Understanding the carbon (C) dynamics of an ecosystem is important for modeling C sequestration and is a critical component of efforts to determine global net primary production (NPP) incorporating both above- and below-ground elements. Within the past twenty years, the eddy covariance technique has become an accepted method for estimating carbon dioxide exchange occurring within forests. This method of investigation, however, requires a precise microclimate sensing system and must take into account major assumptions (e.g., that the forest being measured is flat and uniform and that the measurements are representing an upwind area). Our study used gas exchange measurements made on different vegetation types to estimate the above-ground carbon assimilation in a southern California forest. We also made ground-level  measurements of carbon dioxide flux from the soil to compare with photosynthesis. We calculated the leaf area index for the forest and determined that this was mostly accounted for by manzanita (26%), cedar (18%), pine (41%) and oak (15%) trees. Light response and A-Ci curves were parameterized to model photosynthesis on a daily basis together with the environmental inputs of photosynthetically active radiation (PAR), soil moisture, and air temperature.

Results/Conclusions

Based on the different vegetation groups, carbon assimilation was found to be 3.3 and 3.1 mmol/m²-s for the summer and winter respectively. The soil flux rates were comparable at 1-3 mmol/m²-s. Continuously measured PAR (2006-present) was used to model photosynthesis on a monthly basis. When this was compared with soil flux measurements, net assimilation was positive indicating this southern California forest is currently acting as a carbon sink. Shifts in temperature and precipitation regimes will likely change the direction of this process. This method of estimating carbon assimilation represents a lower cost alternative to the standard eddy covariance technique.