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INTERDYNAMIK thematic workshop on vegetation dynamics Workshop rationale The geographical distribution of dominant land plant forms is mainly controlled by climate. If climate alters, the vegetation cover closely follows the change in the climatic patterns. In turn, changes in spatial distribution and composition of terrestrial vegetation alter the climate through modifications of heat and water fluxes, atmospheric gas and aerosol composition. Biogeochemical and biogeophysical mechanisms form numerous feedbacks between biosphere and climate that were active during the Earth’s geological past, continue to operate at present, and will be important in the future. Analysis of climate-biosphere feedbacks and their role in the climate system dynamics during Quaternary is a new, rapidly developing scientific field that relies on climate system modeling and proxy data analysis. Within INTERDYNAMIK, many projects (COIN, DAMOCLES, GLUES, HOBIMED, HOLOPARC, MISO, Veg-Clim-Man) use coupled climate-vegetation models which are applied to different questions including a role of vegetation cover in climate change during interglacials. Several dynamic vegetation models are used in these projects: two different versions of LPJ, NCAR DGVM, VECODE and TRIFFID. An overview of the comparative performance of these models is needed. On the data gathering side, different terrestrial proxy data, mostly pollen, have been assembled for time-slice reconstruction of vegetation cover during Holocene. New data are going to be collected, especially for the earlier interglacials. A question of whether and how to compare model with proxy data is essential and urgent for both modelling and data communities. In summary, the workshop aimed at a discussion of opportunities for synergy between different projects with a focus on vegetation cover in interglacials as well at exploration of perspectives of further cooperation between modeling and data gathering communities. 1 Workshop participants Nr. Name University/Institute 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Prof. Gerhard Schmiedl Dr. Ulrich Kotthoff Dr. Andre Paul Dian Handiani Dr. Matthias Prange Vidya Varma Dr. Ute Merkel Dr. Peter Köhler Dr. Pavel Tarasov Stefanie Müller Dr. Thomas Kleinen Dr. Stephen Sitch Dr. Carsten Lemmen Dr. Uwe Mikolaewicz Dr. Victor Brovkin Freja Vamborg Juliane Otto Robert Getzieh University of Hamburg University of Hamburg University of Bremen University of Bremen University of Bremen University of Bremen University of Bremen AWI Bremerhaven Free University of Berlin Free University of Berlin PIK, Potsdam Met Office, UK GKSS MPI for Meteorology MPI for Meteorology MPI for Meteorology MPI for Meteorology MPI for Meteorology 2 INTERDYNAMIK project HOBIMED HOBIMED DAMOCLES Guest COIN, HOLOPARC HOLOPARC Guest COIN COIN COIN COIN Guest GLUES MISO, HOBIMED COIN Guest Guest Guest Workshop agenda Tuesday, 14 October 10:30 Start of workshop 10:30-10:40 Introduction by Victor Brovkin 10:40-11:00 Uwe Mikolajewicz, MPI-M. ECHAM-MPIOM-LPJ model 11:00-11:15 Victor Brovkin, MPI-M. New scheme for vegetation dynamics in the ECHAM-MPIOM-JSBACH model Matthias Prange, Uni-Bremen. Introducing the CCSM3-DGVM for interglacial simulations 11:15-11:30 11:30-11:50 Thomas Kleinen, PIK. CLIMBER-LPJ model for interglacial simulations 11:50-12:10 Andre Paul, Uni-Bremen. Simulating the effect of abrupt climate changes on vegetation using the UVic ESCM-TRIFFID coupled climate-dynamic vegetation model Carsten Lemmen, GKSS. User's perspective on vegetation modeling 12:10-12:30 12:30-13:00 Stephen Sitch, Met Office. Modelling global biogeography and biogeochemical cycling using DGVMs: how well do DGVMs perform? 13:00-14:00 Lunch 14:00-14:20 14:40-17:00 Pavel Tarasov, Uni-Berlin. Quantitative reconstructions of the Late Quaternary vegetation using pollen data from northern Eurasia Ulrich Kotthoff, Uni-Hamburg. Quantitative analyses of climate events during interglacials using pollen data Discussion 17:00 End of workshop 14:20-14:40 3 Report on workshop discussion Rapporteur: Thomas Kleinen As part of the SPP INTERDYNAMIK, a workshop on vegetation dynamics and modelling was organised by Victor Brovkin and took place on 14/10/2008 at the Max Planck Institute for Meteorology in Hamburg. All in all there were 18 participants from a number of different INTERDYNAMIK projects and one IODP project, including Stephen Sitch from the UK MetOffice as a guest and a number of PhD students from MPI for Meteorology interested in vegetation feedbacks in past climates. The workshop addressed various questions that would be cross-cutting issues across a number of INTERDYNAMIK projects: 1. Modelling questions: • What is common and what is different between vegetation models used in INTERDYNAMIK? • What are climate-vegetation coupling approaches, model resolution, expected data format? • What important processes which drives vegetation dynamics are missing (e.g. permafrost, grazing) and how these limitations affect model results? 2. Data questions: • What proxy data are already available for the data-model comparison? • What databases are expected to become available soon (e.g. Williams/ Tarasov/Brewer/’s initiative on the Holocene tree cover reconstruction)? • How reliable are climate reconstructions (temperature, precipitation) derived from pollen spectra? 