The taxonomic status and relationships of bracken ferns (Pteridium: Dennstaedtiaceae) from sub-Saharan Africa

Two morphotypes of bracken fern in the genus Pteridium Gled. ex Scop. occur in sub-Saharan Africa. The more common and widespread of these is assigned in contemporary systematic treatments to the European subspecies, P. aquilinum (L.) Kuhn ssp. aquilinum . We show that this African form is separable morphometrically and genomically from the European subspecies, and reinstate its earlier name P. aquilinum ssp. capense (Thunb.) C.Chr. The second African bracken, with a more localized tropical distribution mainly in the drainage basins of the Congo and Zambezi River systems, is confirmed as P. aquilinum ssp. centrali-africanum Hieron. ex R.E. Fr. We reject suggestions that this taxon be treated as a full species. Phenetic cluster analysis based on use of Arbitrarily Primed Polymerase Chain Reaction (A-P PCR) and Inter Simple Sequence Repeat (ISSR) fingerprinting of the nuclear genome provides evidence that the two African brackens are more closely related to each other than to other taxa, and sister to a grouping of the European sspp. aquilinum and pinetorum . The two African subspecies share solely with ssp. aquilinum a distinctive chloroplast haplotype carrying a 5-base direct repeat in the rps 4 –trn S region, confirming the close phyletic relationship between sspp. aquilinum , capense and centrali-africanum .  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 148 , 311–321.     

Plant performance and morpho-functional differentiation in response to edaphic variation in Iberian columbines: cues for range distribution?

Aims Experimental studies of the response to environmental variation of closely related taxa are needed to understand the mechanisms underlying phenotypic divergence, habitat segregation and range span within a radiating genus. We explored the magnitude of phenotypic differentiation and adaptive plasticity in relation to edaphic variation and its significance for habitat segregation and range span in Iberian columbines.Methods We performed a common garden experiment varying soil type (calcareous vs. siliceous) and depth (rocky-shallow vs. deep) with two pairs of widespread and narrowly distributed subspecies of Iberian columbines: Aquilegia vulgaris (subspp. vulgaris and nevadensis) and Aquilegia pyrenaica (subspp. pyrenaica and cazorlensis). We compared tolerance to edaphic variation, trait differentiation and adaptive plasticity of 10 morpho-functional traits between species and distribution ranges. Additionally, we obtained estimates of the competitive environment faced by each taxon in two to four populations per taxa.Important findings Results partially support hypotheses of higher competitive ability and tolerance to edaphic variation in widespread than in narrow endemic taxa. At the species level, the widely distributed taxon was the most tolerant to edaphic variation. Within species, no consistent pattern emerged since the population from the most widespread subspecies was the most tolerant in A. vulgaris but not in A. pyrenaica. Columbines were differentiated in many traits at species and range level. However, the pattern of differentiation does not fully support the hypothesis of higher specialization and stress tolerance in narrow endemics. Although plasticity was generally low, the results support the hypothesis of adaptive plasticity in widespread but not in restricted taxa at least at species level. Ecological differences (adaptive plasticity and competitive ability among others) may have contributed to phenotypic divergence and edaphic niche segregation, as well as to differences in range span among columbines.     

Global chloroplast phylogeny and biogeography of bracken (Pteridium; Dennstaedtiaceae)

Bracken ferns (genus Pteridium) represent an ancient species complex with a natural worldwide distribution. Pteridium has historically been treated as comprising a single species, but recent treatments have recognized several related species. Phenotypic plasticity, geographically structured morphological variation, and geographically biased sampling have all contributed to taxonomic confusion in the genus. We sampled bracken specimens worldwide and used variable regions of the chloroplast genome to investigate phylogeography and reticulate evolution within the genus. Our results distinguish two major clades within Pteridium, a primarily northern hemisphere Laurasian/African clade, which includes all taxa currently assigned to P. aquilinum, and a primarily southern hemisphere Austral/South American clade, which includes P. esculentum and P. arachnoideum. All European accessions of P. aquilinum subsp. aquilinum appear in a monophyletic group and are nested within a clade containing the African P. aquilinum taxa (P. aquilinum subsp. capense and P. aquilinum subsp. centrali-africanum). Our results allow us to hypothesize the maternal progenitors of two allotetraploid bracken species, P. caudatum and P. semihastatum. We also discuss the biogeography of bracken in the context of the chloroplast phylogeny. Our study is one of the first to take a worldwide perspective in addressing variation in a broadly distributed species complex.     

