Post‐fire vegetation dynamics in nutrient‐enriched and non‐enriched sclerophyll woodland

Abstract Exotic plant invasions are a significant problem in urban bushland in Sydney, Australia. In low‐nutrient Hawkesbury Sandstone communities, invasive plants are often associated with urban run‐off and subsequent increases in soil nutrients, particularly phosphorus. Fire is an important aspect of community dynamics in Sydney vegetation, and is sometimes used in bush regeneration projects as a tool for weed control. This study addressed the question: ‘Are there differences in post‐fire resprouting and germination of native and exotic species in nutrient‐enriched communities, compared with communities not disturbed by nutrient enrichment?’ We found that in non‐enriched areas, few exotic species emerged, and those that did were unable to achieve the rapid growth that was seen in exotic plants in the nutrient‐enriched areas. Therefore, fire did not promote the invasion of exotic plants into areas that were not nutrient‐enriched. In nutrient‐enriched areas after fire, the diversity of native species was lower than in the non‐enriched areas. Some native species were able to survive and compete with the exotic species in terms of abundance, per cent cover and plant height. However, these successful species were a different suite of natives to those commonly found in the non‐enriched areas. We suggest that although fire can be a useful tool for short‐term removal of exotic plant biomass from nutrient‐enriched areas, it does not promote establishment of native species that were not already present.     

Urbanization and exotic plants in northern Sydney streams

The vegetation and sediment of urban and non‐urban streams in the northern Sydney region were compared to examine the possible effects of urbanization on within‐stream vegetation. Many sediment characteristics were significantly different in urban streams. At least one exotic plant species was found in each urban stream sampled, but none were found in the non‐urban streams. The presence of exotic species led to the overall number and abundance of plant species being significantly higher in urban streams. Interestingly, the number and abundance of native species at the urban sites were the same as non‐urban sites, but a different suite of species was usually present. This suggests that urban streams favour exotic plants and certain native plants that are adapted to the modified conditions. The differences between the plant communities in the urban and non‐urban streams appeared to be associated with the increased level of nutrients in the urban stream sediment. Several multivariate techniques were used to assess the relative importance of individual nutrients, but no nutrients were directly associated with the observed differences. In particular, total phosphorus levels were less important in explaining the vegetation patterns than a combination of nutrients. It is therefore likely that the general increase of nutrients in stream sediment has enhanced exotic invasion and altered stream plant communities in Sydney streams.     

Influence of experimental,environmental, and geographic factors on nutrient‐diffusing substrate experiments in running waters

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Higher photosynthesis,nutrient‐ and energy‐use efficiencies contribute to invasiveness of exotic plants in a nutrient poor habitat in northeast China

The roles of photosynthesis‐related traits in invasiveness of introduced plant species are still not well elucidated, especially in nutrient‐poor habitats. In addition, little effort has been made to determine the physiological causes and consequences of the difference in these traits between invasive and native plants. To address these problems, we compared the differences in 16 leaf functional traits related to light‐saturated photosynthetic rate (P_max) between 22 invasive and native plants in a nutrient‐poor habitat in northeast China. The invasive plants had significantly higher P_max, photosynthetic nitrogen‐ (PNUE), phosphorus‐ (PPUE), potassium‐ (PKUE) and energy‐use efficiencies (PEUE) than the co‐occurring natives, while leaf nutrient concentrations, construction cost (CC) and specific leaf area were not significantly different between the invasive and native plants. The higher PNUE contributed to higher P_max for the invasive plants, which in turn contributed to higher PPUE, PKUE and PEUE. CC changed independently with other traits such as P_max, PNUE, PPUE, PKUE and PEUE, showing two trait dimensions, which may facilitate acclimation to multifarious niche dimensions. Our results indicate that the invasive plants have a superior resource‐use strategy, i.e. higher photosynthesis under similar resource investments, contributing to invasion success in the barren habitat.     

