Polytrichum strictum as a Nurse-Plant in Peatland Restoration

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1 Polytrichum strictum s Nurse-Plnt in Petlnd Restortion Eliseth V. G. Groeneveld, 1 Arine Mssé, 1 nd Line Rochefort 1, Astrct Polytrichum strictum is pioneer plnt frequently found on re pet sustrte fter perturtions (fire, pet extrction). Cn this moss fcilitte the return of Sphgnum species or other orel plnts fter disturnces? Field surveys of ndoned petlnds fter pet extrction reveled tht Sphgnum ws lwys found in ssocition with P. strictum crpets. We conducted field experiments in ndoned petlnds nd showed tht P. strictum crpets were le to keep Sphgnum frgments more humid thn re pet ut only when the P. strictum crpets were not totlly one dry. In generl, dytime tempertures eneth P. strictum crpets nd frgments were reduced during the dy nd incresed during the night compred to re pet. Polytrichum strictum crpets cted s seed trp, retining more rtificil seeds thn re pet. Polytrichum strictum cn e nurse-plnt: fter 16 months, vsculr plnts trnsplnted in the P. strictum crpet were helthier thn the ones plnted on re pet. The use of P. strictum s nurse-plnt in orel forest or petlnd restortion is recommended for sites prone to frost heving nd with hrsh microclimtic conditions. Key words: cutover petlnds, ecologicl restortion, fcilittion, microclimte, milled vcuum-hrvested petlnd, pioneer species, recoloniztion, Sphgnum. Introduction Sphgnum mosses do not redily recolonize re orgnic soils fter fire or nthropogenic mechnicl disturnce (Joosten 1995; Poulin et l. 5). Former vcuumhrvested petlnds re hostile sustrtes, rren res, nd mostly devoid of Sphgnum mosses (Lvoie et l. 5). As Sphgnum is the min keystone plnt of northern petlnd ecosystems (Vn Breemen 1995), it is judged necessry to reintroduce the pet mosses during restortion (Poschlod 1995; Rochefort ; Rochefort et l. 3). Plnt coloniztion of re pet is severely curtiled for severl resons, including the sence of seed nk of petlnd species (Slonen 1987; Huoplinen et l. 1998), nd limited concentrtion of criticl nutrients such s phosphorus (Wind-Mulder et l. 1996). During summer, invding plnts fce poor hydrologicl conditions (Schouwenrs 1993; Price & Whitehed 1; Price et l. 3) nd hrsh microclimte with potentilly lethl tempertures (Sgot & Rochefort 1996; Price et l. 1998; Boudreu & Rochefort 1999). Although soil moisture conditions re improved in the spring nd fll when the ditches of the former dringe system re locked, the incresed moisture, together with elow-freezing tempertures, led to frost heving tht cn e devstting for newly estlished plnts (Rietveld & Heidmnn 1976; 1 Groupe de recherche en écologie des tourières (GRET) et Centre d études nordiques, Pvillon Pul-Comtois, Université Lvl, Quéec, Cnd G1K 7P Address correspondence to L. Rochefort, emil line.rochefort@fs.ulvl.c Ó 7 Society for Ecologicl Restortion Interntionl Grtner et l. 1986; Groeneveld & Rochefort, 5). In ddition to frost heve, invding plnts suffer from pet instility due to overlnd flow during snowmelt nd wind low (Anderson 1997; Cmpell et l. ). In spite of these difficult conditions, some tough pioneer plnts re commonly found on postmilled sites such s cotton grsses, irches, nd Polytrichum mosses (Tuitill et l. ; see ppendix 1 in Poulin et l. 5; Lvoie et l. 5,). Polytrichum species re pioneer mosses well dpted for growth on re sustrte fter fire (Johnson 1981; Morneu & Pyette 1989; Mlty et l. 199; Corrdini & Clément 1999) or on cutover ogs (Grosversnier et l. 1995; Lvoie et l. 5,). Specilized leves nd n internl wter conduction system llow Polytrichum mosses to photosynthesize t higher light levels nd lower wter content thn do other mosses (Byfield 1973; Cllghn et l. 1978; Silvol 1991). Resistnce to uril nd frost heve, s well s the ility to stilize loose soil, llows Polytrichum mosses to colonize unstle sustrtes (Lech 1931; Birse et l. 1957; Collins 1969; Fenton & Lewis Smith 198; Fuert & Rochefort ; Groeneveld & Rochefort 5). Once estlished, P. strictum ppers to ct s nurseplnt, fcilitting the growth of other species including Sphgnum (Buttler et l. 1996; Roert et l. 1999). Prker et l. (1997) demonstrted tht P. commune increses drought survivl of white spruce. Groeneveld nd Rochefort (5) demonstrted tht P. strictum drmticlly reduced the frost heving of fir seedlings. Other suggested enefits include fcilittion of seed germintion, meliortion of the microclimte, nd the prevention of crust formtion, which could isolte dispores DECEMBER 7 Restortion Ecology Vol. 15, No., pp