The ecological status of bracken

Information about the morphology of bracken ( Pteridium aquilinum (L.) Kuhn var. aquilinum ) and the chief nutrients in the frond at different times of the year introduce an account of litter production and its accumulation in relation to the behaviour of frond, root and rhizome systems.
Where litter gain exceeds loss there is a correlation between the thickness and/or kind of litter and the level of the root and rhizome systems in relation to the mineral soil surface: with increase of litter the bracken becomes progressively more dependent for its physical and chemical soil environment on its own débris and less on the underlying mineral soil. An example of the limit of complete dependence has not been examined, but degeneration of the community can take place before that stage is reached.
From a review of the chief factors affecting bracken the conclusion is reached that the woodland habitat is both favourable and restrictive: in it bracken is in equilibrium with its environment, at a high social status. The relationship with other plant communities depends largely on the degree of human interference to which each is subject. Dominant bracken when left alone, and where gain of litter exceeds loss, becomes the victim of its own success; local degeneration opens the way for entry by other species.     

Orchid biogeography and factors associated with rarity in a biodiversity hotspot,the Southwest Australian Floristic Region

Aim The causes of orchid diversification and intrinsic rarity are poorly resolved. The Orchidaceae of the Southwest Australian Floristic Region use a diversity of pollination strategies and sites of mycorrhizal infection, and occupy a diversity of habitats. We combined a biogeographic analysis with analysis of factors associated with rarity to establish: (1) the landscape features correlated with taxon turnover and speciation, and (2) the possible role in taxon rarity of geographic region, pollination strategy, edaphic habitat and site of mycorrhizal infection. Location Southwest Australian Floristic Region. Methods The distributions of 407 orchid taxa (species and subspecies) were mapped at the quarter‐degree scale using 13,267 collections in the Western Australian Herbarium. This database was used to map taxon richness, for a biogeographic analysis and to quantify rarity of taxa. Using herbarium records, rarity was expressed as mean abundance, mean distribution and incidence of rarity based on abundance and distribution for each genus. We tested for differences in rarity of species between pollination strategies, edaphic habitats and sites of mycorrhizal infection. Results Taxon richness was highest in the High Rainfall Province. Biogeographic provincial boundaries for orchids were aligned with rainfall, while district boundaries tended to follow geological formations. When rarity was defined as either low abundance or small distribution, the greatest number of rare taxa occurred in areas of high taxon richness and naturally fragmented edaphic environments. For both abundance and distributional extent, sexual deception had a significantly higher incidence of rarity than food‐rewarding taxa. There was no significant difference in rarity with site of mycorrhizal infection. Main conclusions While large‐scale edaphic and climatic variation are correlated with orchid taxon turnover and speciation in a similar fashion to the flora in general, the processes responsible for patterns of diversity may differ. Fragmented edaphic environments appear to be associated with a higher incidence of rare species due to limited dispersal/colonization opportunities or radiations of taxa in allopatry. The high incidence of rarity in sexually deceptive taxa could be due to either low fruit set or the risk of specializing on a single pollinator species.     

Factors Affecting the Restoration of Heathland and Acid Grassland on Pteridium aquilinum–Infested Land across the United Kingdom: A Multisite Study

