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Recovery Succession of Native Plant Communities at Big Meadow Bog After Disturbance and Eutrophication from Herring Gulls
Abstract
On Brier Island in eastern Canada, ditching of Big Meadow Bog resulted in predictable shifts in the plant community as shrubs and trees proliferated across the drained landscape. The attraction of nesting gulls to the ditched bog and their daily foraging at distant mink farms was not predictable. To better understand the impacts of the nesting gulls, we performed a retrospective analysis of plant community disruption and recovery succession. Areas of gull-bared peat within Big Meadow Bog were colonized in succession by exotic grasses (e.g. Holcus lanatus) and annuals; raspberry (Rubus spp.) canes and biotically-dispersed Rosaceae shrubs; and wind-dispersed Ericaceae, fern, and tree species. There was no evidence of a persisting bog-species seedbank in the soil to aid post-gull revegetation, nor was there evidence of an exotic-weed seedbank to interfere with plant-community regeneration. The exotic-weed seedbank (6 thousand seeds m⁻²) declined to zero after 33 years (half-life = 3.6 years). In contrast, a secondary, native (mainly Juncus spp.) seedbank of similar magnitude, persisted. The recovered plant communities had swamp features (i.e., trees, ferns, tall shrubs). Whether or not Big Meadow Bog regains ombrotrophy will depend upon whether blocked ditches and reinstated water tables deter nesting gulls.
... The bog was historically ombrotrophic, and after ditching it dried. Thereafter, woody plants, including raspberries (Rubus spp.) and fireweed (Epilobium spp.) increased (Hill et al., 2019;Spooner et al., 2017). Due to these changes in hydrology and vegetation, herring and great black-backed gulls (Larus marinus) colonized the site in the 1980s (Environment and Change Canada, 2018;Hill et al., 2019). ...
... Thereafter, woody plants, including raspberries (Rubus spp.) and fireweed (Epilobium spp.) increased (Hill et al., 2019;Spooner et al., 2017). Due to these changes in hydrology and vegetation, herring and great black-backed gulls (Larus marinus) colonized the site in the 1980s (Environment and Change Canada, 2018;Hill et al., 2019). The population at this colony has increased since drying and recently was estimated to host 3200 nesting pairs (Toms, 2016). ...
... The reference bog for this study colony is located ~0.8 km northwest of Big Meadow Bog. The reference bog is ombrotrophic, conditions that likely characterise Big Meadow Bog before it was ditched, and has no nesting herring gulls (Hill et al., 2019). ...
Seabirds are important biovectors of contaminants, like mercury, moving them from marine to terrestrial environments around breeding colonies. This transfer of materials can have marked impacts on receiving environments and biota. Less is known about biotransport of contaminants by generalist seabirds that exploit anthropogenic wastes compared to other seabird species. In this study, we measured total mercury (THg) in O-horizon soils at four herring gull (Larus smithsoniansus) breeding colonies in southern Nova Scotia, Canada. At colonies with dry substrate, THg was significantly higher in soils collected from gull colonies compared to nearby reference soils with no nesting gulls. Further, THg was distinct in soils among study colonies and was likely influenced by biotransport from other nesting seabird species, most notably Leach's storm-petrels (Hydrobates leucorhous). Our research suggests gulls that scavenge on anthropogenic wastes at local industrial sites are biovectors moving THg acquired at these sites to their colonies and may increase the spatial footprint of contaminants from these industries.
... The DI extractable PO 4 3observed in guano in this study suggests avian biovectors are the cause of nutrient enrichment in Bog Meadow Bog, Brier Island. While the specific effects of increased soluble PO 4 3concentrations in the bog are unclear, nutrients may affect competition within the local plant community which will be a concern for sensitive plant species such as the endangered eastern mountain avens (Geum peckii) present at this site (Hill et al., 2019;Limpens et al., 2004). Kickbush et al. (2018) has shown shallow ground water PO 4 3concentrations at this site are hyper-eutrophic. ...
Avian biovector transport is an important mechanism for the movement of contaminants and nutrients to remote locations, usually bird colonies, through excretion, molting and decomposition of carcasses. Methylmercury (MeHg) is a bioaccumulative neurotoxin and endocrine disruptor which is present in many remote ecosystems. We collected guano samples biweekly from a herring gull (Larus smithsoniansus) colony over two summers and analyzed MeHg, total mercury (THg), water-extractable phosphate (PO4³⁻) and sulfate (SO4²⁻). Concentrations of THg in guano declined through the summer months while %MeHg significantly increased (ranging from 12% to 100% of THg), suggesting a switch in diet as gull nutritional needs or food availability changed through the summer. The percentage of PO4³⁻ in dry guano increased throughout the summer (ranging from 2.8% to 4.4% of dry weight) and SO4²⁻ varied throughout the season (ranging from 0.1% to 0.8% of dry weight). These data indicate that gulls are transporting considerable amounts of MeHg, PO4³⁻, and likely other contaminants to Big Meadow Bog, Nova Scotia.
The local distribution of Prasiola crispa is reported for the first time in Nova Scotia. It was common on emergent basalt outcrops in a coastal wetland on the Bay of Fundy shores of Brier Island. The alga was present on 19 of 102 basalt outcrops in one of the breeding colonies of the Herring Gull, Larus argentatus, and was only associated with basalt outcrops with gull feces. Patches of P. crispa were typically associated with the north facing slopes of the rock or were present in depressions or parts of the rock shaded by adjacent vegetation. At Western Light, the gulls are both facilitating the presence of P. crispa and acting as ecosystem engineers by nesting in the adjoining vegetation where their trampling and nutrient inputs are modifying the surrounding wetland ecosystem.
Long-term monitoring of the rare southern twayblade (Neottia bifolia; Orchidaceae) and understorey of a black spruce-tamarack, swamp community was conducted after construction of an airport taxiway above the swamp edge. Succession documented over 14 years in the treed swamp edge following disturbance involved loss of the twayblade, invasion by guilds of annuals, and shifts between original and new guilds of wetland plants. Despite changes, the twayblade was the only plant extirpated from the swamp edge community although fungal richness (mainly basidiomycete) was substantially lower in the edge than the swamp interior. NMS ordination indicated that the orchid niche is an undisturbed swamp with an intact tree canopy and unbroken mat of Sphagnum which supports a diverse mycorrhizal community adapted to summer water table drawdown and low ionic groundwater chemistry (e.g. Cl<10 mg/L). Fourteen years after the disturbance, southern twayblade populations in the swamp interior, 100 m from the edge were in steady decline. The woodland orchids may be sentinels of ecosystem integrity. Loss of the twayblade signalled the onset of a post-development, disturbance-cascade: installation of an impervious surface restricted infiltration, altered run-off quantity and quality which subsequently resulted in loss of the ecosystem engineer, Sphagnum, and keystone mycorrhizal fungi.
