2010jg001393-txts01

Text S1 Contents 1 The Dynamic Land Ecosystem Model key processes and sub models 2 2 Model validation 9 2 1 Model validation for CO2 fluxes 9 2 2 Model validation for CH4 fluxes 11 2 3 Model validatio[.]

Text S1

Contents

1 The Dynamic Land Ecosystem Model: key processes and sub models 2

2 Model validation 9

2.1 Model validation for CO2 fluxes 9

2.2 Model validation for CH4 fluxes 11

2.3 Model validation for N2O fluxes 16

3 Model uncertainty analysis method 17

Reference cited 19

1 The Dynamic Land Ecosystem Model: key processes and sub models

The Dynamic Land Ecosystem Model (DLEM) is a highly integrated, process-basedterrestrial ecosystem model that aims at simulating the structural and functional dynamics of landecosystems affected by multiple factors including climate, atmospheric compositions (CO2, O3),precipitation chemistry (nitrogen composition), natural disturbance (fire, insect/disease,hurricane, etc), land-use/cover change, and land management practices (harvest, rotation,fertilization, irrigation, etc) DLEM consists of five vegetation, three soil, and seven debrisboxes, and couples major biogeochemical cycles, hydrological cycle, and vegetation dynamics tomake daily, spatially-explicit estimates of water, carbon (CO2, CH4) and nitrogen fluxes (N2O)and pool sizes (C and N) in terrestrial ecosystems

DLEM includes five core components: 1) biophysics, 2) plant physiology, 3) soilbiogeochemistry, 4) dynamic vegetation, and 5) land use and management (Figure 1) Briefly, thebiophysics component simulates the instantaneous fluxes of energy, water, and momentumwithin the land ecosystem and their exchanges with the surrounding environment Plantphysiology component simulates major physiological processes, such as plant phenology, C and

N assimilation, respiration, allocation, and turnover Soil biogeochemistry component simulatesthe dynamics of nutrient compositions and the fundamental microbe’s activities Thebiogeochemical processes, including the mineralization/immobilization,nitrification/denitrification, decomposition, methane production/oxidation are considered in thiscomponent The dynamic vegetation component simulates the structural dynamics of vegetationcaused by natural and human disturbances Two processes are considered: the biogeographyredistribution when climate change, and the recovery and succession of vegetation afterdisturbances Like most dynamic global vegetation models, DLEM builds on the concept of plantfunctional types (PFT) to describe vegetation distributions The land use and managementcomponent simulates cropland conversion, reforestation (cropland abandonment), forestmanagements (harvest, thinning, fertilization and prescribed fire) DLEM emphasizes themodeling and simulation of managed ecosystems including agricultural ecosystems, plantationforests and pastures The spatially-explicit management data sets, such as irrigation, fertilization,rotation, and harvest can be used as input information for controlling the ecosystems It alsosimulates urbanization processes, and can be used to estimate the impacts of urban impervioussurface and urban lawn management on ecosystem processes

The basic simulation unit of DLEM is a single grid with corresponding coverage area

In this unit, vegetated land surfaces are comprised of one of the natural vegetation functionaltypes (forests, grassland and shrubs) or urban area or cropping system The classification ofnatural forests is based on the leaf structure (needle and broadleaf), leaf phenology (evergreen

and deciduous), climate zones (tropical, temperate, and boreal) The grasslands are divided intoC3 grass, C4 grass, and meadow The shrubs have evergreen and deciduous types Besides, otherplant functional types including desert, wetland and tundra are also considered.

