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Dynamics of root systems in native grasslands: effects of elevated atmospheric CO2

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Dynamics of root systems in native grasslands: effects of elevated atmospheric CO2
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  RESEARCH  New Phytol  . (2000),  147 , 73–85 Dynamics of root systems in nativegrasslands: effects of elevated atmosphericCO  J. A. ARNONE III  ,  *, J. G. ZALLER  ,  , E. M. SPEHN  , P. A. NIKLAUS  ,C. E. WELLS     C. KO          RNER  Department of Integrative Biology ,  University of Basel – Botanical Institute , Scho     nbeinstrasse 6 ,  CH-4056 Basel  ,  Switzerland   Desert Research Institute ,  Division of Earth and Ecosystem Sciences , 2215 Raggio Parkway ,  Reno ,  NV 89512  ,  USA  The Ecology Center ,  Utah State University ,  Logan ,  UT 84322–5230 ,  USA  Department of Environmental and Resource Sciences ,  University of Nevada - Reno ,  Reno , NV 89557  ,  USAReceived 4 November 1999; accepted 8 March 2000  The objectives of this paper were to review the literature on the responses of root systems to elevated CO   in intact,native grassland ecosystems, and to present the results from a 2-yr study of root production and mortality in anintact calcareous grassland in Switzerland. Previous work in intact native grassland systems has revealed thatsignificant stimulation of the size of root systems (biomass, length density or root number) is not a universalresponse to elevated CO  . Of the 12 studies reviewed, seven showed little or no change in root-system size underelevated CO  , while five showed marked increases (average increase 38%). Insufficient data are available on theeffects of elevated CO   on root production, mortality and life span to allow generalization about effects. Thediversity of experimental techniques employed in these native grassland studies also makes generalization difficult.In the present study, root production and mortality were monitored  in situ  in a species-rich calcareous grasslandcommunity using minirhizotrons in order to test the hypothesis that an increase in these two measures would helpexplain the increase in net ecosystem CO   uptake (net ecosystem exchange) previously observed under elevatedCO   at this site (600 vs 350  µ l CO   l −  ; eight 1.2-m   experimental plots per CO   level using the screen-aided CO  control method). However, results from the first 2 yr showed no difference in overall root production or mortalityin the top 18 cm of soil, where 80–90% of the roots occur. Elevated CO   was associated with an upward shift inroot length density: under elevated CO   a greater proportion of roots were found in the upper 0–6-cm soil layer,and a lower proportion of roots in the lower 12–18 cm, than under ambient CO  . Elevated CO   was also associatedwith an increase in root survival probability (RSP; e.g. for roots still alive 280 d after they were produced underambient CO  , RSP  0.30; elevated CO  , RSP  0.56) and an increase (48%) in median root life span in thedeepest (12–18 cm) soil layer. The factors driving changes in root distribution and longevity with depth underelevated CO   were not clear, but might have been related to increases in soil moisture under elevated CO  interacting with vertical patterns in soil temperatures. Thus extra CO   taken up in this grassland ecosystem duringthe growing season under elevated CO   could not be explained by changes in root production and mortality.However, C and nutrient cycling might be shifted closer to the soil surface, which could potentially have asubstantial effect on the activities of soil heterotrophic organisms as CO   levels rise.Key words: root turnover, root production, root mortality, CO   enrichment, global change, species-rich grassland,missing carbon, root life span.  The central aims of this paper are to synthesize thecurrent literature on the responses of root systems of intact,nativegrasslandecosystemstoelevatedatmos- *Author for correspondence (fax   1 775 673 7485; e-mail jarnone  dri.edu). pheric CO  , with special emphasis on root dynamics;and to present empirical results from the first 2 yr of a study of root dynamics (especially of productionand mortality), conducted in a species-rich lowlandcalcareous grassland in Switzerland, exploring thepossibility that increased root mortality might helpto explain the fate of the ‘extra’ carbon apparently   7  4   RE S E AR CH   J    . A  . Ar  no n e I  I  I    e t   al     . Table 1.  