An understanding of the morphological stability and succession of open water and wetland ecosystems in Nova Scotia is a priority for informing the conservation management of critical habitats for a complex of nationally listed, rare, disjunct wetland species. Baltzer Bog and Big Meadow Bog in southwestern Nova Scotia contain stratigraphic records of late Holocene moisture variability. Baltzer Bog is a shrub bog that formed in an elevated, enclosed kettle basin. Excavated sections exposed by peat mining revealed 2 distinct wood-rich horizons that are located above a well-developed soil and wood horizon that yielded a radiocarbon-dated age of 3260 cal. BP from an upright stump. The overlying wood-rich horizons were dated at 1640 and 1045 cal. BP and were overlain by Sphagnum species transitions indicative of increasing wetness. At Big Meadow Bog, a thin wood mat in Sphagnum at 90 cm depth was dated at 1760 cal. BP. These records are broadly correlative with pollen and stratigraphic data from Pleasant River Fen in central Nova Scotia that indicate periods of high and low productivity and a fluctuating water table from 1950 cal. BP until present. Though other high-resolution paleoclimate records from the region indicate that the late Holocene was a time of increasing precipitation and cooler air temperatures, these wetland records demonstrate that in Nova Scotia this time period was characterized by rapid variations in effective moisture and that significant and sustained dry periods likely occurred. This record of late Holocene moisture variability and its influence on habitat structure serves to better establish the potential for long-term residency of threatened and endangered species at wetland sites.
Soil seedbanks drive infestations of annual weeds, yet weed management focuses largely on seedling mortality. As weed seedbanks increasingly become reservoirs of herbicide resistance, species‐specific seedbank management approaches will be essential to weed control. However, the development of seedbank management strategies can only develop from an understanding of how seed traits affect persistence.
We quantified interspecific trade‐offs among physiological, chemical, and physical traits of weed seeds and their persistence in the soil seedbank in a common garden study. Seeds of 11 annual weed species were buried in Savoy, IL, from 2007 through 2012. Seedling recruitment was measured weekly and seed viability measured annually. Seed physiological (dormancy), chemical (phenolic compound diversity and concentration; invertebrate toxicity), and physical traits (seed coat mass, thickness, and rupture resistance) were measured.
Seed half‐life in the soil (t0.5) showed strong interspecific variation (F10,30 = 15, p < .0001), ranging from 0.25 years (Bassia scoparia) to 2.22 years (Abutilon theophrasti). Modeling covariances among seed traits and seedbank persistence quantified support for two putative defense syndromes (physiological–chemical and physical–chemical) and highlighted the central role of seed dormancy in controlling seed persistence.
A quantitative comparison between our results and other published work indicated that weed seed dormancy and seedbank persistence are linked across diverse environments and agroecosystems. Moreover, among seedbank‐forming early successional plant species, relative investment in chemical and physical seed defense varies with seedbank persistence.
Synthesis and applications. Strong covariance among weed seed traits and persistence in the soil seedbank indicates potential for seedbank management practices tailored to specific weed species. In particular, species with high t0.5 values tend to invest less in chemical defenses. This makes them highly vulnerable to physical harvest weed seed control strategies, with small amounts of damage resulting in their full decay.
We used 116 years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull ( Larus glaucescens ). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we tested several predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted that on Mandarte and nearby islands with gull colonies, we should observe higher nutrient loading and exotic plant species richness and cover than on nearby islands without gull colonies, as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results support earlier suggestions that nesting seabirds can drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation, and that human activities that affect seabird abundance may therefore indirectly affect plant community composition on islands with seabird colonies.
Rumex acetosella L., sheep sorrel, is a perennial herb originating from Europe and southwestern Asia, now found in at least 70 countries worldwide. A common weed, it appears in grasslands, pastures, rangelands, waste areas, and along roadsides across North America. It frequently occurs as a weed in lowbush blueberry fields in Atlantic Canada, particularly in Nova Scotia. Rumex acetosella thrives on poor, acidic, and disturbed
soils across a broad range of climatic conditions. A persistent seed bank and vegetative reproduction from creeping roots contribute to the ability of R. acetosella to tolerate various methods of control including certain herbicides, fire, and tillage. Tart-tasting leaves, rich in vitamin C have led to the use of R. acetosella in European and North American cultures as both a source of food and a component of herbal medicines. Excessive consumption of the leaves, which contain oxalic acid, can be poisonous to horses and sheep. In addition, R. acetosella is a possible host of the tomato spotted wilt virus, which is detrimental to a variety of vegetable crops.
Peat forming Sphagnum mosses are able to prevent the dominance of vascular plants under ombrotrophic conditions by efficiently scavenging atmospherically deposited nitrogen (N). N-uptake kinetics of these mosses are therefore expected to play a key role in differential N availability, plant competition, and carbon sequestration in Sphagnum peatlands. The interacting effects of rain N concentration and exposure time on moss N-uptake rates are, however, poorly understood.
We investigated the effects of N-concentration (1, 5, 10, 50, 100, 500 µM), N-form (15N - ammonium or nitrate) and exposure time (0.5, 2, 72 h) on uptake kinetics for Sphagnum magellanicum from a pristine bog in Patagonia (Argentina) and from a Dutch bog exposed to decades of N-pollution.
Uptake rates for ammonium were higher than for nitrate, and N-binding at adsorption sites was negligible. During the first 0.5 h, N-uptake followed saturation kinetics revealing a high affinity (Km 3.5–6.5 µM). Ammonium was taken up 8 times faster than nitrate, whereas over 72 hours this was only 2 times. Uptake rates decreased drastically with increasing exposure times, which implies that many short-term N-uptake experiments in literature may well have overestimated long-term uptake rates and ecosystem retention. Sphagnum from the polluted site (i.e. long-term N exposure) showed lower uptake rates than mosses from the pristine site, indicating an adaptive response.
