potential effects of climate and land use change on soil ......modifications in new zealand's...

Weather and Climate (1995) 15-2: 47-56

POTENTIAL EFFECTS OF CLIMATE AND LAND USE CHANGE ONSOIL CARBON AND CO, EMISSIONS FROM NEW ZEALAND'SINDIGENOUS FORESTS AND UNIMPROVED GRASSLANDS

A. Parshotaml, K.R. Tate' and D.J. Giltrap2

' Landcare Research New Zealand Ltd., Private Bag 11 052, Palmerston North, New Zealand.2 Small Office Systems Ltd., P 0 Box 46 024, Lower Hutt, New Zealand.

ABSTRACT

Present understanding o f the relat ivesensitivity to climate change of the naturaland managed land systems of New Zealand,and feedbacks to the atmosphere, is generallypoor and largely qualitative. T h i s paperdescribes the development and application ofa process-based ecological simulation modelto estimate increases in soil CO2 emissionsresulting from human-induced ecosystemmodifications in New Zealand's indigenousforests and unimproved grasslands undercurrent c l imates, a n d f u t u r e c l ima tewarming. F o r these New Zealand land usesat steady state, plant residue carbon inputsof 53 and 73 Mt C y4 respectively, are neededto maintain standing stocks of 547 and 671Mt C (soil carbon) to 23 cm depth. L o w e rinputs resulting from ecosystem degradationcould result in significant net CO2 emissions.An additional source of soil C09-C of about0.52 a n d 0.76 M t y-1 f r o m forests a n dgrasslands could resul t f rom accelerateddecomposition of soil organic matter. Thesesimulated releases of CO2 from decomposingsoil organic mat te r, resu l t i ng f rom thecombined effects of ecosystem degradationand climate change, would accelerate thebui ldup o f a tmospher ic CO2, f u r t h e renhancing the warming trend.

INTRODUCTION

Indigenous fo res ts a n d un imp rovedgrasslands which cover 49% of the total NewZealand land area, store ca 3300 Mt C (Tateet al., 1993), and are likely to be susceptibleto fu ture climate change (e.g. Leathwick,

47

1995). Much of the carbon in these ecosystemsis in soil organic matter (ca. 1600 Mt C to lmdepth, Tate et al., 1993), a dynamic reservoirwhose size is largely determined by plantresidue additions to soil and concomitantlosses to the atmosphere, mainly as CO2, fromheterotrophic soil respiration. Changes inclimate can directly affect this balance, bychanging net primary productivity and litterinput quality and the biomass and activity ofthe soil decomposers. Global warming couldthreaten the future product iv i ty o f theseecosystems through the loss of soil organicmatter, with a resultant significant feedbackof CO,, to the atmosphere (e.g. Jenkinson etal., 19-91).

In addition to promoting water retention,infiltration, soil tilth, and reducing wind andwater erosion, organic mat ter is a majornutrient reservoir in most soils. Soils alsosustain a large and diverse population oforganisms (Tate et al., 1985) which contributesignif icantly t o decomposition processes.Apar t f r o m t h e l a r g e r a n i m a l s (e .g .earthworms), m o s t o r g a n i s m s a r econcentrated in the dead organic matter nearthe soil surface, in a zone most vulnerable tothe effects of changes in climate and land use.

An understanding of the extent and timingof changes in soil organic matter under awarmer climate requires knowledge of boththe inventory o f carbon i n soils and i t sturnover rate. A further requirement is anunderstanding of how current patterns ofclimate i n New Zealand w i l l change i nresponse to regional climate change scenarios(Tate et al., 1996).

48 P o t e n t i a l Effects of Climate and Land Use Change

We describe f i r s t t h e development o fdatabases for spatially extensive simulationof soil carbon turnover. These databases areused in a process-based soil carbon turnovermodel under the current climate to assesspotential CO, emissions f rom ecosystemmodification in New Zealand's native forestsand unimproved grasslands. Second, wecompare these CO, emissions w i t h thoseexpected under future climate warming.

To assess future changes in soil organicmatter, n a t i o n a l l a n d u s e , s o i l , a n dtopoclimate databases were combined (Figure1) in a soil-carbon turnover model based onthe Rothamsted (Roth 26.2) model (Jenkinsonet al., 1992).

