Model parameterization to simulate and compare the PAR absorption potential of two competing plant species

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 aims of the current paper are to (1) 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, (2) to initialize the properly parameterized radiation scheme with realistic global irradiation conditions of the Rio San Francisco Valley in the Andes of southern Ecuador, and (3) 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 characterized 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, (1) the relation of incoming diffuse and direct irradiance, and (2) 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. (approximately 74 MJ m(-2) year(-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 favoring a higher PAR absorptance of bracken occur in approximately 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.

Fig. 1

The research area showing fractional cover by southern bracken as derived from Landsat TM data with the probability guided spectral unmixing technique (Göttlicher et al. 2009); BS experimental bracken site and micrometeorological station, ECSF Estación Científica San Francisco, ECSF met Meteorological station of the Estación, TS1 and Cerro met are meteorological stations located at 2,660 and 3,180 m asl, respectively. Grey shades indicate different bracken coverage per pixel, white means bracken-free pixel

Fig. 2

a Average diurnal course of global and derived diffuse radiation at ECSF met station (1998–2005). b The diffuse fraction from six decomposition functions is also shown as a function of the clearness index (bottom). For abbreviations, see text

Fig. 3

Partitioning into reflectance (albedo), transmittance and absorptance of spectral radiation incident on Setaria sphacelata, Pteridium arachnoideum (as an average of three representative samples, LAI=1) and bare soil. The traced line represents the division between visible (PAR) and near infra red (>700 nm)

Fig. 4

Sunlit and shaded leaf fractions of bracken and Setaria for solar elevations between 5 and 85° (solar elevation=90°-solar zenith)

Fig. 5

Leaf area index (m²·m⁻²) of sunlit and shaded leaves of bracken and Setaria as depending on the solar zenith angle (solar elevation=90°-solar zenith)

Fig. 6

a Leaf area indices (m²·m⁻²) of sunlit and shaded portions of the leaves of bracken and Setaria in the course of a day with regard to the average leaf–sun geometry of the study area. b Annual average of the diurnal course of irradiance at the ECSF meteorological station (1998–2005; Bendix et al. 2008a), sum of absorbed direct and diffuse PAR by sunlit and shaded leaves of bracken and Setaria plotted on the left ordinate axis and typical solar elevation in degrees on the right ordinate axis

Fig. 7

PAR absorption of southern bracken and Setaria for (left) a sunny day (5 December 2007) and (right) a typical overcast day (5 January 2008). Radiation data for model initialization are taken from the micrometeorological station at the bracken experimental site (BS in Fig. 1)

Fig. 8

Different PAR components absorbed by the sunlit canopy of bracken (left) and Setaria (right) on the sunny day (Fig. 7, left), 5 December 2007, based on the radiative transfer scheme

Fig. 9

Frequency and intensity of the daily irradiance maximum between 1200 and 1300 hours for the ECSF meteorological station (1998–2005) and total daily PAR absorption by bracken and Setaria, respectively, based on the relative diurnal course of radiation from 5 December 2007 (see Figs. 7 and 8)