3. Synergy questions: • What can be expected as a synergy between INTERDYNAMIK projects regarding vegetation cover? • What could be a synergy between INTERDYNAMIK and PMIP-2 (Paleo Model Intercomparison Project, Phase 2)? 4. Cross-cutting issues: • Can we calibrate time scales of vegetation dynamics from the pollen/climate datasets? • What is a scale of vegetation-climate feedback during interglacials? • What is an effect of humans on climate through landuse during the Holocene (socalled “Ruddiman hypothesis”)? 4 Discussion on modelling questions Plant functional types During the workshop it emerged that most projects use the model LPJ-DGVM (LundPotsdam-Jena Dynamic Global Vegetation model, Sitch et al., 2003), or one of its derivatives, as a vegetation model. The other models used are TRIFFID and VECODE (for model comparison, see Cramer et al., 2001). One crucial difference between these models is the number of Plant Functional Types (PFTs) employed: VECODE includes 2, TRIFFID uses 5, whereas LPJ utilizies between 9 and 11 PFTs. The number of PFTs would have a strong effect in some cases, for example if boreal forest is replaced by temperate forest. This would lead to a strong release of carbon to the atmosphere if these types of forest are described by different PFTs, while it would have no effect on the carbon balance in the case of a single PFT (Sitch et al., 2008). The number and parameterizations of PFTs also constitute an important issue in data-model comparisons, where the model PFTs would ideally be suited to be compared to the pollen data available. A question posed for further analysis was “what is the critical number of PFTs?” Climate-vegetation coupling In most projects, the vegetation model is coupled to the atmosphere directly, and in these cases the DGVM would run at the resolution of the atmospheric model, roughly 4° on average. The exception to this rule is the CLIMBER-LPJ model, where the vegetation runs at a much higher resolution than the atmospheric model and the coupling is done via an anomaly approach. Issues of the asynchronous coupling and use of a parallel land-surface scheme in ECHAM were discussed. It was discussed whether an intercomparison of the different models used in INTERDYNAMIK would make sense, but the general consensus was that a further intercomparison in addition to the ones in the published literature would make little sense, especially since the projects present are manpower-limited. Missing processes There are some important processes missing in all DGVMs present. None of the models has a representation of permafrost, which would affect hydrology and plant growth at high latitudes, and the representation of stochastic disturbance processes like fire or grazing pressure is also rather limited. The latter two processes would affect the edge between forest and grassland areas, while the missing permafrost would affect the northern edge of the boreal forest. In addition to these processes, the resolution of DGVMs is too low to properly resolve topography-dependent land surface properties. Wetland area, for example, would be underestimated at grid resolutions used in the DGVMs present. 5 Discussion on data questions Calibration of models against data The calibration of models against pollen data is a complex problem. It was discussed whether the models should be compared to the data at the species, PFT or biome level, but the workshop did not arrive at a conclusive answer. Most DGVMs were developed with the modern climate in mind, and in this context the main question is whether carbon fluxes are represented correctly. For palaeoclimate applications, on the other hand, the distribution of PFTs would appear more important, and only the latter can be compared against pollen data. The number and exact type of PFTs chosen for the model would therefore affect strongly whether a calibration against pollen data is possible or not. In some cases it may also be useful to introduce additional PFTs to facilitate the comparison. In some cases it may be useful to aggregate the model output to more generic vegetation types, i.e. combine all types of trees into a single class, to facilitate the data-model comparison. Similarly, a biome classification may lead to an easier comparison between model and data, but here the pollen data may be inconclusive and not allow a clear classification to biomes. Two further problems pointed out in the discussion were the temporal scales of forest expansion, and the effects of heat stress. Heat stress is important for the vegetation dynamics under a future warmer climate, and current DGVM parameterisations are untested, since little validation data exists (yet). The timescales of forest expansion, or succession of PFTs, under changing climate are rather uncertain as well. Here, model calibration using pollen data from carefully selected sites, especially if proxies for both climate and vegetation were present, seems potentially very useful. Large-scale data synthesis With regard to the pollen data itself, the workshop participants pointed out that there appears to be quite a large number of pollen cores from single sites, but that syntheses of these data on larger scales are currently missing. An aggregation of these data to a regional or even hemispheric scale