For simulating vegetation dynamics, DLEM model uses a strategy similar to the dynamicglobal vegetation model (DGVM) LPJ The time step for vegetation dynamic processes is 1 year.Two kinds of vegetation dynamic processes can be simulated by DLEM: the biogeographyredistribution when climate change, and the plant competition and succession during vegetationrecovery after disturbances Like most DGVMs, DLEM builds on the concept of plant functionaltypes (PFT) to describe vegetation distributions Many different PFTs that adapt to local climatecan coexist in the same grid, competing light, water, and nutrient resources For the historicalsimulations, which focus on historical variations of carbon, nitrogen, and water cycles affected

by changing climate, land-use, and atmospheric compositions, we used prescribed plantfunctional type for natural vegetation cover or human-managed system, like cropland and urban

Figure S1 Structure and Key Processes of Dynamic Land Ecosystem Model (DLEM)

1.1 Biophysical Processes and Radiation Transmissions

Radiation is the major energy source driving hydrological and biogeochemical cycles ofterrestrial ecosystems Daily incident shortwave radiation (SRAD) is used to estimate thephotosynthesis, transpiration, evaporation, and snow melt SRAD can be put into the model asuser’s input, or generated by the model The algorisms for generating this variable and relativehumidity are from MT-CLIM 4.3 developed by Thornton et al (1998) Daily temperature range,precipitation, DEM, and information of geographical location are used to generate SRAD

SRAD is partitioned into reflectance, absorptions by plants, and absorptions by soilsurface The canopy is divided into sunlit and shaded fractions in which photosynthesis and ETwere estimated independently

1.2 The hydrological cycle

In the DLEM, water pools in terrestrial ecosystems were classified into six boxes: thecanopy intercepted snow and intercepted water; the ground surface snow; the litter interceptedwater; the upper layer soil water (0 ~ 50 cm), and the lower layer soil water (50 ~ 150 mm) Thewater content in each boxes are updated daily based on water input (precipitation and dew) andthe water losses (evaporation, transpiration, sublimation, surface runoff and drainage runoff)(unit mm/day) driven by the solar radiation and the plant’s physiological processes

The major processes include the partition of precipitation, canopy interception of rain andsnow, canopy snow sublimation, snowmelt and sublimation from ground snowpack, canopyevapotranspiration, litter interception of rain or snow, soil surface evaporation, surface runoff andinfiltration, and soil moisture movement

1.3 The Carbon cycle

The carbon cycle is the most important process in the DLEM; it serves framework forbiogeochemical processes of other nutrients and hydrological processes The DLEM mainlysimulates carbon fluxes through various pools

The vegetation carbon pool has six components for trees and shrubs (storage organ, leaf,heartwood, sapwood, fine root, and coarse root), and five components for herbaceous vegetation(storage organ, leaf, stem, fine root, and coarse root) Vegetation carbon pool gains carbonthrough photosynthesis (Gross Primary Production, GPP), loses carbon through autotrophicrespiration (including maintenance respiration and growth respiration), litter fall, mortality, anddisturbances such as land conversion and fire The effects of ozone pollution and nitrogendeposition on photosynthesis are also simulated in the DLEM The litter carbon pool includesseven pools for trees and shrubs: coarse woody debris, aboveground very active litter,aboveground middle active litter, aboveground resistant litter, belowground very active litter,belowground middle active litter, and belowground resistant litter The sources of litter poolsinclude litterfalls from leaves and roots, debris from mortalities, harvest, and land use change

Litter carbon pools can be converted into soil organic matter (SOM) pools and emits CO2 to theatmosphere through decomposition

Soil Organic Matter has three pools with different decomposition base rate: very active,middle active, resistant SOM, and dissolved organic carbon The balance of SOM depends on thetransformation of litter to SOM, the fractions of conversion from GPP to the dissolved organiccarbon (DOC), the returned organic matter from production decay (e.g manure), the growth ofmicrobes, the methane production from DOC, and the decomposition rate Meanwhile, theDLEM simulates the life cycles of microbe, and production It should be noted that the methanemodule in the DLEM mainly simulates the production, consumption, and transport of CH4 [Tian

et al., 2010] Due to relatively small contribution from other substrates [Conrad, 1996; Mer andRoger, 2001], DLEM only considers the CH4 production from DOC, which is indirectlycontrolled by environmental factors including soil pH, temperature and soil moisture content.The DOC was produced through three pathways, GPP allocation, and decomposition byproductsfrom soil organic matter and litterfall CH4 oxidation, including the oxidation during CH4