Effects of elevated CO   ( mean difference as percentage change from ambient CO  )  on fine - root biomass ,  length density production and mortality in native  (in situ) or intact  ( monoliths )  grasslands GrasslandExperimentalsystemCO   treatments( µ l CO   l −  ) andenvironmentalconditions*Stage of experimentObservedsoil depth(cm) MethodStanding rootbiomass, numberor lengthRootproductionRootmortality ReferenceTallgrass prairieC   Kansas,USA In situ  345, 685ventilatedclosedchambersSecond yr 0–40 Coring No difference Mo  et al  . (1992)Tallgrass prairieC   C   mixKansas, USA In situ  350, 700 OTC First 2 yr 0–15 Ingrowth cores   125% (Year 1)  17% (Year 2)Owensby  et al  .(1993)Shortgrasssteppe C   or C  Colorado,USALarge cores 350, 700 growthchambers(PFD  550 µ mol m −   s −  )Second growingseason0–45 Entire coreharvested  23% (C  )No difference (C  )Hunt  et al  .(1996)MediterraneanannualMontpellier,FranceMonoliths 350, 700glasshouse (fullsun)First growingseason0–20 Entire monolithharvestedNo difference Navas  et al  .(1995)Pasture turf New ZealandMonoliths 350, 700 growthchambers(PFD  480 µ mol m −   s −  )217 d 0–25 Coring   50% Newton  et al  .(1994)Pasture turf (sandy)New ZealandMonoliths 350, 525, 700growthchambers(PFD  500 µ mol m −   s −  )324 dAdditional 105 d(to 429 d)0–30 Sequential coresCoring   39% (wet soil)  21% (dry soil)350–700 ppm CO   72% 350–525  0% 525–700Newton  et al  .(1995)Newton  et al  .(1996)  RE S E AR CH C O         a n d  n a t   i    v e   gr  a s  s  l    a n d r oo t   s   y s  t   e  m s   7   5   Natural turf  Nardus Juncus on peaty gleyUKMonolith 350, 600glasshouse (fullsun)Two growingseasons0–25 Entire monolithharvested  minirhizotrons  41%   20%(cumulativebirths)  21%(cumulativedeaths)Fitter  et al  .(1996, 1997)CalcareousUKMonoliths 350, 600glasshouse (fullsun)Two growingseasons0–25 Coring  minirhizotrons  48%   30%   29% Fitter  et al  .(1996, 1997)CalcareousSweden In situ  360, 700 OTC 3 yr 0–20 Minirhizotrons   25% (dry years)No difference (wetyears)Sindhøj  et al  .(2000)CalcareousSwitzerland(550 m) In situ  350, 600 SACC First 4 yrYears 1 and 20–80–18CoringMinirhizotronNo differenceNo difference No differenceYear 1No differenceYear 1Leadley  et al  .(1999); Arnone et al  . (this study)CalcareousSwitzerland(550 m)Monoliths 350, 600glasshouse (fullsun)Second growingseason0–20 Coring  entiremonolithNo difference Sto    cklin  et al  .(1998)Salt marshMaryland,USA In situ C   ( Scirpus )C   ( Spartina )Mixed C   C  343, 681 OTC Second yr 0–15 Ingrowth cores   83% (C  )No difference(C  )No difference(mixed)Curtis  et al  .(1990)AlpineSwitzerland(2480 m) In situ  350, 600 OTC Year 2 (cool)Year 3 (warmer)Year 50–100–20Ingrowth coresSmall andingrowth coresLarge coresNo difference(Year 3)  12% (Year 5)No difference(Year 2)  27% (Year 3)Scha    ppi &Ko    rner (1996)Ko    rner  et al  .(1997)*PFD, photon flux density; OTC, open-top chamber; SACC, screen-aided CO   control.  76  RESEARCH  J  .  A .  Arnone III   et al.taken up by ecosystems under elevated CO   duringthe growing season (Stocker  et al  ., 1997). Theliterature on the effects of CO   enrichment on rootsystems of individual grass species growing innonnative systems (e.g. agricultural fields) or underrelatively artificial conditions (e.g. single plants inpots on potting mix, or planted communities onhomogenized soils) has been reviewed extensively byRogers  et al  . (1994, 1996) and by Pritchard & Rogers(2000).Nonmanaged and extensively managed nativegrasslands of the world account for more than half of the grassland biome, and store tremendous amountsof C in their soils (Schimel, 1995). On a global scale,the grassland biome accounts for 25% of allterrestrial land surface and 10% of global C stores(mostly in soils; Schimel, 1995; Schlesinger, 1997).Because most (60–90%) of the net primary pro-ductivity in grassland systems occurs below ground,and root biomass pools often exceed above-groundstanding biomass pools by as much as two to fivetimes (Speidel, 1976; Stanton, 1988), the contri-bution of root litter to the pool of soil organic mattercould greatly exceed the contribution of shoot litter.Thus the way in which root systems of nativegrasslands respond to elevated atmospheric CO   willprobably play a large role in defining ecosystem Ccycling in the future, and in determining thepotential for grassland ecosystems to sequester C.The literature on responses of intact nativegrassland root systems to elevated CO   is limited. Of the 12 studies we identified (Table 1; with the studyby Hunt  et al  . counted twice because of separateevaluation of C   and C   systems, but not includingthe study presented in the current paper), six wereconducted in the field ( in situ ), and six in intactmonoliths that were removed from the field andexposed to CO   treatments in glasshouses or growthchambers. With so few studies, and with so manydifferent experimental methodologies (CO   levels;study duration; soil depth