature limitation may constrain the importance of these organisms in temperate aridlands, particularly in the southern hemisphere.     

Do litter manipulations affect leaf functional traits of savanna woody plants?

Plant litter is the layer composed of dead plant material that covers soil surfaces in terrestrial ecosystems. It is an important pool of essential nutrients for soil and plants, serving also as a protective layer on the soil surface. In this study, we investigated the effects of litter addition and removal on leaf functional traits of woody Neotropical savanna trees. We measured maximum photosynthesis (A _max), stomatal conductance (g _s), leaf transpiration (E), intrinsic water use efficiency (IWUE), specific leaf area (SLA), and chlorophyll content (CCI) in 15 species belonging to three different phenological groups (evergreen, briefly deciduous, and deciduous species) that were subjected to three distinct litter availability treatments (addition, removal, and control plots) in a Neotropical Savanna site in Brazil. Although SLA and CCI differed among phenology groups, they were not affected by the litter treatments. In contrast, when considered at the community level, we found that the availability of litter affected the leaf traits linked with the water status of the plants (E, g _s and IWUE). Plants in the litter removal plots exhibited lower g _s and E (25 % of reduction in comparison with control group) but higher IWUE, while plants in the litter addition plots had a 10 % decrease in IWUE but a 12 % increase in g _s and E compared with plants in control plots. Savanna woody plants responded promptly to litter manipulation by adjusting leaf water loss, which suggests that in the short term, changes in the amount of litter in Cerrado ecosystems can affect the soil water availability to the plant community.     

Do climate and soil influence phenotypic variability in leaf litter,microbial decomposition and shredder consumption?

We tested the hypothesis that water stress and soil nutrient availability drive leaf-litter quality for decomposers and detritivores by relating chemical and physical leaf-litter properties and decomposability of Alnus glutinosa and Quercus robur, sampled together with edaphic parameters, across wide European climatic gradients. By regressing principal components analysis of leaf traits [N, P, condensed tannins, lignin, specific leaf area (SLA)] against environmental and soil parameters, we found that: (1) In Q. robur the condensed tannin and lignin contents increased and SLA decreased with precipitation, annual range of temperature, and soil N content, whereas leaf P increased with soil P and temperature; (2) In A. glutinosa leaves N, P, and SLA decreased and condensed tannins increased with temperature, annual range of temperature, and decreasing soil P. On the other hand, leaf P and condensed tannins increased and SLA decreased with minimum annual precipitation and towards sites with low temperature. We selected contrasting leaves in terms of quality to test decomposition and invertebrate consumption. There were intraspecific differences in microbial decomposition rates (field, Q. robur) and consumption by shredders (laboratory, A. glutinosa). We conclude that decomposition rates across ecosystems could be partially governed by climate and soil properties, affecting litter quality and therefore decomposers and detritivores. Under scenarios of global warming and increased nutrients, these results suggest we can expect species-specific changes in leaf-litter properties most likely resulting in slow decomposition with increased variance in temperatures and accelerated decomposition with P increase.     

Do intestinal nematodes affect productivity in adulthood?

Intestinal nematode infections have been associated with many physical and mental developmental insults. These include anaemia, wasting, stunting, cognitive impairment and lowered educational achievement, all of which have in turn been shown to interfere with productivity and wage-earning capacity in adults. Although there is no direct evidence for an effect of intestinal nematodes on productivity, circumstantial evidence suggests such an effect. Here, Helen Guyatt reviews the indirect evidence for an effect of intestinal nematodes on productivity in adults through current infection and associated morbidity, and on early ill-health in children, which might affect productivity later in life.     

Do vine species in neotropical forests see the forest or the trees?

