What are the challenges in modelling isoprene and monoterpene emission dynamics for subarctic plants?
Jing Tang, Postdoc, BIO/CENPERM.
Arctic is warming at twice the global average speed, and the warming-induced increases in biogenic volatile organic compounds (BVOC) emissions from arctic plants are expected to be drastic. The current global models’ estimations of minimal BVOC emissions from the Arctic are based on very few observations and have been challenged by increasing field data. Recent observations from the Arctic have indicated higher emission potentials than anticipated in large-scale models and field experiments focusing on the effects of climate warming on BVOC emissions have found unexpectedly high responses of BVOC release to a few degrees of warming. These observed emission features for arctic plants have not been specifically considered in models.
This study used a dynamic ecosystem model, LPJ-GUESS, as a platform to investigate short-term and long-term BVOC emission responses to climate warming. Field observations in a subarctic tundra heath with long-term (13 years) warming treatments (Abisko, northern Sweden, 68o21’N, 18o49’E) were extensively used for parameterizing and evaluating BVOC related processes in LPJ-GUESS. We proposed an adjusted temperature (T) response curve of BVOC emissions for arctic plants with much stronger T sensitivity than the commonly-used algorithms for large-scale modelling. The simulated emission responses to 2 oC warming between the adjusted and original T response curves in the model were evaluated against the observed warming responses at short-term scales. Moreover, the modelled annual emission responses to 13 years’ summertime warming by 2 °C, 4 °C and 8 oC were also compared.
The model was able to reproduce vegetation CO2 fluxes as well as day-to-day variability of isoprene and monoterpene emissions. The modelled BVOC warming responses, especially for isoprene, were better captured by using the adjusted T response curve. A few days’ underestimation of leaf T led to the underestimated emission rates as well as warming responses. During 1999-2012, the modelled annual mean isoprene and monoterpene emissions were 17 and 7 mg C m-2 yr-1, with an increase in emission by 60 % and 57 % for 2 °C warming, respectively. Warming by 4 °C and 8 °C further elevated isoprene emission for all years compared with 2 °C warming, but the impacts on monoterpene emissions were levelled off for a decreased coverage of monoterpene-emitters in evergreen prostrate shrubs.
The captured high warming responses by the suggested new T response curve highlight the strong T sensitivity of arctic plants. At short-term scale, the warming responses seem to be strongly impacted by leaf T; while at long-term scale, the warming responses are a combined effect of plant functional type (PFT) dynamics as well as the instantaneous BVOC responses to warming. The identified essential issues associated with estimating arctic BVOC emissions are: (1) leaf T extrapolation based on air T; (2) PFT parameterization accounting for BVOC emission features as well as their responses to warming; and (3) representation of vegetation dynamics in the past and the future.
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