transport to the atmosphere, CH4 oxidation in the soil/water, and atmospheric CH4 oxidation onthe soil surface, is determined by CH4 concentrations in the air or soil/water, as well as soilmoisture, pH, and temperature Most CH4-related biogeochemical reactions in the DLEM weredescribed as the Michaelis-Menten equation with two coefficients: maximum reaction rate andhalf-saturated coefficient Three pathways for CH4 transport from soil to the atmosphere-ebullition, diffusion, and plant-mediated transport-are considered in the DLEM [Tian et al.,2010]

1.4 The Nitrogen cycle

DLEM simulates C-N interaction with “wide-open” N cycle (Rastetter et al., 1997),where nitrogen inside ecosystem unlimitedly exchanges with exterior environment throughexternal N supply and N export on atmosphere-land interface (such as N deposition, N fixation,nitrous trace gas emission etc.) and hydrosphere-land interface (e.g N leaching loss) In DLEM,nitrogen cycles are intimately coupled with the carbon cycle by constant C/N ratios of differentbiomass pools and SOM Available nitrogen status can affect the carbon cycle directly throughphotosynthesis, respiration, allocation and decomposition processes Meanwhile, carbon cyclecouples with the nitrogen cycle directly through providing litterfalls with various quantities andqualities and affecting nitrogen uptake by plants DLEM simulates the nitrogen in several forms:organic nitrogen stored in biomass, labile nitrogen stored in plants, organic nitrogen stored in soilorganic matter and litter/woody debris, dissolved organic nitrogen (DON), and inorganicnitrogen ions such as NH4+ or NO3-

The major nitrogen processes in terrestrial ecosystem includes nitrogen input from theatmosphere (through nitrogen deposition and nitrogen fixation), fertilization, nitrogen

immobilization/mineralization, plant N uptake, nitrification/denitrification,adsorption/desorption, nitrogen leaching, and unknown nitrogen loss through fire or otherdisturbances.

It should be noted that both denitrification and nitrification processes are simulated asone-step processes as we do not consider the mid-products in each process Nitrification, aprocess converting ammonium into nitrate, is simulated as a function of soil temperature,moisture, and the NH4+ concentration [Lin et al., 2000] Denitrification, through which the nitrate

is converted into nitrous gases, is simulated in the DLEM as a function of soil temperature,moisture, and the NO3- concentration [Lin et al., 2000] All the products of nitrification anddenitrification that leave the system are N-containing gases The empirical equation reported byDavidson et al [Davidson et al., 2000] is used to separate N2O from other gases (mainly NO and

N2)

1.5 Carbon and nitrogen allocation

DLEM simulates carbon and nitrogen allocation by combining the ideas from thefunctional equilibrium models (e.g., Friendlingstein et al., 1999), sink regulation models (e.g.,Marcelis, 1994), and descriptive allometry models (e.g., Wilson, 1988; Marcelis & Heuvelink,2007) DLEM takes the following assumptions as foundations of carbon and nitrogen partition

at daily time step:

1) Plants have a potential to store proportional carbon and nitrogen to sustain its growth

in harsh situations If the stored carbon is less than the minimum requirement, 50% of availableGPP (AGPP) (i.e total GPP minus growth respiration loss) will go to the storage pool until itgets to minimum storage

2) Plants have a potential to optimize the capture of limiting resources (Cannell &Dewar, 1994; Litton et al., 2007) DLEM uses some equations revised from Friendlingstein et al.(1999) to represent limitation effects of light (leaf area index), water, and nitrogen on allocation

