Process based inventory of isoprenoid emissions from European forests: model comparisons, current knowledge and uncertainties

Abstract

Abstract. Large uncertainties exist in our knowledge of regional emissions of non-methane biogenic volatile organic compounds (BVOC). We address these uncertainties through a two-pronged approach by compiling a state of the art database of the emissions potentials for 80 European forest species, and by a model assessment and inter-comparison, both at the local and regional scale, under present and projected future climatic conditions. We coupled three contrasting isoprenoid models with the ecophysiological forest model GOTILWA+ to evaluate leaf and ecosystem isoprenoid emissions, build an emissions inventory for European forests, and to consider model behaviour in present climate and under projected future climate change conditions. Hourly, daily and annual isoprene emissions as simulated by the models were evaluated against flux measurements. The validation highlighted a general model capacity to capture gross fluxes but inefficiencies in capturing short term variability. A regional inventory of isoprenoid emissions for European forests was created using each of the three modelling approaches. The models agreed on an average European emissions budget of 1.03 TgC a −1 for isoprene and 0.97 TgC a −1 for monoterpenes for the period 1960-1990, which was dominated by a few species with largest aerial coverage. Species contribution to total emissions depended both on species emission potential and geographical distribution. For projected future climate conditions, however, emissions budgets proved highly model dependent, illustrating the current uncertainty associated with isoprenoid emissions responses to potential future conditions.Correspondence to: T. Keenan (t.keenan@creaf.uab.es) These results suggest that current model estimates of isoprenoid emissions concur well, but future estimates are highly uncertain. We conclude that development of reliable models is highly urgent, but for the time being, future BVOC emission scenario estimates should consider results from an ensemble of available emission models.

Isoprene emission potentials from European oak forests derived from canopy flux measurements: an assessment of uncertainties and inter-algorithm variability

“…At the Ispra site the derived emission potential was a composite of the two known isoprene-emitting species, Quercus robur and Populus alba, which represented 80 and 5 % of the forest composition, respectively. According to Keenan et al (2009) the emission potentials of these two species on an area basis are 6820 and 5109 µg m −2 h −1 , respectively. Based on the known species composition and relative emission potentials of these two species, we scaled our ecosystem emission potential to assume a canopy composed of 94 % oak and 6 % poplar.…”

“…This lower emission rate is attributable to not only a lower percentage of oak species present (27 %) within the canopy, but to the particular species of oak present. At Castelporziano two evergreen oak species, Quercus ilex and Quercus suber, account for 27 % of the forest canopy, but both species are relatively minor emitters of isoprene (Keenan et al, 2009). …”

“…The percentage leaf dry mass assumed for each oak species was taken from Keenan et al (2009) for each of the tree species and given an assumed uncertainty of 25 %. The process of converting from fluxes made on a "per area" to a "per mass" basis is clearly a source of uncertainty, but it is worth noting that this uncertainty could be eliminated if investigators making leaf-level measurements were to report their emission potentials on both a per mass and per area basis (Niinemets et al, 2011).…”

“…Based on the known species composition and relative emission potentials of these two species, we scaled our ecosystem emission potential to assume a canopy composed of 94 % oak and 6 % poplar. Leaf-level equivalent emission potentials were subsequently calculated for each site by dividing the whole-oak canopy emission potentials by values of leaf dry mass per unit area obtained from Keenan et al (2009) for each species. This converts the canopy-scale emission potentials which assume an emission rate on a per area basis to units of micrograms of isoprene per gram of dry leaf per hour.…”

“…Numerous laboratory and field studies have contributed to an extensive database of isoprene emission potentials (IEPs) from individual plant species which have been used to assign emission potentials to differing plant functional types (PFTs). Keenan et al (2009) compiled a database of leaf-level isoprene emission potentials for 80 European plant species which they used in conjunction with three separate BVOC (biogenic volatile organic compounds) emission models (Niinemets et al, 1999;Martin et al, 2000;Guenther et al, 1993Guenther et al, , 2006 to generate a comprehensive regional isoprene emission inventory for European forests. Their work highlighted the importance of oak trees, which, when averaged over the three models were shown to account for 70 % of the total isoprene emissions within Europe, with the bulk (∼ 66 % of the total) attributed to just three oak species, Quercus robur, Quercus pubescens and Quercus petraea.…”