brome grass {Bromus rigidus). Dead fronds collected 

 before fall rains were the most toxic. Average 48-hour 

 radicle length of seed soaked 2 hours in bracken leachate 

 was 82.2 percent of the control groups. Quadrupling the 

 leachate concentration decreased radicle elongation to 



31.4 percent of the control. Leachate from green fronds 

 was not toxic; yellowing fronds were toxic only at concen- 

 trated levels. Leachate did not reduce germination of the 

 brome grass, but viald oat {Avena fatua) germination was 

 reduced to 76 percent of the control. 



Gliessman and Muller (1978) also reported on experi- 

 ments excluding other factors thought to be responsible 

 for bracken's dominance. Eliminated were animals, soil 

 differences, and competition for light, moisture, and 

 nutrients. These factors account for some differences 

 between bracken-dominated land and adjacent grasslands 

 but were not the major reasons for the differences. They 

 conclude that allelopathy, in the form of water-soluble 

 phytotoxins from dead fronds, is the primary mechanism 

 accounting for bracken dominance. 



A few studies have reported on allelopathic effects of 

 bracken on conifers. Stewart (1975) found that extracts 

 from senescent bracken fronds did not reduce germination 

 of Douglas-fir {Pseudotsuga menziesii) but did reduce ger- 

 mination of western thimbleberry {Rubus parviflorvs) and 

 delayed germination of salmonberry (Rubtis spectabilis). In 

 Stewart's study, seeds moistened with either distilled 

 water or bracken leachate were stratified in vermiculite 

 and germinated on sterile pumice. 



In a corresponding greenhouse experiment, Stewart ger- 

 minated seeds on untreated forest soil, on forest soil with 

 seeds covered by bracken frond material, and on forest 

 soil in which bracken frond material was mixed with the 

 soil. When seeds were covered by bracken, germination of 

 Douglas-fir was 58 percent versus 65 percent for untreated 

 soil. Germination of salmonberry and thimbleberry was 

 also reduced in pots having seeds covered by bracken 

 (2 percent germination for salmonberry versus 14 percent 

 in the control and 29 percent germination for thimbleberry 

 versus 65 percent in the control). Shoot length and dry 

 weight of thimbleberry seedlings were reduced in pots 

 where bracken was mixed with the soil. 



Del Moral and Gates (1971) showed that water extracts 

 from bracken fern reduced growth of Douglas-fir, but the 

 effect was barely significant. Results are not conclusive in 

 this instance because green frond material was used, 

 which Gliessman and Muller (1972) later found to be 

 nontoxic. 



Seedlings have been successfully planted in cutover 

 forests dominated by bracken fern. Dimock (1964) 

 reported 79 percent survival of Douglas-fir seedlings 

 planted on a productive site in western Washington. But 

 growth was extremely slow, with trees averaging only 



29.5 cm tall after four growing seasons. A planting study 

 by Boyd and Znerold (in preparation) was confounded by 

 high losses of planted trees to pocket gophers {Thomomys 

 sp.). Survival after 7 years was only 15 percent, leaving 

 463 trees available for height measurement. Heights of 

 trees planted in plots where bracken had been eliminated 

 were nearly twice the heights of trees growing in bracken- 

 dominated plots. 



Allelopathic effects can also be indirect. Chemicals 

 released by plants could stimulate harmful microorganisms 

 or suppress helpful ones (Rice 1979). The relationship 

 between bracken fern and beneficial mycorrhizal fungi has 

 been explored. Acsai and Largent (1983) found no effect 

 of bracken phytotoxins on ectomycorrhizae formation with 

 white fir {Abies concolor) or Douglas-fLr in northern 

 California. Rose and others (1983) found mixed effects 

 during in vitro studies on gro\vth of four mycorrhizal 

 fungi. One fungus was stimulated, two were inhibited, and 

 the other was not affected. 



Buildup of phytotoxins in the soil depends on the 

 amount produced, the rate at which the compounds are 

 deactivated, and soil characteristics (Rice 1979). Phj1;o- 

 toxic effects seem most pronounced in soils that are hea\'y 

 in texture, poorly aerated, moist a good portion of the 

 year, and often cool in temperature (DeBell 1970; Fisher 

 1978; Horsley 1977; Rietveld and others 1983). For exam- 

 ple, Rietveld and others felt that juglone from black 

 walnut {Juglans nigra) was not harmful until the walnut 

 trees were large enough to produce substantial amounts 

 which, in combination with wet soOs that reduced 

 microbial activity, allowed juglone to accumulate in the 

 soil. 



Phenolic acids have been identified as the most probable 

 cause of bracken's allelopathic effect. PhenoHc acids are 

 water soluble and thus able to leach from fronds into the 

 soil. Cinnamic and benzoic acids are the two most fre- 

 quently mentioned phenoHc acids associated with bracken. 

 Derivatives of cinnamic acid include p-coumaric acid, 

 caffeic acid, and ferulic acid while derivatives of benzoic 

 acid include p-hydroxybenzoic acid, vanillic acid, and proto- 

 catechuic acid (Bohm and Tryon 1967; Glass and Bohm 

 1969; Wliitehead 1964). 



Many studies show a statistically significant reduction in 

 germination or growth attributable to bracken fern 

 allelopathy, but the magnitude of these results does not 

 demonstrate how woody species could be \'irtually 

 eliminated from bracken glades in northern Idaho. When 

 one considers seedfall from species adjacent to bracken 

 glades, even a low percentage germination should result in 

 higher coverage of these species over time. 



Some light has been shed on the reason for differences 

 between field observations and results of greenhouse and 

 laboratory tests. Moving, drjdng, or mixing the soil may 

 result in detoxification of plant residues. Microbial activity 

 might be altered by changes in the temperature or mois- 

 ture regime of the soil; and if the soil is mixed, allelo- 

 pathic chemicals might be diluted. BrowTi (1967) suggests 

 that soil compounds quickly break down once the soil is 

 removed from the field site to the greenhouse. Fisher 

 (1978) found that soil containing juglone was detoxified if 

 allowed to dry before bioassay tests were started, whereas 

 the inhibitory effects remained strong when the soil was 

 not dried. 



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