Sphagnum therefore appears to be highly efficient in using short N pulses (e.g. rainfall in pristine areas). This strategy has important ecological and evolutionary implications: at high N input rates, the risk of N-toxicity seems to be reduced by lower uptake rates of Sphagnum, at the expense of its long-term filter capacity and related competitive advantage over vascular plants. As shown by our conceptual model, interacting effects of N-deposition and climate change (changes in rainfall) will seriously alter the functioning of Sphagnum peatlands.
The aim of this overview paper is to analyse the inclusion and use of the ecosystem services concept in scientific studies of degraded peatlands and peatland restoration. Publications indexed by the Institute of Science Information (ISI) Web of Science (WoS) from 1980 to October 2009 were analysed. Word combinations relevant to peatland ecosystem services in the title, keywords and abstract were used. We followed the division of ecosystem services into four categories: supporting, regulating, provisioning and cultural, as provided by the Millennium Ecosystem Assessment (2005). The analysis indicated that the concept of ecosystem services is not referred to explicitly in ISI WoS studies on peatland restoration. The interpretation of the content identified using search phrases related to various beneficial functions of peatlands showed that they mainly include information on regulating and supporting ecosystem services critical to sustaining vital ecosystem functions that deliver benefits to people. There are only a few articles addressing provisioning and cultural ecosystem services. One of the key issues concerning the effect of peatland restoration in the provisioning of ecosystem services is the balance of greenhouse gases and their role in global climate regulation.
The spatial development and vegetation history of a large boreal peatland complex in east-central Alberta was reconstructed to examine factors that control peatland development in continental regions. Peat depth throughout the site was interpolated from over 300 depth measurements, and basal radiocarbon dates were obtained from 16 cores. Peat first initiated about 7400 calibrated 14C years BP (cal. BP), and early peat-forming communities were wet fens or marshes. Rates of expansion from these nucleation sites were dependent on both moisture availability and topography, with asynchronous expansion in different regions. Basal macrofossil assemblages suggest that paludification on slopes of large basins was the result of flooding caused by rising peatland water tables. In many areas that initiated after 3000 cal. BP, paludification involved invasion of upland forest by Sphagnum. Long-term apparent rates of peat accumulation were fastest in wet, moderate-rich fen areas where little community change has occurred over time. Macrofossil analysis of core profiles reveals a tendency for sites that initiated wet and minerotrophic to eventually be colonized by Sphagnum. However, the thickness of surficial Sphagnum layers differs between cores, and there are several examples of minimal or apparently reverse successional development.Key words: peatlands, boreal, paleoecology, vegetation succession, peat accumulation, paludification.
The management of eutrophication has been impeded by reliance on short-term experimental additions of nutrients to bottles and mesocosms. These measures of proximate nutrient limitation fail to account for the gradual changes in biogeochemical nutrient cycles and nutrient fluxes from sediments, and succession of communities that are important components of whole-ecosystem responses. Erroneous assumptions about ecosystem processes and lack of accounting for hysteresis during lake recovery have further confused management of eutrophication. I conclude that long-term, whole-ecosystem experiments and case histories of lake recovery provide the only reliable evidence for policies to reduce eutrophication. The only method that has had proven success in reducing the eutrophication of lakes is reducing input of phosphorus. There are no case histories or long-term ecosystem-scale experiments to support recent claims that to reduce eutrophication of lakes, nitrogen must be controlled instead of or in addition to phosphorus. Before expensive policies to reduce nitrogen input are implemented, they require ecosystem-scale verification. The recent claim that the 'phosphorus paradigm' for recovering lakes from eutrophication has been 'eroded' has no basis. Instead, the case for phosphorus control has been strengthened by numerous case histories and large-scale experiments spanning several decades.
Ericaceae (the heath family) are widely distributed calcifuges inhabiting soils with inherently poor nutrient status. Ericaceae overcome nutrient limitation through symbiosis with ericoid mycorrhizal (ErM) fungi that mobilize nutrients complexed in recalcitrant organic matter. At present, recognized ErM fungi include a narrow taxonomic range within the Ascomycota, and the Sebacinales, basal Hymenomycetes with unclamped hyphae and imperforate parenthesomes. Here we describe a novel type of basidiomycetous ErM symbiosis, termed 'sheathed ericoid mycorrhiza', discovered in two habitats in mid-Norway as a co-dominant mycorrhizal symbiosis in Vaccinium spp. The basidiomycete forming sheathed ErM possesses clamped hyphae with perforate parenthesomes, produces 1- to 3-layer sheaths around terminal parts of hair roots and colonizes their rhizodermis intracellularly forming hyphal coils typical for ErM symbiosis. Two basidiomycetous isolates were obtained from sheathed ErM and molecular and phylogenetic tools were used to determine their identity; they were also examined for the ability to form sheathed ErM and lignocellulolytic potential. Surprisingly, ITS rDNA of both conspecific isolates failed to amplify with the most commonly used primer pairs, including ITS1 and ITS1F + ITS4. Phylogenetic analysis of nuclear LSU, SSU and 5.8S rDNA indicates that the basidiomycete occupies a long branch residing in the proximity of Trechisporales and Hymenochaetales, but lacks a clear sequence relationship (>90% similarity) to fungi currently placed in these orders. The basidiomycete formed the characteristic sheathed ErM symbiosis and enhanced growth of Vaccinium spp. in vitro, and degraded a recalcitrant aromatic substrate that was left unaltered by common ErM ascomycetes. Our findings provide coherent evidence that this hitherto undescribed basidiomycete forms a morphologically distinct ErM symbiosis that may occur at significant levels under natural conditions, yet remain undetected when subject to amplification by 'universal' primers. The lignocellulolytic assay suggests the basidiomycete may confer host adaptations distinct from those provisioned by the so far investigated ascomycetous ErM fungi.
