The effects of precipitation timing on sagebrush steppe vegetation

Transcription

1 Journl of Arid Environments Journl of Arid Environments 64 (26) The effets of preipittion timing on sgerush steppe vegettion J.D. Btes, T. Svejr, R.F. Miller, R.A. Angell Estern Oregon Agriulturl Reserh Center, A Hwy, 25 Burns, OR 9772, USA Reeived 16 Otoer 23; reeived in revised form 22 April 25; epted 16 June 25 Aville online 11 August 25 Astrt Chnges in preipittion ptterns nd inputs hve the potentil to use mjor hnges in produtivity, omposition, nd diversity of terrestril plnt ommunities. Vegettion response to ltered timing of preipittion ws ssessed during 7-yer experiment in n Artemisi tridentt spp. wyomingensis ommunity in the northern Gret Bsin, USA. Four permnent rinout shelters exluded nturl rinfll, with sesonl distriution of preipittion ontrolled with the use of n overhed sprinkler system. Preipittion tretments under eh shelter were WINTER, SPRING, nd CURRENT. The WINTER tretment reeived 8% of its wter etween Otoer nd Mrh; in the SPRING tretment 8% of totl wter ws pplied etween April nd July; nd the CURRENT tretment reeived preipittion mthing the site s long-term (5 yers) distriution pttern. A CONTROL tretment, pled outside eh shelter replite, reeived nturl preipittion inputs. CURRENT, WINTER, nd CONTROL tretments hd similr in soil wter-ontent ptterns nd thus, there were few onsistent differenes in vegettion response. The SPRING tretment resulted in more reground nd lower plnt produtivity ompred to other shelter tretments. This result ontrsted with our initil hypothesis tht shllower-rooted grsses would gin ompetitive dvntge over shrus if preipittion ws shifted from winter to spring. Our results lso demonstrted the resiliene of these ommunities to limte perturtion s mny of the vegettion shifts did not egin until the fourth yer fter tretments were pplied. r 25 Elsevier Ltd. All rights reserved. Keywords: Climte hnge; Chetgrss; Gret sin; Preipittion ptterns; Plnt suession Corresponding uthor. E-mil ddress: jon.tes@oregonstte.edu (J.D. Btes) /$ - see front mtter r 25 Elsevier Ltd. All rights reserved. doi:1.116/j.jridenv

2 1. Introdution ARTICLE IN PRESS J.D. Btes et l. / Journl of Arid Environments 64 (26) Preipittion exerts primry ontrol of plnt produtivity nd omposition in semi-rid nd rid lnd plnt ommunities (Pehne et l., 1937; Noy-Meier, 1973; Pssey et l., 1982; Gretz et l., 1988). The frequeny nd sesonl distriution of preipittion ply mjor role in the vilility of wter within soil profiles, thus, strongly influening rid lnd plnt omposition nd dynmis (Gretz et l., 1988; Comstok nd Ehleringer, 1992; Ojim et l., 1993; Ehleringer et l., 1999). Winter preipittion is more likely to perolte deeper into the soil profile, wheres, summer preipittion my evporte efore infiltrting (Shwinning et l., 23). In the northern Gret Bsin of the United Sttes, the mjority of nnul preipittion is reeived during the winter nd erly spring. This limti regime fvors growth nd development of deep-rooted shrus nd ool seson plnts using the C 3 photosyntheti pthwy (Comstok nd Ehleringer, 1992). The prevlene of summer preipittion on the Colordo Plteu nd Gret Plins results in higher proportion of shllower-rooted summer tive speies using the C 4 photosyntheti pthwy (Comstok nd Ehleringer, 1992; Cook nd Irwin, 1992). Climte hnge rought on y nthropogeni tivities forests not only glol wrming ut ltertion of preipittion regimes y ffeting timing, frequeny, nd intensity of preipittion events (Esterling et l., 2; NAST, 2; IPCC, 21). Altertion of preipittion ptterns hs the potentil to use mjor hnges in vegettion, soils, iodiversity, nd eologil proesses in terrestril eosystems (Neilson et l., 1989; Brown et l., 1997; Ehleringer et l., 21). Experimentl evidene indites tht reduing storm frequeny nd inresing rinfll quntity per storm inreses temporl vriility of soil moisture nd dereses ove-ground prodution in tll grss pririe of the entrl United Sttes (Fy et l., 22; Knpp et l., 22). Shifting from winter to spring preipittion pttern redued produtivity in the sgerush steppe of the northern Gret Bsin (Svejr et l., 23, pp. 9 16). Atmospheri inreses in CO 2 nd limte hnge lso hve the potentil to inrese the ompetitiveness of invsive weeds in rid lnds s doumented y inresed Bromus tetorum produtivity in response to elevted CO 2 in southern Nevd (Smith et l., 2). We investigted the effets of ltered timing of preipittion to vegettion dynmis in n Artemisi tridentt spp. wyomingensis ommunity in the northern Gret Bsin over 7 yers. We hypothesized tht hereous plnts, prtiulrly perennil grsses, would etter dpt to shift towrd spring preipittion distriution nd produtivity nd omposition would either remin stle or inrese. Perennil grsses in this system hve more shllow-firous roots ompred to A. tridentt (Brnson et l., 1976; Reynolds nd Frley, 1989; Dorowolski et l., 199). Evidene from eologil studies onduted in our re indited tht ool seson speies would respond to lte spring nd summer moisture with inresed growth (Btes et l., 2). We lso hypothesized tht shift to greter perentge of winter preipittion would inrese over nd reruitment of A. tridentt ompred to hereous perennils. Inresed winter preipittion ws expeted to enhne wter rehrge in the lower prt of the soil profile nd thus fvor

