Process-based estimates of terrestrial ecosystem isoprene emissions: incorporating the effects of a direct CO2-isoprene interaction

[摘要] In recent years evidence has emerged that the amount of isoprene emittedfrom a leaf is affected by the CO₂ growth environment. Many – though notall – laboratory experiments indicate that emissions increase significantlyat below-ambient CO₂ concentrations and decrease when concentrationsare raised to above-ambient. A small number of process-based leaf isopreneemission models can reproduce this CO₂ stimulation and inhibition.These models are briefly reviewed, and their performance in standardconditions compared with each other and to an empirical algorithm. One ofthe models was judged particularly useful for incorporation into a dynamicvegetation model framework, LPJ-GUESS, yielding a tool that allows theinteractive effects of climate and increasing CO₂ concentration onvegetation distribution, productivity, and leaf and ecosystem isopreneemissions to be explored. The coupled vegetation dynamics-isoprene model isdescribed and used here in a mode particularly suited for the ecosystemscale, but it can be employed at the global level as well.

Annual and/or daily isoprene emissions simulated by the model were evaluatedagainst flux measurements (or model estimates that had previously beenevaluated with flux data) from a wide range of environments, and agreementbetween modelled and simulated values was generally good. By using a dynamicvegetation model, effects of canopy composition, disturbance history, ortrends in CO₂ concentration can be assessed. We show here for fivemodel test sites that the suggested CO₂-inhibition of leaf-isoprenemetabolism can be large enough to offset increases in emissions due toCO₂-stimulation of vegetation productivity and leaf area growth. Wheneffects of climate change are considered atop the effects of atmosphericcomposition the interactions between the relevant processes will become evenmore complex. The CO₂-isoprene inhibition may have the potential tosignificantly dampen the expected steep increase of ecosystem isopreneemission in a future, warmer atmosphere with higher CO₂ levels; thiseffect raises important questions for projections of future atmosphericchemistry, and its connection to the terrestrial vegetation and carboncycle.

[学科分类] 大气科学