Summary • In many ombrotrophic bog areas the invasion of grass (e.g. Molinia caerulea) and tree (e.g. Betula pubescens) species has become a major problem. We investigated whether the invasion of such species is due to high atmospheric nitrogen (N) deposition by conducting a fertilization experiment. • The effects of experimentally increased N input on Molinia, Betula and Eriophorum vaginatum were studied in desiccated bog vegetation in Ireland, where there is relatively low background N deposition. Four different N treatments were applied for 3 years: 0 (control), 2, 4 and 8 g m−2 year−1. • Ammonium and nitrate concentrations in the peat moisture increased at high N addition rates, leading to significantly higher carbon : nitrogen (C : N) and nitrogen : phosphorus (N : P) ratios in the top layer of the peat. The potential CO2 production rate of the peat was not stimulated at high N addition rates due to severe acidification of the peat. • Despite high tissue N : P ratios (above 40), above-ground biomass production by Molinia was stimulated at high N addition rates, and foliar nutrient concentrations were unaffected. In contrast to Molinia, Betula and Eriophorum were unable to increase their above-ground biomass, probably due to P limitation. Regrowth of the lichen Cladonia portentosa was suppressed at high N addition rates. • Synthesis and applications. We conclude that the invasion of bogs by Molinia and Betula is likely to be less affected by desiccation than by increased N availability. Apparently, Molinia is well adapted to P-limiting conditions, which may explain its success in regions with increased N deposition levels. The high availability of P in many Dutch bogs compared with Irish bogs, together with prolonged high N deposition levels, may explain the strong increase in both Molinia and Betula observed in the Netherlands. As long as N and P availabilities in Dutch bogs are too high to prevent invasion of Betula and/or Molinia, management measures stimulating growth of Sphagnum mosses could probably reduce the negative effects of high N deposition levels. Journal of Applied Ecology (2004) 41, 139–150
Even though Sphagnum mosses are not easy plants to manipulate on artificial substrates or in nonnatural environments, it is possible to revegetate
large expanses of cutover peatland at a relatively low cost (in the range of US $900–1400 per hectare). Only long term monitoring
of the current restoration projects will confirm if it is possible to restore the ecological functions of the cutover peatland
to bring it back to a peat-accumulating ecosystem. Fen restoration of peat fields used for agriculture has been mostly studied
in central Europe but much research is needed to develop sound restoration procedures for cutover peatlands and learn how
to grow true mosses. Sphagnum farming (cultivation in nurseries) is promising and research in that area should be promoted.Not only would it be useful
for supplying plant material for reintroduction in countries with low supply, but it could prove a useful source of biomass
to ameliorate growing substrates.
The recent development of peatlandrestoration activities indicates thatmodern peat mining techniques seriouslyhamper the natural capacity of bogecosystems to regenerate after adisturbance. However, some plants have theability to colonize dry peat deposits, andseem to help stabilize the soil surface andfacilitate the establishment of other plantspecies. In this paper, we review studiesregarding the spontaneous regeneration ofombrotrophic peatlands. There are numerousexamples throughout North America andEurope showing that spontaneousrevegetation of mined peatlands by typicalbog plants, and particularly by Sphagnum species, is possible. However,this phenomenon is much more common inblock-cut peatlands than in vacuum-minedsites. The slow recovery of vacuum-minedpeatlands compared to block-cut sites isprobably related to intense drainagenecessary for the use of tractor-drawnvacuum machines. There are some cases,however, where the spontaneous revegetationof vacuum-mined sites is successful,particularly for vascular plants. Thesesites are mainly dominated by cotton-grass(Eriophorum vaginatum L.). Itseems that the microclimatic conditionscreated by cotton-grass tussocks improveestablishment conditions for other vascularand non-vascular plants. Restorationactivities should be minimal in minedpeatlands already invaded by cotton-grass,and spontaneous revegetation processescould and should be integrated intopeatland restoration programs.
Methylmercury (MeHg) is a neurotoxin and endocrine disruptor that bioaccumulates and biomagnifies through trophic levels, resulting in potentially hazardous concentrations. Although wetlands are known hotspots for mercury (Hg) methylation, the effects of avian biovectors on these processes are poorly understood.We examined Hg speciation and distribution in shallow groundwater and surface water from a raised-bog with over 30 years of avian biovector (herring gulls Larus argentatus and great black-backed gulls Larus marinus) colonization and guano input. Compared to the reference site, the avian-impacted bog had elevated concentrations of total dissolved organic carbon (TOC), total Hg, MeHg, phosphate (PO4 3−), and other trace metals, notably Pb, As, Cd and Ni. Spatial interpolation showed that the densest area of gull nesting was co-located with areas that had the highest concentrations of PO4 3−, MeHg, As and Cd, but not total mercury (THg), and models suggested that Mn, PO4 3−, and dissolved TOC were strong predictors of MeHg. Our findings suggest that while these gulls may not be a significant source of Hg, the excess of PO4 3− (a well recognised component of guano) and the subsequent changes in water chemistry due to avian biovector subsidies may increase net Hg methylation.
Nutrient loading is a subtle, yet serious threat to the preservation of high diversity wetlands such as peatlands. Pathways of nutrient loading and impacts on plant diversity in a small peatland in New York State, USA were determined by collecting and analyzing a suite of hydrogeological, hydro-chemical, soil, and vegetation data. Piezometer clusters within an intensive network constituted hydro-chemical sampling points and focal points for randomly selected vegetation quadrats and soil-coring locations. Hydrogeological data and nutrient analyses showed that P and K loading occurred chiefly by means of overland flow from an adjacent farm field, whereas N loading occurred predominantly through ground-water flow from the farm field. Redundancy analysis and polynomial regression showed that nutrients, particularly total P in peat, total K in peat, extractable NH4-N, and NO3-N flux in ground water, were strongly negatively correlated with plant diversity measures at the site. No other environmental variables except vegetation measures associated with eutrophication demonstrated such a strong relationship with plant diversity. Nitrate loading over 4 mg m⁻² day⁻¹ was associated with low plant diversity, and Ca fluxes between 80 and 130 mg m⁻² day⁻¹ were associated with high plant diversity. Areas in the site with particularly low vascular plant and bryophyte species richness and Shannon-Wiener diversity (H′) occurred adjacent to the farm field and near a hillside spring. High H′ and species richness of vascular plants and bryophytes occurred in areas that were further removed from agriculture, contained no highly dominant vegetation, and were situated directly along the ground-water flow paths of springs. These areas were characterized by relatively constant water levels and consistent, yet moderate fluxes of base cations and nutrients. Overall, this study demonstrates that knowledge of site hydrogeology is crucial for determining potential pathways of nutrient loading and for developing relationships between nutrient inflows and wetland plant diversity.
Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).