The variability in the success of Pteridium aquilinum (bracken) control and vegetation restoration has been highlighted as a major issue in the United Kingdom. Experiments were set up at four different regional locations to assess bracken control at the national scale and the impact of restoration practices at the local scale. Bracken control treatments (cutting once or twice per year, a combination of cutting and asulam spraying, and asulam in year 1) were combined with site‐specific treatments designed to restore appropriate heathland or acid grassland vegetation. This article considers the effects on the developing understorey vegetation, testing the following hypotheses: (1) local differences between sites would affect community change; (2) treatments applied to control Pt. aquilinum (same at all sites) influences community change; and (3) treatments applied at the individual site level to restore vegetation influences community change toward the target vegetation. There were a considerable number of spatial effects. It is, therefore, difficult to develop a one‐size‐fits‐all policy for vegetation restoration within a national Pt. aquilinum control strategy. Few bracken control treatment effects were found, and, where they were detected, it was only at single sites. Thus, the development of target vegetation requires a combination of control and restoration treatments that take into consideration the aspects of that site. Only three species, Deschampsia flexuosa, Galium saxatile, and Campylopus introflexus, increased as a direct effect of the control treatments. Vegetation restoration was most successful in the cutting‐twice‐per‐year plots, the treatment with the greatest reduction in Pt. aquilinum cover.     

Genetic structure among and within populations of the serpentine endemic Heteropappus hispidus ssp. leptocladus (Compositae)

Heteropappus hispidus ssp. leptocladus is an edaphic endemic taxon that is confined to serpentine and limestone-derived soils and is allopatrically distributed in three regions of western Japan. In this study, we attempted to detect genomic signatures of seven H. hispidus ssp. leptocladus populations along with eight other subspecies populations using eight nuclear microsatellite loci. The Mantel test supported an isolation by distance model across all H. hispidus populations, thus implying the possibility of parallel evolution for each subspecies. Results from AMOVA recognized relatively larger differentiations in geographic distribution compared to intraspecific taxonomy. Relationships indicated by neighbor-joining phylogenetic tree analysis and population structure generally did not reflect an intraspecific taxonomy. Populations from limestone-derived soil harbored a homogeneous genetic structure with neighboring populations from serpentine-derived soils. These results suggest that the edaphic ecotype may have derived allopatrically while a lack of edaphic constraint existed between serpentine and limestone soils.     

The Control of Bracken

Abstract

Bracken is a weed of economic importance to hill farming, conservation, and forestry in many parts of the world. It is an aggressive competitor, capable of rapid spread by means of its persistent underground rhizomatous system, and it is highly toxic and carcinogenic to domestic stock. In addition, it harbors large populations of ticks that transmit disease to animals and humans.

Control by husbandry or by mechanical frond destruction is labor-intensive and often inadequate. Chemical control continues to be a subject of intensive research, with particular emphasis on the use of adjuvants, including surfactants. There is evidence that formulation of systemic herbicides such as asulam or glyphosate with low concentrations (e.g., 0.05%) of certain nonionic surfactants can effectively increase foliar absorption and translocation to the underground rhizome buds.

Biological methods of control using either insects or fungi are under investigation. Certain bracken-feeding South African insects are specific to the variety of bracken (Pteridium aquilinum ssp. aquilinum var. aquilinum) that is also common in the U.K. It is proposed that the moth Conservula consigna would occupy a vacant ecological niche if introduced into the U.K. bracken ecosystem and thereby give effective control. The insect feeds on the pinnae and completes its life cycle on bracken. The fungus Ascochyta pteridis is indigenous to the U.K., where, under certain climatic conditions, it causes the disease of bracken known as curl-tip. The potential of a mycoherbicidal formulation, in which the spores are mixed with adjuvants designed to give the fungus a competitive advantage, is under investigation. Oil and water emulsions, for example, are expected to be of particular importance: they assist adhesion of spores to the host surface while providing water for their germination.

This review examines the problems posed by bracken and the progress of research toward the development of economically and environmentally acceptable methods of control of this noxious and aggressive weed.     

Estimates of gene flow, genetic substructure and population heterogeneity in bracken (Pteridium aquilinum)