3 672 J.D. Btes et l. / Journl of Arid Environments 64 (26) deeper-rooted speies suh s A. tridentt. Higher thn verge winter preipittion in the 199s in the south-western United Sttes my hve een responsile for the three-fold inrese in shru densities oserved y Brown et l. (1997). Beuse the preipittion pttern pplied to the CURRENT tretment onformed to long-term trends, we did not expet vegettion in this tretment to hnge reltive to mient onditions. Further, we expeted tht totl vegettion over nd re ground would not e influened y rinfll distriution. 2. Methods 2.1. Study re nd experimentl design The study ws onduted on the Northern Gret Bsin Experimentl Rnge ( W, N), 67 km west of Burns, Oregon, USA. The study site is odominted y A. tridentt spp. wyomingensis 1 nd perennil unh grsses: Stip thurerin, A. spitum, nd Po sndergii. All hereous speies present were ool seson plnts tht utilize the C 3 photosyntheti pthwy. Elevtion is 138 m nd slope is 1%. Soils were lssed s Vil-Dentl, Vrint-Rtto omplex nd re well drined with duripn eginning t 3 5 m (Lentz nd Simonson, 1986). Grvimetri field pity of soils ws 23% ( 15 m) nd 25% (15 3 m). Annul preipittion hs verged 3 mm sine mesurements egn in the 193s. Historil distriution of preipittion ws 6% from Otoer to Mrh, 3% from April to July, nd 1% in Septemer. Annul preipittion is highly vrile yer to yer. The wettest yer on reord in 1993 (53 mm preipittion) ws followed y the driest yer on reord in 1994 (14 mm preipittion). Winter preipittion tends to e in the form of snow, lthough during the study period there ws little snow umultion during the winter months. Four, fixed lotion rinout shelters were uilt in lte summer The design of the shelters nd sprinkler pplition system ws desried y Svejr et l. (1999). Rinout shelters were 3 m 12 m in size nd were open on ll sides. The originl trnsprent fierglss roof ws repled in summer 1998 with Dynglss s, ler polyronte mteril. 2 Preipittion tretments egn in fll Tretment plots were m 2 in size with 2 m uffer strip ordering eh plot. Shelter effets to environmentl onditions (soil nd ir temperture, PAR, wind speed, nd RH) re reported in Svejr et l. (1999). Averge soil tempertures were out 1 2 1C wrmer nd wind speeds were 25% less under the shelters thn in CONTROL plots (Svejr et l., 1999, 23). Tretments were rndomly ssigned s WINTER, SPRING, nd CURRENT under eh shelter. The WINTER tretment reeived 8% of its wter etween Otoer nd Mrh; the SPRING tretment reeived 8% of its wter etween April nd July; nd the CURRENT tretment reeived preipittion orresponding to 1 Nomenlture used is from Hithok nd Cronquist (1987). 2 Mention of trde nmes does not indite n endorsement y USDA-ARS or Oregon Stte University.

4 J.D. Btes et l. / Journl of Arid Environments 64 (26) Tle 1 Preipittion distriution for the shelter tretments (WINTER, SPRING, CURRENT) nd CONTROL Yer nd tretment Winter (Ot. April) Applition trget (mm) Spring Summer (My July) Fll (August Sept.) Preipittion totl Control Current Winter Spring yer preipittion distriution verge. 35 Long term preipittion verge 3 25 Amount pplied to eh shelter tretment (28 mm) Preipittion (mm) Yer Fig. 1. Annul preipittion (mm) from 1994 to 2 in the CONTROL nd showing the long-term verge for the site (3 mm) nd the level pplied to eh shelter tretment (23 mm). long-term (5 yers) distriution ptterns. The wtering distriution shedule is shown in Tle 1. CONTROL plots of identil size were loted 1 m south of eh shelter nd reeived nturl preipittion (Fig. 1). All shelter tretments reeived totl of 23 mm of wter nnully, whih is 68% of the site s long-term nnul verge. In nturl setting this would e onsidered drought. However, it ws

5 674 J.D. Btes et l. / Journl of Arid Environments 64 (26) determined in the first yer of the study tht pplying 3 mm of wter ws exessive, s soils eme super-sturted. Thus, the mount of wter input ws redued, lleviting the sturted onditions. Beuse ll shelter tretments reeived the sme mount of wter, the study provides n indition of effets to vegettion with shifts in sesonlity of preipittion. Though less wter ws pplied in totl, more ws pplied in fewer events nd ws effetive t wetting the soil profile for the WINTER nd CURRENT tretments when ompred to CONTROLS (Svejr et l., 23). Typilly, shelter tretments reeived nnul preipittion totls in 13 2 events eh yer (verging etween 1 nd 15 mm per event ut osionlly up to 25 mm). In nturl setting, there re mny smller storm events during given yer. These smll events re not tht effetive t wetting the soil, espeilly y lte spring when evportive demnd inreses. For exmple, in 1999 the ontrol tretment experiened 9 storm events, ut 67% of the events were reorded s eing less thn 2.5 mm of wter. In the study it ws not fesile, logistilly nd euse of environmentl onditions, to duplite this numer of events under the shelters Preipittion pplition nd soil wter-ontent Wter pplied to shelter tretments ws olleted using five rin guges pled in eh experimentl unit. Guges were onstruted of 2-liter plsti soft drink ontiners nd nhored to the ground with steel rods (Wrge et l., 1994). Colleted wter ws mesured immeditely fter pplition. Preipittion in the CONTROL ws determined from tipping uket rin guge. Under the shelters, wter pplition ws not fesile etween mid-novemer nd mid-ferury euse of old ir tempertures whih froze sprinkler heds or used hevy umultions of ie on plnts, with the potentil to use signifint mehnil dmge. Grvimetri soil wter-ontent ws mesured iweekly in the 1998 nd 1999 growing seson (April Septemer) t 15 nd 15 3 m. Two susmples, rndomly pled in re interspes were olleted for eh depth in eh tretment replite. Soils were weighed, dried t 16 1C for 48 h, nd re-weighed to determine grvimetri wter-ontent Vegettion mesurements Plnt response prmeters mesured were shru over nd density, hereous iomss, hereous over nd density, reprodutive suess, nd reprodutive shoot density nd weight. Shru over ws estimted y the line interept method (Cnfield, 1941). Three, 8-m trnsets, sped 2-m prt were permnently estlished in eh experimentl unit. Density of mture shrus ws determined y ounting ll rooted plnts in 8 2m 2 elt trnsets. Hereous over nd density were determined inside.2 m 2 (4 5 m 2 ) frmes. Frmes were pled every meter long the 8-m trnset lines (strting t.5 m). Cover of hereous plnts, litter, rok, nd re ground were estimted visully. Densities of hereous speies nd shru seedlings were determined y ounting ll individuls rooted

