Geographic variation in growth and phenology of two dominant central US grasses: consequences for climate change

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1 Journl of Plnt Ecology Volume 7, Number 3, Pges June 214 doi:1.193/jpe/rtt36 Advnce Access publiction 7 August 213 vilble online t Geogrphic vrition in growth nd phenology of two dominnt centrl US grsses: consequences for climte chnge Amnd L. Giulini 1, Eugene F. Kelly 2,3 nd Aln K. Knpp 1,3, * 1 Deprtment of Biology, Colordo Stte University, Fort Collins, CO 8523, USA 2 Deprtment of Soils nd Crops Science, Colordo Stte University, Fort Collins, CO 8523, USA 3 Grdute Degree Progrm in Ecology nd Deprtment of Biology, Colordo Stte University, Fort Collins, CO 8523, USA *Correspondence ddress. Deprtment of Biology, Colordo Stte University, Fort Collins, CO 8523, USA. Tel: ; F: ; E-mil: knpp@colostte.edu Abstrct Aims The rte of climte chnge my eceed mny plnt species migrtion rtes, prticulrly for long-lived perennil species tht dominte most ecosystems. If bioclimtic envelopes shift more rpidly thn dominnt species cn migrte, individuls locted peripherl to biomes or in djcent biomes my become significnt source of trits for future dominnt popultions (DPs). Thus, trits of individuls from peripherl popultions (PPs) my ffect future ecosystem functioning more thn those of tody s DPs. Methods We ssessed key trits of individuls collected from popultions tht currently dominte two centrl US grsslnds, the shortgrss steppe (Boutelou grcilis) nd the tllgrss pririe (Andropogon gerrdii). We compred these to individuls from PPs in reciprocl-trnsplnt common grden eperiment with grdens t the Shortgrss Steppe Long Term Ecologicl Reserch site in Colordo nd the Konz Pririe Biologicl Sttion Long Term Ecologicl Reserch site in Knss. DPs nd PPs were subjected to high nd reduced wter vilbility in common grdens locted in ech biome. Trits mesured included the following: individul plnt biomss, reproductive lloction, specific lef re (SLA) nd plnt wter reltions. We focused on the climte-chnge relevnt comprisons of trits from PPs versus DPs epressed under the climte of DPs. Importnt Findings PPs of B. grcilis differed from DPs primrily in phenologicl trits. Under semirid shortgrss steppe climte, PPs initited flowering lter in the seson, produced fewer reproductive tillers nd were more sensitive to wter stress. Biomss differences between popultions were miniml. For A. gerrdii, biomss in PPs ws 5% lower thn in DPs under the mesic tllgrss pririe climte nd reproductive tillers were considerbly smller, despite higher SLA in PPs. Biomss of PPs ws less sensitive to wter stress, however. From these results, we conclude tht key trits of PPs differed from DPs in both grsslnd types, but potentil effects on reproductive phenology were greter for the bioclimtic shift in which mesic biome becomes rid, wheres boveground productivity my be ffected more when semirid biome becomes more mesic. Keywords: grsslnd, phenology, productivity, trits, wter stress Received: 16 Jnury 213, Revised: 1 My 213, Accepted: 27 June 213 Introduction Biomes re defined by rnge of climtic prmeters (principlly temperture nd precipittion) nd chrcterized by reltively few dominnt (or foundtion) plnt species (Ellison et l. 25; Grime 1998; Whittker 1965). Such dominnt species, though widely distributed, cn be reltively rre tonomiclly. In grsslnds, e.g. only c. 5% of the pproimte 11 species in the Pocee re cpble of functioning s dominnt species (Edwrds et l. 21). Nonetheless, dominnt species contribute most to the totl plnt biomss in n ecosystem nd their trits re closely linked to ecosystem function (Arssen 1997; Grime 1998; Huston 1997). Thus, dominnt species will determine mny community nd ecosystem responses to environmentl chnge (Smith nd Knpp 23). Although dominnt species re epected to be bundnt throughout their biomes, their physiogrphic rnges lmost lwys etend fr beyond where they function ecologiclly The Author 213. Published by Oford University Press on behlf of the Institute of Botny, Chinese Acdemy of Sciences nd the Botnicl Society of Chin. All rights reserved. For permissions, plese emil: journls.permissions@oup.com