2 Polytrichum strictum s Nurse-Plnt from the soil wter (Mrsh & Koerner 197; Grosvernier et l. 1995; Prker et l. 1997). However, the microclimte within P. strictum crpet hs not hitherto een chrcterized nd the nursing functions of Polytrichum moss re poorly understood. The ojective of this study ws to determine how P. strictum nurses Sphgnum. First, systemtic field survey on spontneously revegetting ndoned pet fields ws done to evlute the presence of Sphgnum in reltion to P. strictum. Second, field experiment ws designed to chrcterize the microclimte of P. strictum crpet, in terms of irrdince, temperture, nd moisture. Third, the role of P. strictum s seed trp nd s nurse-plnt fcilitting plnt survivl nd growth ws lso exmined. Methods Field Survey on the Co-occurrence of Polytrichum strictum nd Sphgnum To verify if Sphgnum mosses were more likely to grow with Polytrichum strictum, two ndoned pet fields (3 3 m) were surveyed. The pet fields ndoned 1 yers prior to investigtion were chosen ecuse they were spontneously revegetting. The sites were locted in Lc St-Jen, Quéec (lt 8 79N, long 7 19W), in 6,-h petlnd. The sites were surrounded y nturl petlnds t distnce of 3 m nd dispores, moss frgments, or seeds, could esily disseminte onto the re et. Systemtic smpling surveys were crried out in to mke sure tht we smpled the whole pet field s opposed to rndom smpling. The distnce etween the smpling sttions ws set to 1 m cross the width of ech field nd to m long the length of ech field nd gve totl of 11 points smpled. At ech smpling point, wter well ws ored using n uger. The wter tle nd the presence of P. strictum nd Sphgnum were noted t ech smpling point on the two ndoned pet fields five dys fter the wells were ored. Nursing Effect Experiment An importnt ojective of this study ws to test whether there ws cusl effect reltionship for the oserved P. strictum Sphgnum ssocition on spontneously revegetting pet fields. An experimentl design ws used to chrcterize the microclimte generted y P. strictum, in terms of irrdince, temperture, nd moisture nd to evlute the role of P. strictum s seed trp for dispores nd s nurse-plnt fcilitting plnt growth. The experimentl design ws implemented in pet field locted ner Rivière-du-Loup, Quéec (lt 7 89N, long 69 89W), tht ws hrvested y the vcuum method for pproximtely 1 yers, nd then ndoned in 198. Spontneous regenertion of plnt cover on the ndoned pet fields hs een poor likely due to the hrsh microclimtic conditions. The site nd experimentl design re descried in detil in Groeneveld nd Rochefort (5). In rief, three levels of P. strictum covers were tested for their nursing effect: (1) P. strictum trnsplnted crpet; () P. strictum frgments stems of P. strictum moss (verge length ¼ 7.9 cm) were hrvested from helthy colony nd spred over the re pet; nd (3) no cover (re pet). Strw hs een shown to promote the growth of newly estlishing og vegettion (Price et l. 1998) nd is used in the North Americn method of og restortion. We tested the three levels of P. strictum cover in fctoril rrngement with strw (two levels: present, sent) pplied t rte of 3, kg/h, in complete rndomized lock design with six locks. Ech experimentl unit mesured m nd ws seprted y t lest 1 m from its nerest neighor. Although very sprse, ll existing vegettion (including roots) nd deris were removed from the locks on the ndoned pet field, nd the pet surfce ws smoothed with grden rke efore pplying the tretments. The resulting smooth pet surfce hd the ppernce of freshly ndoned pet field. A wooden ordwlk ws instlled etween the plots to minimize trmpling. A vrile numer of tretments nd locks were used for the three different ojectives of the study (Tle 1). The experimentl design ws put into plce in My. Dt were collected from 3 July to 16 Octoer 1. The Microclimte in Polytrichum strictum Crpet. The microclimtic conditions generted under the following four different tretments were tested during July nd August : P. strictum crpet (without strw), P. strictum frgments (without strw), strw on re pet, nd re pet (Tle 1). Mesures were tken successively during mny smpling periods to do fine-description P. strictum microclimte over the plnt growing seson. Irrdince. Irrdince mesures were tken in ech tretment with photometer (SunScn Anlysis System Delt-T Devices Ltd., Cmridge, United Kingdom; 1-m-long proe contining 6 photodiodes). Mesures were tken in midsummer on 1 July nd in lte-summer on 1 August, under the following