6 J.D. Btes et l. / Journl of Arid Environments 64 (26) within frmes. Vegettion ws smpled in mid-june in nd erly August The June smpling ptured over nd density t pek prodution for the understory omponent. The August smpling ws designed to pture ontinued growth nd ny hnges in over through the summer in ll tretments. The two smpling dtes proved unneessry for ompring mong tretments. Plnts in the SPRING tretment styed tive longer ut tretment reltionships for over, density, nd iomss were onsistent ross smpling dtes. Thus, presented dt will fous on the June smpling. Hereous iomss ws determined in Septemer 1998, June 1999, nd June 2. Plnts were lipped to 2 m stule height inside five, 1 m 2 frmes in eh tretment replite. Biomss ws seprted into five funtionl groups; perennil grsses, P. sndergii, perennil fors, B. tetorum (nnul grss), nd nnul fors. P. sndergii is ommon perennil grss ut is shllow rooted, nd grows nd develops erlier in the spring thn other unhgrss speies (Rikrd nd Vughn, 1988; Link et l., 199). Reprodutive suess ws determined y trking plnt phenology from growth initition to seed dispersl over six growing sesons (1995 2). Phenology ws olleted on weekly to iweekly shedule in 1995 nd 1996 nd on weekly sis from 1997 through 2. Reprodution ws rted suessful if plnts ompleted ll growth phses from initition or germintion through seed dispersl. Plnts monitored were A. tridentt spp. wyomingensis, S. thurerin, Sitnion hystrix, Collinsi prviflor, nd, s group, we monitored the perennil fors Agoseris glu nd Crepis umint. Three individuls (A. tridentt, S. thurerin, S. hystrix) or groups of individuls (e.g. A. glu nd C. prviflor) of eh speies were monitored in eh tretment replite. Reprodutive density nd shoot weights were estimted for A. tridentt spp. wyomingensis, S. thurerin, nd S. hystrix in 1998, 1999, nd 2. Artemisi tridentt reprodutive shoots were ounted within two 2 m 2 frmes on three plnts per replite. All reprodutive shoots were then hrvested, dried (t 48 1C) nd weighed to otin n verge shoot weight. Stip thurerin nd S. hystrix reprodutive tillers were ounted on four plnts per replite in July 1998, 1999, nd 2. Plnts were lipped to 2-m stule height nd dried; then seprted into ded, live, nd reprodutive omponents nd weighed Sttistil nlysis nd dt presenttion A rndomized lok design ws used with four replites of eh tretment. A repeted mesures nlysis of vrine (ANOVA) ws used to ssess yer (df ¼ 6), tretment (df ¼ 3), intertion effets on hereous nd shru over, density, nd reprodutive suess (df ¼ 18, error df ¼ 81). Pre-tretment dt (plnt over nd density) were nlysed to test for pre-existing differenes mong tretments. Vegettion over nd density were lso nlysed y yer using ANOVA to ssist in explining intertions. Biomss (hereous nd reprodutive) nd reprodutive densities were ompred mong tretments using repeted mesure ANOVA for rndomized lok design. Min effets were yer (df ¼ 2) nd tretment (df ¼ 3) nd

7 676 J.D. Btes et l. / Journl of Arid Environments 64 (26) the yer y tretment intertion (df ¼ 6, error df ¼ 21). Soil wter-ontent ws nlysed within yers using repeted mesures ANOVA. Min effets for soil wterontent were tretment (df ¼ 3), soil depth (df ¼ 1), nd time (df ¼ 19). All sttistil nlyses were performed using the Sttistil Anlysis System (SAS Institute, 21). Arsine squre root trnsformtions of over nd density dt were used to meet ANOVA ssumptions of normlity nd homogeneity of vrine. Nontrnsformed mens re reported from sttistil omprisons of the trnsformed mens. Sttistil signifine of ll tests were set t po:5. Tretment mens nd their intertions were seprted using Fisher s proteted LSD proedure. 3. Results 3.1. Preipittion pplition nd soil wter-ontent The CONTROL tretment illustrtes the vriility of preipittion mong (Fig. 1) nd within yers (Fig. 2) tht re hrteristi of the sgerush eosystem. Annul preipittion mounts for the CONTROL in nd were signifintly less thn the long-term verge (Fig. 1, po:1). Preipittion in the CONTROL ws lso less thn the shelters in the lst 2 yers of the study. The CONTROL reeived five times s muh wter in the spring summer (Mrh July) period in ompred to the sme period in Exept for winter , there ws little snow umultion during the winter months of the study. Winter preipittion during most of the study rrived s rin or wet snow tht melted quikly. CURRENT nd WINTER tretments tended to hve equivlent soil wterontent t the strt of eh growing seson when ompred to the CONTROL in oth 1998 nd 1999 (Fig. 3A nd B). CURRENT nd WINTER tretments resulted in higher soil profile wter-ontents thn the SPRING tretment etween the months of Mrh nd My in oth 1998 nd Soil wter-ontent in the SPRING tretment ws greter thn the other tretments from lte June until mid- August in 1998 nd However, soils t oth depths in the SPRING tretment never rehed field pity Hereous iomss Hereous iomss prodution ws influened y the different preipittion tretments. There ws yer y tretment intertion for P. sndergii iomss (Tle 2). The intertion resulted from the CONTROL tretment hving greter P. sndergii iomss thn the WINTER nd CURRENT tretments in 1999, ut in 2 the WINTER ws greter thn CURRENT nd CONTROL tretments (Fig. 4A). In oth yers, the SPRING tretment ws signifintly less thn other tretments for P. sndergii iomss. Mens seprtion indited tht the SPRING tretment ws less thn one or more of the other tretments for iomss of perennil grsses, perennil fors,

8 J.D. Btes et l. / Journl of Arid Environments 64 (26) Preipittion (mm) CURRENT WINTER SPRING CONTROL 2 (A) Sep Ot Nov De Jn Fe Mr Apr My Jun Jul Aug Sep CURRENT WINTER SPRING CONTROL Preipittion (mm) (B) Sep Ot Nov De Jn Fe Mr Apr My Jun Jul Aug Sep Fig. 2. Monthly wter pplitions (mm) for the shelter tretments nd preipittion (mm) in CONTROL in (A) nd (B) B. tetorum, nd totl hereous when ompred ross the three smple yers (Tle 2). However, when nlysed y yer, onsistent trend for these response vriles ws not ovious, exept for totl iomss (Fig. 4B E). Among the three

9 678 J.D. Btes et l. / Journl of Arid Environments 64 (26) CONTROL CURRENT SPRING WINTER -15 m Soil Wter Content (%) (A) m 25 Soil Wter Content (%) (B) Apr My Jun Jul Aug Sep Apr My Jun Jul Aug Sep Fig. 3. Soil wter-ontent (% grvimetri) for the shelter tretments nd CONTROL t (A) 15 m nd (B) 15 3 m for the 1998 nd 1999 growing sesons. Vlues re in mens7one stndrd error (N ¼ 4).