2 212 Journl of Plnt Ecology s dominnt species. For emple, Andropogon gerrdii (big bluestem) is dominnt grss throughout much of the tllgrss pririe in the centrl USA, but this species cn be found 8 km west of this grsslnd type nd throughout much of the estern two-thirds of the USA ( usd.gov). Thus, it my be useful to distinguish between popultions found in res where they function s the dominnt species (dominnt popultions [DPs]) from those popultions where they re simply components of the flor (peripherl popultions [PPs]). Not surprisingly, PPs re less often studied thn DPs becuse they re minor components of the community or re only loclly bundnt. Ltitudinl nd ltitudinl rnge shifts hve been documented for numerous species s result of climte chnge (Prmesn nd Yohe 23; Root et l. 23; Wlther et l. 22), with these predicted to be more prevlent in the future (Morin et l. 28). Although dynmic globl vegettion models hve been limited in their bility to ccurtely ssess plnt migrtion nd locl vegettion chnge (Neilson et l. 25; Thuiller et l. 28), most studies suggest tht plnt migrtion rtes fster thn those observed during post-glcil times will be required to keep pce with the projected rte of globl climte chnge (Mlcolm et l. 22; Solomon nd Kirilenko 1997), prticulrly in biomes with little topogrphic relief (Jump et l. 29; Lorie et l. 29). If present-dy popultions of dominnt species cnnot keep pce with shifting climtic envelopes, those popultions tht dominte future (geogrphiclly shifted) biomes re more likely to reflect trits of individuls from PPs lredy present rther thn those from the DPs left behind in other words, trits from PP individuls my be more importnt thn those of DPs for predicting ecosystem function s biomes epnd into new res. For emple, with forecsts of continued wrming nd ltered precipittion mounts nd regimes in the Gret Plins (Giorgi nd Diffenbugh 28; IPCC 27), the climtic envelope tht defines the current distribution of the shortgrss steppe ecosystem (dominted by Boutelou grcilis, blue grm) my etend estwrd leving behind migrtion-limited DP individuls. Thus, those PP individuls tht lredy occur frther est would serve s the founder popultion for future shortgrss steppe ecosystems dominted by B. grcilis. If individuls from DPs nd PPs re functionlly similr, then there my be little ltertion in ecosystem function s biome distributions shift. But if they re functionlly distinct, then the new DPs (derived from former PP individuls) my drive importnt chnges in ecosystem function nd behvior, t lest until locl dpttion occurs (Jump nd Penuels 25). In this study, we compred trits from geogrphiclly distinct DP nd PP individuls for two dominnt C 4 Gret Plins grsses, B. grcilis nd A. gerrdii (big bluestem). We used reciprocl-trnsplnt common grden pproch to ddress this issue (Clusen et l. 1941; Turesson 1922) nd focused on few key morphologicl nd whole plnt trits. We collected individuls from popultions of B. grcilis nd A. gerrdii tht currently dominte their respective grsslnd types or re found in peripherl ecosystems, set up common grdens for both species nd subjected ll popultions to two levels of wter vilbility t ech grden. This design llowed us to ssess differences between individuls collected from DPs nd PPs nd their responses to wter stress under common environmentl conditions. Although reciprocl-trnsplnt eperiments represent well-estblished pproch for ssessing dpttion nd responses to locl environmentl conditions (e.g. Brdshw 1984; Linhrt nd Grnt 1996), in this cse, our focus ws on climte-chnge relevnt comprisons of trits of individuls from present-dy PPs versus those from DPs, when grown together in n environment pproprite for tht species to dominte. Moreover, lthough drying in generl is forecst for much of this region of the world, some res re predicted to become wetter (Chou et l. 29). Thus, this reciprocl design llowed us to focus on scenrios of both mesic PPs influencing future eric grsslnds nd eric PPs impcting future mesic grsslnds. We hypothesized tht individuls from DPs nd PPs of both species would hve trits tht differed nd tht responses to wter stress would lso differ. We lso epected tht popultions originting from more wter-stressed environments (DPs of B. grcilis nd PPs of A. gerrdii) would be impcted reltively less by wter stress compred to popultions from