The ecological implications of bracken control to plant/animal systems

The importance of considering bracken as part of a dynamic soil/plant/animal system is emphasized and the extent, distribution and successional relationships of the fern in the uplands are discussed. Pteridietum is classified into six types according to its pattern of local distribution. Bracken communities as animal habitat are considered in relation to distribution types, frond density and canopy cover. As a direct source of food for the vertebrates bracken is considered to be of little or no value. As an indirect source of food bracken communities may be important in providing habitats for invertebrates, though in terms of numbers of species the insect fauna of vigorous Pteridietum is not great compared with that of other communities that may replace it. Pteridietum provides shelter and cover for a number of small mammals but in these respects it is of very limited value to the larger grazing animals. It is estimated that in Scotland, under the economic conditions of recent years, bracken herbicide is unlikely to be used on more than about 40,470 ha (100,000 acres) in the combined land uses of agriculture, forestry and sport; i.e. less than 25% of the Scottish bracken coverage is at risk. At this level of control, provided some precautions are taken, it is considered that the wild vertebrates are unlikely to be seriously affected; indeed in certain situations some benefits may be derived.     

An ecological and historical review of bracken (

New Zealand bracken (Pteridium esculentum) belongs to a group of closely related fern species of near global extent. Pteridium species worldwide are aggressive, highly productive, seral plants, functionally more akin to shrubs than ferns. Their deeply buried starch-rich rhizomes allow them to survive repeated fire and their efficient nutrient uptake permits exploitation of a wide range of soils. They are limited by cool annual temperatures, frost, wind, and shallow, poorly drained and acidic soils. Bracken stands accumulate large amounts of inflammable dead fronds and deep litter and often persist by facilitating fire that removes woody competitors. Bracken was present but not abundant in New Zealand before the arrival of humans. Occasional fire or other disturbances created transient opportunities for it. Rhyolitic volcanic eruptions led to short-lived expansions of bracken, and it briefly became dominant over ash-affected areas of the central North Island after the large AD 186 Taupo Tephra eruption. Andesitic eruptions had limited effects. Bracken became one of the most abundant plants in the country after the arrival of Maori in the 13th century, when massive deforestation affected most of the lowland landscape. The bracken-dominated vegetation that formed in most places immediately after burning gave way with time to fire-maintained mosaics in which bracken dominated on deeper soils and under moist, mild climates. Although Maori relied on bracken rhizome starch as a major element of their diet, food-quality rhizomes were obtained only on deep, moderately fertile soils. The dominance of bracken over very large areas was mainly a result of burning to create open landscapes for access and ease of travel. Bracken remained a troublesome weed through the European pastoral period and well into the 20th century. Bracken has a problematical role in conservation as it can form a persistent, fire-prone, low-diversity cover in drier regions. However, it is an indigenous plant that is effective in preventing erosion and, in wetter areas, it will easily suppress exotic grasses and facilitate regeneration to forest. It should be considered an essential component of landscapes conserved for their historical significance to Maori.     

The effects of fungi on bracken

Laboratory experiments on fungal pathogens of the gametophyte generation are described and the possible causative agents associated with disease of bracken colonies in the field discussed.     

The history and ethnobotany of bracken

The fossil record shows that Pteridium has been present in all the temperate stages of the Quaternary, but that for most of this period it formed a component of the herb layer of deciduous woodlands. Bracken did not begin to reach its present abundance and importance until widespread forest clearance began with the arrival of Neolithic man about 5000 years ago. Some of the factors that have played a role in the history of bracken are illustrated by a discussion of the spread of bracken in Scotland that occurred contemporaneously with the change from cattle- to sheep-farming in the 18th century. It is shown that man has played a dominant role in influencing the spread of bracken in Scotland, but that bracken was considered of great positive economic value. The ethnobotany of bracken is discussed, and it is suggested that it was once an important source of potash for the glass, soap and bleaching industries. The use of the plant as fuel, thatch, litter, compost and food, and for medicinal purposes is considered, and mention is made of various minor uses of the plant. Unfortunately it is difficult to estimate the amount of bracken consumed by these various employments, but it is suggested that the increased abundance of the plant may have been exaggerated because what was once a useful resource has now become a pest.     

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.     

Control of bracken

Mechanical methods of bracken control are now being replaced by the use of chemicals. Trials indicate the value of asulam and glyphosate in reducing frond numbers of bracken when sprayed in late July or early August in the west of Scotland. There is still a need to find a bracken eradicant chemical rather than a control chemical. Research is also required to investigate the results of removing the bracken cover on the ecology of the treated areas. The effects of sudden exposure of the hitherto protected grass to extremes of climate are not known.     

The structure of the arthropod community on bracken

The species of herbivorous arthropods known to feed on bracken in Britain are listed and the seasonal changes in the structure of this arthropod community are discussed.     

