Modelling the Papyrus Ecosystem Carbon Flux

Macroclimate (weather)

The WIMOVAC macroclimate module uses established methods to calculate direct and diffuse solar radiation components from atmospheric conditions, latitude and time of year (Monteith and Unsworth, 1991; Norman and Campbell, 1989; Norman, 1980).

Diurnal temperature variations were simulated according to the equations of Spain (1992) which express temperature as a sinusoidal function of time. This approach conforms to that proposed by Webb et al., (1975) in which extremes of temperature are more important than means and allows a definable offset between the time of maximum incident solar radiation and maximum temperature.

Although rainfall is not considered an important direct determinant of production in papyrus, due to its wetland location, the impact of cloudiness on atmospheric transmissivity and incident solar radiation is likely to have a large affect on productivity, particularly during the rainy season. The stochastic rainfall model described by Thornley (1990) was parameterised with the 10 year mean meteorological data of Muthuri (1985) and used here to determine the rain/no rain status of each simulated day. Days which received no rain were assumed to have an atmospheric transmissivity of 0.85 whilst days with rainfall had an assumed value of 0.55. These values gave daily integrated solar radiation values very closely comparable (Fig 7(iii), R2=0.99) to the ten year means reported by Muthuri (1985).

Bracteole photosynthesis

The principles developed by Farquhar & von Caemmerer (1980) for C3 plants have been incorporated into a model of C4 photosynthesis by Collatz et al. (1992) and are included in WIMOVAC. In their model, Collatz et al. (1992) derive a simple biochemical intercellular transport model that includes inorganic carbon fixation by PEP carboxylase, light dependent generation of PEP and RuBP, rubisco reaction kinetics, and the diffusion of inorganic carbon and oxygen between the bundle sheath and mesophyll. The C4 model couples a modified version of the Ball et al., (1987) stomatal conductance calculations directly with the photosynthesis model to give an analytical solution that does not require iteration.

The key model properties of initial slope of photosynthetic CO2 response, maximum rubisco capacity, slope of photosynthetic light response, and dark respiration rate were estimated for papyrus from the bracteole light and CO2 response curves measured as described above. Other parameters for the biochemical and stomatal models were as given by Collatz et al. (1992). Culm and umbel structures were assumed to have identical photosynthetic and stomatal properties.

Although the papyrus canopy roots and rhizome extend either directly into water or into water logged peaty detritus, experimental measurements at the study site after 2 p.m. routinely showed a marked decline in photosynthetic activity due to stomatal closure. This was supported by bracteole water potential measurements made by Jones and Muthuri (1984) in which mid day water potentials fell to -1500 kPa. Transient water stress effects were incorporated into the model using the approach of Campbell (1991).

Canopy light interception

In order to scale from organ gas exchange to the canopy level it is necessary to predict radiation distribution in the canopy. Papyrus canopy radiation microclimate was simulated using the multiple layer model proposed by Reynolds et al. (1992) in which there is one sunlit and one shaded leaf class per layer. The model canopy was assumed to consist of an upper level of umbels and a lower level of culms. Each level was represented by 5 layers of sunlit and shaded leaves with culms assuming a largely vertical orientation and umbels a spherical one.

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