Dayto Day Variability in Solar Radiation and Photosynthesis

The most important short-term control of photosynthesis of tundra plants at Toolik Lake is the amount of solar radiation. This is illustrated in figure 5.7 by the process-based model of net ecosystem production (NEP) developed by Williams et al. (2000). The model, the solid line in the top panel of this figure, is driven by the hourly amount of photosynthetically active radiation (PAR), by hourly air temper

Figure 5.7 The top panel is the net ecosystem production (NEP) for acidic tundra at Toolik Lake measured by eddy covariance and modeled (continuous line) by Williams et al. (2000, 2001). The bottom panel is the photosynthetically active radiation (PAR) at the same site (Chapin data base at National Snow and Ice Data Center).

Julian day of year, 1995

Figure 5.7 The top panel is the net ecosystem production (NEP) for acidic tundra at Toolik Lake measured by eddy covariance and modeled (continuous line) by Williams et al. (2000, 2001). The bottom panel is the photosynthetically active radiation (PAR) at the same site (Chapin data base at National Snow and Ice Data Center).

atures, and by the leaf nitrogen content. Only a small change in photosynthesis occurred over the temperature ranges for the 12 days in summer, and the leaf nitrogen did not change appreciably. Accordingly, the changes in photosynthesis are mostly due to the changes in PAR, shown in the bottom panel. How close is the model to the real world? We were able to compare the model runs for acidic tussock tundra with actual measurements of NEP by the eddy covariance method of measuring CO2 exchange with the atmosphere. These measurements, shown as small open squares in the top panel, were carried out on the acidic tundra at Toolik Lake by Eugster et al. (1997) and corroborate the model. Note especially the low values of NEP and PAR on days 201 and 202, followed by high values of both for day 204. Also, the variable PAR values of days 211 and 212 are closely reflected in the multiple peaks of photosynthesis for the eddy covariation data.

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