Soil seed banks offer plants the possibility to disperse through time. This has implications for population and community dynamics, as recognised by ecological and evolutionary theory. In contrast, the conservation and restoration literature often find seed banks to be depauperate, weedy and without much conservation value or restoration potential. One explanation for these contrasting views might lie in a systematic bias in the sampling of seed banks versus established plant communities. We use the species–area relationship as a tool to assess and compare the per-area species richness and spatial structuring of the diversity of the established plant community versus soil seed banks. To allow this direct comparison we extensively survey the species–area relationship of the vegetation and underlying seed bank of a grassland community across twelve sites spanning regional bioclimatic gradients. We also compile a global dataset of established vegetation and seed banks from published sources. We find that seed banks have consistently higher intercepts and slopes of the relationship, and hence higher diversity at any given spatial scale, than the vegetation both in the field and literature study. This is consistent across habitat types, climate gradients, and biomes. Similarity indices are commonly used to compare vegetation and seed bank, and we find that sampling effort (% of the vegetation area sampled for seed bank) was the strongest predictor of vegetation-seed bank similarity for both the Sørensen (R2 = 0.70) and the Raup–Crick (R2 = 0.25) index. Our study suggests that the perception that seed banks are intrinsically less diverse than established plant communities has been based more on inadequate sampling than on biological reality. Across a range of ecosystems and climatic settings, we find high diversity in seed banks relative to the established community, suggesting potentially important roles of seed banks in population dynamics and diversity maintenance.
1. The effects of increased nitrogen or phosphorus supply on the productivity of Sphagnum-dominated ombrotrophic bogs in northern and southern Sweden were studied. Atmospheric nitrogen deposition in southern Sweden (high-N site) exceeds that in northern Sweden (low-N site) by about tenfold. 2. Vertical height growth of the Sphagnum carpet was measured by the cranked-wire method. Length growth of individuals was measured by autoradiography after labelling with 14CO2. The results of both methods were significantly correlated, but the cranked-wire data were systematically lower. 3. Productivity of Sphagnum at the low-N site increased almost fourfold after additional nitrogen supply (4 g N m-2 year-1), but no increase was found after additional phosphorus supply (0.4 g P m-2 year-1). At the high-N site, phosphorus supply caused an almost threefold productivity increase, but nitrogen supply did not result in any productivity increase. Thus, in an area with a high atmospheric nitrogen supply, plant productivity is P-limited instead of N-limited. 4. At an intermediate nitrogen supply (2 g N m-2 year-1) the recovery of the supplied nitrogen in the Sphagnum carpet was not different for both sites (60% and 69%, respectively). However, at a high nitrogen supply (4 g N m-2 year-1) nitrogen recovery at the low-N site significantly exceeded that at the high-N site (73% and 47%, respectively). At the low-N site, the supplied phosphorus was not recovered, but at the high-N site the phosphorus recovery was 85% (intermediate phosphorus supply) and 100% (high phosphorus supply), respectively. 5. It is suggested that a high atmospheric nitrogen supply may affect the carbon balance of ombrotrophic bogs, because productivity is under these circumstances not N-limited, but decomposition is probably increased by high loads of nitrogen. In the end, this may turn these C-accumulating systems into C-emitting systems.
We grew four perennial grass species (Poa pratensis, Agropyron repens, Agrostis scabra, and Schizachyrium scoparium) for 5 yr in monocultures and in pairwise competition plots on an experimental nitrogen gradient. The gradient contained plots ranging from 100% sand to 100% black soil, plus plots that received additional N fertilizer. To examine the impact of initial conditions on the long-term outcome of interspecific competition, three competitive situations were created: seed vs. seed competition (both species planned simultaneously), seed invasions (each species added as seed to year-old monocultures of the other), and vegetative invasions (dividers separating adjacent monocultures of two species removed after 1 yr). Extractable soil NO"3^- and NH"4^+ were measured to test if species differences in the concentration of available soil N in monoculture (i.e., R* for N, Tilman 1982) could predict the long-term outcome of competition. By year 5, Schizachyrium displaced or greatly reduced the biomass of both Poa and Agropyron on the soil mixture gradient (the mixed soils but not the added-N plots) independent of the wide range of starting conditions. On these soils, Schizachyrium monocultures had significantly lower soil concentrations of both NO"3^- and NH"4^+ than either Poa or Agropyron monocultures. Similarly, Agropyron displaced or greatly reduced the biomass of Agrostis by year 5. Agropyron monocultures had significantly lower concentrations of NO"3^- and NO"3^- + NH"4^+, but not NH"4^+, than Agrostis monocultures. In contrast, no competitive displacement occurred in competition between Poa and Agropyron, and initial differences persisted over 5 yr. Monocultures of these two species did not differ in NO"3^- concentration, but did differ for NH"4^+ and NO"3^- + NH"4^+. Thus, species differences in ability to deplete soil NO"3^- successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource preemption or asymmetric competition had been the mechanism of competition, initial conditions would have affected the long-term outcome of competition. Rather, these results support the R* (i.e., resource reduction) model for competition for soil N. In the added-N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and Agropyron. However, neither Agropyron nor Poa appeared to have an advantage when they competed with each other in the added-N plots. For these three species pairs, the 5-yr results of competition in the added-N plots, which had greatly reduced light availability because of increased production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all competition treatments in the added-N plots. Thus, we cannot reject the hypothesis that resource preemption (i.e., asymmetric competition) is important in light competition.
This article presents a reconstruction of the development of the ombrotrophic Ga˛zwa peatland
based on a high-resolution macrofossil analysis and AMS dating. Successional changes in the plant
assemblages dominated by Sphagnum were influenced mainly by climate changes. Local fires occurred in the
peatland, but did not have a substantial impact on the Sphagnum succession. The disappearance of the first
stage of the ombrotrophic peatland, which was dominated by Sphagnum magellanicum and Sphagnum
angustifolium, resulted from a decrease in the water level that was also recorded at a number of sites in
Europe in approximately 3750 BC. The development of Sphagnum fuscum/rubellum assemblages in
approximately 350 BC corresponds with a climate cooling that resulted from low solar activity. The reappearance
of Sphagnum magellanicum in approximately AD 1650 corresponds with the Maunder
Minimum of the Little Ice Age. The recent dominance of Sphagnum fallax is a result of the drainage of the
peatland and of changes in the nearby vegetation. The paleoecology of Sphagnum contortum is also
discussed. In the Ga˛zwa peatland, S. contortum occurred during two periods, 3700–3300 BC and 2850–2000
BC. The fossil occurrence of S. contortum in the Ga˛zwa peatland corresponds with its modern ecological
requirements in the northern hemisphere. S. contortum occurred in the rich fen phase with Carex lasiocarpa,
Comarum palustre, Menyanthes trifoliata and Meesia triquetra. The disappearance of S. contortum is assumed
to have been caused by a trophic shift and the succession of a more acidophilic species, Sphagnum obtusum.