10 J.D. Btes et l. / Journl of Arid Environments 64 (26) Tle 2 Anlysis of vrine P vlues for iomss, over, nd density y speies nd funtionl group for intertion nd min effets Funtionl group/speies Biomss (kg h 1 ) Cover (%) Density (# plnts m 2 ) Yer y tretment Yer Tret. Yer y Tret. Yer Tret. Yer y tretment Yer Tret. A. tridentt spp. wyomingensis Po sndergii.251*.23*.2*.41* o.1* o.1* o.1* o.1*.142* Agropyron spitum Sitnion hystrix.678 o.1*.9*.8*.5*.375* Stip thurerin o.1* o.1* o.1*.622 o.1*.724 Totl perennil grsses *.69* o.1* o.1*.258* * Bromus tetorum (Annul grss) * o.1* o.1* o.1* o.1* o.1*.261* Agoseris glu *.52*.743 o.1*.5296 Astrglus lentiginousus * *.7124 Astrglus osurum *.13*.3866 o.1*.437 Crepis umint o.1*.5 o.1*.94 Delphinium ndersonii * Lomtium mrorpum.824 o.1*.2*.185 o.1*.58* Lupinus leuophyllus * *.444* Phlox longifoli.7154 o.1*.89* o.1* o.1*.1632 Other perennil fors * * Perennil for totl.476.8*.4*.3219 o.1*.2* o.1* o.1*.292* Alyssum desertorum.2413 o.1*.31* o.1* o.1*.3216 Collinsi prviflor o.1* o.1*.14* o.1* o.1*.2* Desurini pinnt *.3216 Miroseris grilis.63* o.1*.73* o.1* o.1*.28* Other nnul fors *.7732* o.1* o.1*.236* Annul for totl o.1*.23* o.1* o.1*.22* Totl hereous o.1*.4* o.1* o.1* o.1* o.1*.16* Crust * o.1*.478 Moss o.1*.42* o.1*.74* Litter o.1*.2*.2* Bre-ground o.1* o.1* o.1* All re ool-seson (C3) speies. Asterisks indite signifine t the.5 level. Biomss ws grouped y funtionl group (speies or groups re underlined), so individul speies nlyses re not ville.

11 68 J.D. Btes et l. / Journl of Arid Environments 64 (26) P. sndergii* 4 kg h (A) 5 4 Totl Hereous kg h (B) 1 5 Perennil Grss* kg h -1 (C) Perennil Fors 4 kg h -1 (D) kg h -1 5 B. tetorum CONTROL (E) YEAR CURRENT SPRING WINTER Fig. 4. Biomss (kg h 1 ) y funtionl groups nd totl hereous for the shelter nd CONTROL tretments: (A) P. sndergii; (B) totl hereous; (C) perennil grsses; (D) perennil fors; nd (E) B. tetorum. Dt re in mens7one stndrd error. Signifint differenes (P4:5) mong the tretments, y yer, re indited y different lower se letters.

12 J.D. Btes et l. / Journl of Arid Environments 64 (26) shelter tretments, totl iomss ws greter in the WINTER nd CURRENT tretments ompred to the SPRING of the lst 2 yers of the study Plnt over funtionl group response Cover of hereous funtionl groups ws lso influened y the different preipittion tretments. There were yer y tretment intertions for over of P. sndergii, totl perennil grsses, B. tetorum, totl hereous, litter, nd re ground (Tle 2). The tretment differenes for these response vriles did not mterilize until the third (1997) or fourth growing seson (1998) fter tretment pplition (Figs. 5 nd 6). Perennil grss over only hd 1 yer (1997) tht showed strong tretment differene (Fig. 5A). Cover of P. sndergii nd perennil fors were less in the SPRING ompred to the other tretments in 1998 nd 1999 (Fig. 5B C). Cover of B. tetorum ppered to e estlishing trend of greter over in the WINTER ompred to the other tretments y study s end (Fig. 5D). At orser sle, tretments eme more distint t the onlusion of the study, prtiulrly totl hereous over. Hereous over in 2 ws gretest in the WINTER followed in desending order y CURRENT, CONTROL, nd SPRING tretments (Fig. 6A). The lower over in the SPRING tretment ws minly result of redued over of P. sndergii nd perennil fors thn the other tretments. Cover provided y litter ws lowest in the CONTROL ompred to the shelter tretments (Fig. 6B). Bre ground in CURRENT nd WINTER tretments delined over time while not hnging in SPRING nd CONTROL tretments (Fig. 6C). As result, re ground ws greter in the SPRING nd CONTROL tretments ompred to the CURRENT nd WINTER tretments. Artemisi tridentt over nd density did not hnge nor were ny tretment differenes deteted (Tle 2) Plnt over nd density, speies response Cover nd density of individul speies were influened y the different preipittion tretments. There were yer y tretment intertions for over nd/ or densities of S. hystrix, S. thurerin, P. sndergii, B. tetorum, C. umint, Lomtium nevdensis, A. desertorum, C. prviflor, nd Miroseris grilis (Tle 2). For severl speies some ler trends eme ovious nd re illustrted y hnges in plnt densities (Fig. 7A F). P. sndergii densities inresed in the WINTER nd CURRENT tretments etween 1997 nd 1999 nd were greter thn the SPRING nd CONTROL tretments (Fig. 7A). B. tetorum nd Crepis umint densities inresed in the WINTER tretments nd y 1998 were greter thn the other tretments (Fig. 7B nd C). However, for S. hystrix, S. thurerin, A. desertorum, C. prviflor, nd M. grilis the response to preipittion pplitions ws highly vrile ross yers nd no ler trends emerged. This is illustrted y the density dynmis of A. desertorum nd C. prviflor (Fig. 7D nd E). Min effets (yer nd/or tretment) for over nd density were signifint for most of the remining plnt speies (Tle 2, e.g. over of C. umint nd L. nevdensis, nd densities of S. thuerin nd A. glu). Cover nd/or density

13 682 J.D. Btes et l. / Journl of Arid Environments 64 (26) COVER (%) Perennil Grsses 3 (A) 18 P. sndergii (B) COVER (%) 12 Perennil Fors COVER (%) (C) COVER (%) B. tetorum CONTROL CURRENT SPRING WINTER,, (D), 8 Annul Fors COVER (%) (E) YEAR Fig. 5. Funtionl group over (%) for the shelter nd CONTROL tretments, : (A) perennil grsses; (B) P. sndergii; (C) B. tetorum; (D) perennil fors; nd (E) nnul fors. Dt re in mens7one stndrd error. Signifint differenes (P4:5) etween tretments, y yer, re indited y different lower se letters.