Vegetative cover databaseThe vegetative database employed in this

work was the Vegetative Cover Map of NewZealand (VCM) (Newsome, 1987). This is anational survey mapped at a scale of 1: 1 000000, and records vegetative cover in 49 classes.The survey was undertaken between 1981 and1985 from computer-generated vegetationmaps, published maps, unpublished surveysand records, with extensive field checks. Thesource information was available in map formand from a computerised data achive. A total

climatechangescenario

METHODS

of 14 f o r e s t classes and 13 c lasses o funimproved grassland were aggregated toprovide t h e l oca t i ons a n d a r e a s f o r' indigenous fo res t ' a n d ' u n i m p r o v e dgrassland'. These areas were respectively 5.2and 7.3 M ha, and are shown in Figure 2.

Soil carbon inventoryEstimates o f soil carbon and i ts spatial

distribution (Figure 3), were reported recently(Tate e t a l . , 1993). So i l carbon contentestimates for two depth ranges (0-25, 0-100cm)were derived from the NZ Land ResourceInventory (NZLRI ) (DSIR Land Resources,1992), the Soil Map of Stewart Island (Leamyet al., 1974) and the National Soils Database(McDonald et al., 1988). The NZLRI gives aspatial coverage of North and South Islandwith attribute information which includes soil.This was combined with the Stewart Islanddata to give complete national coverage. TheNat ional S o i l s D a t a b a s e i n c l u d e smorphological, chemical and physical data onsoil profiles and a reference soil l ist givingtaxonomic data and lists of similar soils. Themethods used, and the main sources of errorare fully described by Tate et al. (1993)

Topoclimate regionsAn unsupervised cluster analysis was used

to divide New Zealand into 32 regions basedon topographic var iab les cor re la ted t ocontemporary c l ima te . T h i s app roach

changes ini s 4 soil organicmatter

Figure 1: Impacts of climate change on soil organic matter. The soil compartments in the model are: DPM, decomposableplant material; RPM, resistant plant material; B10, microbial biomass; HUM, humic substances; IOM, inert organicmatter.

Potential Effects of Climate and Land Use Change 4 9

38'S

43'S

a170*E 1 7 6 ' E

LEGEND

a l INDIGENOUSFORESTS

INDIGENOUSGRASSLANDS

STUDY SITES

SOUTH ISLAND

TrE 1 7 6 - E

Scale 1:10 000 000

38'S

43'S

Figure 2: Map showing the area of indigenous forest and unimproved grasslands, and the locationsof 9 study sites used, to obtain estimates of soil carbon storage and turnover (Tate et al., 1995).


Figure 3: Estimates of topsoil carbon and its spatial distribution.


simplified the model simulation processes asonly 32 regions had to be processed ratherthan many thousands of separate points onthe landscape (e.g. >10 ,000 5 k m cells).Defining the regions in this way also meantthat i) the regions were objectively definedand had minimum combined variance withrespect to the topographic variables, and ii)the regions were no t d i rec t l y based oncontemporary climate and would remain valideven i f there were major shifts i n futurec i rcu lat ion pa t te rns . T h e topograph icvariables used were i) latitude and longitude;i i ) d is tance f r om the coast and i t s logtransform; i i i ) elevation above sea level, andiv) "shadow" elevations (i.e. the elevation ofthe highest point between the current pointand the sea in the nominated direction) ineach of 12 directions.

Multiple regression analysis based on datafor 1436 New Zealand climate and rainfallstations (Giltrap, 1994) showed that the abovevariables accounted for 90.8 - 95.5% of thevariance in mean monthly air temperaturesand for 44.0 - 71.8% of the variance in logtransform monthly rainfall. A simpler modelwhich depended only on latitude, month andelevation accounted for 97.6% of the variancein solar radiation. An updated version of themodel is currently under preparation.

The whole „country was divided into 5 kmgrid (10,084 cells) and a cluster analysis wasused to divide this sample into 32 clusters withmin imum in t ra -c lus te r var iance i n thetopographic variables. T h e following minormodifications were made to these clusters -

i )A cluster which predominantly comprisedthe North land peninsula also included anumber of small coastal areas in other partsof t he count ry. These inc lus ions werereclassif ied t o t he same class as t h e i rimmediate neighbours. A total o f 74 cells(1850 km2) were affected;

ii) A small cluster which contained only 15cells (375 km2) on the Fiordland coast waseliminated and combined w i t h a s imi larneighbouring cluster;

iii) A new cluster (92 cells, 2300 km2) wasgenerated by separating out the cells wi thpredicted ( f rom the mu l t i p le regressionanalysis) annual rainfall of less than 600 min.This was done to ensure that simulation inthese regions sampled the dry extreme o fconditions encountered in New Zealand, and

iv) The clusters (units) were renumberedin order of increasing mean latitude.

