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VALIDITY OF EXTRAPOLATING FIELD CO 2 EXPERIMENTS TO PREDICT CARBON SEQUESTRATION IN NATURAL ECOSYSTEMS
Author(s) -
Luo Yiqi,
Reynolds James F.
Publication year - 1999
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(1999)080[1568:voefce]2.0.co;2
Subject(s) - ecosystem , carbon sequestration , environmental science , terrestrial ecosystem , carbon dioxide , primary production , atmospheric sciences , productivity , acclimatization , ecology , carbon dioxide in earth's atmosphere , ecosystem model , biology , geology , macroeconomics , economics
One of the ultimate goals of ecosystem carbon dioxide (CO 2 ) experiments is to infer the capacity of terrestrial ecosystems to sequester carbon (C) in a CO 2 ‐enriched world. This modeling study examines C sequestration (C seq ) in natural ecosystems based on CO 2 experiments. Most experiments are conducted by a step increase in CO 2 concentration, whereas natural ecosystems are experiencing a gradual increase in atmospheric CO 2 (C a ). To examine the effects of a step vs. gradual CO 2 increase on ecosystem responses, we have developed a terrestrial C sequestration (TCS) model that focuses on C and nitrogen (N) interactions in regulating C seq . We used the model to: (1) compare C seq and N demand in response to the step vs. gradual increase in CO 2 ; (2) identify mechanisms underlying different ecosystem responses to the step vs. gradual CO 2 forcing; (3) examine key parameters in controlling C seq ; and (4) explore three hypothesized N supply mechanisms in regulating photosynthetic acclimation and C seq . Application of this model to simulate responses of a forest ecosystem with gross primary productivity of 1200 g C·m −2 ·yr −1 suggested that a step increase in CO 2 from 350 to 700 ppm resulted in C seq of 263 g C·m −2 ·yr −1 in the first year. A gradual C a increase led to the C seq rates of 27 and 58 g C·m −2 ·yr −1 in 1987 and 2085 when CO 2 reached 350 and 700 ppm, respectively. The model predicted that N demand required to balance the additional C influx was 4.1 g N·m −2 ·yr −1 in the step CO 2 increase and only 0.6 and 1.7 g N·m −2 ·yr −1 in 1987 and 2085, respectively, in the gradual C a increase. The contrasting differences in C seq and N demand between the two increase scenarios reflected the nature of C fluxes that were controlled by the sizes of donor pools (i.e., donor‐controlled system). Our modeling analysis of four ecosystems (forest with high productivity [HP]; grassland with HP; forest with low productivity [LP]; and grassland with LP) indicated that additional C influx and C relaxation time are the key parameters in determining ecosystem C seq . The additional C influx varied with ecosystem productivity and N regulation, while C relaxation time differed between the forests and grasslands due to woody tissues and litter in the forests. We conclude that in spite of the fact that the step experiment is one of the most effective approaches in ecosystem studies, its results cannot be directly extrapolated to predict terrestrial C seq in natural ecosystems responding to a gradual C a increase. In order to develop predictive understanding from the step experiments, we need not only to improve experimental design and measurement plans, but also to develop new approaches, such as deconvolution and inverse modeling, for data analysis and interpretation.