Bracken Fern Inhibition of 

 Conifer Regeneration 

 in Northern Idaho 



Dennis E. Ferguson 

 Raymond J. Boyd 



INTRODUCTION 



Bracken fern {Pteridium aquilinum [L.] Kuhn) is the 

 most widely distributed vascular plant in the world (Page 

 1982), and Taylor (1985) calls bracken the most successful 

 international weed of the 20th century. It is a highly com- 

 petitive species in grazing areas and disturbed woodlands. 

 Invasion can be through spore germination or expansion of 

 rhizomes. Bracken's success has been attributed to its 

 ability to compete for moisture, nutrients, and light, 

 smothering of other plants by senescing fronds, unpalat- 

 ability of bracken to animals and insects, lack of disease 

 problems, a rhizome system that readily sprouts following 

 disturbance, and chemical interference with other plants 

 (allelopathy). 



In northern Idaho, bracken fern can be a vigorous 

 competitor in forests and pastures. Bracken also occupies 

 1/5- to 6-ha glades where woody vegetation is virtually ex- 

 cluded and only a few forb and grass species grow. The 

 absence of woody species and charcoal fragments in glade 

 soils suggests bracken has been dominant for perhaps 

 hundreds of years. 



Phytotoxins released by bracken fern can inhibit the 

 establishment and growth of associated species. The 

 allelopathic effect of bracken has been reported in other 

 parts of the world and could be a factor operating in 

 bracken-dominated areas of northern Idaho. We conducted 

 investigations to test the allelopathic potential of bracken 

 fern in northern Idaho and gain an understanding of the 

 mechanism(s) causing regeneration failures. 



LITERATURE REVIEW 



Bracken fern is a widely distributed weed species, caus- 

 ing problems in places such as Africa, India, Australia, 

 New Zealand, North America, South America, and the 

 British Isles (Cody and Crompton 1975; Gliessman 1976). 

 Published information on bracken is quite extensive, and 

 excellent review articles are available (Braid 1959; Cody 

 and Crompton 1975). Increases in abundance of bracken 

 are correlated with clearing of farm and forest by man 

 (Page 1982; Rymer 1976). Bracken has a cumulative 

 toxic effect on livestock (Cody and Crompton 1975; 

 Cooper-Driver 1976) and is linked to cancer in humans 

 (Evans 1976; Hirono 1981; Hirono and others 1972). In 

 addition, studies have shown bracken produces phytotoxins 

 that interfere with establishment and growth of other 

 plants. 



Bracken fern is usually a minor component of native 

 forests but increases with disturbance by fire, harvesting. 



or grazing. Windblown spores disperse bracken over long 

 distances, although the main reproductive method is ex- 

 pansion of underground rhizomes. Spores evidently need 

 nearly sterile soil conditions before germination will occur 

 (Cody and Crompton 1975; Page 1982). Cody and 

 Crompton (1975) note that there are no published reports 

 of spores germinating under natural conditions in Canada. 

 Sporelings have been observed in Britain and elsewhere, 

 but not in abundance. 



Bracken fern's extensive rhizome system spreads 

 rapidly. Conway (1952) grew a young plant in a trough to 

 document vegetative reproduction. Forty-five fronds were 

 produced the second growing season, 136 the third, and 

 297 the fourth season. Znerold (1979) calculated frond den- 

 sity of 286,300 stems/ha at a bracken glade in northern 

 Idaho. 



About one-fifth of the underground buds produce fronds 

 in the spring. Dormant buds provide a reservoir of poten- 

 tial fronds to replace those killed by frost, fire, or other 

 agents. The deep rhizome system is impervious to most 

 damaging agents and readily resprouts. 



Disturbances favor bracken fern over associated species. 

 Bracken is difficult to control and nearly impossible to 

 eradicate. The best long-term control is by repeated cut- 

 tings, cultivation, or herbicides (Martin 1976; Preest 1975). 

 Herbicides are probably the most effective method of con- 

 trol because the rhizomes are killed. 



Livestock have been used to control bracken, but they 

 do not prefer bracken as a food source. Bracken is toxic to 

 livestock if eaten for several weeks or months. Poisoning, 

 which has been frequently documented, affects cattle, 

 horses, and sheep differently (Hirono 1981). Pamukcu and 

 others (1978) have shown that the milk of cows fed 

 bracken fern has toxic, carcinogenic, and mutagenic 

 properties. 



Bracken fern also gains a competitive advantage over 

 other plants through chemical interference. Allelopathic 

 compounds in many plants, including bracken, are thought 

 to be metabolic byproducts that have accumulated in the 

 plant and are often stored in localized plant parts such as 

 vacuoles and cell walls (DeBell 1970; Fisher 1979; 

 Whittaker and Feeny 1971). Leaching and decomposition 

 release phytotoxins into the environment. Microbial action 

 breaks chemicals down into secondary plant chemicals, 

 which are also potential phytotoxins. 



Rice (1979, 1984) reviews the subject of allelopathy and 

 discusses bracken and other ferns. The remainder of this 

 review discusses the allelopathic potential of bracken. 



Chessman and Muller (1972) demonstrated that water 

 leachate from bracken fronds reduces radicle growth of 



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