Figure 4 shows the boundaries of the 32clusters and gives t h e mean predic ted(contemporary) annual rainfall and Februaryand July air temperatures for these clusters.

The Rothamsted soil-carbon turnover modelThe Rothamsted soil-carbon turnover

model represents soil organic matter as acompartmental system (see the cent ra lcomponent of Figure 1), where the transferof carbon from compartment to compartmentby decomposition processes is by first-orderprocesses (Jenkinson e t a l . , 1992). Thecompartments DPM, RPM, BIO and HUMrepresent fractions of soil organic matter thatconsist of decomposable plant material (DPM),resistant plant material (RPM), microbialbiomass (BIO) and humified organic matter(HUM). Organic carbon inputs are assumedto enter the DPM or the RPM pool, both ofwhich decompose to form CO2, microbialbiomass and humified organic matter. I naddition, an ine r t organic mat te r ( I 0 M )compartment is included which is assumedto be biologically inactive. Decompositionrates are modified by climatic factors (suchas rainfall and temperature), clay content andthe presence or absence o f plants. Thesefactors are expressed in the model as a yearly-periodic, rate modifier, which is empiricallydetermined (Jenkinson et al., 1992). Modelequations for this dynamical system wi thconstant organic input rate may be expressedin the form of a linear system of first orderd i f ferent ia l e q u a t i o n s w i t h p e r i o d i ccoefficients (Parshotam, 1995).

An average soi l carbon value may bedefined over t ime which can considerablyreduce computa t iona l t i m e . A l i n e a rrelationship exists between average soi lcarbon and plant inputs at steady state, sothat inputs required to maintain soil carbonat a steady state may be obtained by matrixinversion.

Model InputsThe topoclimate and soil carbon maps were

overlaid to produce a polygon map of inputvariables for each representative vegetativeclass. In addition to soil carbon, clay contentsused in the simulation were also obtained fromthe National Soils Database. W h e r e soilscontained the short range order clay minerals


10 1 6 0 08 1 6 5 07 1 9 5 05 1 4 0 04 1 8 5 07 1 2 0 05 1 6 5 08 1 7 5 06 1 1 5 06 1 8 5 04 2 5 5 05 1 6 5 06 1 3 5 02 2 3 5 00 2 2 0 05 2 2 0 06 1 4 0 03 1 3 0 0

5 2 4 5 05 8 0 0

- 2 2 7 0 01 3 2 5 00 1 2 0 01 1 0 5 03 5 5 00 2 2 5 03 2 2 5 02 1 0 0 05 1 7 5 03 9 0 05 9 0 09 0

U N I T A I R / T E M P R A I NNo F e b J u l A n n

I 1 92 1 83 1 74 1 75 1 56 1 87 1 68 1 79 1 7

10 1 611 1 412 I S13 1 714 1 315 1 216 1 517 1 518 1 419 1 6 S 2 5 0 02 0 1 421 1 62 2 92 3 1 22 4 1 32 5 1 32 6 1 52 7 1 12 8 1 22 9 1 43 0 1 331 1 43 2 1 4

2 0

3 04 - I I

. 20

19

10

17

11

Figure 4: Topoclimate regions from cluster analysis. The 32 clusters (units) are in order of increasing latitudeand represent regions with minimum intra -cluster variance in the topographic variables.


forest 5.2 547 53 9.9 7+3

grassland 7.3 671 73 8.0 3+1

allophane and ferrihydrite, which stronglyretard decomposition rates (Saggar et al.,1994), a n 'e ffect ive ' c l a y con ten t wascalculated from the 'raw' clay contents fromthe soils database using the relationship (BK G Theng and J J Claydon, unpublishedresults):

Effective c l a y (%) r a w c l ay (%) +3(allophane + ferrihydrite) (%).

A water balance was calculated from thePriestley and Tay lo r (1972) method fo restimating evapotranspiration. Model outputwas then mapped to the appropriate polygonsand spatially averaged. The computer sourcecode for the Rothamsted model (written inFor t ran 7 7 ) w a s r e w r i t t e n u s i n g t h emathematical simulation package MATLABto deal with large matrices of data from thedatabases and to perform matrix exponentialand matrix inverse computations.

Several other assumptions were made,including the decomposibility factors for thedifferent plant residues (Jenkinson et al.,1992). These were 0.25 for indigenous forestsand 0.67 for unimproved grasslands. Soilswere assumed to be under steady stateconditions, with the 'inert' fraction of soil Cbeing taken as 7 t ha-1(Jenkinsonet al., 1992).

