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.     

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.     

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 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.     

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.     

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.     

Botanical expeditions of The New York Botanical Garden

A list is given of the botanical expeditions undertaken by staff members, graduate students, and close collaborators of The New York Botanical Garden from 1897 to 1994.     

Diurnal variations in lipids of bracken fronds

The galactolipids, sulpholipid and phosphatidyl glycerol in the fronds of bracken show diurnal variations of 50–100%, with a maximum at about 1600 hr.     

The effects of control on the biomass, carbohydrate content and bud reserves of bracken (Pteridium aquilinum (L.) Kuhn), and an evaluation of a bracken growth model

This paper examines the initial effects of bracken control on frond numbers and biomass, and the biomass, carbohydrate reserves and bud densities of bracken stands cut once per year, twice per year, subject to a single application of asulam or left untreated. The seasonal dynamics of these parameters are displayed; carbohydrate and biomass are both removed from the rhizome system to produce frond tissue, and are replenished at the end of the growing season. Asulam application reduced densities of both active and dormant buds, and both frond biomass and density. It did not significantly reduce rhizome biomass or carbohydrate reserves in the two years after treatment. Cutting, either once or twice per year reduced both rhizome biomass and rhizome carbohydrate reserves, as well as bud densities, though the latter were reduced in proportion to biomass. Cutting twice a year reduced the production of fronds, both in numbers and biomass. The collected data were used to evaluate a model of bracken growth, and subsequently to improve estimates of some of the model parameters. The model simulations of control treatments were compared to field data. The effects of cutting once per year and spraying with asulam were predicted accurately, but the bracken stand was more resilient to cutting twice per year than would be expected from model predictions. The combination of cutting and spraying is discussed as a potential tool in land management and the deficiencies of the model are discussed in relation to the need for future research into the biology of bracken.