Abstract:
Abstract Mountain pastures dominated by the pasture grass Setaria sphacelata in the Andes
of southern Ecuador are heavily infested by southern bracken (Pteridium arachnoideum), a major
problem for pasture management. Field observations suggest that bracken might outcompete the grass
due to its competitive strength with regard to the absorption of photosynthetically active radiation
(PAR). To understand the PAR absorption potential of both species, the main aim of the current paper
is to (i) parameterize a radiation scheme of a two-big leaf model by deriving structural (LAI, leaf angle
parameter) and optical(leaf albedo, transmittance) plant traits for average individuals from field
surveys, (ii) to initialise the properly parameterized radiation scheme with realistic global irradiation
conditions of the Rio San Francisco Valley in the Andes of southern Ecuador, and (iii) to compare the
PAR absorption capabilities of both species under typical local weather conditions. Field data show
that bracken reveals a slightly higher average leaf area index (LAI) and more horizontally oriented
leaves in comparison to Setaria. Spectrometer measurements reveal that bracken and Setaria are
characterised by a similar average leaf absorptance. Simulations with the average diurnal course of
incoming solar radiation (1998-2005) and the mean leaf-sun geometry reveal that PAR absorption is
fairly equal for both species. However, the comparison of typical clear and overcast days show that two
parameters, (i) the relation of incoming diffuse and direct irradiance, and (ii) the leaf-sun geometry
play a major role for PAR absorption in the two-big leaf approach: Under cloudy sky conditions (mainly
diffuse irradiance), PAR absorption is slightly higher for Setaria while under clear sky conditions
(mainly direct irradiance), the average bracken individual is characterized by a higher PAR absorption
potential. (~74 MJ m-2 a-1) . The latter situation which occurs if the maximum daily irradiance exceeds
615 W m-2 is mainly due to the nearly orthogonal incidence of the direct solar beam onto the
horizontally oriented frond area which implies a high amount of direct PAR absorption during the
noon maximum of direct irradiance. Such situations of solar irradiance favouring a higher PAR
absorptance of bracken occur in ~36% of the observation period (1998-2005). By considering the
annual course of PAR irradiance in the San Francisco Valley, the clear advantage of bracken on clear
days (36% of all days) is completely compensated by the slight but more frequent advantage of Setaria
under overcast conditions (64% of all days). This means that neither bracken nor Setaria show a
distinct advantage in PAR absorption capability under the current climatic conditions of the study area.
