Comparative spatial distribution, size, biomass and growth rate of two varieties of bracken fern (Pteridium aquilinum L. Kuhn) in a neotropical montane habitat

A comparative field study of caudatum and arachnoideum, the two Pteridium aquilinum varieties of the caudatum subspecies known to grow in the neotropics, was performed in a montane savanna habitat of the Venezuelan Andes that was affected by wildfire. Frond size, ramet density and spatial distribution, blade and rhizome biomass, and frond elongation and expansion rates were measured in separate, isolated stands each containing only one bracken variety and covering approximately the same area (540 m²). In addition to clearly discernible morphological differences, caudatum and arachnoideum were found to possess distinct features: caudatum tends to develop open stands of relatively shorter blades of 76.6±0.89 cm (±SE) of rachis length and lower ramet density (1.6 fronds m^-2, max.=7 fronds m^-2) whereas arachnoideum grows into longer, more expanded fronds 124±1.6 cm tall and significantly higher ramet density (5.1 fronds/m², max.=14.6 fronds m^-2). The sum of aerial and underground biomass was found to be notably larger for arachnoideum (8522±614 Kg/ha) than for caudatum (1929±131 Kg ha^-1) in stands growing under the same habitat conditions. Therefore the spatial distribution of arachnoideum appeared considerably more compact than that of caudatum. Blade growth rates and development time were also very different. Newly emerged caudatum croziers developed into mature blades within 42 to 48 days following an inverse exponential curve whereas arachnoideum blades required 70 to 75 days to reach maturity following a linear development. All the above dissimilarities are interpreted as the hitherto unreported indication of diverging growth strategies of two cohabitant bracken varieties following fire.     

Comparative dynamics of ptaquiloside and pterosin B in the two varieties (caudatum and arachnoideum) of neotropical Bracken Fern (Pteridium aquilinum L. Kuhn)

Significant chemical differences have been observed among the two neotropical varieties of bracken, caudatum and arachnoideum. Trans-o-hydroxycinnamic acid and coumarin have been found only in the caudatum variety. The concentration changes of ptaquiloside and pterosin B with blade growth were markedly different in caudatum and arachnoideum. In the newly emerged croziers where the concentration of these compounds was highest the former exhibited from 1.98 to 3.9 mg of ptaquiloside per gram of biomass (mg/gbm) while arachnoideum possessed only between 0.032 and 0.66 mg/gbm. These concentrations decreased consistently with the unfurling of the blade. Pterosin B was found in approximately one-tenth of the ptaquiloside concentration in most stages, suggesting that these compounds are chemically related in vivo. Caudatum, in retaining a much higher concentration of these toxins than arachnoideum in the full grown frond poses a higher risk of exposure to farm grazing animals that the latter, where ptaquiloside and pterosin B are almost absent.