more mesic environments. Mterils nd Methods We ssessed key trits in individuls from PPs nd DPs of B. grcilis nd A. gerrdii in reciprocl trnsplnt common grden eperiment with two levels of wter tretments, s pproprite for lrgely wter-limited grsslnd region (Humn et l. 24). Our focus ws on the difference between PPs nd DPs when grown in environments where the species currently domintes. We were prticulrly interested in the following two scenrios: (i) mesic grsslnd (tllgrss pririe in Knss) drying to the etent tht it becomes semirid nd thus B. grcilis might be epected to dominte (with trits from PPs currently in Knss). And (ii) semirid grsslnd (shortgrss steppe in Colordo) becoming more mesic nd thus A. gerrdii (with trits from PPs currently in Colordo) would be epected to dominte. Although the former is more likely to occur thn the ltter in this prticulr region, there re res of the world tht re forecst to become wetter with climte chnge (Chou et l. 29; IPCC 27), providing relevnce for the ltter scenrio. Popultion nd grden site descriptions Individuls from DPs nd PPs of A. gerrdii nd B. grcilis were collected from n rry of undisturbed sites in Colordo nd Knss, USA (Tble 1). A totl of 12 popultions were smpled, si for ech species (three DP nd three PP). Popultions were collected from rnge of sites to cpture nturl vribility within the popultions. For B. grcilis, DPs were collected from different sites tht vried in grzing intensity t the

3 Giulini et l. Geogrphic vrition nd climte chnge 213 Tble 1: generl description of the collection sites for the DPs nd PPs of the semirid grsslnd dominnt B. grcilis nd the tllgrss pririe dominnt A. gerrdii DPs B. grcilis From Colordo From Knss Ltitude N N Longitude W W 29 growing 254 mm 528 mm seson precipittion Men growing 18.6 C 23. C seson temperture 29 growing 17.2 C 21.2 C seson temperture A. gerrdii From Knss From Colordo Ltitude N N Longitude W W 29 growing 528 mm mm seson precipittion Men growing 23. C 18.5 C seson temperture 29 growing seson temperture 21.2 C 18.9 C Men growing seson precipittion nd temperture re 3-yer verges (1971 2) for the months of My through September. 29 dt re shown s contet for the common grden results. Averged between Fort Collins, Colordo nd Boulder, Colordo (where popultions were collected). Shortgrss Steppe Long Term Ecologicl Reserch site in Weld County, Colordo (SGS, see description below). The PPs for this species cme from the Konz Pririe Biologicl Sttion Long Term Ecologicl Reserch site in Gery County, Knss (KNZ, description below); to increse vribility in trits, ech popultion ws collected from different KNZ wtershed tht vried in grzing regime nd long-term fire history. For A. gerrdii, the DPs were locted t KNZ (lso in different wtersheds with different grzing nd burning histories), nd the PPs of this species were locted long the Front Rnge of Colordo, two from Boulder County nd one from Lrimer County. These popultions were found within 7 km of ech other. Trnsplnts long with some residul soil to include mycorrhize nd other importnt soil biot were moved to two common grdens t the strt of the 28 growing seson. The SGS common grden, locted t the Centrl Plins Eperimentl Rnge, ws in semirid grsslnd locted in northestern Colordo (4 49 N, W). SGS is primrily dominted by the C 4 grss B. grcilis, with other mjor species including Buchloe dctyloides (bufflo grss), Artemisi frigid (fringed sgewort), Spherlce coccine (scrlet PPs globemllow) nd Opunti polycnth (plins prickly per; Luenroth nd Burke 28). SGS hs men nnul precipittion of 321 mm (Luenroth nd Sl 1992) nd men nnul temperture of 8.6 C (Milchuns nd Luenroth 1995). Averge boveground net primry productivity (ANPP) is estimted t c. 1 g m 2 (Luenroth nd Sl 1992), with B. grcilis comprising up to 9% of the totl biomss (Luenroth et l. 1978). The grden ws estblished ner the site hedqurters on soil with cly lom teture (top 2 cm), which is typicl of SGS (Luenroth nd Burke 28). The KNZ common grden ws estblished t the hedqurters re of the site, which is ntive tllgrss pririe locted in the Flint Hills of northestern Knss (39 5 N, W). KNZ encompsses 3487 hectres of ntive tllgrss pririe nd is dominted by C 4 grsses, primrily A. gerrdii Vitmn nd Sorghstrum nutns (L.) Nsh (Freemn 1998). Averge