Download INTERDYNAMIK thematic workshop on vegetation dynamics

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.
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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
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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”)?
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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.
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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
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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.
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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.
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