Non‐additive effects of litter mixing are suppressed in a nutrient‐enriched stream

Investigations of how species compositional changes interact with other aspects of global change, such as nutrient mobilization, to affect ecosystem processes are currently lacking. Many studies have shown that mixed species plant litters exhibit non‐additive effects on ecosystem functions in terrestrial and aquatic systems. Using a full‐factorial design of three leaf litter species with distinct initial chemistries (carbon:nitrogen; C:N) and breakdown rates (Liriodendron tulipifera, Acer rubrum and Rhododendron maximum), we tested for additive and non‐additive effects of litter species mixing on breakdown in southeastern US streams with and without added nutrients (N and phosphorus). We found a non‐additive (antagonistic) effect of litter mixing on breakdown rates under reference conditions but not when nutrient levels were elevated. Differential responses among single‐species litters to nutrient enrichment contributed to this result. Antagonistic litter mixing effects on breakdown were consistent with trends in litter C:N, which were higher for mixtures than for single species, suggesting lower microbial colonization on mixtures. Nutrient enrichment lowered C:N and had the greatest effect on the lowest‐ (R. maximum) and the least effect on the highest‐quality litter species (L. tulipifera), resulting in lower interspecific variation in C:N. Detritivore abundance was correlated with litter C:N in the reference stream, potentially contributing to variation in breakdown rates. In the nutrient‐enriched stream, detritivore abundance was higher for all litter and was unrelated to C:N. Thus, non‐additive effects of litter mixing were suppressed by elevated streamwater nutrients, which increased nutrient content of all litter, reduced variation in C:N among litter species and increased detritivore abundance. Nutrients reduced interspecific variation among plant litters, the base of important food web pathways in aquatic ecosystems, affecting predicted mixed‐species breakdown rates. More generally, world‐wide mobilization of nutrients may similarly modify other effects of biodiversity on ecosystem processes.     

Effects of plant harvesting and nutrient enrichment on phytoplankton community structure in a shallow urban lake

The utility of shallow water bodies in urban environments is frequently compromised either by dense beds of submerged plants or cyanobacterial blooms associated with nutrient enrichment. Although submerged plants are often harvested to facilitate recreational uses, this activity may alter the phytoplankton community, which in turn, also may restrict the use of the lake. We tested whether (i) plant harvesting reduced the abundance of flagellate algae and increased the abundance of cyanobacteria, and (ii) whether increasing levels of nutrient enrichment caused shifts in the dominance of heterocytous cyanobacteria, non-heterocytous cyanobacteria and Chlorophyta, in a shallow urban lake in Southern Australia as has been observed for shallow Danish lakes in previous studies. These predictions were tested with large (3000 l), replicated mesocosms in a warm, highly productive, shallow lake densely colonised by the submerged angiosperm, Vallisnaria americana Michaux. The heterokont algae, Chlorophyta, Cyanobacteria and Cryptophyta were the most numerous algal divisions in the lake. The Euglenophyta, although uncommon in early summer, became more abundant towards the end of summer. The Dinophyta and Charophyta were rare. The abundance of the heterokont algae and Euglenophyta was significantly reduced by plant harvesting even after plants had partially re-established 18 weeks after initial harvesting. The decline in the Euglenophyta in response to plant harvesting is consistent with earlier findings, that the relative abundance of flagellate algae tends to be greater in the presence of submerged plants. Contrary to our prediction, we found that the Cyanobacteria did not increase in response to plant harvesting, however the response may be altered under higher nutrient levels. Algal responses to nutrient enrichment in the presence of dense V. americana plants generally followed the patterns observed in shallow Danish lakes despite the large differences in climatic conditions. Both studies found that the abundance of heterocytous cyanobacteria declined at higher levels of nutrient enrichment, whereas non-heterocytous cyanobacteria and chlorophytes increased.     

Macroinvertebrate community structure,secondary production and trophic‐level dynamics in urban streams affected by non‐point‐source pollution

1. Despite non‐point‐source (NPS) pollution being perhaps the most ubiquitous stressor affecting urban streams, there is a lack of research assessing how urban NPS pollution affects stream ecosystems. We used a natural experimental design approach to assess how stream macroinvertebrate community structure, secondary production and trophic structure are influenced by urban NPS pollution in six streams. 2. Differences in macroinvertebrate community structure and secondary production among sites were highly correlated with stream‐water specific conductivity and dissolved inorganic phosphorus (DIP) concentrations. Macroinvertebrate richness, the Shannon diversity index and the Shannon evenness index were all negatively correlated with specific conductivity. These patterns were driven by differences in the richness and production of EPT and other intolerant taxa. Production of the five most productive taxa, tolerant taxa, non‐insect taxa and primary consumers were all positively correlated with stream‐water DIP. 3. Despite the positive correlation between primary consumer production and DIP, there was no correlation between macroinvertebrate predator production and either total or primary consumer macroinvertebrate production. This was observed because DIP was positively correlated with the production of non‐insect macroinvertebrate taxa assumed to be relatively unavailable for macroinvertebrate predator consumption. After removing production of these taxa, we observed a strong positive correlation between macroinvertebrate predator production and production of available prey. 4. Our results suggest that urban NPS pollution not only affects macroinvertebrate community structure, but also alters secondary production and trophic‐level dynamics. Differences in taxon production in our study indicate the potential for altered energy flow through stream food webs and potential effects on subsidies of aquatic insect prey to riparian food webs.     