ANPP t KNZ is c. 42 g m 2 (Knpp et l. 1998), with A. gerrdii comprising up to 8% of the totl (Smith nd Knpp 23). The climte consists of cold, dry winters nd wrm, wet summers (Hyden 1998). Men nnul precipittion is 835 mm, nd men nnul temperture is 13 C. The grden ws estblished on soil with silt lom teture (top 2 cm), which is typicl of KNZ (Knpp et l. 1998). Popultion collection nd common grden estblishment From ech popultion, 3 individuls were collected, 15 for ech grden. In these highly clonl species, we defined n individul s ll tillers locted within 2-3-cm dimeter circle within monospecific stnd of the trget species. We collected ll roots nd rhizomes to 15 2-cm soil depth nd ll boveground tissue. Despite the strongly clonl hbit of these species, it is possible tht more thn one genotype ws included in ech individul smple (Avolio et l. 211). Smples were stored in cool, moist environment until they were trnsplnted (within 2 weeks of collection). Trnsplnt survivl success ws >95%. To estblish the common grdens, 1 1-m re ws tilled to 25 cm nd the perimeter ws fenced to eclude grzers. Plstic lwn edging ws formed into 39-cm dimeter circles nd inserted 1 cm into the ground to isolte nd contin the individuls. We trnsplnted 144 individuls into these circulr plots in ech grden; 12 individuls of B. grcilis nd A. gerrdii from ech of the 12 popultions. Trnsplnts were rryed in 6 6 Ltin squre design repeted once for ech species this llowed us to subject hlf the trnsplnts to wter vilbility tretment in the second yer. Intct rhizomes nd tillers were seprted from other vegettion initilly to estblish monospecific stnds in ech plot, nd we removed other species throughout both growing sesons vi weeding by hnd. Three etr individuls from ech popultion were plced round the perimeter of the grdens to provide replcements in the cse of trnsplnt mortlity. The 28 growing seson ws dedicted to the estblishment,

4 214 Journl of Plnt Ecology growth nd cclimtion of the popultions, thus ll plnts were wtered frequently to fcilitte survivl nd growth. In order to crete wet versus dry tretment t ech site, we ecluded precipittion to hlf of the KNZ grden nd irrigted hlf of the SGS grden in 29. Four soil moisture probes were plced in ech hlf of ech grden to monitor tretments. At SGS, mbient conditions constituted the dry side of the grden. We irrigted the other hlf of the grden to reduce wter stress; ech individul received wter twice dily vi drip irrigtion. The mount of wter dded vried depending on rinfll nd the wter sttus of the plnts. Conversely t KNZ, mbient conditions constituted the wet side of the grden, nd rin eclusion shelter ws erected over hlf the grden to reduce soil moisture. The shelter consisted of cler polyethylene greenhouse mteril stretched cross wooden frme ~2 m bove the plnts (similr to the design described in Fy et l. 2). The roof deflected most rinfll off the plot nd could be removed if the plots becme too dry. However, due to the winter rechrge of moisture in the deep soils t this site, the roof remined intct over hlf the grden for most of the growing seson. The shelter ws designed with open sides so tht ltertions to ir movement, temperture nd reltive humidity were minimized. The roof ws slnted to fcilitte wter run off nd the plstic sheeting reduced the trnsmission of light by 19%. In generl, differences in soil moisture between wet versus dry tretments incresed over the growing seson nd were greter t SGS thn the KNZ grden, likely becuse the deep soils t KNZ were nerly sturted prior to erecting the rin eclusion shelter. Averge growing seson soil moisture ws 28.2 ±.5% in the wet versus 17.6 ±.9% in the dry tretment t SGS (P <.5) nd 29. ±.4% in the wet versus 24.7 ±.7% in the dry tretment t KNZ (P <.5). The sesonl mimum tretment differences observed t ech grden were 23.2% t SGS nd 9.3% t KNZ. Lef wter potentil (LWP) mesurements (predwn nd mid-dy) confirmed tht these two grsses eperienced different levels of wter vilbility in both grdens (Tble 2) with the more shllow rooted B. grcilis generlly responding more strongly to the wter tretments thn the deeper rooted A. gerrdii. Dt collection For ll sites (the two grdens nd the collection sites for the 12 popultions), soil teture nd ph were determined from