Questions: Is the occurrence of vine species in neotropical rain forests primarily determined by properties of the forest (environmental factors), by properties of the trees (tree species or tree size) or are vines randomly distributed? Location: Maya Biosphere Reserve, Guatemala. Methods: In five 1‐ha plots that span variation from unlogged forest to forest impacted by recurrent human disturbance we recorded the presence of all climbing vine species on every tree. The presence of all free standing vine species and 11 environmental variables were recorded in 100‐m² subplots. The relationship of host tree diameter and host tree identity on single tree vine species richness was investigated by GLM modelling. Partial redundancy analyses were used to partition the variation in vine species composition on two sources: environmental factors and tree species identity. Results: Single tree vine richness increased with increasing host tree DBH and differed significantly among host species. For climbing vines, the ratio of variation in subplot presence explained by tree species and by environmental variables was ca. 4:1 (in the most disturbed logged plots slightly lower), for free standing vines this ratio varied from 1:2 in the most disturbed logged plots to 9:1 in reserve plots, while a ratio of ca. 1:1 was found for all plots analysed together. Conclusion: Different tree species have different probabilities of being infested by vines. Vines see both the forest and the trees; the environment is more important in earlier developmental stages, properties of individual trees become more important from the time vines start to climb.     

Do fall additions of salmon carcasses benefit food webs in experimental streams?

Research showing that salmon carcasses support the productivity and biodiversity of aquatic and riparian ecosystems has been conducted over a variety of spatial and temporal scales. In some studies, carcasses were manipulated in a single pulse or loading rate or manipulations occurred during summer and early fall, rather than simulating the natural dynamic of an extended spawning period, a gradient of loading rates, or testing carcass effects in late fall-early winter when some salmon stocks in the US Pacific Northwest spawn. To address these discrepancies, we manipulated salmon carcass biomass in 16 experimental channels located in the sunlit floodplain of the Cedar River, WA, USA between mid-September and mid-December, 2006. Total carcass loads ranged from 0?C4.0 kg/m² (0, 0.001, 0.01, 0.1, 0.5, 1.0, 2.0 and 4.0 kg/m², n = 2 per treatment) and were added to mimic the temporal dynamic of an extended spawning period. We found little evidence that carcasses influenced primary producer biomass or fish growth; however, nutrients and some primary consumer populations increased with loading rate. These effects varied through time, however. We hypothesize that the variable effects of carcasses were a result of ambient abiotic condition, such as light, temperature and disturbance that constrained trophic response. There was some evidence to suggest peak responses for primary producers and consumers occurred at a loading rate of ~1.0?C2.0 kg/m², which was similar to other experimental studies conducted during summer.     

Do ectomycorrhizas alter leaf-litter decomposition in monodominant tropical forests of Guyana?

This work tested the hypothesis that ectomycorrhizas (EM) of Dicymbe corymbosa alter leaf-litter decomposition and residual litter quality in tropical forests of Guyana. Mass loss of leaf litter in litter bags was determined on three occasions, in two experiments, during a 12-month period. Paired root-exclusion plots were located randomly within a D. corymbosa forest. Both D. corymbosa and mixed-species leaf litters were reciprocally transplanted into their respective forest types. Elemental analysis was performed on the residual D. corymbosa leaf litter after 1 yr. Leaf litter mass loss in the D. corymbosa forest was not influenced by EM, despite high EM colonization. Elemental analysis of D. corymbosa leaf litter residues demonstrated reduced calcium levels in the presence of EM, which were negatively correlated with EM rootlet-colonizing mass. The lack of EM effect on the litter decomposition rate, coupled with high EM colonization, suggests an important but indirect role in mineral nutrient acquisition. Lowered Ca concentration in leaf litter exposed to EM may suggest a high Ca demand by the ectotroph system.     

Does nanosized plastic affect aquatic fungal litter decomposition?