Relationships among soil fertility,native plant diversity and exotic plant abundance inform restoration of forb‐rich eucalypt woodlands

Aim Water and nutrient availability are major limits to productivity in semi‐arid ecosystems; hence, ecological restoration often focuses on conserving or concentrating soil resources. By contrast, nutrient enrichment can promote invasion by exotic annuals, leading to restoration approaches that target reduction of soil nutrients. We aimed to explore potential biodiversity trade‐offs between these approaches by investigating relationships among soil nutrients, exotic annuals and native plant diversity and composition. In particular, we investigated the hypothesis that native plant diversity in semi‐arid to temperate woodlands reflects the productivity–diversity hypothesis, leading to hump‐backed relationships with soil nutrients such that (1) native plant diversity declines with increasing nutrient enrichment and (2) native diversity is limited at the lowest levels of soil fertility. Location Fragmented, long‐ungrazed Eucalyptus loxophleba subsp. loxophleba (York gum)–Acacia acuminata (jam) woodlands in the wheatbelt of South‐Western Australia. Methods We conducted stratified surveys of floristic composition and topsoil nutrient concentrations in 112 woodland patches. We used generalized linear models, structural equation models and ordinations to characterize relationships among soil nutrients, rainfall, exotic annuals and patch‐scale (100 m²) native plant composition and diversity. Results Patch‐scale native plant diversity declined strongly with increasing exotic abundance. This was partly related to elevated soil nutrient concentrations, particularly total nitrogen and available phosphorus. By contrast, there was little evidence for positive correlations between soil nutrients and native diversity, even at very low soil nutrient concentrations. Main conclusions Minimizing weed invasions is crucial for maximizing native plant diversity in E. loxophleba woodlands and could include nutrient‐depleting treatments without substantially compromising the functional capacity of soils to maintain native plant richness and composition. More broadly we emphasize that understanding relationships among ecosystem productivity, plant diversity and exotic invasions in the context of associated theoretical frameworks is fundamental for informing ecological restoration.     

Nitrogen limitation of the phytobenthos in Alpine lakes: results from nutrient‐diffusing substrata

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Rapid genetic delineation of local provenance seed‐collection zones for effective rehabilitation of an urban bushland remnant

Abstract The rehabilitation of native plant communities in urban bushland remnants is an increasingly important activity requiring the collection of large amounts of seed. Best practice generally identifies that local seed are best, but how far does the local provenance extend? Using the DNA fingerprinting technique amplified fragment length polymorphism, we assessed genetic differentiation between potential seed source populations and the target population, Bold Park, a large and significant bushland remnant in Perth, Western Australia. For each of 15 species, analysis of molecular variance was used to partition genetic variation within and among populations. Genetic differentiation between Bold Park and potential seed source populations was assessed by non‐metric multidimensional scaling ordination, and statistically by Fisher’s exact tests. The partitioning of variation among populations (Φ_ST) varied from 0.66 for Santalum acuminatum to 0.04 for Mesomelaena pseudostygia. For eight of 15 species, Bold Park plants were completely or largely non‐overlapping with other populations in ordinations, suggesting genetic differentiation and a narrow provenance. Five species showed overlap between Bold Park and some other, but not all, populations sampled, with geographically closest populations generally undifferentiated. Only two species, Acanthocarpus preissii and Mesomeleana pseudostygia, showed little genetic differentiation between Bold Park and all other populations, suggesting a regional genetic provenance. These species can be classified into three broad provenance classes – narrow, local and regional – to help guide decisions about appropriate seed‐collection zones for the rehabilitation of urban bushland remnants.     

Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

The study of phosphorus and nitrogen fluxes in enriched isolation columns

Isolation columns positioned in approximately 3.5 m of water in Midmar Dam, were enriched with PO₄-P and NO₃-N, singly and in combination. Increased chlorophyll concentration resulted from all enrichment treatments. Responses showed marked seasonality and varied in intensity and duration. Despite the PO₄-P fixing potential of the sediments, SRP concentrations increased markedly at times and possible reasons for this discussed. The marked response to addition of NO₃-N alone suggested a flux of phosphorus from the sediment through the water to the algal component. The significance of periphyton within the columns is discussed.     