composite smples of three 1-cm deep soil cores (Colordo Stte University Soil, Wter nd Plnt Testing Lbortory). In both grdens in 29, soil moisture (volumetric wter content in the top 2 cm) ws estimted t pproimtely weekly intervls with ECH 2 O soil moisture sensors (Decgon, Pullmn, WA, USA). Eight probes were plced in eight plots (four ech in the dry nd wet portions) rryed in two trnsects through ech grden (northest to southwest nd northwest to southest). Soils in ech grden rnged in ph vlue from nd most of the popultion collection sites fell within this rnge, with soil teture vrying from lom to sndy lom. Tble 2: sesonl miniml LWP when differences between wtering tretments were generlly the lrgest for the growing seson SGS grden KNZ grden B. grcilis A. gerrdii B. grcilis A. gerrdii Predwn wet (MP).87 ±.2.24 ± ± ±.8 NS.34, 6.3.7, , 6.79 Dry (MP).71 ±.2.43 ± ± ±.8 Mid-dy wet (MP) 2.4 ± ± ± ±.11.42, , , NS Dry (MP) 2.99 ± ± ± ±.11 Abbrevition: NS = not significnt. Wter potentils were verged cross popultions to determine if wtering tretments were effective (n = 3). Where results were significnt, the P vlue nd F-sttistic re provided (P vlue, F-sttistic). Sttisticl significnce between wtering tretments t P =.5 level. There were no consistent ptterns noted mong the 12 collection sites or the two grdens in other soil chrcteristics. LWP ws mesured for ech species t 3-week intervls throughout the growing seson (PMS 1 pressure chmber). For both predwn nd mid-dy estimtes, single leves were mesured (n = 3 from ech popultion for ech tretment) from both wet nd dry portions of the grden. At the end of the seson, biomss ws mesured by hrvesting ll individuls in 1 1-cm qudrts, drying the biomss t 6 C for t lest 3 dys nd weighing to the nerest.1 g. To determine reproductive lloction, reproductive biomss ws seprted nd divided by totl biomss to clculte the proportionl lloction. Additionlly for A. gerrdii, 1 individul tillers (both reproductive nd vegettive) were rndomly selected in ech plot, hrvested, dried nd weighed. The totl number of flowering tillers in ech plot ws censused every 2 3 weeks throughout the growing seson for both species to ssess flowering phenology. Additionlly, si stges of reproductive phenology were monitored in ech plot every 2 3 weeks for A. gerrdii (emergent reproductive tiller, covered flower, flower 3 cm eposed, fully eposed flower, nthesis, post-nthesis). Ech stge ws scored 1 6, respectively, nd n verge phenologicl stge ws computed for both DPs nd PPs under wet nd dry tretments. Such fine-scle phonologicl mesurements were not possible in B. grcilis due to the rpid production of flowering stlks in this species. Specific lef re (SLA), the rtio of lef re to dry weight, ws mesured on fully epnded green leves for ech popultion (n = 3) t 3-week intervls during the growing seson. This trit ws chosen becuse SLA is often positively correlted with plnt growth (Grnier et l. 21, 21b; Shipley 22) nd my be indictive of the trde-off between rpid biomss production (high SLA) nd efficient conservtion of nutrients (low SLA; Poorter nd de Jong 1999). Sttisticl nlysis A two-wy mied model nlysis of vrince (ANOVA; SAS 9.2, SAS Institute 25) ws performed for biomss,

5 Giulini et l. Geogrphic vrition nd climte chnge 215 reproductive lloction, individul reproductive tiller mss nd individul vegettive tiller mss for ech species t ech grden. We used two-wy mied model repeted mesures ANOVA to ssess responses for phenology, predwn nd mid-dy LWP nd SLA. Fied effects were wter tretment (wet/dry) nd popultion (DP/PP). Rndom effects were row (wter), column (wter), pop (individul popultion) nd wter*pop. A Ltin Squre nlysis ws initilly performed for ech response vrible to ssess row, column or individul popultion effects. None of these effects were significnt; thus, the si individul popultions were combined into two geogrphic loctions (DP nd PP). When necessry, log (y + c) trnsformtions were used to stisfy homogeneity ssumptions for the ANOVA model (B. grcilis: biomss, reproductive lloction of biomss, predwn, phenology; A. gerrdii: biomss, individul tiller mss, predwn). The constnt c vried depending on the vrible nd ws chosen (reflecting the lower vlues in the dt) so tht zeroes did not dominte the nlyses. Bsed on the