14 J.D. Btes et l. / Journl of Arid Environments 64 (26) Hereous d 35 COVER (%) (A) 1 5 Litter COVER (%) (B) 5 8 Bre Ground COVER (%) CONTROL CURRENT SPRING WINTER 1 (C) YEAR Fig. 6. Cover (%) of; (A) totl hereous; (B) litter; nd (C) re ground, for the shelter nd CONTROL tretments, Dt re in mens7one stndrd error. Signifint differenes (P4:5) mong tretments, y yer, re indited y different lower se letters.

15 684 J.D. Btes et l. / Journl of Arid Environments 64 (26) P. sndergii Plnt m (A) 1 Plnt m (B) B.tetorum CONTROL CURRENT SPRING WINTER Plnt m -2 (C) Plnt m (D) C. umint A. desertorum 1 C. prviflor 8 Plnt m -2 (E) P. longifoli Plnt m -2 (F) YEAR Fig. 7. Densities (plnts m 2 ) of: (A) P. sndergii; (B) B. tetorum; (C) C. umint; (D) A. desertorum; (E) C. prviflor; nd (F) P. longifoli, for the shelter nd CONTROL tretments, Vlues re in mens7one stndrd error. Signifint differenes (P4:5) mong tretments, y yer, re indited y different lower se letters.

16 vlues for these speies inresed for ll or some of the tretments ut the SPRING etween 1996 nd In the SPRING tretment there ws no mesurle hnge in over or densities of remining plnt speies. Severl perennil for speies were prtiulrly sensitive to the preipittion pplitions. Densities of P. longifoli inresed in CONTROL, WINTER nd CURRENT tretments nd were greter thn the SPRING y the end of the study (Fig. 7F) Reprodutive development ARTICLE IN PRESS J.D. Btes et l. / Journl of Arid Environments 64 (26) Reprodutive development ws ffeted y the preipittion tretments (Tle 3, Figs. 8 nd 9). There ws tendeny for A. tridentt to reh more dvned stges of phenology in the shelter tretments ompred to the CONTROL plots (Tle 3), though this ws not onsistent ross yers (Fig. 8A). Reprodutive suess of S. thurerin nd S. hystrix ws signifintly lower in the SPRING tretment versus the other tretments when nlysed over the ourse of the study (Tle 3). However, there ws vrition in reprodutive suess for oth speies ross yers (Figs. 8B nd C), prtiulrly for S. thurerin. Reprodutive suess of the A. glu/ C. umint ws lso highly vrile for CONTROL, WINTER, nd CURRENT tretments, resulting in yer y tretment intertion (Tle 3, Fig. 8D). Nonetheless, A. glu/c. umint reprodutive suess ws lowest in the SPRING when ompred to the other tretments over the ourse of the study (Tle 3). Reprodutive suess for C. prviflor ws lower in the SPRING versus the other tretments (Fig. 8E). In the SPRING tretment, C. prviflor did not dvne eyond erly lef stge. Reprodutive effort y oth S. thurerin nd S. hystrix exhiited no onsistent tretment differenes in tiller densities or weights (Figs. 9A B nd 1A B). Tiller densities nd weights of oth speies were greter in 1999 ompred to other yers for WINTER, CURRENT, nd CONTROL tretments. Reprodutive shoot densities for A. tridentt were gretest in the SPRING ompred to ll other tretments during the study (Tle 6). This reltionship ws prtiulrly in evidene the lst 2 yers of the study (Fig. 9C). Reprodutive shoot weight densities of A. tridentt were gretest in the SPRING ompred to the other tretments, lthough there ws strong yer y tretment intertion (Tle 3, Fig. 1C). 4. Disussion Preipittion timing influened hereous omposition nd dynmis of the A. tridentt steppe ommunity, ut not s hypothesized. We hd ntiipted tht the SPRING wter pplition would result in stle or inresed over nd iomss of grsses nd other hereous speies euse their rooting strutures would llow them to utilize soil wter in the upper profile more effetively thn A. tridentt. We expeted the WINTER tretment to fvor A. tridentt euse of its ility to ess soil wter deeper in the profile. Neither of these hypotheses proved to e orret.

17 686 J.D. Btes et l. / Journl of Arid Environments 64 (26) Tle 3 Anlysis of vrine P vlues for reprodutive suess, reprodutive tiller (shoot) density, nd reprodutive tiller (shoot) weight density for the intertion nd min effets for seleted hereous speies nd A. tridentt spp. wyomingensis Funtionl group/speies Reprodutive suess (%) Reprodutive tiller (shoot) density (# m 2 ) Reprodutive tiller (shoot) weight density (mg m 2 ) Yer y tretment Yer Tret. Yer y tretment Yer Tret. Yer y tretment Yer Tret. A. tridentt spp. wyomingensis.676 o.1*.124* o.1*.75*.118* o.1* Sitnion hystrix o.1*.6419 o.1* o.113*.769 Stip thurerin o.1*.3167 o.1*.1* o.1*.2*.133* o.379* o.64* Agoseris glu/crepis umint o.1* o.1* o.1* Collinsi prviflor o.1* o.82* o.7* Asterisks indite signifine t the.5 level.

18 J.D. Btes et l. / Journl of Arid Environments 64 (26) (A) A. tridentt spp.wyomingensis,,,,,,, (B) S. thurerin,, Perent reprodutive Sueess (%) (C) S. hysterix C. umint,, (D) ,,,,,,,, C. prviflor CONTROL CURRENT SPRING,, WINTER (E) YEAR Fig. 8. Perent reprodutive suess (%) of: (A) A. tridentt spp. wyomingensis; (B) S. thurerin; (C) S. hystrix; (D) C. umint; nd (E) C. prviflor, for the shelter nd CONTROL tretments, Vlues re in mens7one stndrd error. Signifint differenes (P4:5) mong tretments, y yer, re indited y different lower se letters.

19 688 J.D. Btes et l. / Journl of Arid Environments 64 (26) Tillers m S. thurerin CONTROL CURRENT SPRING WINTER. (A) 2. S. hystrix 1.6 Tillers m (B) 2. A. tridentt spp. wyomingensis 1.6 Tillers m d (C) YEAR Fig. 9. Reprodutive tiller density (tillers m 2 ) of: (A) S. thurerin; (B) S. hystrix; nd (C) A. tridentt spp. wyomingensis, for the shelter nd CONTROL tretments, Vlues re in mens7one stndrd error. Signifint differenes (P4:5) mong tretments, y yer, re indited y different lower se letters.