Bracken [ Pteridium aquilinum (L.) Kuhn] is a cosmopolitan species and is a noxious weed in many areas. Because of its abundance, particularly in Britain, bracken affords an ideal system for investigating various aspects of population genetics and evolution. High mobility of dispersal units (spores) suggests that rates of gene flow among distant populations should be high. Gene flow is a major evolutionary force that influences the genetic structure of populations. To examine the effects of gene flow on population heterogeneity and population substructuring in bracken, starch gel electrophoresis of enzymes was used to provide the necessary genetic database. Allele frequency data at 21 loci were obtained for seven British populations, one Majorcan and one from the eastern United States. A model was employed to estimate the amount of gene flow ( Nm ) at several levels. Gene flow among British populations was extremely high ( Nm = 36.51), one of the highest estimates reported for plants. Among eight European populations gene flow was lower (but still considered high) at Nm = 2.47. Trans-Atlantic gene flow was low ( Nm = 0.0926).
F -statistics were used to assess population heterogeneity and substructuring. The data indicate that, compared with other species, there is very little genetic differentiation among British populations of bracken. Indeed, it appears that the whole island is behaving as a single randommating population. This result is consistent with high levels of gene flow. Only one population (on the Isle of Arran) showed statistically significant genetic substructuring. Habitat heterogeneity on the island and age structure are hypothesized as possible causes of this result.
The data reported here support previous studies demonstrating that bracken is genetically polymorphic and is an outcrossing species.     

Bracken (Pteridium aquilinum)-induced DNA adducts in mouse tissues are different from the adduct induced by the activated form of the Bracken carcinogen ptaquiloside

Following treatment with bracken fern (Pteridium aquilinum) extract and bracken spores a number of DNA adducts were detected by (32)P-postlabeling. Three of these adducts have been described previously (Povey et al., Br. J. Cancer (1996) 74, 1342-1348) and in this study, using a slightly different protocol, four new adducts, with higher chromatographic mobility, were detected at levels ranging from 50 to 230% of those previously described. When DNA was treated in vitro with activated ptaquiloside (APT) and analysed by butanol extraction or nuclease P1 treatment, only one adduct was detected by (32)P-postlabeling. This adduct was not present in the DNA from mice treated with bracken fern or spores, suggesting either that bracken contains genotoxins other than ptaquiloside or that the metabolism of ptaquiloside produces genotoxins not reflected by activated ptaquiloside. However, as the ATP-derived adduct has been detected previously in ileal DNA of bracken-fed calves, species-specific differences in the metabolism of bracken genotoxins may exist, thereby leading to differences in their biological outcomes.     

The taxonomy and phytogeography of bracken—a review

Some of the biological problems presented by bracken, Pteridium aquilinum (L.) Kuhn, are posed. Its taxonomic position within the Pteridophyta and the delimitation of entities within the genus are discussed on the basis of morphological and cytological evidence. The geographical ranges of the various brackens world-wide are described and mapped in outline, and emphasis placed on reviewing the natural ecological role of bracken in plant communities throughout the world. Further geographic areas where taxonomic investigation of Pteridium is most needed are indicated, and evidence of the reproductive, dispersal, establishment, colonizing ability and vegetative persistence of bracken is reviewed. Its palaeobiological spread, with associated vegetational history, and the effects on this of anthropogenic influences—better known than are comparable details for any other pteridophyte—are detailed, and the present magnitude of the resulting bracken problem in Britain (and especially in upland Britain) indicated.     

Bracken thiaminase-mediated neurotoxic syndromes

The toxicity of bracken ( Pteridium aquilinum (L.) Kuhn) to animals is complicated because this plant elaborates more than one type of agent harmful to livestock. An enzyme, thiaminase I, which destroys thiamine, is responsible for the neurotoxic syndrome. Using a radiochemical assay, the distribution of thiaminase I activity in bracken throughout the growing season has been ascertained: levels are high in the rhizome and young buds, but fall sharply in the fronds as the aerial parts of the plant unfold.
The so-called thermostable 'antithiamine' factors present in bracken and other plant species are discussed.
The biochemical lesions of thiamine deficiency in animals are briefly outlined, and the clinical syndrome caused by the inclusion of bracken fronds or rhizomes in the diet for simple-stomached animals (rat, horse, pig) and a ruminant (sheep) are described.
All these neurotoxic syndromes respond to thiamine therapy in a dramatic way, if administered during the early stages of the disease.     