Akike Informtion Criterion goodness of fit test, n utoregressive prmeter ws included for severl nlyses t both grdens (B. grcilis: phenology, SLA [SGS only]; A. gerrdii: phenology, SLA). Results Boutelou grcilis Our focus for compring trits from PPs nd DPs for B. grcilis ws in the SGS grden becuse this represents the climte where this species is dominnt. Thus, individuls from PPs from higher resource environment (greter precipittion) were compred to DP individuls from low resource environment. In the SGS grden, we detected no difference between these two popultions in totl end-ofseson biomss (Fig. 1, top), but PPs (from KNZ) llocted less biomss to reproduction thn DPs (Fig. 1, bottom) nd were significntly shorter t the end of the seson (DP verge mimum height = 39.8 ± 1.9 cm vs ± 2.1 cm for PP). PPs lso initited flowering lter nd hd lower reproductive tiller density throughout the growing seson Biomss (g/.1 m 2 ) DP, wet trt DP, dry trt PP, wet trt PP, dry trt SGS = g/.1m 2 b b * wter KNZ = g/.1m 2 y z * wter * pop 2 Reproductive Alloction (%) 1 * wter * wter * pop * pop b y Shortgrss Steppe Konz Pririe Figure 1: Boutelou grcilis end-of-seson biomss (top pnel) nd reproductive lloction (bottom pnel) t Shortgrss Steppe (left) nd Konz Pririe (right) grden sites. There ws significnt difference between the totl biomss t ech grden (top pnel). Significnt min effects (sterisk) nd pirwise comprisons (different letters) re noted in ech pnel (P =.5). Min effect P vlues cn be found in Tble 3; no significnt interctions were detected. Men vlues ± 1 SE re shown.

6 216 Journl of Plnt Ecology Tble 3: results from ANOVAs for the min effects of wter tretment (wet vs. dry) nd popultion (DP vs. PP) t both grdens (SGS nd KNZ) B. grcilis A. gerrdii Wter tretment Popultion Wter tretment Popultion SGS KNZ SGS KNZ SGS KNZ SGS KNZ ANPP <.1*.12*.844.5* , , 72.5, , , , 65.1, , 65 RTM NA NA NA NA *.72* 1.13, 46.96, , , 66 VTM NA NA NA NA.498* *.329* 4.44, 63.16, , , 53 RP.121*.246*.492*.33* *.12* , , , , , , , , 65 RTD <.1*.2* <.1* <.1*.85*.83* <.1* <.1* 6.53, , , , , , , , 325 SLA * * * <.1* 3.11, , , , , 66.32, , , 65 Height <.1*.24*.21* , , , , , 66 Abbrevitions: RP = reproductive percentge, RTD = reproductive tiller density, RTM = reproductive tiller mss, VTM = vegettive tiller mss. P vlues re shown in the first line, followed by F-sttistics nd smple size. ANOVAs were run seprtely for ech trit. With the eception of RTD, ll vlues re for end-of-seson dt. RTD is nlyzed over the growing seson (min effect*time). NA indictes tht dt were not collected. *Sttisticl significnce t P =.5 level. compred to DPs (Fig. 2, top). When wter vilbility ws reduced, biomss nd reproductive tiller density were lso reduced in both popultion types (Figs 1 nd 2, top), but only the PPs eperienced decrese in reproductive lloction (Fig. 1, bottom). There ws no mesureble popultion or wter stress effect on SLA (Fig. 3). There were no significnt differences in predwn or mid-dy LWP between PPs nd DPs under wet or dry conditions (Tble 2). Although trit responses for B. grcilis in the reciprocl mesic environment grden (KNZ) were not the primry focus of this study, individuls from PPs produced significntly more biomss growing in Knss thn those from the more rid DPs (Fig. 1, top), the dry tretment reduced SLA for PPs (Fig. 3), nd in contrst to responses in the SGS environment, reproductive lloction in DPs ws negtively ffected by reduction in wter vilbility (Fig. 1, bottom). Ptterns in reproductive phenology were similr for both grdens, with the eception tht PPs surpssed DPs in reproductive tiller density by the end of the seson t KNZ (Fig. 2, bottom). Andropogon gerrdii For A. gerrdii, we focused on trits of individuls from PPs versus DPs growing in the tllgrss pririe climte (KNZ grden). Under wet conditions t KNZ, PPs (from Colordo) produced only hlf s much biomss s DPs (Fig. 4, top) nd lthough the popultions did not show significnt differences in reproductive lloction (Fig. 4, middle), individul reproductive tiller size ws reduced by c. 6% in individuls from PPs compred to DPs (Fig. 4, bottom). Vegettive tiller size ws similrly reduced (dt not shown). Despite their smll individul size, PPs incresed their reproductive tiller density more rpidly nd