3) Leaves have the first priorities to use the products of photosynthesis and theremaining carbon is assigned into other tissues (reproduction, root, and stem, accordingly) InDLEM, the requirement for daily leaf growth is calculated with the leaf phenology and themaximum leaf carbon content estimated by the size of sapwood pools (for natural vegetation)and the theoretical maximum leaf carbon (for crops) There are several scenarios for carbonpartitioning:

a If the available GPP (after being extracted by storage pool) is greater than therequirement of leaf growth during the growing season, proportional carbon will flow tothe leaf pool to meet the requirements Then, some constant part (vegetation type-specific) will flow to reproduction pool The left carbon will flow to stem and roots withthe ratio calculated by section (2)

b If the available GPP (after being extracted by storage pool) is less than therequirement of leaf growth during the growing season, all of this available GPP will flow

to the leaf pool If current LAI is less than 1 and the soil moisture scalar forphotosynthesis is greater than 0.05, the storage pool will be activated to join theallocation process to meet the leaf carbon requirement Some constant proportion of thistotal available carbon will flow to root This situation normally occurs at the beginning ofgrowing season for deciduous vegetation and crops

c If current leaf carbon is greater than the potential leaf carbon (calculated withphenology and maximum LAI), or it is the leaf-off season (phenology period from theclimax to the minimum) of deciduous vegetation, the carbon partition is based on theratios calculated in section (2) The extra carbon beyond potential leaf carbon will flow tostem and roots according to their relative partition coefficients

4) Availability of nitrogen can limit the overall carbon partitions to different organs Weassume that all the nitrogen used for carbon allocation to maintain tissue-specific C/N ratiocomes from the storage pool For leaf allocation, we take its maximum C/N ratio as initialbaseline to estimate its nitrogen requirement In nitrogen uptake process, the leaf C/N ration can

be changed according to the contemporary nitrogen status of the ecosystem (see detail in thenitrogen section) If the total nitrogen requirement cannot be met, the extra GPP will flow tostorage pool till its maximum storage capacity Then the allocation process will feed back to theformer simulation step to adjust GPP as we assume that plants cannot produce morephotosynthate than what they can use in forming biomass and material storage

1.6 Nutrient export from landscape to coastal region

By incorporating the processes of soil erosion with modified universal soil loss equation(MUSLE) , water routine process based on global river network data sets (GTN30) [Vörösmarty

et al., 2000], and simplified nitrogen removal process in the river systems [Alexander et al.,2000; Wollheim et al., 2006; Wollheim et al., 2008], we expanded DLEM model with NutrientExport (NE) component to track the leached nutrients from terrestrial ecosystems to freshwateraquatic systems and eventually to the coastal regions The DLEM-NE model is capable ofassessing the impacts of natural and anthropogenic driving forces on nutrients (currently weconsider forms of DOC, POC, DON, DIN, and PON) leaching and delivering from terrestrialsystem to coastal regions The export of nutrients from landscape to coastal area includes threemajor processes: the productions of nutrients in inland watersheds; the leaching of nutrients fromland along with overland flow and base flow; and the transport of nutrients through rivernetworks to river outlets in coastal region

1.7 Disturbances and plantation managements

DLEM simulates the impacts of intensive land management and disturbances on theecosystem structure and functions The land management includes: forest harvest, forestthinning, fertilization, weed control, insect & disease control and prescribed fire; the naturaldisturbances include: hurricane & storm, wild fire, and pest & disease Forest management anddisturbance will not completely change the vegetation functional types, but directly influence thecarbon, nitrogen and water flux, forest biomass, litter composition and decomposition, soilorganic matter, wood and wood products and CO2 mitigation by replacing fossil fuel with wood-based bio-fuel, and indirectly influence other processes of forest carbon, nitrogen and watercycles DLEM estimates the wildfire-caused CO2 and non-CO2 GHG emissions from terrestrialecosystems The carbon emission is based on the equations utilized in Seiler and Crutzen [1980];Thonicke et al [2001], and Lu et al [2006].