20 J.D. Btes et l. / Journl of Arid Environments 64 (26) S. thurerin CONTROL CURRENT SPRING WINTER (A) S. hystrix Tiller Weight Density (mg m -2 ) (B) A. tridentt spp. wyomingensis,, (C) YEAR Fig. 1. Reprodutive tiller weight density (mg m 2 ) of: (A) S. thurerin; (B) S. hystrix; nd (C) A. tridentt spp. wyomingensis, for the shelter nd CONTROL tretments, Vlues re in mens7one stndrd error. Signifint differenes (P4:5) mong tretments, y yer, re indited y different lower se letters.

21 69 J.D. Btes et l. / Journl of Arid Environments 64 (26) Sgerush Artemisi tridentt ws unresponsive to the preipittion shifts in terms of over nd density, nd thus ppered less likely to e influened in the short term y limti shifts thn ssoited hereous speies. However, visully, sgerush in the SPRING tretment did not pper to produe s muh vegettive iomss s in the other tretments. We did not mesure A. tridentt iomss euse we wnted to minimize destrutive smpling. We ttempted to use llometri equtions developed for this lotion y Rittenhouse nd Snev (1977) to estimte sgerush iomss ut these models were not sensitive enough to detet differenes mong tretments. The SPRING tretment did onfer some dvntges to reprodutive suess, stem density, nd stem weight density of A. tridentt (Figs. 8A, 9C nd 1C). The summer wter pplition in the SPRING tretment my provide n explntion. At the Hnford fility in estern Wshington, A. tridentt spp. wyomingensis hs een shown to respond to summer preipittion with inresed reprodutive shoot development (Evns et l., 1991). A. tridentt reprodutive shoots initite growth in mid-june nd this is the period the SPRING tretment reeived frequent wter pplitions. This would explin the high reprodutive suess of A. tridentt nd hevier shoot weights in the SPRING ompred to the other preipittion tretments. The greter reprodutive effort did not result in the reruitment of new individuls Effets of spring preipittion pttern: hereous response nd soil moisture Hereous plnts were detrimentlly ffeted y the SPRING preipittion tretment s indited y pttern of lower hereous iomss, over, nd densities ompred to the other tretments. The hereous response in the SPRING tretment likely resulted from redued vilility of soil wter during the most tive growth period (April My) (Fig. 2A nd B). Even when wter ws mde ville in June nd July, soils never eme thoroughly wetted; thus effetive preipittion ws less in the SPRING when ompred to the other tretments. Soil wter-ontent never rose ove 15% grvimetri, even with pplitions s high s 25 mm. This level of soil wter-ontent ws indequte for stimulting hereous growth nd development in the spring nd summer. Oserving the development of plnts in the other tretments t this level of soil wter-ontent my explin the lk of response in the SPRING. In WINTER, CURRENT, nd CONTROL tretments, y the time soil wter ontent hd een drwn down to 15% grvimetri, most hereous plnts hd either entered dormny (e.g. P. sndergii, nnul fors) or re in lter growth stges (seed development) nd re not dding dditionl oveground iomss Eologil implitions of spring preipittion pttern Shifting preipittion distriution to spring/summer pttern (SPRING tretment) hs the gretest potentil for ltering produtivity, omposition, nd struture

22 J.D. Btes et l. / Journl of Arid Environments 64 (26) of A. tridentt steppe vegettion. Annul nd perennil fors ntive to the system were the lest resistnt to timing hnge, delining in density, over, nd iomss. A long-term shift to spring/summer dominted preipittion pttern would led to the for omponent eing lost or severely redued, with the potentil to redue eosystem iodiversity. Reprodutive mesurements demonstrted the utility of using ertin speies s inditors of environmentl hnge in ontrolled or field experiments. Collinsi prviflor nd perennil fors were prtiulrly sensitive to soil wter onditions in lte winter/erly spring. In the SPRING tretment, C. prviflor rrely emerged or ompleted its life yle nd reprodutive efforts of perennil fors were lso redued. Though individuls emerged, the lk of seed development suggests tht the seed nks for C. prviflor nd other fors re eing depleted. When ompred to the other shelter tretments the results lso suggest n inrese in re ground with spring/summer preipittion pttern. Greter levels of re ground ould potentilly inrese soil erosion in this environment nd inrese heterogeneity of soil resoures. Aording to Shlesinger et l. (199), this results in positive feedk linkge exerting desertifition proesses nd further reduing eosystem produtivity. In retrospet, we proly should not hve een surprised y the ommunity dynmis in the SPRING tretment. There is mple doumenttion in the literture, from experimentl evidene (e.g. Cldwell et l., 1977; Snev, 1982) nd from reviews (Comstok nd Ehleringer, 1992) tht produtivity of old desert C 3 speies of the Gret Bsin re keyed to the rehrging of soil moisture during winter. Reent physiologil studies indite tht ool seson-c 3 speies my utilize summer moisture ut do not inrese ron ssimiltion, prtiulrly following winter drought (Shwinning et l., 22; Shwinning et l., 25). Our study tends to onfirm these results t the plnt ommunity level. Though plnt speies styed green longer into the summer in the SPRING tretment, they did not dd dditionl ove-ground strutures nd there ws no reruitment of new individuls. When plnts do not respond to summer preipittion events it is euse there is no inentive to invest in dditionl strutures to exploit ville wter, s the osts re outweighed y ny potentil returns in ron gin (Shwinning et l., 23; Shwinning nd Sl, 24). There is evidene tht suggests plnts in this system will respond to pulses of preipittion in the summer. Btes et l. (2) oserved extensive tillering of C 3 perennil grsses nd seond set of reprodutive tillers were produed y P. sndergii nd S. hystrix following 7 mm (3-dy event) nd 2 mm storm events ourring over 3-week period in June nd mid-july. There re two ftors tht we elieve ontriuted to this response; (1) there ws dequte winter rehrge in soils, thus, the root net ws proly well developed nd le to rpidly exploit ville wter, nd/or (2) the preipittion pulses were lrge enough to rehrge the soil profile suffiiently for plnts to mke dditionl investments in new growth. In the SPRING tretment, we suspet tht root development ws urtiled euse of winter drought, thus when wter rrived in the lte spring nd summer, plnts did not hve the pity to fully exploit ville wter. In ddition, wtering

