Answers to Analyzing Data 20.1 Questions
1. Whether an ecosystem is a carbon sink (takes up more C than it releases) is determined by net ecosystem exchange (NEE). NEE is equal to NPP minus heterotrophic respiration. Prior to the beetle outbreak, NEE was equal to 440 g C∕m2∕year - 408 g C∕m2∕year = a net uptake (sink) of 32 g C∕m2∕year.
2. Following the beetle outbreak, NEE was 400 g C∕m2∕year - 424 g C∕m2∕year = -24 g C∕m2∕year, or a net source of 24 g C∕m2∕year. As tree regrowth occurs during secondary succession, the forest will again revert to a net sink of C, so the trend will reverse over the next 100 years.
3. NEE is equal to GPP minus the total (autotrophic and heterotrophic) respiration. For the pasture, NEE is equal to 2,345 g C∕m2∕year - 2,606 g C∕m2∕year = -262 g C∕m2∕year (net source), and for the second- growth forest, NEE is equal to 2,082 g C∕m2∕year - 1,640 C∕m2∕year = 442 C∕m2∕year (net sink). Thus, despite higher GPP in the pasture than in the second-growth forest, the higher respiratory losses in the pasture result in a net loss of C from the system.
4. Currently tropical rainforests account for around 3 Pg C∕yr times 0.35 (35%) = 1.05 Pg C∕yr. Converting half of the tropical rainforests to pasture would result in a decrease of NEE to 0.5 (-262 g C∕m2∕year) + 0.5 (442 g C∕m2∕year) = 90 g C∕m2∕year. This is an 80% reduction in NEE by tropical rainforests, or a 28% reduction in C uptake by the terrestrial land surface. Note that this scenario is a gross oversimplification of what would actually happen, and does not take into account biotic and functional variation among tropical rainforests and pastures.