Botanical implications of bracken control

In addition to bracken, Pteridium aquilinum , and other ferns, only a restricted range of species among the natural flora suffer severe damage if sprayed with asulam in late summer. Of the plants associated with bracken in relatively well-drained situations those affected most by the herbicide include the three Ulex species, Rumex acetosella , young plants of Calluna vulgaris , some Compositae and grasses of the genera Poa, Holcus and Agrostis (though not A. setacea ). In damp and wet upland habitats, Cirsium palustre, Juncus effusus, Glyceria fluitans and a few other species are moderately susceptible to asulam but the majority of plants appear to be resistant or only slightly damaged.
Areas from which bracken has largely been removed tend to be invaded by such species as Deschampsia flexuosa, Holcus mollis, Chamaenerion augustifolium, Digitalis purpurea and Urtica dioica . If grazing pressure is low the absence of competition enables woody species to regenerate from seed. Colonization and subsequent spread of the perennial plants is influenced by what was present as ground flora before the bracken was killed and by the depth of bracken litter.
Asulam is considered to be a useful aid to the management of nature reserves where encroachment by bracken needs to be checked. Its most widespread use for bracken control is likely to be on upland farms to clear bracken from marginal land with the aim of increasing and improving pasture. Adjoining woodland and moorland habitats may suffer from resulting changes in land management. Moreover, indiscriminate aerial spraying of asulam over large areas may affect local populations of plants, notably ferns, in the more open areas not protected by a canopy of bracken.     

Bracken (Pteridium aquilinum L.) frond biomass and rhizosphere microbial community characteristics are correlated to edaphic factors

Bracken is a broadly distributed weedy fern common in disturbed habitats. Frond and rhizosphere soil samples were obtained from bracken growing in three clearcut locations in the Willamette National Forest in western Oregon. The highest frond biomass was correlated with soil having the highest total %N, lowest Fe content and oldest geological age. Based on analysis of variance of principal component scores for patterns of utilization of substrates on Biolog GN plates, metabolic profiles of rhizosphere microbial communities of bracken differed significantly between locations. Utilization of carbohydrates and phosphorylated compounds was positively correlated with organic matter (OM) and total N and negatively correlated with extractable Fe and Mn content of soil. Carboxylic acid utilization was positively correlated with pH and OM and negatively correlated with extractable Mn and P content of soils. Pseudomonas rDNA fingerprints of bracken rhizosphere samples suggested that the diversity of pseudomonads at the location with the most acidic (pH 5.5) soil (Burnside Road) differed from those at less acidic (pH 6.2 and 6.1) locations (Falls Creek and Toad Road). Mycorrhizal infection of bracken was lowest at Falls Creek, the location with the highest %N soil content. Our results suggest that bracken frond biomass and rhizosphere microbial community characteristics are correlated with local edaphic factors such as soil chemistry and geological age.     

The morphology of bracken (Pteridium aquilinum (L.) Kuhn) in the North York Moors—a comparison of the mature stand and the interface with heather (Calluna vulgaris (L.) Hull) 2. The rhizome

The rhizome system of mature bracken (Pteridium aquilinum (L.) Kuhn) contains large reserves of both biomass (mean 8.63 kg m?² fr. wt) and buds (mean 565 m^-2) which are largely responsible for both its persistence and its often rapid rates of vegetative encroachment. Within areas such as the North York Moors the spread of bracken into areas previously dominated by heather and grass is considered undesirable because of reduced land value in terms of both agriculture and ecological diversity. In this paper we describe the morphology of bracken rhizome within a mature bracken stand, and at advancing and stationary stand margins where bracken-heather interfaces occur. Stationary margins, i.e. those where bracken is not encroaching into heather at a significant rate, often have morphological characteristics intermediate to those of a mature stand and an advancing margin. In the mature stand rhizome biomass is dominated by carbohydrate-storing long shoots which comprise 63% of the total fresh weight, whilst the majority of rhizome buds (89% of all active and 86% of all dormant buds) are found on frond-bearing short shoots. At the margins of a bracken stand the proportion of rhizome which is composed of long shoots is even greater, and that of short shoots small relative to that in the mature stand. More transitional shoots are also found at the stand margins. Hence close to the margin a greater proportion of fronds is found on transitional rhizome than is the case in more mature parts of the stand. The majority of buds on all types of rhizome are in a dormant state. The proportion of buds which are active is, however, greater on long and transitional shoots than on short shoots. Hence, a larger proportion of buds are active close to the margin where the rhizome is composed less of short shoots than is the case further into a mature stand. The differences in the morphology of bracken in a mature stand and at the stand margins which are identified here support the idea of controlling bracken at stand margins in preference to the spraying of large areas of dense, mature bracken. Morphological differences include an increased proportion of active buds, greater frequency of fronds per unit rhizome biomass, reduced biomass reserves. Improved conditions for the re-invasion and re-establishment of alternative vegetation are also available at stand margins in comparison with the centre of a dense bracken stand.     