23 692 J.D. Btes et l. / Journl of Arid Environments 64 (26) tretments were never suffiient to rehrge the soil profile so there ws likely little inentive to inrese rooting tivity to pture ville wter. The most wter pplied in 1-month period ws out 6 mm with mximum of 25 mm for single event. There is likely threshold wherey ool seson speies will respond to summer moisture ut we did not rrive t tht level in the study WINTER, CURRENT, nd CONTROL tretments: soil wter nd vegettion response Generlly, we mesured few onsistent differenes mong the WINTER, CURRENT, nd CONTROL tretments. We hd predited tht shift to greter winter preipittion oupled with redued spring nd summer preipittion would result in deep peroltion of wter nd inrese growth of deep-rooted A. tridentt ompred to more shllow rooted hereous plnts. The similrity mong tretments in soil wter-ontent (Fig. 3) proly explins the resemlne in plnt ommunity response. In ddition, soils on our site re reltively shllow for the A. tridentt spp. wyomingensis lline; 3 5 m in depth efore striking hrd pn. Dvies et l. (24, pp ) quntified soil hrteristis on 15 sites in the lline sttered ross estern Oregon nd northern Nevd. Soils in their study verged 85 m in depth with rnge from 28 to 215 m. Beuse soils on our site re shllow nd fine-textured, wter nnot esily penetrte elow rooting depths of perennil grsses nd fors. For A. tridentt to tke dvntge of dditionl winter moisture proly requires deeper nd/or orser textured soils. Nonetheless, our results ontrst with reent studies tht doumented inresed shru density nd over with greter long-term winter preipittion in south-estern Arizon (Brown et l., 1997), nd onlusions developed y Shwinning et l. (25) found tht inresed summer drought would fvor survivorship of deep-rooted shrus over shllow rooted plnts Shelter effets nd vegettion response Differenes in vegettion, litter, nd re ground mong WINTER, CURRENT nd CONTROL tretments we ttriuted to the effets of the shelters rther thn the wtering tretments. The wrmer tempertures nd redued diurnl vrition under the shelters (Svejr et l., 1999) my explin the higher densities of P. sndergii nd B. tetorum in CURRENT nd WINTER tretments. Frost heving nd pedestling t the soil surfe ws ommonly oserved in lte winter nd erly spring (Ferury April) in the CONTROL tretment. Under the shelters, frost heving ws not ftor, whih my prtilly explin the inresed estlishment y reduing stress of P. sndergii nd B. tetorum seedlings during the lte winter erly spring period. Frost heving in soils hs een reported to redue B. tetorum estlishment (Sheley nd Lrsen, 1994). The wrmer tempertures omined with winter moisture pplition were prole ftors for the lrger B. tetorum iomss vlues mesured in the WINTER tretment. With wrmer soil tempertures, B. tetorum produes greter shoot iomss (Nsri nd Doesher, 1995).

24 J.D. Btes et l. / Journl of Arid Environments 64 (26) The inrese in B. tetorum is of onern s its presene hs ltered histori fire regimes nd permnently modified Gret Bsin plnt ommunities (Young et l., 1987). If tempertures inrese s predited y generl irultion models (NAST, 2; IPCC, 21), the potentil exists for inresed nnul grss estlishment into res where it is still minor omponent of the A. tridentt eosystem. Reserh hs indited B. tetorum n spred, estlish, nd inrese in undistured nd intt sgerush ommunities (Pssey et l., 1982; Svejr nd Tush, 1991). B. tetorum domintes extensive res in the Intermountin region, prtiulrly in res only slightly wrmer thn our study site (Young et l., 1987; Pellnt, 199). These res hve onverted to systems dominted y nnul grss nd re now mintined y more frequent fire disturnes. The ltered disturne regime hs lrgely removed or prevented reestlishment of ntive flor. There re lso inditions tht B. tetorum is more ompetitive with ntive speies with elevted CO 2 levels (Smith et l., 2). A wrmer environment oupled with winter preipittion regime nd greter CO 2 levels would likely permit invsion nd dominne y B. tetorum, prtiulrly if fire disturnes inrese. Litter rekdown nd deomposition ppered to e redued under the shelters whih my explin the higher litter overs in the shelter tretments. Litter mteril in the CONTROL ws more exposed nd ppered more redily degrded. Litter in the CONTROL rpidly ssumed lehed pperne fter the growing seson, while litter under the shelters retined freshly ured pperne. This my e relted to ultrviolet rdition inputs whih re n importnt ftor in degrdtion of plnt mterils in rid lnd systems (MKy et l., 1994; Moorehed nd Cllghn, 1994; Rozem et l., 1997). Ultrviolet rdition inputs were not mesured ut would e redued under the shelters due to the filtering of UV rdition y the roofing mteril. 5. Conlusions In other studies, experimentlly shifting preipittion ptterns, ltering storm frequeny, or djusting nnul wter inputs hve hd vrile effets to plnt ommunity produtivity nd omposition. Produtivity nd omposition in plnt ommunities hs inresed, deresed, or not hnged sustntilly in response to experimentl mnipultion of preipittion (Grime et l., 2; Knpp et l., 22; Svejr et l., 23). A shift in preipittion tht does not stry fr from historil ptterns does not pper to use mjor disruptions to eosystem omposition or produtivity, nd in some ses my enhne produtivity (Grime et l., 2). As indited y our study, inresed winter preipittion omined with summer drought ppers unlikely to use mjor hnges to vegettion omposition or produtivity of A. tridentt ommunities in the northern Gret Bsin. Shwinning et l. (25) suggested tht inresing summer drought in old desert plnt ommunities will potentilly redue fitness of ntive perennil plnts nd open sites up to invsion y exoti nnuls. We mesured inreses in nnul grss B. tetorum ut there ws no indition tht the fitness of ntive perennils deresed under the

