Carbon cycle and natural vegetation

Component is implemented in:

LPJmL model (version IMAGE-LPJmL 3) Components: Carbon cycle and natural vegetation Crops and grass Water

Related IMAGE components

Agriculture and land use Atmospheric composition and climate Carbon, vegetation, agriculture and water Ecosystem services Land cover and land use

Models/Databases

HYDE database

Key publications

Müller et al., 2016a Sitch et al., 2003

References

Ballantyne et al., 2012 Bondeau et al., 2007 Braakhekke et al., 2019 Doelman et al., 2019 Friedlingstein et al., 2019 Gerten et al., 2004 Klein Goldewijk et al., 1994 Müller et al., 2007 Schaphoff et al., 2018a Schaphoff et al., 2018b Van Minnen et al., 2000 Von Bloh et al., 2018

Flowchart Carbon cycle and natural vegetation. See also the Input/Output Table on the introduction page.

Key policy issues

• What is the role of the terrestrial biosphere in the global carbon cycle, how will it change in time as a result of climate and land-use change?
• To what extent can the terrestrial biosphere contribute to reducing the accumulation of CO₂ in the atmosphere and what are viable mechanisms?
• What opportunities exist to reduce land-use related carbon emissions (e.g. REDD) and even enhance the carbon uptake through the establishment of new forests.
• What are the contributions of land-use change, climate change and CO₂ fertilization on the future carbon cycle and how can these be considered in climate policies?

Introduction

The terrestrial biosphere plays a key role in global and regional carbon cycles and thus in the climate system. Large amounts of carbon (between 2000 and 3000 PgC) are stored in the vegetation and soil components. Currently, the terrestrial biosphere absorbs about 30% of emitted CO₂ (Ballantyne et al., 2012), and this carbon sink can be maintained and even enhanced by, for instance, protecting established forests and by establishing new forests (Doelman et al., 2019). However, deforestation and other land use changes in the last few centuries have contributed considerably to the build-up of atmospheric carbon dioxide (Friedlingstein et al., 2019) and this trend is projected to continue (Müller et al., 2007).

Regardless of land cover and land use, the net carbon sink in the terrestrial biosphere is affected by a range of environmental conditions such as climate, atmospheric CO₂ concentration and moisture. These conditions influence processes that take up and release CO₂ from the terrestrial biosphere such as photosynthesis, plant and soil respiration, transpiration, carbon allocation and turnover, and disturbances such as fires.

In plant photosynthesis, CO₂ is taken from the atmosphere and converted to organic carbon compounds. This CO₂ conversion is referred to as gross primary production (GPP). The sequestered carbon is needed for plant maintenance and growth (autotrophic respiration), and for the development of new plant tissues, forming live biomass carbon pools. All plant parts (including leaf fall and mortality) are ultimately stored as carbon in carbon pools in the soil and atmosphere.

Terrestrial carbon cycle and vegetation models contribute to better understanding of the dynamics of the terrestrial biosphere in relation to these underlying processes and to the terrestrial water cycle (see Component Water) and land use (see Component Agriculture and land use).

The IMAGE-2 carbon cycle and biome model (Klein Goldewijk et al., 1994; Van Minnen et al., 2000) have been replaced by the Lund-Potsdam-Jena model with Managed Land (LPJmL) model (Sitch et al., 2003; Gerten et al., 2004; Bondeau et al., 2007). An overview of the LPJmL model in the IMAGE context with regard to carbon and biome dynamics is presented here; the model and a sensitivity analysis is described in detail by Müller et al. (2016).

Input/Output Table

Input Carbon cycle and natural vegetation component

IMAGE model drivers and variables	Description	Source
Number of wet days - grid (historical data)	Number of days with a rain event, per month; assumed constant after the historical period	CRU database
Cloudiness - grid (historical data)	Percentage of cloudiness per month; assumed constant after the historical period	CRU database
Temperature - grid	Monthly average temperature.	Atmospheric composition and climate
Timber use fraction	Fractions of harvested timber entering the fast-decaying timber pool, the slow-decaying timber pool, or burnt as traditional biofuels.	Forest management
Precipitation - grid	Monthly total precipitation.	Atmospheric composition and climate
Forest management type - grid	Forest management type: clear cut, selective logging, forest plantation or additional deforestation.	Forest management
Irrigation water supply - grid	Water supplied to irrigated fields; equal to irrigation water withdrawal minus water lost during transport, depending on the conveyance efficiency.
CO2 concentration	Atmospheric CO2 concentration.	Atmospheric composition and climate
Land cover, land use - grid	Multi-dimensional map describing all aspects of land cover and land use per grid cell, such as type of natural vegetation, crop and grass fraction, crop management, fertiliser and manure input, livestock density.	Land cover and land use

External datasets	Description	Source
Soil properties - grid	Soil properties that have an effect on vegetation growth and hydrology. These characteristics differ between soil types. Relevant characteristics are soil texture and depth and water holding capacity	HWSD database

Output Carbon cycle and natural vegetation component

IMAGE model variables	Description	Use
NPP (net primary production) - grid	CO2 sequestered by plants and incorporated in new tissue in plant carbon pools.	Land cover and land use
Carbon pools in vegetation - grid	Carbon pools in leaves, stems, branches and roots).	Forest management Land cover and land use
Carbon pools in soil and timber - grid	Carbon biomass in three soil pools (litter, humus and charcoal) and two timber pools (slow decaying, and fast decaying).	Land cover and land use
Land-use CO2 emissions - grid	Land-use CO2 emissions from deforestation, wood harvest, agricultural harvest, bioenergy plantations and timber decay.	Atmospheric composition and climate Climate policy
NEP (net ecosystem production) - grid	Net natural exchange of CO2 between biosphere and atmosphere (NPP minus soil respiration), excluding human induced fluxes such as decay of wood products.	Atmospheric composition and climate Ecosystem services
Potential natural vegetation - grid	Potential natural vegetation type/biome, based on distribution of plant functional types.	Land cover and land use
Soil respiration - grid	CO2 release from soils into the atmosphere due to the decay of soil carbon pools and respiration of soil organisms.	Final output