Microbial film formation on metals in an enriched arctic river

Attachment of microorganisms to metal surfaces was investigated in a nutrient enriched pristine arctic river. Biofilms were characterised in terms of bacterial population size and metabolic activity. Metals were placed in the river at an untreated and a phosphate + nitrate enriched site. After 25 days, poorly developed bacterial films were formed on stainless steel and titanium surfaces at the control site. However, at the enriched site thick algal/bacterial films developed on both of these metals, as shown by scanning and transmission electron microscopy. The bacteria in these films were highly active, with high levels of polyhydroxy alkanoic acid synthesis. Even after 25 days, adhesion was minimal on copper alloy surfaces at both sites. The data are discussed in relation to the pristine nature of this river system and the implications for eutrophic rivers.     

外来入侵植物的根系觅养行为研究进展

土壤养分分布具有高度空间异质性, 植物的根系觅养行为是其对土壤养分异质性的一种适应。不同植物为了适应养分异质性会产生不同的根系觅养行为, 通过调整自身的根系觅养范围、觅养精度和觅养速度来更好地吸收利用土壤中的养分。外来植物与本地植物的竞争是决定其成功入侵的重要因素, 土壤养分等环境因素会影响它们之间的竞争关系。近年来, 外来入侵植物的觅养行为逐渐受到人们的关注, 关于入侵植物根系觅养行为的研究成果陆续出现: (1)总体来看, 外来入侵植物具有较强的根系觅养能力, 但根系觅养范围与觅养精度之间的权衡关系还不确定; (2)营养异质性会影响入侵植物与本地植物之间的竞争, 反过来, 二者之间的竞争也会影响根系觅养行为对营养异质性的响应; (3)丛枝菌根真菌(arbuscular mycorrhizal fungi, AMF)能够提高入侵植物的根系觅养能力, 外来植物入侵能够改变入侵植物对AMF的偏好性, 形成AMF对入侵的正反馈作用, 而本地植物与AMF的相互作用也会影响入侵植物的竞争力。未来还应加强营养异质环境下种间竞争和AMF共生对入侵植物根系觅养行为的影响机制研究, 以及全球变化背景下入侵植物根系觅养行为的变化与机制方面的研究, 可以更深入地认识外来植物的觅养行为在其成功入侵中的作用, 并为利用营养调控来防控入侵植物提供理论依据。     

Use of SSCP to improve the efficiency of microsatellite identification from microsatellite‐enriched libraries

An inefficient aspect of marker identification from microsatellite‐enriched libraries is the proportion of clones with identical sequences. This can substantially increase the number of clones that need to be sequenced in order to identify a sufficient number of microsatellite loci. We propose the use of single‐stranded conformation polymorphism (SSCP) analysis to identify unique clones prior to sequencing. We used this approach prior to sequencing from microsatellite‐enriched libraries for three marine invertebrate species and were able to obtain a given number of unique clone sequences for only 28% of the sequencing effort that would have been required without SSCP screening.     

Responses of Phytoplankton to in‐situ Nutrient Enrichment; Potential Influences on Species Dominance in a River

The Hawkesbury River at Sackville, New South Wales, Australia is fresh and vigorously mixed by tidal movement. The location has frequent blooms of Microcystis aeruginosa, which have been recorded occurring throughout the year, including winter temperatures as low as 13 °C. Nutrient enrichment tests were performed in‐situ on the natural phytoplankton population in 1997 and 1998 while Microcystis aeruginosa dominated (covering both summer and winter periods). These experiments compared population changes under the ambient nutrient regime with those after additions of ortho‐phosphate, nitrate, ammonia and various combinations of these nutrients. Under ambient conditions, the Microcystis population was able to grow significantly (P < 0.05) while most non‐cyanobacterial phytoplankton did not. Nutrient additions induced a variety of nutrient limitation responses that often varied between genera of major groups i.e. in the Chlorophyceae (Actinastrum sp. responded to phosphorus while Psephonema sp. responded to nitrogen). The possibility that shifts in population dominance from Chlorophyceae to the Cyanobacteria (M. aeruginosa) at Sackville are in response to competition for limiting nutrients is discussed.     

Ecological filtering of exotic plants in an Australian sub‐alpine environment

We investigated some of the factors influencing exotic invasion of native sub‐alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non‐invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus‐Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broad‐leaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross‐pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub‐alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub‐alpine environment.     

Effects of warming on annual production and nutrient‐use efficiency of aquatic mosses in a high Arctic lake

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