Nanosized plastics are an emerging concern in freshwater ecosystems, raising the question whether they put freshwater ecological processes at risk. Litter decomposition is a major ecological function in forested streams which is mainly driven by aquatic hyphomycetes. Here we investigated whether increasing concentrations (up to 102.4 mg/L) of nanosized polystyrene plastics (NPPs; 100nm) affect litter decomposition by five widely distributed species of aquatic hyphomycetes. Results showed that average litter decomposition decreased by 8% relative to the control when exposed to 102.4 mg/L NPPs. Aquatic hyphomycete species differed in their sensitivity to NPPs. The greatest inhibition of litter decomposition was found with Tetracladium marchalianum, where it dropped from 37 (control) to 16% (102.4 mg/L of NPP). Overall our study highlights the emerging risks and potential dangers of NPPs to freshwater ecosystem functioning. It also indicates that the impact of NPPs may be species specific.     

Do holly leaf spines really deter herbivory?

Summary Although spinose teeth of holly leaves have been widely cited as an example of a physical defense against herbivores, this assumption is based largely on circumstantial evidence and on general misinterpretation of a single, earlier experiment. We studied the response of third and fifth instar larvae of the fall webworm, Hyphantria cunea Drury, a generalist, edge-feeding caterpillar, to intact American holly leaves and to leaves that had been modified by blunting the spines, by removing sections of leaf margin between the spines, or by removing the entire leaf margin. The results suggest that the thick glabrous cuticle and tough leaf margin of Ilex opaca are more important than the spinose teeth in deterring edge-feeding caterpillars. Microscopic examination of mature leaves revealed that the epidermis is thickened at the leaf margin, and that the leaf is cirucumscribed by a pair of fibrous veins. In simple choice tests neither domesticated rabbits nor captive whitetailed deer discriminated between spinescent holly foliage and foliage from which spines were removed. Nevertheless, we found little evidence of herbivory by mammals in the field, either on small experimental trees or in the forest understory. While it is possible that spinose teeth contribute to defense by reducing acceptibility of holly relative to other palatable plant species, we suggest that the high concentrations of saponins and poor nutritional quality of holly foliage may be more important than spines in deterring vertebrate herbivores. The degree of leaf spinescence and herbivory was compared at different heights with the tree canopy to test the prediction that lower leaves should be more spinescent as a deterrent to browsers. Leaves on lower branches of mature forest trees were slightly more spinescent than were upper leaves, and juvenile trees were slightly more spinescent than were mature trees. However, there was no relationship between degree of spinescence and feeding damage. The greater spinescence of holly leaves low in the canopy is probably an ontogenetic phenomenon rather than a facultative defense against browsers.The investigation reported in this paper (No. 87-7-8-77) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Dirctor     

Do changes in rainfall patterns affect semiarid annual plant communities?

Question: Climate change models forecast a reduction in annual precipitation and more extreme events (less rainy days and longer drought periods between rainfall events), which may have profound effects on terrestrial ecosystems. Plant growth, population and community dynamics in dry environments are likely to be affected by these changes since productivity is already limited by water availability. We tested the effects of reduced precipitation and fewer rain events on three semiarid plant communities dominated by annual species. Location: Three semiarid plant communities from Almería province (SE Spain). Methods: Rain‐out shelters were set up in each community and watering quantity and frequency were manipulated from autumn to early summer. Plant productivity, cover and diversity were measured at the end of the experimental period. Results: We found that a 50% reduction in watering reduced productivity, plant cover and diversity in all three communities. However, neither the 25% reduction in watering nor changes in the frequency of watering events affected these parameters. Conclusions: The lack of response to small reductions in water could be due to the identity and resistance of the plant communities involved, which are adapted to rainfall variability characteristic of arid environments. Therefore, a rainfall reduction of 25% or less may not affect these plant communities in the short term, although higher reductions or long‐term changes in water availability would probably reduce productivity and diversity in these communities.     

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species‐specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home‐field advantage–HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T‐RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species‐specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.     

Do non-native earthworms in Southeast Alaska use streams as invasional corridors in watersheds harvested for timber?