hd much higher density t KNZ throughout the seson (Fig. 5, top). When emining phenology, PPs were more phenologiclly dvnced thn DPs (dt not shown). SLA ws greter in PPs t KNZ (Fig. 6, left). Individuls from PPs were less ffected by reduced wter vilbility in biomss, reproductive lloction nd individul reproductive tiller size thn those from DPs (Fig. 4, left). In the reciprocl grden (t SGS), no popultion or tretment effects were observed in totl biomss (Fig. 4, top). However, contrry to the KNZ grden, individuls from PPs llocted significntly more biomss to reproduction (Fig. 4, middle). The density of reproductive tillers t the end of the seson ws significntly greter in PPs (Fig. 5) nd the individul reproductive tillers of PPs were gin reduced in size (Fig. 4, bottom). Differences in SLA between popultions t SGS were similr to ptterns t KNZ (Fig. 6, right). Discussion There hs been much recent interest in the ttributes nd trits of orgnisms found in the core versus edge of species rnge

7 Giulini et l. Geogrphic vrition nd climte chnge 217 Reproductive tiller density (m -2 ) DP, wet trt DP, dry trt PP, wet trt PP, dry trt Shortgrss Steppe * * * * Reproductive tiller density (m -2 ) min effects: * wter effect popultion effect Konz Pririe * Jun Jul Aug Sep Oct * * * Figure 2: Boutelou grcilis reproductive tiller density (in meters per squre) t Shortgrss Steppe (top pnel) nd Konz Pririe (bottom pnel) grden sites throughout the growing seson. Significnt min effects re indicted by sterisk (wter effect) nd/or dgger (popultion effect) directly bove the dte where significnce occurred (P =.5). Min effect P vlues cn be found in Tble 3. Some points were offset slightly so error brs cn be seen clerly. Note the chnge in scle between pnels; KNZ ws much more productive thn SGS despite the similr trends t both sites. Men vlues ± 1 SE re included. Specific Lef Are (cm 2 g -1 ) DP, wet trt DP, dry trt PP, wet trt PP, dry trt y * wter Shortgrss Steppe Konz Pririe Figure 3: Boutelou grcilis end-of-seson SLA t Shortgrss Steppe (28 August 29; left) nd Konz Pririe (21 August 29; right) grden sites. Significnt min effects (sterisk) nd pirwise comprisons (different letters) re noted (P =.5). Min effect P vlues cn be found in Tble 3; no significnt interctions were detected. Men vlues ± 1 SE re included.

8 218 Journl of Plnt Ecology Figure 4: Andropogon gerrdii end-of-seson biomss (top pnel), reproductive lloction (middle pnel) nd individul reproductive tiller size (bottom pnel) t Shortgrss Steppe (left) nd Konz Pririe (right) grden sites. There ws significnt difference between the totl biomss t ech grden (top pnel). Significnt min effects nd interctions (sterisk) nd pirwise comprisons (different letters) re noted in ech pnel (P =.5). Min effect P vlues cn be found in Tble 3; significnt interction P vlues re indicted by pproprite sterisks. Men vlues ± 1 SE re included. (Dudniec et l. 212; Jrem et l. 29; Munwes et l. 21; Villells et l. 212), the strength of locl dpttion in core popultions which my limit their bility to respond to climte chnge (Bennington et l. 212; Hereford 29) nd potentil climte chnge brriers to migrtion (Chen et l. 211; Jlili et l. 21). We hve rgued tht if the rte of climte chnge is greter thn the rte of migrtion for species tht dominte biomes, individuls from PPs my ply significnt role in the future in determining initil ecosystem structure nd function. This is becuse it is the dominnt species tht lrgely determine ecosystem structure nd function (Ellison et l. 25; Grime 1998; Whittker 1965). Thus, the gol of this study ws to better understnd the potentil consequences of ecosystems being dominted by individuls with trits of formerly PPs. As noted previously, we focused on two scenrios: (i) mesic grsslnd drying to the etent tht it becomes semirid nd (ii) semirid grsslnd becoming more mesic. For the first scenrio, lthough there were no significnt differences between DPs nd PPs in totl biomss of B. grcilis in the shortgrss pririe environment ( climte similr to tht the PPs would eperience in the future), differences were noted in reproductive lloction, reproductive tiller density, flower initition, verge mimum end-of-seson height nd sensitivity to wter stress (Figs 1 3). For the second scenrio, A. gerrdii