Riparian wetlands are under heavy pressure from hydrological changes produced by dam construction and water diversion projects. There has been ample documentation of the relationship between the extent of flooding and the composition of shoreline plant communities, yet we have few models that allow us to predict the impact of altered flooding regimes on riparian wetlands. In the humid temperate zone, river regulation commonly affects the distribution of two major vegetation types: wooded wetland and herbaceous wetland. The practice of reducing peak floods and augmenting minimum river flows is often followed by the succession of herbaceous to wooded wetland. We used logistic regression models to describe the distribution of wooded wetland as a function of all possible combinations of seven hydrological variables. The variables were chosen to reflect the depth, duration, and time of flooding and were calculated for four different time intervals (3, 7, 12, and 18 growing seasons). Our best model was a combination of two variables: the last day of the first flood and the time of the second flood. For three of the four time intervals, the vegetation type was correctly identified as herbaceous or wooded for > 80% of the sample points. Our results suggest that models based on a few key environmental variables can be valuable tools in the conservation management of the vegetation of temperate and boreal zone wetlands.
It is suggested that evolution in plants may be associated with the emergence of three primary strategies, each of which may be identified by reference to a number of characteristics including morphological features, resource allocation, phenology, and response to stress. The competitive strategy prevails in productive, relatively undisturbed vegetation, the stress-tolerant strategy is associated with continuously unproductive conditions, and the ruderal strategy is characteristic of severely disturbed but potentially productive habitats. A triangular model based upon the three strategies may be reconciled with the theory of r- and K-selection, provides an insight into the processes of vegetation succession and dominance, and appears to be capable of extension to fungi and to animals.
The effects of nesting colonies of the Ring-billed Gull (Larus delawarensis Ord.) on vegetation and soil nutrients were monitored over a 4-year period on Barrier Island and other islands in Georgian Bay and Lake Huron, in the Great Lakes region of Canada. Nesting had a profound effect on the vegetation and species composition. Two distinct communities of annual alien species dominated the nesting sites, one flowering in May and June and the other from late August to October. Native species were rare in the nesting sites, in contrast to adjacent areas, where perennial native grasses were dominant. Factors favouring the replacement of indigenous vegetation by annual aliens include physical disturbance by gulls during nesting and deposition of guano, which caused dramatic increases in the levels of soil nutrients. Plant succession in nesting sites that had been abandoned for 3 and 4 years was characterized by a rapid decline in the abundance of annual aliens and their replacement by perennials. After the 2nd year native perennial grasses were dominant. Nutrient levels in the soil rapidly returned to near normal, except for phosphorus, which persisted at high levels. Herring Gull (Larus argentatus Pontoppidan) nests were also studied. Their density was much lower, and hence their effects were less pronounced. However, they supported a similar annual alien flora. Herring Gulls frequently nested on unvegetated cobble or gravel shores where the humus from the nests permitted the establishment of a permanent vegetation.
Seedling emergence from organic and mineral soil layers was measured for nine study sites at the Acadia Forest Experiment Station near Fredericton, New Brunswick. The number of viable seeds showed a decrease from deciduous-dominated forest, to conifer-dominated forest, to organic soil study sites. Viable seed number varied from 3400/m2 for a deciduous-dominated forest study site to zero for a bog study site. Most seeds germinated from the upper organic soil layers of all study sites and were predominantly Rubus strigosus Michx. After the germination experiment, ungerminated seeds, which showed no viability by the tetrazolium test, were separated from the soil. These seeds were almost entirely Betula spp. and seed numbers were as high as 4200–9400/m2 for a deciduous-dominated forest. The applicability of the results to differing types of postdisturbance revegetation is discussed.
Question: Does the seed density of invasive species affect establishment by native species in a bare ground context (following invasive species control efforts), and is it possible to promote transition to a native species dominated state by manipulating sowing density of the native community?
Location: Experimental wetland basin in Chanhassen, Minnesota, USA.
Methods: A mesocosm experiment investigated the influence of Phalaris arundinacea (invasive species) propagule pressure on establishment of native wet meadow species in the context of a newly restored wetland. Mesocosms were sown with P. arundinacea (0, 10, 50, 100, or 500 seeds/m2) and a mix of native species (3000 or 15000 seeds/m2).
Results: When planted at densities > 100 seeds/m2, P. arundinacea increased suppression of native species. Also, high native seed density suppressed P. arundinacea biomass production. This effect was more pronounced when P. arundinacea seed density was high (> 100 seeds/m2), but high native seed density (15000 seeds/m2) did not suppress recruitment of P. arundinacea from seed.
Conclusions: The transition from post-control bare ground (a common result of efforts to control invasive species) to native species establishment depends on both native species and invader seed density. These results suggest that a threshold of P. arundinacea propagule pressure exists, beyond which transition to a native community is less likely without management intervention. P. arundinacea can establish in the presence of a newly developing native plant community, even at very low densities of P. arundinacea seed. Invader control (following initial site clearing efforts) is essential to native species establishment.