sampled; samplingmethod; parameters measured; Table 1), it isextremely difficult to discern general patterns of CO  response, even with respect to the two most commonmeasures of root response – standing biomass, lengthor numbers (size) of roots (all 12 studies); and rootproduction (five studies). Seven of the 12 studiesreported little (  20%) or no increase in the size of root systems under elevated CO   (C   grasslands: Mo et al  ., 1992, 0%; Hunt  et al  ., 1996, 0%; C  grassland: Navas  et al  ., 1995, 0%; Sto    cklin  et al  .,1998, 0%; Leadley  et al  ., 1999, 0%); Scha    ppi &Ko    rner, 1996 and Ko    rner  et al  ., 1997,  6%, 2-yrmean; Sindhøj  et al  ., 2000,  12%, 2-yr mean). Theaverage stimulatory response to elevated CO   inthese low-responding systems (excluding the C  grassland of Mo  et al  ., 1992 and Hunt  et al  ., 1996) isapprox.  4%. The average stimulatory effect of theother five studies reporting stronger effects was 38%(C   grassland: Hunt  et al  ., 1996,   23%; Newton et al  ., 1994,   50%; Newton  et al  ., 1996,   30%,2-yr mean; Fitter  et al  ., 1996, 1997,  41 and  48%in  Nardus   Juncus  turf and calcareous grassland,respectively). The explanation for low- versus high-responding systems is unclear, but might be relatedto length of growing season or overall soil resourceavailability. For example, four of the seven low-responding systems were conducted in the field(possibly a shorter growing season), whereas all thehigh responding systems were conducted in growthchambers or glasshouses (possibly a longer growingseason). This last piece of speculation makes theneed for more field studies abundantly clear. Finally,only two studies (Fitter  et al  ., 1996, 1997) havedirectly measured the effects of elevated CO   on rootmortality in intact native grasslands (  21 and  29%). No complete data set exists for nativegrasslands that addresses the dynamic responses of roots (production, mortality and fluctuations in root-length density) to elevated atmospheric CO   in situ .Yet the available data clearly indicate that under-standing the dynamic responses of grassland rootsystems to elevated CO   is key to understandingecosystem and global C cycling (Curtis  et al  ., 1994;Jackson  et al  ., 1996, 1997).Two almost universal sets of responses of plantsand plant communities to elevated atmospheric CO  are(1)stimulationofgrowing-seasonleafandcanopyphotosynthesis and net ecosystem CO   exchange(NEE; Jackson  et al  ., 1994; Wolfenden & Diggle,1995; Drake  et al  ., 1996; Owensby  et al  ., 1996;Stocker  et al  ., 1997); and (2) a reduction in leaf diffusive conductance (Farquhar & Wong, 1984;Morison & Gifford, 1984; Morison, 1985; Tyree& Alexander, 1993; Knapp  et al  ., 1996; Lauber &Ko    rner, 1997) and some reduction in plant canopyevapotranspiration (Jackson  et al  ., 1994; Field  et al  .,1995; Samarakoon & Gifford, 1995; Owensby  et al  .,1997; Stocker  et al  ., 1997; Arnone & Bohlen, 1998).CO  -induced increases in net ecosystem CO   ex-changehavealsobeenassociatedwithincreasesinnetprimary productivity (NPP, net biomass productionplus all biomass losses, e.g. litter, exudation, herbi-vory). Numerous studies have reported that muchof the increase in NPP might occur below ground, asevidenced by increases in allocation to root systems(Rogers  et al  ., 1994) and soils via increases in live-root rhizodeposition (e.g. cell sloughing, exudation(O’Neill, 1994; Cardon, 1996; Darrah, 1996) andfine root mortality (Pregitzer  et al  ., 1995; Canadell  etal  ., 1996; Fitter  et al  ., 1997). The positive effect of high CO   on overall soil water availability (Knapp  etal  ., 1996; Owensby  et al  ., 1997; Arnone & Bohlen,1998; Niklaus  et al  ., 1998b) might also stimulateabove-ground (ANPP) and below-ground (BNPP)productivity by enhancing soil nutrient availability,particularly in seasonally dry grassland systemswhere high CO   might extend water availability into  RESEARCH  CO   and native grassland root systems  77dry periods (Hungate  et al  ., 1997a). In fact, increasesinsoilmoistureunderelevatedCO  mightcontributemost to the stimulation of biomass production underelevated CO   in natural ecosystems (Hungate  et al  .,1997b; Owensby  et al  ., 1997; Leadley  et al  ., 1999).We began our root dynamics studies in 1994 withhopes of explaining the fate of the ‘extra’ Capparently fixed during the growing season byecosystems under elevated CO   (Stocker  et al  ., 1997)by finding increased root mortality under elevatedCO  . Since then only two other studies have assessedthis directly, using minirhizotrons. Pregitzer  et al  .(1995) reported a 250% increase in fine rootmortality in young stands of   Populus  under elevatedCO   in plots with no additional N, while Fitter  et al  .