Abstract:
Growing land scarcity due to population increase promotes the conversion of natural ecosystems into areas for food and energy production, especially in the tropics. On the other hand, high proportions of degraded land are present. In the research area in the south-eastern Andes of Ecuador about 40% of the potential pastureland has been abandoned because of infestation by bracken (Pteridium spp.). This rhizomatous plant is one of the most aggressive weeds worldwide especially where fire is used for forest clearing or in agriculture. Taxonomically, it represents a complex that is grouped into a northern, temperate, and a southern, tropical clade. In the research area, two species of the southern bracken complex, P. arachnoideum and P. caudatum, co-occur.
We analysed the bracken species composition on active and abandoned pastures at 7 altitudinal levels between 1,000 and 3,000 m. The 2 species could be differentiated using morphological and molecular characters. Depending on the altitudinal and climate conditions, the shares of the two species in the bracken cover changed: P. caudatum preferred the lower altitudes, where P. arachnoideum was hardly found. The opposite was observed in the upper regions. At 1800 m an extensive study of the composition of the bracken population was performed using microsatellite markers. As bracken propagates readily via rhizome branching and disaggregation, a high degree of homogeneity of the ramets was expected. However, the percentage of identical ramets of both species was very low, as were distances at which genetically identical samples were found. Maximum extension of a genet (consisting of several ramets) was less than 150 m which contrasts with findings of northern bracken species. The high diversity was attributed to a high frequency of bushfires whose heat could stimulate sexual reproduction. After such fire fast colonization of the bare soil by bracken was observed. The entire developmental cycle from spore formation until ample growth of young sporophytes required less than 2 months. In addition, regrowth of bracken fronds from rhizomes was also stimulated by the heat pulse from such fires.