is highly desirable, but currently underrepresented within INTERDYNAMIK. Links to the other international initiatives and projects, such as QUEST QUATERNARY, were discussed. Climatic reconstructions from pollen data The discussion then focused on the reliability of climate reconstructions from pollen data. Here, the reconstruction of the late Quaternary (e.g. Holocene, Eemian) climate seems to be 6 less problematic since modern analogues exist. For earlier interglacials this situation might be different, e.g. there may have been a different species composition despite similar climates. Though pollen-based climate reconstructions are questionable further back in time, they are not impossible. The necessity of an evaluation of the reconstruction results by a regional expert and the error bars associated with the reconstruction should not be underestimated. The need for climate reconstructions from pollen data for a data-model comparison was also questioned during the discussion. Since DGVMs would allow a direct comparison of modelled palaeovegetation to pollen-based vegetation reconstruction, climate reconstructions could be seen as superfluous. However, since pollen data are point data, they may be more representative of localised conditions, and it may therefore be very useful to use these data to reconstruct climate in addition to comparing it to model output. Land use effect in pollen records Finally it was pointed out that pollen data from the Holocene would also contain an anthropogenic land use change component. If DGVMs were reliable, this effect could be used to determine the anthropogenic influence on vegetation by determining the difference between modelled and reconstructed vegetation, but it would also hinder model calibration from Holocene pollen data. Discussion on synergy within INTERDYNAMIK and with external projects The final part of the discussion focused on the synergy between INTERDYNAMIK and the currently ongoing PMIP2 (Paleo Model Intercomparison Project Phase 2). In PMIP2, model simulations of both mid-Holocene and Eemian climate are to be intercompared. A participation of some INTERDYNAMIK modelling groups could therefore be worthwhile. In addition, the data sets produced for PMIP2 could be useful in a number of INTERDYNAMIK projects. Within INTERDYNAMIK, some synergy between the different projects can be expected as well. A dynamic vegetation model intercomparison for a selected time slice could still be carried out at the EMIC level, at which simulations are not that costly. Furthermore, a new data synthesis effort would be beneficial for all projects concerned with modelling vegetation cover. Then, methodological questions of model-data comparisons are similar in all INTERDYNAMIK projects, some synergy is therefore to be expected here as well. Finally, the question of uncertainties in models and vegetation reconstructions is a cross-cutting issue. 7 The way forward In the course of the discussion a number of activities emerged that would appear useful for improving our understanding of vegetation dynamics in the past, as well as quantifying these. 1. Intercomparisons of simulated vegetation cover for both the mid-Holocene and the Eemian climate will be carried out within ongoing activities at the international level. The state-of-the-art models used within INTERDYNAMIK are already involved in this process. Beyond this, the intermediate complexity models can be intercompared as well. A necessary condition for a meaningful intercomparison is the existence of large-scale syntheses of vegetation cover in the Holocene, as well as dataset for the last interglacial. A compilation of these datasets is planned within the PMIP-2 project and a contribution from the INTERDYNAMIK projects is expected. 2. The INTERDYNAMIK groups will contribute to a further development of metrics of datamodel intercomparison. Discrete classes metrics could be based on biome or mega-biome classification (Wohlfahrt et al., 2008), but metrics based on continuous vegetation classes (PFTs) should be investigated as well. 3. An approach for flexible PFT classifications within the vegetation models used for paleo applications, especially in the case of a no present analogue situation, requires further consideration. References Cramer W, Bondeau A, Woodward FI, Prentice IC, Betts RA, Brovkin V, Cox PM, Fisher V, Foley JA, Friend AD, Kucharik C, Lomas MR, Ramankutty N, Sitch S, Smith B, White A, Young-Molling C, 2001. Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biology, 7(4), 357-373. Sitch S, Smith B, Prentice IC, Arneth A, Bondeau A, Cramer W, Kaplan JO, Levis S, Lucht W, Sykes MT, Thonicke K, Venevsky S, 2003. Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model, Global Change Biology, 9 (2), 161-185. Sitch, S., Huntingford, C, Gedney, N. et al., 2008. Evaluation of the terrestrial carbon cycle, future plant geography and climate-carbon cycle feedbacks using five Dynamic Global Vegetation Models (DGVMs), Global Change Biology, 14(9), 2015-2039. Wohlfahrt, J., S.P. Harrison, P. Braconnot, C.D. Hewitt, A. Kitoh, U. Mikolajewicz, B.L. OttoBliesner and S.L. Weber, 2008. Evaluation of coupled ocean-atmosphere simulations of the mid-Holocene using palaeovegetation data from the northern hemisphere extratropics, Climate Dyn, in press. doi:10.1007/s00382-008-0415-5. 8