Factors contributing to the regeneration of bracken

• Main Author: Wynn, J. M.
• Published: University of Manchester 2002
• Subjects: 587
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520557

Bracken (Pteridium aquilinum (L. ) Kuhn) is considered a problem plant in many countries, so an understanding of its method of reproduction is important to those trying to control it, prevent its spread or to manage rangelands sustainably. It is commonly believed that in the UK, bracken mainly reproduces asexually via its rhizome system. However, spores are produced in many locations in some years and in some places apparently every year. What stimulates the production of these spores and whether they contribute to the spread of the plant is unknown. The spores as allergens are hazardous to the health of animals including humans, and may be carcinogenic when ingested in sufficient dosage. The aims of this study were therefore to identify both the conditions conducive to spore production, and the conditions that may promote the growth and survival of gametophytes and sporelings. To test what conditions were associated with spore production, three experiments using bracken sporophytes of different genotypes grown in pots in controlled environment growth chambers were carried out. The experiments differed in their designs and the range of variables tested, but all were multifactorial. The environmental variables were: two different levels of photosynthetically active radiation (PAR) and light quality (simulated broad-leaf canopy shade versus simulated full sun), two temperature regimes (each resembling one of those at the sites inhabited by the source plants), two nutrient addition rates, two water addition treatments, and rhizome disturbance. A novel quantitative method of scoring sporangial development was developed and used to assess the effects of these factors. Genotypeh ad a significant effect on sporangial development, but not on the rate of that development. Nutrient and water additions affected vegetative growth, but had no significant effects on sporangial development. The higher of the two rates of PAR and temperature increased both the rate and the maximum sporangial development attained. Rhizomes that were excavated and cut to length immediately before the experiment produced fronds with sporangia that developed at a slower rate and attained a lower degree of maturity than those rhizomes that had been excavated and cut prior to the previous growing season. There was no correlation of biomass of the sporophyte with sporangial development, or development of sporangial tissue on fronds from rhizomes in a second experiment, with sporangial development of the same rhizomes in the previous year. This shows that environmental factors influence sporangial development in bracken, with genotype having an influence on maximum spore output, but that the rate of development depends entirely on some environmental variables. Spores of bracken were cultured in a wide range of different conditions. The variables included temperature, PAR; state of media; calcium content; pH; soil water content, and relative humidity. Of these, only temperature significantly affected germination, with 20°C producing the highest germination of 5,10,15,18,20,25 and 30°C. Gametophytes were more likely to reach a state of 2-dimensional growth (transition) at 20 or 25°C than at 18 or 30°C. Lower levels of PAR decreased the likelihood of gametophytes reaching transition. Of the calcium levels tested, only complete lack of added calcium had a significant effect on gametophytes, with increased gametophyte mortality and a decrease in numbers reaching transition. Media at pH 4 and 8 reduced numbers of gametophytes reaching transition compared to pH 6.25. There was increased mortality at pH3 Water content of the substratum outwith the range 50 - 150% field capacity, significantly reduced the number of cordate gametophytes. Humidities in the range 10 - 100% relative humidity did not have any significant effect on gametophyte growth. This may have been due to the constant availability of water from the substratum. Conditions conducive to gametophyte development to transition were; a temperature above 8°C, the presence of calcium, the absence of deep shade, a pH near to 6.25 and adequate soil moisture. Data from the growth chamber and spore culture experiments involving temperature were used to create a model of bracken sporulation and spread in the British Isles. Resultant maps show that the south east is at the highest risk of bracken spread by spores, however, when climate change predictions are used in conjunction with the model, it can be seen that northerly areas may be at risk in the future. This research shows that while genotype does play a part in determining spore production of bracken, environment plays the bigger role. It is likely that with the predicted increase in temperatures in the UK, more bracken plants will begin to spore in more areas of the country, putting more animals and people at risk from health problems and potentially further degrading susceptible land. This work does not explain why bracken gametophytes are rarely encountered in field situations. Since the conditions that were found to be conducive to gametophyte establishment are often found in the UK, either one or more factors that were not included in these experiments play a key role in determining the success of sporal regeneration, or colonisation by spores occurs more frequently than is widely believed.