biomss nd growth trits differed significntly between DPs nd PPs. Individuls from PPs hd much reduced biomss under well-wtered conditions compred with DPs (despite greter SLA), s well s significntly smller reproductive tillers (lthough t higher density). Biomss of individuls from PPs ws less sensitive to wter stress however. Thus in both scenrios, under common grden conditions, the trits of individuls from PPs differed in number of ecologiclly meningful wys from individuls collected from tody s DPs. Although production responses might be epected to hve the gretest rry of ecosystem consequences, ltertions in reproductive phenology could be very importnt for the cpbility of these dominnt species to loclly dpt to new climtic conditions (Jump nd Penuels 25). Although it would be unwise to generlize from these two specific scenrios, our results led to two importnt conclusions. First, reltively brod rnge of trits differed between PPs nd DPs from phenologicl, to those relted to growth, nd to sensitivity to resource limittion (wter stress). Second, these trit differences between popultions were distinctive they were lrgely relted to reproduction for the first scenrio versus growth in the second. Both of these support the contention tht present-dy ecosystem functioning, s determined by trits of the dominnt species, my be ltered significntly s function of these popultion-level trit differences. An ssumption mde in this study is tht the dispersl distnce needed for trits to trck climte chnge is too gret for seed nd pollen dispersl. An lterntive perspective is tht pollen-medited gene flow could led to lower differentition mong core, intermedite nd leding-edge popultions (Shi nd Chen 212), lthough this did not pper to be the cse in these grsses. Thus, intrspecific geogrphic vrition in the trits of dominnt species dds nother, more subtle lyer of uncertinty to our bility to predict future ecosystem function bsed on our etnt knowledge. Previous studies hve lredy identified no-nlog environmentl conditions nd novel community composition s mechnisms driving chnges in future ecosystem structure nd function (Nippert et l. 26; Sestedt et l. 28; Veloz

9 Giulini et l. Geogrphic vrition nd climte chnge 219 Figure 5: Andropogon gerrdii reproductive tiller density (in meters per squre) t Konz Pririe (top) nd Shortgrss Steppe (bottom) grden sites throughout the growing seson. Significnt min effects re indicted by sterisk (wter effect) nd/or dgger (popultion effect) directly bove the dte where significnce ws found (P =.5). Min effect P vlues cn be found in Tble 3. Some points were offset slightly so error brs cn be seen clerly. Note the chnge in scle between pnels. Although similr trends were seen between sites, KNZ ws much more productive thn SGS. Men vlues ± 1 SE re included. 25 DP, wet trt DP, dry trt * pop * wter * pop Specific Lef Are (cm 2 g -1 ) PP, wet trt PP, dry trt b b y Konz Pririe Shortgrss Steppe Figure 6: Andropogon gerrdii end-of-seson SLA t Konz Pririe (21 August 29; left) nd Shortgrss Steppe (28 August 29; right) sites. Significnt min effects (sterisk) nd pirwise comprisons (different letters) re noted (P =.5). Min effect P vlues cn be found in Tble 3; no significnt interction P vlues were observed. Men vlues ± 1 SE re included.

10 22 Journl of Plnt Ecology et l. 212; Willims nd Jckson 27). The results of this reserch suggest tht even without these two mechnisms, forecsting the future of ecosystems nd the services they provide will require knowledge, currently lcking, on the migrtion rtes (nd brriers to migrtion) of key species. If tody s popultions of dominnt plnt species cnnot keep pce with climte chnge, then greter understnding of genetic nd phenotypic vrition throughout species rnge, nd the rtes of dpttion for these dominnt species must be investigted, prticulrly under conditions where there re relistic interspecific interctions (Eckert et l. 28; Hoffmnn nd Blows 1994; Lesic nd Allendorf 1995; Moser et l. 211). Funding Ntionl Science Foundtion (127319, nd ). Acknowledgements We thnk Dr Phillip Chpmn (Deprtment of Sttistics, Colordo Stte University) for consulttion with sttisticl nlyses. Conflict of interest sttement. None declred. 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