Biological and environmental variables tracking seasonality of CO2 exchange in boreal forests and links to modeling
Abstract
The CO2 exchange of boreal forests is characterized by a strong seasonal cycle, including strong winter dormancy, active summer season and transition periods between these two. In order to estimate changes imposed by the future climate on the boreal forests' carbon balance it is important to understand the dynamics and controls of this seasonal variation. In this work we studied proxies that were used in tracking the seasonal cycle of the boreal forests and how the seasonal cycle can be taken into account in modeling. We studied different biological and meteorological variables to track beginning and ending of the growing season. We used data from four coniferous sites located in Finland and Sweden. Micrometeorological CO2 flux measurements were used as a determinant for the growing season that other methods were referenced to. The variables used included different indices based on air temperature, the chlorophyll fluorescence Fv/Fm (i.e., the maximal photochemical efficiency), surface albedo, and the CO2 tropospheric concentration. One year was used as a reference to set a threshold values to these variables in the start and end of the growing season and these threshold values were used in estimation of the growing season during other years. Air temperature indices and Fv/Fm were useful in estimation of both start and end of the growing season. Decrease of surface albedo indicated the start of the snow melt and this coincided with the beginning of the growing season. The CO2 concentration was studied in two aspects, using threshold method as well as derivative estimation method. The first one was successful in prediction of the growing season. The latter enabled also capturing larger-scale spring recovery. The use of temperature indices was more feasible in the northern sites. In addition to the coniferous sites, one deciduous forest in Finnish Lapland was studied. The transition period from winter to summer creates a challenge for the biochemical models. A canopy level model based on the biochemical model developed by Farquhar et al. was used to study these transition periods of the coniferous forests. Two parameters, maximum rate of carboxylation ( Vc(max)) and maximum electron transport rate (Jmax), were examined. They were estimated by fitting temperature dependencies to half-hourly values obtained by inversion with micrometeorological CO2 flux data. It was found that better modeling results were obtained when the temperature responses of the parameters were allowed to change during the transition period. Using calendar-tied changing dates for other years did not provide as good results as when these dates were tied to temperature indices. Thus these temperature indices can be used in modeling of the seasonal development of photosynthesis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B52C..05T
- Keywords:
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- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0428 BIOGEOSCIENCES / Carbon cycling