25 694 J.D. Btes et l. / Journl of Arid Environments 64 (26) WINTER regime. However, s disussed, the trend of inresing B. tetorum is of onern, prtiulrly if regionl tempertures inrese s predited. Mjor shifts wy from historil preipittion ptterns hve the gretest potentil to lter eosystem funtion nd produtivity. Altering the temporl distriution nd size of rinfll events in the tll grss pririe of the Gret Plins redued oveground produtivity y 11% (Knpp et l., 22). In our study, the shift to spring summer preipittion pttern pproximtely hlved hereous prodution ompred to the other tretments, nd re ground ws 2% nd 4% greter in the SPRING thn CURRENT nd WINTER tretments, respetively. In the pst, gret del of emphsis hs een pled on totl nnul preipittion reltive to hnges in produtivity nd omposition of semi-rid plnt ommunities. Our results demonstrted tht speies omposition nd produtivity in the sgerush steppe n e ltered y the timing of preipittion. Others hve lso estlished tht ommunity response to ltered preipittion ptterns is diret result of the timing of wter vilility within the soil profile (Brown et l., 1997; Shwinning et l., 25). This eomes prtiulrly importnt in mking eologil ssessments nd deteting trends in rngelnd ondition. Lnd mngers re often presented with the hllenge of seprting wether effets from those relted to mngement tions. Experimentl results from this nd other studies should egin to provide mngers with knowledge they require to evlute wether-relted hnges in semi-rid nd rid eosystems. Aknowledgments The reserh ws funded y the Vegettion Diversity Projet, United Sttes Geologil Survey, Forest nd Rngelnd Eosystem Siene Center, Corvllis, Oregon. We thnk Dr. H. Wyne Polley nd Dr. Der Peters for onstrutive reviews of the mnusript. The uthors knowledge the tehnil/field ssistne of Roxnne Biley nd Kr Pintner nd numerous summer student tehniins. The Estern Oregon Agriulturl Reserh Center is jointly funded y the Oregon Agriulturl Experiment Sttion nd USDA-Agriulturl Reserh Servie. Referenes Btes, J., Miller, R.F., Svejr, T.S., 2. Understory dynmis in ut nd unut western juniper woodlnds. Journl of Rnge Mngement 53, Brnson, F.A., Miller, R.F., MQueen, I.S., Moisture reltionships in twelve northern desert shru ommunities ner Grnd Juntion, Colordo. Eology 57, Brown, J.H., Vlone, T.J., Curtin, C.G., Reorgniztion of n rid eosystem in response to reent limte hnge. Proeedings of the Ntionl Ademy of Siene 94, Cldwell, M.M., White, R.S., Moore, R.T., Cmp, L.B., Cron lne, produtivity: wter use of old-winter desert shru ommunities dominted y C 3 nd C 4 speies. Oeologi 29, Cnfield, R.H., Applition of the line interept in smpling rnge vegettion. Journl of Forestry 39,

26 J.D. Btes et l. / Journl of Arid Environments 64 (26) Comstok, J.P., Ehleringer, J.R., Plnt dpttion in the Gret sin nd Colordo Plteu. Gret Bsin Nturlist 52, Cook, J.G., Irwin, L.L., Climte vegettion reltionships etween the Gret Plins nd Gret Bsin. Amerin Midlnd Nturlist 127, Dvies, K.W., Btes, J.D., Miller, R.F., 24. Vegettion nd environmentl hrteristis of the Wyoming ig sgerush lline in Oregon. In: Btes, J. (Ed.), Eology of the Wyoming Big Sgerush Alline in the Northern Gret Bsin: 24 Speil Report. Estern Oregon Agriulturl Reserh Center, Burns, Oregon 65pp. Dorowolski, J.P., Cldwell, M.M., Rihrds, J.H., 199. Bsin hydrology nd plnt root systems. In: Ormond, C.B., Pitelk, L.F., Hidy, G.M. (Eds.), Plnt Biology of the Bsin nd Rnge. Eologil Studies 8. Springer, Berlin. Esterling, D.R., Meehl, G.A., Prmesn, C., Chngnon, S.A., Krl, T.R., Merns, L.O., 2. Climte Extremes: Oservtions, Modeling, nd Impts. Siene 289, Ehleringer, J.R., Shwinning, S., Geuer, R., Wter use in rid lnd eosystems. In: Press, M.C., Sholes, J.D., Brker, M.G. (Eds.), Physiologil Plnt Eology. Blkwell Siene, Boston, USA, pp Ehleringer, J.R., Cerling, T.E., Flngn, L.B., 21. Glol hnges nd the linkges etween physiologil eology nd eosystem eology. In: Press, M., Huntly, N., Levin, S. (Eds.), Eology: Ahievement nd Chllenge. Blkwell, Oxford, pp Evns, R.D., Blk, R.A., Link, S.O., Reprodutive growth during drought in Artemisi tridentt. Funtionl Eology 5, Fy, P.A., Crlisle, J.D., Dnner, B.T., Lett, M.S., MCrron, J.K., Stewrt, C., Knpp, A.K., Blir, J.M., Collins, S.L., 22. Altered rinfll ptterns, gs exhnge, nd growth in grsses nd fors. Interntionl Journl of Plnt Sienes 163, Gretz, R.D., Wlker, B.H., Wlker, P.A., The onsequene of limte hnge for seventy perent of Austrli. Nturl Environment, Grime, J.P., Brown, V.K., Thompson, K., Msters, G.J., Hillier, S.H., Clrke, I.P., Askew, A.P., Corker, D., Kielty, J.P., 2. The response of two ontrsting limestone grsslnds to simulted limte hnge. Siene 289, Hithok, C.L., Cronquist, A., Flor of the Pifi Northwest, Sixth Printing. University of Wshington Press, Settle, WA, USA. IPCC, 21. Climte hnge 21: the sientifi sis. Contriution of Working Group I to the Third Assessment Report of the Interntionl Pnel on Climte Chnge, Cmridge University Press, Cmridge, 881pp. Knpp, A.K., Fy, P.A., Blir, J.M., Collins, S.L., Smith, M.D., Crlisle, J.D., Hrper, C.W., Dnner, B.T., Lett, M.S., MCrron, J.K., 22. Rinfll vriility, ron yling, nd plnt speies diversity in mesi grsslnd. Siene 298, Lentz, D.R., Simonson, G.H., A detiled soils inventory nd ssoited vegettion of Squw Butte Rnge Experiment Sttion. Speil Report 76, Agriulturl Experiment Sttion, Oregon Stte University, Corvllis, 184pp. Link, S.O., Gee, G.W., Downs, J.L., 199. The effets of wter stress on phenologil nd eophysiologil hrteristis of hetgrss nd Snderg s luegrss. Journl of Rnge Mngement 43, MKy, P., Johnson, L., Billow, C., Miller, J., Pu, R., Ftors ffeting loss in mss of reosoteush ush lef litter in the northern Chihuhun desert. Southwest Nturlist 39, Moorehed, D.L., Cllghn, J.F., Effets of inresing ultrviolet-b rdition on deomposition nd soil orgni mtter dynmis: synthesis nd modeling study. Biology nd Fertility of Soils 18, Nsri, M., Doesher, P.S., Effet of temperture on growth of hetgrss nd Idho fesue. Journl of Rnge Mngement 48, Ntionl Assessment Synthesis Tem (NAST), 2. Climte Chnge Impts on the United Sttes: Potentil Consequenes of Climte Vriility nd Chnge. US Glol Chnge Reserh Progrm, Wshington, DC. Cmridge University Press, New York, USA 154pp.

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