(1996, 1997) found a 29% increase in root mortalityin intact monoliths of calcareous grassland and a21% increase in native turf on peaty soil.The objectives of the present study were todetermine: (1) whether root mortality increases innative calcareous grassland communities (Leadley &Korner, 1996; Huovinen-Hufschmid & Ko    rner,1998) exposed to elevated atmospheric CO   in thefield, and whether this increase could help to explainthe apparent sizable increases in net ecosystem CO  exchange observed during the growing season atelevated CO   (Stocker  et al  ., 1997; but also seeNiklaus  et al  ., 2000); (2) whether elevated CO   altersthe size and vertical distribution of plant-communityroot-length density (RLD) and how this variesacross seasons; and (3) whether root longevity isaffected by high CO   and by depth. By addressingthese objectives we aimed to elucidate and quantifyroot-system responses that might affect ecosystemfunction (e.g. C and nutrient cycling).    Field site and experimental design A full description of the lowland grassland site(500–530 m above sea level) in northwesternSwitzerland and details of the experimental design,are given by Leadley & Ko    rner (1996) and Leadley et al  . (1999). We used 16 1.2-m   research plotscontaining intact, native, species-rich grasslandcommunities. Each plot was equipped with low-stature screen-aided CO   control windscreens(Leadley  et al  ., 1997) for controlling atmosphericconcentrations. Beginning 19 March 1994, CO  concentrations in eight of these plots were main-tained at 360  µ l CO   l −   and in the remaining eight at600  µ l CO   l −  . The plots were arranged in arandomized complete block design over a 0.2-ha areaon a south-facing 20   slope. The aspect of the slopecombined with warm summer temperatures (Fig. 1)resulted in high evapotranspiration and, combinedwith the relatively shallow, calcareous soil (transitionRendzina with a 15 cm topsoil underlain with anextremely rocky subsoil: Ogermann  et al  ., 1994),contributed to strong topsoil drying in June–August(Niklaus  et al  ., 1998b). Plant communities in eachplot consisted of an average of 31 plant species(graminoids, forbs and leguminous forbs). The grassspecies Bromuserectus dominatesthesite(Houvinen-Hufschmid & Ko    rner, 1998), which for manydecades before 1993 was grazed by cattle (Schla    pfer et al  ., 1998). From early 1993, however, the parcel of land used in our study was fenced off from grazingand mown (clipped) twice a year, in June andOctober. The switch to clipping had very little effecton plant-species composition or abundance(Schla    pfer  et al  ., 1998).In late February 1994 we installed one transparentbutyrate minirhizotron tube 5 cm in diameter  100cm long in each experimental plot at an angle of  c . 35   to the plane of the soil surface. The tubeswere inserted through the 15-cm deep A horizon andintotheupper3cmoftherockysubsoil.Thisallowedus to observe roots in the top 18 cm of soil, the layerin which 80–90% of all roots occur (J. Arnone,unpublished). Before installing the tubes we etchedan observation track (18 mm wide  54 cm long) onthe outside upper surface of each tube. We thendivided each track into 45 frames 12 mm high and 18mm wide. The tube bottoms were capped beforeinsertion into pre-cored cylindrical holes, and thetop 10 cm of each tube wrapped in opaque tape andstoppered to prevent light penetration and entry of debris and insects.We were unable to distinguish among roots of themore than 30 species growing in each experimentalplot and thus only considered responses of the rootsystem of the entire plant community. On 28 April1994, we recorded video images of roots in all 45frames in each minirhizotron using a Bartz BCT-2Minirhizotron Camera (Bartz Technology Co.,Santa Barbara, CA, USA) attached to a Hi-8 SonyCamcorder (all mounted on a backpack). We re-peated this on 15 more dates up to 6 April 1996. All45 frames were used to quantify RLD through thesoil profile. This was accomplished by viewingundigitized video tapes and counting intersects withgridlines drawn on an overhead transparency andplaced over the video monitor (Tennant, 1975). Wealso digitized three of the 45 video images per tubefor each of the first 10 sampling dates (first year of CO   treatment) to quantify root production andmortality (based on both number of roots and rootlength). On each sampling date we recorded eachnew root that appeared in (production) and each rootthat disappeared from (mortality) the three frames,which were located at depths of 3, 9 and 15 cm, andwere chosen to represent the 0–6, 6–12 and 12–18 cmsoil layers, respectively. We alsomeasuredthe lengthand diameter of new and disappearing roots using
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