Abstract
In tropical agriculture, the vigorously growing Bracken fern causes severe problems by invading pastures and out-competing the common pasture grasses. Due to infestation by that weed, pastures are abandoned after a few years, and as a fatal consequence, the biodiversity-rich tropical forest is progressively cleared for new grazing areas. Here we present a broad physiological comparison of the two plant species that are the main competitors on the pastures in the tropical Ecuadorian Andes, the planted forage grass Setaria sphacelata and the weed Bracken (Pteridium arachnoideum). With increasing elevation, the competitive power of Bracken increases as shown by satellite data of the study region. Using data obtained from field measurements, the annual biomass production of both plant species, as a measure of their competitive strength, was modeled over an elevational gradient from 1800 to 2800 m. The model shows that with increasing elevation, biomass production of the two species shifts in favor of Bracken which, above 1800 m, is capable of outgrowing the grass. In greenhouse experiments, the effects on plant growth of the presumed key variables of the elevational gradient, temperature and UV radiation, were separately analyzed. Low temperature, as well as UV irradiation, inhibited carbon uptake of the C4-grass more than that of the C3-plant Bracken. The less temperature-sensitive photosynthesis of Bracken and its effective protection from UV radiation contribute to the success of the weed on the highland pastures. In field samples of Bracken but not of Setaria, the content of flavonoids as UV-scavengers increased with the elevation. Combining modeling with measurements in greenhouse and field allowed to explain the invasive growth of a common weed in upland pastures. The performance of Setaria decreases with elevation due to suboptimal photosynthesis at lower temperatures and the inability to adapt its cellular UV screen.
Highlights
In order to obtain grazing land in the humid tropical Andes of Ecuador, natural forest has widely been cleared by “slash and burn” up to an elevation of 3000 m
We worked with the diploid species P. arachnoideum which dominates in the uplands [1], whereas its allotetraploid relative P. caudatum is known as a lowland species [2]
This work, to the best of our knowledge, is the first attempt to portray the differential success of two plant species along an environmental gradient by a model that has been parameterized with measured ecological and physiological data
Summary
In order to obtain grazing land in the humid tropical Andes of Ecuador, natural forest has widely been cleared by “slash and burn” up to an elevation of 3000 m. Southern Bracken fern (Pteridium arachnoideum and P. caudatum), indigenous to tropical South America [1], rarely occurs in the undisturbed forest, but rapidly invades areas on which fire has been used to clear the natural vegetation It propagates by spores which are distributed by the wind. Regarding the higher temperature optima of C4-plants (Setaria) relative to C3-plants (Bracken) [9], temperature, in particular, may be an ecologically important factor that can shift the competitive equilibrium between the species Another ecophysiological trait that could affect the balance between both species in these pasture communities is the capability to accumulate protective, especially UV-absorbing compounds which shield the photosynthetic apparatus from excessive radiation [10]. Quantitative responses of phenolic and flavonoid contents have been correlated with the elevation and the intensity of UV-irradiation [13,14,15,16]
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