Freshly-collected seeds of twenty species of annual weeds were uniformly mixed with 2.5, 7.5 or 15 cm of soil confined in cylinders sunk in the ground outdoors. The soil was either cultivated four times a year or left undisturbed, seedling emergence was recorded, and after 5 years the numbers of viable seeds remaining were determined.Total seedling emergence from seeds incorporated to 2·5, 7·5 and 15 cm amounted to 75, 65 and 54% respectively of those added when the soil was cultivated, and 58, 36 and 21% where it was left undisturbed. The corresponding numbers of viable seeds remaining after 5 years were 2·3, 4·0 and 7·7% respectively for cultivated soil, and 6·8, 16·5 and 31·6% of the seeds initially added and then not disturbed. Individual species varied in their responses to increasing depth of incorporation and to cultivation; these differences are discussed in relation to the dormancy characteritics of the seeds.Devenir des semences de quelques mauvaises herbes annuelles à différentes profondeur dans un sol cultivé et non cultivéRésumé. Des semences récoltées récemmeni sur vingt espèces de mauvaises herbes an-nuelles furent uniformément mélangées avec 2,5; 7,5 ou 15 cm de sol contenu dans des cylindres stockés dans le sol en dehors du laboratoire. Le sol fut travaillé quatre fois dans l'année, ou au contraire laissé sans facon culturale, la levée dts plantules fut enregistrée, et, après 5 ans, les semences viables restantes furent dénombrées.La levée totale des semences incorporées a 2,5; 7,5 et 15 cm fut respectivement de 75, 65 et 54% pour les semences ajoutées au sol travaillé et de 58, 36, et 21 % pour les semences en sol non travaillé. Les nombres correspondants de semences restées viables aprés 5 années représentèrent respectivement 2,3; 4,0 et 7,7% du nombre initial dans le sol travaillé et 6,8; 16,5 et 31,6% dans le sol non travaillé, Les espèces, considérées individuellement, présentèrent des réactions variées à l'accroissement de la profondeur d'enfouissement et au travail du sol; ces différences sont examinées en relation avec les caractératiques des dormances des semences.Verhalten von Samen einiger annueller Unkräauter in versckiedenen Tiefen eines bearbeiteten und nicht bearbeiteten BodensZusammenfassung. Frisrh gesammelte Samen von zwanzig Arten annueller Unkräuter wurden mit Boden gleichiormig gemischt und In Zylindern in 2,5; 7,5 oder 15 cm Tiefe draussen im Boden gclagert. Der Boden wurde entweder viermal im Jahr bearbeitet oder blieb unbcarbeitet. Das Auflaufen der Samen wurde ermittelt, und nach 5 Jahren wurde die Zahl der lebensfähig gebliebenen Samen bestimmt.Der gesamte Auflauf der Samen, die in 2,5; 7,5 und 15 cm Tiefe eingebracht worden waren, belief sich bei dem bearbeiteten Boden auf 75; 65 und 54% der untergemischten Samen. Im nicht bearbeiteten Boden waren cs 58; 36 und 21%. Die entsprechende Anzahl der nach 5 Jahren noch lebensfäAhig gebliebenen Samen belief sich im bearbeiteten Boden auf 2,3; 4,0 und 7,7% und im nicht bearbeiteten Boden auf 6,8; 16,5 und 31,6% der ausgcbrachten Samen. Vcrsrhiedene Arten zeigten in ihrer Reaktion aufdie zunehmende Tiefe ihrer Aussaat und aufdie Bearbeitung ein abweichendes Verhalten. Diese Unterschiede werden in Bczug zu den Dormanzcharaktc Hstika der Samen diskutiert.
1. Characterization of the seed bank is one of the most important demographic assessments that can be undertaken for a plant community. Overlapping generations, evidence of past above-ground vegetation and histories of invasion and disturbance are recorded in the seed bank.
2. Two broad approaches have been used to elucidate seed bank components – sifting-sorting techniques and germinability assays. The utility of these approaches varies with community type and habitat although a common theme among studies has been the quest for an efficacious method. Here, we compare the two approaches for semi-arid ephemeral wetlands: seed extraction through flotation and seedling emergence.
3. Species composition of the soil seed bank differed dramatically depending on the technique, with only 19 species common to both methods and a total of 66 species detected using both procedures.
4. Both techniques provided similar estimates of seed density and species richness of the seed bank in the top 5 cm of soil. However, samples collected from 5 cm to 20 cm had lower seed densities using the flotation technique than with the seedling emergence technique.
5. Differences in seed detectability between the two approaches may be related to seed size, seed dormancy and specific germination requirements.
6. The community composition of soil seed banks for ephemeral wetlands depends on the choice of technique.
The interference between vascular plants and peat mosses with respect to nitrogen and phosphorus was studied in a fertilization experiment and with respect to competition for light in a removal experiment in poor fens with either soligenous or topogenous hydrology using Nartheciumossifragum (L.) Huds. and three species of Sphagnum sect. Sphagnum as targets. Adding fertilizer either on the moss surface or below it confirmed the hypotheses of an asymmetric competition for nutrients, viz. that the Sphagnum mosses relied on the atmospheric supply while Narthecium depended on mineralization in the peat. The results of the removal experiments and the negatively correlated growth of Narthecium and Sphagnum mosses demonstrated a symmetric competition for light. The intensity of the competition for light increased as the availability of N and P increased. The nutrient resources in the total biomass decreased with decreasing standing crop of Narthecium. Only with a considerable amount of mineral nutrients in the biomass has Narthecium the capacity to grow ahead of Sphagnum, because the asymmetric competition for N and P gives Sphagnum the capacity to reduce the performance of vascular plants. The mosses are more efficient in their use of nutrients and produce a decay-resistant litter inducing low mineralization and increasing the peat accumulation rate, and that withdraws N and P from the rhizosphere. The Sphagnum mosses thus act as ecological engineers structuring the plant community and determining the carbon balance of the system. The development of ombrotrophic conditions through peat accumulation seems less probable on soligenous than on topogenous mires owing to the higher mineralization rate there supporting the growth of the vascular plants. Correspondingly, disturbances of the Sphagnum cover, such as through airborne pollutants, increase the productivity of the vascular plants and decrease the capacity for carbon accumulation.
It is notable that Themeda reduced soil nitrate at Windermere even though other native perennial grasses already dominated these plots. This could reflect the small overall increase in native perennials on these plots by autumn 2004 (Fig. 4), or Themeda may have a greater effect than other native grasses on soil nitrate. The latter is supported by evidence for consistently lower nitrate levels beneath Themeda swards than beneath other native grass swards (Prober, Thiele & Lunt 2002) and evidence that perennial grass species can differ greatly in their effects on soil nitrate (Tilman & Wedin 1990). Studies comparing effects of relevant native perennial grasses on soil nutrients are thus needed to guide selection of species for restoring ecosystem function in temperate Australian woodland. Of particular interest is the influence of Poa, once codominant with Themeda in woodland understoreys; this cool-season C3 perennial may influence soil nitrogen during winter and early spring, when Themeda is dormant and exotic annuals are active. The contribution of subsidiary forbs and grasses to this process also requires assessment.