Bracken, ants and extrafloral nectaries. III. How insect herbivores avoid ant predation

Abstract. 1. Ants (Myrmica spp. and Formica lemani) visiting the extrafloral nectaries of bracken, Pteridium aquilinum, imposed heavy mortality on caterpillars of a novel moth species experimentally introduced onto bracken fronds. However, the exclusion of ants from bracken fronds had no significant effect on adapted bracken-feeding herbivores in Britain.
2. The feeding stages of British bracken-feeding insects are immune to, or can largely avoid, predation by ants in a variety of ways. Gall formers and miners cannot be attacked by these ants. Two other species hide, one inside tied leaves, the other in a mass of 'spittle'. Another group of species jumps away from, or falls off the plant when touched by ants. Sawfly caterpillars in the genera Strongylogaster, Aneugmenus and Tenthredo have viscous, distasteful haemolymph that repels ants.
3. No species of bracken herbivore has an absolute temporal refuge from ants; during their development they all overlap with ants to some degree.
4. Ant predation appears to have played a significant role in determining the contemporary structure of British bracken-feeding insect communities. Distasteful haemolymph in sawfly caterpillars may have evolved in response to selection from ant predation. Other species may fortuitously possess characteristics, evolved in response to a variety of selective forces, that also reduce the impact of ants; without such characteristics, however, we postulate that they would be unable to live on this plant. An absence of external, foliage feeding Lepidoptera early in the spring, a high proportion of sawfly species, and a high proportion of gall-formers and miners may all be characteristics of the bracken herbivore community which have been influenced by ant predation.     

Bracken-induced increase in soil P availability,along with its high P acquisition efficiency,enables it to invade P-deficient meadows

欧洲蕨入侵提高了土壤磷有效性和高磷获取效率 对于欧洲蕨(Pteridium aquilinum)入侵荒地后土壤化学的变化,已经有了相关的研究,但是对于欧洲蕨入侵草地的研究却比较缺乏。本研究探讨了欧洲蕨入侵缺磷草地是否会改变土壤养分资源库,以及与土壤过程和欧洲蕨营养相关的机制。此外,还研究了欧洲蕨入侵前后群落组成对土壤化学差异的反应。在蕨菜生物量高峰期间,我们进行了土壤和植物取样以及植被调查。数据分析包括方差分析(ANOVA)和典型对应分析(CCA)。研究结果表明,欧洲蕨入侵提高了土壤磷有效性、土壤有机碳浓度以及碳氮比、碳硫比和氮硫比,同时降低了铁和钴的浓度。欧洲蕨羽片富含磷,根茎富含钾,而羽片 和根茎的氮磷比很低。典型对应分析(CCA)显示了与磷和钾有效性相关的常见草地植物物种的不同丰度模式。在磷有效性极低的条件下,绒毛草(Holcus lanatus)表现出竞争优势。欧洲蕨通过促进铁和铝的浸出提高了磷有效性。从土壤资源生态位的角度来看,欧洲蕨通过提高自身生长所需的磷有效性并且增加对其他物种的氮限制,从而获得了竞争优势。欧洲蕨提高土壤磷有效性的能力,以及其高磷获取效率背后的生理机制,似乎将欧洲蕨与其他竞争生态策略的物种区分开来,因为其他物种的生长主要局限于营养丰富的环境,因此欧洲蕨更容易入侵缺磷的草地。     

Chemotaxonomy and phytochemical ecology of bracken

Variations in flavonoids extracted from fronds suggest that Pteridium aquilinum (L.) Kuhn is a monospecific genus. A wide range of secondary plant substances also occur in bracken including sesquiterpenoids, echydones, cyanogenic glycosides, tannins and phenolic acids.