Exotic earthworms from Europe and Asia have invaded previously earthworm-free areas of North America where they consume leaf litter, mix soil horizons, and alter nutrient cycling. Primarily, earthworm introductions occur through human activities; we hypothesized that the combination of logging (i.e., road construction and soil disturbance) and stream transport (i.e., hydrochory) allows earthworms to invade new ecosystems and spread within watersheds. On Prince of Wales Island, AK, we surveyed riparian zones in 11 watersheds with varying timber harvest intensity for terrestrial oligochaetes. Additionally, common invasive earthworms were experimentally submerged in a local stream to test for tolerance to prolonged immersion: all taxa survived immersion for at least 6 days. Using principal components analysis, watershed and harvest variables describing the watersheds upstream of our sampled riparian areas were reduced to two principal components describing harvest intensity (PC1) and harvest style (PC2). Logistic models successfully predicted earthworm abundance (r ²  = 0.70) from PC1, which indicated that watersheds with older, intense upstream timber harvest contained larger earthworm populations. Earthworm species richness was best predicted by PC2 (r ²  = 0.39), which suggested that earthworm communities in watersheds containing large clear-cut stands were more species-rich. Collectively, these results suggest that (1) invasive earthworms may use streams for dispersal and (2) upstream introductions via timber harvest can initiate downstream earthworm invasions. Hydrochory would allow invasive earthworms to spread at rates (tens of km d⁻¹) that are much greater than previously reported rates of terrestrial spread (5–10 m y⁻¹). Effective control of exotic earthworms in riparian zones will require watershed-level management and surveillance.     

Testing for seasonality in alpine streams: How does altitude affect freshwater insect life cycles?

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Does elevated atmospheric CO2 concentrations affect wood decomposition?

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric [CO₂] have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech (Fagus sylvatica L.) twigs grown under ambient and elevated [CO₂] in open top chambers. Elevated [CO₂] significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient [CO₂]. The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated [CO₂]. However, the elevated [CO₂] treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the [CO₂] treatment, despite initial changes in N and lignin concentrations between the ambient and elevated [CO₂] beech wood. This revised version was published online in June 2006 with corrections to the Cover Date.     

Do foraging methods in winter affect morphology during growth in juvenile snow geese?

Physical exertion during growth can affect ultimate size and density of skeletal structures. Such changes from different exercise regimes may explain morphological differences between groups, such as those exhibited by lesser snow geese (Chen caerulescens caerulescens; hereafter snow geese) foraging in southwest Louisiana. In rice‐prairie habitats (hereafter rice‐prairies), snow geese bite off or graze aboveground vegetation, whereas they dig or grub for subterranean plant parts in adjacent coastal marshes. Grubbing involves considerably more muscular exertion than does grazing. Thus, we hypothesized that rates of bone formation and growth would be lower for juveniles wintering in rice‐prairies than those in coastal marshes, resulting in smaller bill and skull features at adulthood. First, we tested this exertion hypothesis by measuring bills, skulls, and associated musculature from arrival to departure (November–February) in both habitats in southwest Louisiana, using both banded birds and collected specimens. Second, we used the morphological data to test an alternative hypothesis, which states that smaller bill dimensions in rice‐prairies evolved because of hybridization with Ross's geese (C. rossii). Under the exertion hypothesis, we predicted that bill and skull bones of juveniles would grow at different rates between habitats. However, we found that bill and skull bones of juveniles grew similarly between habitats, thus failing to support the exertion hypothesis. Morphometrics were more likely to differ by sex or change with sampling date than to differ by habitat. We predicted that significant, consistent skewness toward smaller birds could indicate hybridization with Ross's geese, but no skewness was observed in our morphological data, which fails to support the hybridization hypothesis. Further research is needed to clarify whether snow geese wintering in Louisiana represent a single polymorphic population that segregates into individually preferred habitats, which we believe at present to be more likely as an explanation than two ecologically and spatially distinct morphotypes.