Most of North America’s northern Great Plains have been cultivated for crop production, leaving remnants of natural mixed-grass prairie fragmented and threatened by alien plant invasions. The region’s most widespread alien perennial forage crop, crested wheatgrass (Agropyron cristatum sensu amplo), has invaded native grassland and raised concerns regarding its ecological impact. To evaluate impacts at multiple scales of organization, adjacent invaded and uninvaded mixed-grass prairie were sampled at eight widely separated locations. At the population level, native C3 mid-grasses and forbs were less abundant in invaded grasslands, while native C3 and C4 short-grass abundance was not different. At community and landscape levels, diversity was lower in invaded grasslands largely because of lower forb species richness and cover, and crested wheatgrass dominance of both cover (14% basal cover) and seedbank (404 seeds m−2). At the ecosystem level, both vegetation and litter biomass were greater in invaded grasslands, however, below ground organic matter (roots and litter), soil organic carbon, total nitrogen and phosphorus were not different. Crested wheatgrass invasion of mixed-grass prairie was associated with lower diversity within and among plant communities, and appears to simplify the composition of mixed-grass prairie landscapes. Hypotheses for crested wheatgrass dominance and persistence following invasion are suggested.
Bogs and fens are rare communities in the southern Appalachians of the USA. Many have been degraded, and little ecological
information beyond cursory floral inventories is available to help guide conservation and restoration efforts. The seedling
emergence technique was used to examine the soil seed banks of open and closed canopy regions of a southern mountain fen in
North Carolina. We also examined the seed bank of an adjoining portion of the floodplain, which had been drained and cleared
for a golf fairway and is now slated for restoration. A total of 32 taxa emerged, with graminoids (particularlyJuncus spp.) dominating all three seed banks. Seedlings were assigned to one of five plant types: woody, rush, sedge, grass, or
forb. Significantly more woody seedlings emerged in soils from the closed canopy fen than in soils from the other two areas.
Most rush seedlings emerged in open canopy fen soils, more sedge and forb seedlings emerged in floodplain soils, and more
grass seedlings emerged in floodplain soils than in closed canopy fen soils. A discriminant function analysis separated the
open canopy fen from the closed canopy fen and floodplain by seedlings of woody plants and rushes. The floodplain was separated
from the open and closed regions of the fen by sedge and grass seedlings. These patterns in seed bank composition bore little
similarity to the standing vegetation in the three areas. Restoration activities planned for the floodplain are intended to
restore its hydrology and microtopography, which will strongly influence recruitment from the seed bank and surrounding seed
sources.
We examined the natural colonization by vascular plants of 11 created wetlands in southeastern Wisconsin. The wetlands studied
were small depressional wetlands that were isolated from other wetland sites. Wetlands were sampled over a two-year period,
providing samples of wetlands aged one to three years. The development of wetland vegetation in these 11 naturally colonized
sites was compared to that in five wetlands to which we introduced 22 species of native wetland plants. We identified 142
species of vascular plants in the naturally colonized wetlands. Of these, 82 (58%) were native, obligate or facultative wetland
plants. The diversity and richness of native wetland plants and the proportion of total plant cover that was comprised of
native wetland species increased from one-to three-year-old wetlands. The diversity and richness of native wetland species
increased with wetland age, wetland size, and with proximity to the nearest established wetland. Distance to the nearest seed
source had a particularly strong effect on the number of native wetland species present. Cattail (Typha spp.) accounted for 15% of the cover of native wetland species in one-year-old wetlands; this increased to 55% in three-year-old
wetlands. We predict that the naturally colonized wetlands will develop into near monocultures of cattail with a fringe of
willows (Salix spp.) and cottonwood (Populus deltoides) at the wetland/upland margin. Wetlands seeded with native wetland species had much higher diversity and richness of native
wetland species than unseeded wetlands after two years. Seventeen of the 22 seeded species became established in at least
two wetlands after simple introduction of seed to the sites. Cattail cover after two years was lower in seeded sites, both
as an absolute cover and as a proportion of native wetland plant cover. Early introduction of a diversity of wetland plants
may enhance the long-term diversity of vegetation in created wetlands.
Nutrient loading is a subtle, yet serious threat to the preservation of high diversity wetlands such as peatlands. Pathways
of nutrient loading and impacts on plant diversity in a small peatland in New York State, USA were determined by collecting
and analyzing a suite of hydrogeological, hydro-chemical, soil, and vegetation data. Piezometer clusters within an intensive
network constituted hydro-chemical sampling points and focal points for randomly selected vegetation quadrats and soil-coring
locations. Hydrogeological data and nutrient analyses showed that P and K loading occurred chiefly by means of overland flow
from an adjacent farm field, whereas N loading occurred predominantly through ground-water flow from the farm field. Redundancy
analysis and polynomial regression showed that nutrients, particularly total P in peat, total K in peat, extractable NH4-N, and NO3-N flux in ground water, were strongly negatively correlated with plant diversity measures at the site. No other environmental
variables except vegetation measures associated with eutrophication demonstrated such a strong relationship with plant diversity.
Nitrate loading over 4 mg m−2 day−1 was associated with low plant diversity, and Ca fluxes between 80 and 130 mg m−2 day−1 were associated with high plant diversity. Areas in the site with particularly low vascular plant and bryophyte species richness
and Shannon-Wiener diversity (H′) occurred adjacent to the farm field and near a hillside spring. High H′ and species richness
of vascular plants and bryophytes occurred in areas that were further removed from agriculture, contained no highly dominant
vegetation, and were situated directly along the ground-water flow paths of springs. These areas were characterized by relatively
constant water levels and consistent, yet moderate fluxes of base cations and nutrients. Overall, this study demonstrates
that knowledge of site hydrogeology is crucial for determining potential pathways of nutrient loading and for developing relationships
between nutrient inflows and wetland plant diversity.
A three year fertilization experiment was conducted in which nitrogen (N series: 20 g N m–2 yr–1), phosphorus (P series: 4 g P m–2 yr–1) and potassium (K series: 20 g K m–2 yr–1) were added to a mixed vegetation of Erica tetralix and Molinia caerulea. At the end of each growing season the percentage cover of each species was determined. At the end of the experiment percentage cover of each species was found to be positively correlated with the harvested biomass. In the unfertilized control series the cover of Erica and Molinia did not change significantly during the experiment. In all fertilized series however, especially in the P series, cover of Erica decreased significantly. The cover of Molinia increased significantly in the P series only.In the fertilized series the biomass of Erica and total biomass per plot did not change significantly compared with the control series. In the P series the biomass of Molinia increased significantly.It is suggested that with increasing phosphorus or nitrogen availability Molinia outcompetes Erica because the former invests more biomass in leaves which in turn permits more carbon to be allocated to the root system, which thereupon leads to a higher nutrient uptake.