Decomposition of bracken litter

Investigations on the decomposition of bracken petioles, over a five-year period on six adjacent soil types, including moder-type humus, mull and peat, are reviewed. Changes in gross physical features, chemical composition, pH and dry weight are outlined. The succession of colonizing fungi is described and related to fungal activities.
Until the petioles were buried in the litter layer, decomposition occurred at different rates on the various sites, the rate on moder > mull > peat, but the sequence of events was similar. Large proportions of readily leached components were removed in the first few months, but 95% loss of dry matter was estimated to occur only after 11–23 years. The majority of fungi were species cosmopolitan on litter, the population becoming less specialized as decay advanced. The succession resembled those on some other woody tissues, lignin and cellulose decomposers predominating before sugar fungi. From field observations and laboratory experiments, the Basidiomycete Mycena galopus (Pers. ex Fr.) Kummer appeared to be the most active of the fungal decomposers.
Some ecological and economic implications of the decomposition of bracken litter are briefly discussed, including its effect on soil type, and advantages of bracken compared with straw as bedding for farm animals.     

The hemicelluloses of bracken Part II. A galactoglucomannan

A galactoglucomannan has been isolated from the stem tissues of a fern, bracken (Pteridium aquilinum). It had [] −35° and, on hydrolysis, yielded mannose, glucose, and galactose in the molar ratios of 60:15:1. Mild hydrolysis with acid released galactose only. The d.p., determined by periodate-oxidation and Smith-degradation studies, was 43–45 for a doubly branched molecule. The methylated galactoglucomannan had a d.p. of 25–30 by ebulliometry. Methylation analysis, in combination with other evidence, indicated that the hemicellulose had β-(1→4)-linked -glucopyranose and -mannopyranose residues in the ratio of 1 to 4, with residues of the latter contiguous. -Mannopyranosidic and -galactopyranosidic residues were present as non-reducing end-groups in the ratio of 7 to 1, but there was no evidence of non-reducing -glucopyranosidic residues. The branching was to 6-positions on the main glucomannan chain and was probably -(1→6). The galactoglucomannan is similar to hemicelluloses isolated from softwoods, but the former has a lower d.p.     

The taxa of Scottish bracken in a European perspective

Summary

In contrast to the traditional view that Scottish Bracken (Pteridium Scop., Hypolepidaceae sensu Page, 1976) is a single, uniform taxon, evidence is presented that two species, each represented by multiple subspecies, are present in Scotland. These belong to two different species complexes within Pteridium, the P. aquilinum complex and the P. latiusculum complex. The P. aquilinum complex is represented by P. aquilinum (L.) Kuhn, with at least three subspecies: subsp. aquilinum, subsp. atlanticum C.N. Page and subsp. fulvum C.N. Page. The P. latiusculum complex is represented by P. pinetorum C.N. Page & R.R. Mill with at least two subspecies, subsp. pinetorum and subsp. osmundaceum (Christ) C.N. Page. Only P. aquilinum subsp. aquilinum is an aggressive weed which is a widespread problem to man; the others are rare. The paper establishes the nomenclatural priorities, typification, taxonomy, morphology, known distribution and ecology of all bracken taxa so far known from Scotland. The typification of P. aquilinum is discussed. There is no conflict between the two lectotypifications of Try on (1941) and Sheffield et al. (1989); both relate to the common bracken of the mid- and southern-latitudes of Europe, for which the existing epithet aquilinum must therefore be retained. It is pointed out that some of the other European taxa of bracken might well also yet be found within the British Isles, as well as, perhaps, other (especially possible diploid) taxa as yet undescribed.     

Evolution and current status of ecological phytochemistry

Phytochemical studies have experienced a great deal of change during the last century, not only regarding the number of compounds described, but also in the concept of phytochemistry itself. This change has mainly been related to two key points: the methodologies used in phytochemical studies and the questions regarding 'why secondary metabolites appeared in plants and in other living organisms?' and 'what is their role?'. This transformation in the field has led to new questions concerning such different subjects as evolution, paleobotany, biochemistry, plant physiology and ethnography. However, the main issue is to clarify the role that secondary metabolites play in the plant (and other organisms) and whether the resources invested in their production (C and N allocation, genes encoding their biogenetic pathways, specific enzymes, energy-rich molecules such as ATP and NADPH) have or lack a reasonable reward in terms of advantages for survival. Consequently, in this review the main emphasis will be placed on two subjects related to the evolution of phytochemical studies. The first aim is to describe briefly the influence that the development of the methodologies needed for compound isolation and structure elucidation have had on the field of phytochemistry. The second area to be covered concerns the new theories addressing the role of secondary metabolites from an ecological point of view: co-evolution of plants and their potential enemies (phytophagous insects, microbes, herbivores and other plants), chemical plant defence, adaptative strategies of phytophagues to plant toxins (among them sequestration will be briefly mentioned), and models and theories for carbon and nitrogen allocation. Some final remarks are made to summarize our opinion about the immediate future of phytochemical ecology and phytochemical studies.     

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.     

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.     

Relationship between bracken and cancer

The production of tumours in animals by the carcinogenic action of bracken fern ( Pteridium aquilinum (L.) Kuhn) is described. One of the substances responsible has been identified as shikimic acid, and evidence is presented that it is a powerful mutagen. The isolation of another toxin which gives rise to malignancies is nearing completion. The possibility of an environmental human hazard is considered.