Model estimates of annual inputs of organic carbon to 'topsoils' inNew Zealand's indigenous forests and unimproved grasslands

Land use A r e a T o t a l soil Ct

(Mha) ( M t )

t soil depth of 25 cm, measured from litter-mineral soil interface

t means + SD from 6 grassland and 4 forest sites

RESULTS

Current soil CO2 emissions from nativeforests and unimproved grasslands

Initially, the inputs of plant residues neededto maintain the organic carbon contents to25cm depths for each cl imatic zone wereestablished for the indigenous forest andunimproved grassland areas in North Island,South Is land and Stewart Is land, underpresent climatic conditions. A s a test of ourassumptions, estimates of carbon inputs atfour forest and six grassland sites (Tate et al.,1995) were compared w i th the simulatedannual inputs averaged over the areas underindigenous f o r e s t s a n d u n i m p r o v e dgrasslands. Good agreement was obtained forthe forest inputs. The averaged inputs for thegrasslands were higher than those from thespecific sites (Table 1) (Tate et al., 1996) butthe agreement was nevertheless encouraging,given t h e w ide range o f cl imo/edaphicconditions represented in the areas underboth land systems, and the small number ofspecific sites used. An additional assessmentof within-site variability of C inputs at thegrassland sites was not possible because ofthe unreplicated soil profile data.

I t is likely from the long history of burningand overgrazing in New Zealand's indigenous

Total C input C input estimated from:

Total area S p e c i f i c sites t(Mty-I) ( t C

Table 1: Model estimates of annual inputs of organic carbon to 'topsoils' in New Zealand'sindigenous forests and unimproved grasslands.


grasslands ( A n o n . , 1 9 9 5 ) t h a t t h e s eecosystems have lost much soil carbon overthe past 150 years (Parshotam and Hewitt,1995). Th is process s t i l l continues today(Basher et al., 1990). I n addition, over thepast century and especially the past f ivedecades, native forest decline has occurred asa result of introduced herbivore (brushtailpossum, deer, goats) impacts (Rogers, 1995).Forest regeneration is also occurring, at leastpartly offsetting this loss in vegetation carbon.Although little information is available, apartfrom accompanying increased erosion in steephill country (Blaschke et al., 1992), loss of soilorganic matter is an inevitable consequenceof deforestation. Net losses of soil carbon occurwhen the ne t p r imary product ion o f anecosystem, and consequently plant residue-Cinput decreases. Based on recent studies (Tateet al., 1996), we could not expect to detect animbalance of less than ca 10% between totalC inputs and CO2 emissions for the total areaof forest and grassland (ca 12.5 Mha) (Table1). An imbalance of this magnitude (i.e., totalannual C inputs and losses of 113 Mt and 126Mt, respectively) could result in about 13 MtC being added to the atmosphere from forestand grassland soils, and would exceed currentestimates (Min is t ry for the Environment,1994) for CO„ emissions from use of fossil fuels.Large uncertainties currently exist in the netcarbon balance f o r the vegetation and soilsin N e w Zea lands n a t i v e f o res t s a n dgrasslands. I n contrast to the vegetationhowever, net emissions o f CO2 from soilsappear t o b e m o r e l i k e l y , becausesequestration rates of soil carbon are generallymuch slower than loss rates from land usechanges (Houghton, 1995).

Future CO2 emissionsFuture climate conditions were based onthe IS92a scenario f o r greenhouse gasemissions (Wigley and Raper, 1992), andfuture p a t t e r n s o f t e m p e r a t u r e a n dprecipitation for New Zealand (Mullan, 1994).The other parameters used in the model weretotal carbon inputs (Table 1), and the spatially-referenced soil carbon and clay content.

Estimates of the sensitivity of soil carbonto p r o j e c t i o n s o f t e m p e r a t u r e a n dprecipitation changes (Tate e t al . , 1996)suggested reductions of 5 - 6% for an increaseof 1°C i n global mean temperature f rompresent. These estimates are similar to the

reductions based on using a soil sequence asa spatial analogue of climate change (Tate,1992). Nationally, this reduction in soil carbonwould release between 120-150 Mt CO2-C fromenhanced decomposition of soil organic matter(Tate et al . , 1996), by the year ca 2037.Calculated on an annual basis fo r NewZealand's indigenous forests and grasslands,however, these emission rates from the effectsof global warming would only be ca 0.52 and0.76 M t Cy- ' respectively, assuming nocompensating effects from CO2 fertilizationand i n c r e a s e d n u t r i e n t a v a i l a b i l i t y.Consequently, CO2 emissions resulting fromenhanced decomposition of soil organic matterwith global warming could be considerablyless than those estimated as a consequence ofecosystem modif ications i n nat ive forestforests and grasslands.

