Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Comparison of the bottom-up and top-down approach results

Forest inventory data and the biomass functions gathered and developed during the CarboInvent project were integrated in two ways: in a top-down approach (WP 6) using aggregated inventory data, and in a bottom-up approach using inventory data at the plot level (WP 7). We compared biomass allocation and tree carbon stocks of the bottom-up and top-down approaches (deliverable 6.4) for the test regions located in Austria (Salzburg), Finland (Hyytiala) and Germany (Thuringia). In the bottom-up approach, biomass functions were applied directly to the dimensions of the inventoried trees, mostly diameter and height, and then age- and tree species-specific carbon estimates were calculated. The singletree results were aggregated by tree species and age class.

In the top-down approach, no singletree dimension was available, only aggregated information from the inventory. Growth and yield tables were used in order to assign diameter and height to the age class information of the forest inventory data. Only for the Finnish test region, biomass allocation could be derived directly from the applied biomass functions. The same biomass functions were used in both approaches for the test regions in Austria and Germany. For the Finnish test region, compartment-specific biomass function were used in the top-down approach, while the bottom-up approach used functions from the same source that expanded to total tree biomass directly, or only aboveground biomass in the case of broadleaves. For each of the three test regions, we compared carbon stocks stratified by tree species and age class for the top-down and bottom up approach. Total tree carbon stocks differed only slightly (1-2%) in the Austrian and German test regions, where the same biomass functions were used in both the bottom-up and the top-down approach. In the Finnish test region, the deviation was 6%. The deviation between the two approaches was highest for the youngest age class in the Austrian and German test regions. In the Finnish test region, the deviation between the two approaches increased with age.

A more detailed comparison of the top-down and the bottom-up approaches was made for one of the test regions, Thuringia (Germany). For both approaches, the share of each biomass compartment was calculated as percentage of the total tree biomass for stem, branches, foliage, coarse roots, and fine roots separately for the main tree species. The differences between the approaches were generally larger at the species level. For beech, the bottom-up and top-down approach gave very similar biomass allocations. Due to slightly higher stem shares in the top-down approach for older age classes, tree carbon stocks were 5-8% lower than in the bottom-up approach for stands older than 80 years. The difference in stem shares for older stands was even more pronounced for oak, where tree carbon stocks were estimated 19% smaller (35 Mg C ha-1) in the top-down approach. The deviation in biomass allocation for spruce was highest in the first two age classes and decreased for stands older than 30 years. The top-down approach calculated 17% larger tree carbon stocks for the youngest age class, whereas for the oldest age class the result was 8% smaller than with the bottom-up approach. The differences between biomass allocations followed the same pattern also for pine, but the deviation in tree carbon stocks for the youngest age class was with +43% even more pronounced.

Deviations in biomass allocation between the bottom-up and top-down approach were caused by differences in tree dimensions between growth and yield tables and the measured tree diameter and height from the most recent national forest inventory in Germany. The effect of using different growth and yield tables in the top-down approach was assessed for the German test region for the case of spruce, the most abundant tree species in Germany. The growth and yield tables varied in the intensity of thinning and in yield class. The deviation in biomass allocation was highest in the first age class of most species, where the stem share of total biomass ranged between 36% and 56% depending on the used yield table. The deviation decreased with increasing age, and the allocation for older stands matched very well. Since the carbon stock is still very low in young stands, the difference in biomass allocation and carbon stocks in young stands between the two approaches had only a small effect when comparing the carbon stock for the total test regions.

It can be concluded that the two approaches gave very similar results at the regional level, whereas species-specific tree carbon stocks for different age classes showed larger variation, particularly for the youngest one or two age classes.

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