The fates of wood products are tracked by the DLEM plantation module In the same year

as the harvest takes place, several intermediate processing and allocation steps are done, until thecarbon resides in the end products, the millsite dump, or is transferred to the bioenergy module.When the end products are discarded at the end of their lifespan, they can be recycled, deposited

in a landfill, or can be used for bioenergy in the specific module Carbon is released to theatmosphere through decomposition at the millsite dump, at the landfill, or via the bioenergymodule The fates of the harvested wood include: sawnwood, boards wood, paper wood andfirewood The initial product fractions (Default values) for these three fates are given asSchelhaas, et al [2004]

1.8 Agricultural ecosystem module

The DLEM agricultural module enhances the ability of DLEM to simulate the interactiveeffects of agronomic /land management practices and other environmental factors on cropgrowth, phenology and biogeochemical cycles in croplands It aims to simulate crop growth andyield, as well as carbon, nitrogen, and water cycles in agricultural ecosystems All the processes

of crop growth (e.g photosynthesis, respiration, allocation) and soil biogeochemistry (e.g.decomposition, nitrification, fermentation) are simulated in the same way as in DLEM for allnatural functional types and with a daily time-step, but different crops are specificallyparameterized according to each crop type Besides natural environmental driving factors, themodule pays special attention to the role of agronomic practices, including irrigation, fertilizationapplication, tillage, genetic improvement and rotation on crop growth and soil biogeochemicalcycles

1.9 Urbanization processes

Unlike most of current biogeochemical models that either ignore the urbanization processes

or simply treat the urban ecosystem as cropland or grassland, DLEM includes a concise but

effective urban module that can simulate the impacts of urbanization on ecosystem structure andfunctions The model treats urban as a mosaic of three different land-use fractions: urbanimpervious surface (UIS), urban lawn (ULW), and urban natural (unmanaged) vegetation(UNG) For simplification, the relative fractions of these three land-use types will not changeafter urbanization in DLEM, while the regional coverage of UIS and ULW will change as theresults of urban expansion Urbanization takes place when the land-use type of the current year is

changed from non-urban types (i.e the potential vegetation or cropland) into an urban type.

During the urbanization, the land-conversion takes place in UIS and ULW fractions, the priorlocal vegetations will keep undisturbed in the UNG Since trees that are removed from urbanareas are not normally developed into wood products for long-term carbon storage [Nowak andCrane, 2002], unlike the impacts of land-use change during cropland-conversion, DLEMassumes that all wood products during urbanization will decompose in one year

2 Model validation

2.1 Model validation for CO2 fluxes

Consistency between model results and field measurements is essential for establishingthe credibility of biogeochemistry models such as DLEM To evaluate model capabilities, wecompared our model estimates of net ecosystem production (NEP) to short-term measurements

of net ecosystem exchange (NEE) at six eddy co-variance sites in China (Table S1) The eddycovariance technique has been recognized as one of the most reliable approaches for estimatingthe net exchange of carbon dioxide between land ecosystems and the atmosphere We ran ourmodels in site-specific mode, using the driving variables specific to the grid cell in which thefield study was conducted The model results were in reasonable agreement with themeasurements for all sites, with the modeled annual NEE estimates falling within +/- 20 percent

of the eddy flux measurements (Table S1) In addition, the NEP estimated by DLEM alsocaptured the daily/seasonal variations in the field NEE estimates at eddy covariance sites inwhich daily NEE measurements and climate data were available (Figure S2 and S3)

Figure S2 Comparison of daily net ecosystem production simulated by DLEM against observeddata in dry farmland of Yucheng, northern China (116o36’ E, 36o57’ N) (Cited from Tian et al.,

2011)

Figure S3 Comparison of daily net ecosystem production simulated by DLEM in temperate

evergreen needleleaf forest in Qianyanzhou, Southeastern China (26o45’ N, 115o04’ E) (Cited

from Tian et al., 2010)

Table S1 Comparison of model and eddy covariance net ecosystem exchange estimates (Anegative value represents a terrestrial carbon sink whereas a positive value represents a carbon

source) (Cited from Tian et al., 2010)

( g C m -2 yr -1 )

DLEM (g C

m -2 yr -1 ) RefsTemperate mixed forest/Changbaishan