There a r e severa l m a j o r sources o funcerta inty associated w i t h es t imat ingpotential CO, emissions due to changes in landuse or climate. First , there is uncertainty inthe future regional climate change scenarios(Mullan, 1994; Tate et al., 1996), which shouldbe reduced with improved capability to useregional climate information from GCM's tosimulate c l ima te v a r i a b i l i t y over N e wZealand. Second, there is uncertainty in thespatial extent of native forests and grasslands,and the spatial variability of soil C in theseecosystems is poorly understood. The mostserious source of uncertainty in the soil Cturnover model is the value used for therecalcitrant IOM fraction (Figure 1). Thisfraction can vary markedly between andwithin sites (Tate et al., 1995), possibly as aresult of the spatial variation in soil hydrology(K R Tate, N A Scott, A Parshotam and DRoss, unpublished results). Understandingthe proportion and variability of recalcitrantC in soils is essential for determining whethersoils w i l l behave as sources o r s inks o fatmospheric CO2 (Schlesinger, 1995). Finally,a fur ther major source o f uncertainty iswhether CO, fe r t i l i za t i on and increasednutrient availability wi l l cause soil organicmatter to accumulate (Tate et al. , 1996).There is no clear consensus on whetherorganic matter will increase or not (e.g. Diazet al., 1993), but in nutrient-limited naturalecosystems such as those reported here, theseeffects could be qui te small (Schlesinger,1995).


Despite the uncertainty, there is little doubtthat rates of soil respiration will increase withglobal warming. A 1% increase in soil CO2respired f r o m so i l s g l oba l l y w o u l d beequivalent to about 14% of the annual CO2flux to the atmosphere from fossil fuel use(Schlesinger, 1995). I n New Zealand, smallchanges in soil carbon wil l also have a largeeffect on t he na t i ona l carbon balance.Compared with global estimates, a potentiallymuch larger proportion of total annual CO,emissions in New Zealand could be sourcedto soi l changes from the ongoing human-induced ecosystem modifications o f nativeforests and grassland. T h e accuracy of thisassertion awai ts the resu l ts o f cur rentresearch on the carbon balance of these majorecosystems.

The combined effects on the atmosphere ofthese changes i n soil C , and enhanced soilrespiration f rom cl imate warming, couldrepresent substantial future additions to thetotal CO, emissions from the New Zealandland surface.

CONCLUSIONS

We analysed the effect of human-inducedecosystem modifications and global warmingon changes in soil organic carbon in NewZealand's nat ive forests and unimprovedgrasslands. Spatially referenced data for landuse, soil C and climate were used in a process-based soil carbon turnover model to simulateannual plant C inputs needed to maintaincurrent soil C stocks. U s i n g these inputs,climate change effects on soil carbon weresimulated based on the IS92a emissionsscenario (IPCC, 1990). The resulting increasein soil CO2 emissions, ranging between 120-150 Mt C per °C, was compared with thosefrom the potent ia l effects f rom on-goingecosystem modifications. Land use changefrom ongoing forest decline and grasslanddegradation may increase CO2emissions fromsoil by an amount comparable with emissionsfrom fossil fuel use, bu t confirmation w i l lrequire further research. Simulated losses ofsoil C of c a 1.3 M t Cy ' averaged over thenext 40 years are suggested for indigenousforests and unimproved grasslands; theselosses could be reduced from the combinedeffects o f CO2 fert i l izat ion and increasednutr ients released f rom organic matter,

although these effects are likely to be smallfor natural ecosystems. Large uncertaintiesexist in these estimates, but increases in soilCO2 emissions f rom one o r both o f thesemechanisms could reduce considerably thebenefits to the national C balance o f CO2sequestration by planted forests i n NewZealand. F u t u r e work must provide morerobust cl imate change scenarios for NewZealand, and a better understanding of spatialand temporal variation in the net primaryproduct iv i ty o f indigenous forests a n dgrasslands.

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ACKNOWLEDGMENT

We wish to thank Dr NA Scott of LandcareResearch for his valuable suggestions in thepreparation of this paper.

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