conditions: it ws middy, there ws t lest lmol m second 1 of light nd there were no rupt chnges from sunny to cloudy (Potter et l. 1996). In the re pet tretment, the 1-m-long proe ws plced on the pet surfce cross the center of ech plot. Where there were frgments nd strw, the proe ws inserted eneth the cover in the center of the plot. For mesures in the crpet, long nrrow furrow ws cut pproximtely 15 cm from the edge of ech crpet. When the proe ws lid in the furrow, it ws t the sme height s the rhizoid cot of the P. strictum stems (where Sphgnum is often found growing). The surfce of the proe ws then covered with freshly clipped P. strictum so s to recrete the ppernce of nturl crpet. This method is similr to tht used y Keizer et l. (1985). Within ech lock, the order of dt collection etween experimentl units ws rndomized. 71 Restortion Ecology DECEMBER 7

3 Polytrichum strictum s Nurse-Plnt Tle 1. Nme, purpose, nd description of tretments used for field experiments on Polytrichum strictum s nurse-plnt t n ndoned vcuum-hrvested og ner Rivière-du-Loup, Quéec. Tretment Numer* Tretment P. strictum Strw Tretment Purpose Tretment Description 1 P. strictum crpet Strw sent To simulte dense P. strictum colony Squres of P. strictum were removed intct from nturlly occurring colonies nd trnsplnted into the experimentl plots P. strictum crpet Strw present As tretment 1, ut with 3, kg/h of strw spred over the crpet 3 P. strictum frgments Strw sent A tretment to simulte the reintroduction of plnt mteril ccording to the usul North Americn restortion techniques P. strictum frgments were clipped from helthy colony nd spred over ech experimentl plot of re pet in rtio of 1:1, which resulted frgments cover of pproximtely 75% of the re pet P. strictum frgments Strw present As tretment 3, ut with 3, kg/h 5 Bre pet Strw sent To test conditions on surfce left re fter hrvesting 6 Bre pet Strw present To test the effect of strw mulch compred to Polytrichum crpets of strw spred over the frgments Bre pet 3, kg/h of strw ws evenly spred over re pet * Vrying numers of tretments nd locks were used for ech experiment, s follows microclimte experiment: tretments 1, 3, 5, nd 6, nd n ¼ 6; seed trp experiment: tretments 1 nd 5, nd n ¼ 3, n ¼ 6; nd nurse-plnt experiment: tretments 1,, 3,, 5, nd 6, nd n ¼ 6. Air Temperture. Air temperture ws mesured every 3 minutes during three periods: 1 19 July, 3 July to 3 August, nd 8 August. StowAwy dtloggers (Onset Computer Corportion, Pocsset, MA, U.S.A.) with externl sensors were plced in ech plot. The sensor ws plced in the center of ech plot within the rhizoids of the P. strictum crpet, eneth the frgments, on re pet, or eneth the strw. One logger ws plced per experimentl unit, in ech of the six locks. Cumultive curves of the temperture differences etween the tretments nd re pet were then clculted. Wter Content of Sphgnum. For the moisture contents, we chose to use Sphgnum moss itself (individul stems with cpitulum, herefter clled shoot) s method for mesuring the moisture provided y P. strictum crpet. Sphgnum hs no physiologicl mechnism to prevent wter loss nd therefore tends to e in equilirium with its environment. Sphgnum fuscum (Schimp.) Klinggr. ws chosen for this mesure. Sphgnum fuscum ws hrvested in the evening preceding its use, cut into.5-cm-long shoots then stored for 1 hours t C. Prior to eing inserted in the tretments, the Sphgnum shoots were sturted with og wter. Ten Sphgnum shoots per plot were plced t rndom loctions within the P. strictum crpet, eneth the P. strictum frgments, on the re pet, nd eneth the strw. Becuse the P. strictum frgments hd een spred in 1:1 rtio (1 m of collected mteril spred over 1 m ), the Sphgnum shoots were not necessrily eneth frgments t every loction. In the P. strictum crpets they were horizontlly inserted in the zone where the stem ws covered with the white tomentum. Sphgnum shoots were plced out once week from July to August for totl of six time periods: July, 18 July, 5 8 July, 1 August, 8 11 August, nd 5 7 August. The Sphgnum shoots were retrieved on rinless dys, 3 5 dys following their insertion. The order of retrievl ws rndomized within ech lock. The 1 Sphgnum shoots were collected, plced in screw-top ottle (one ottle per shoot, 1 ottles per plot, ut men vlue ws retined for further clcultion) nd stored t C for mximum of hours. The wet weight ws then mesured. Dry weight ws determined following hours of oven drying t 15 C nd percent wter content ws clculted. A pluviometer ws instlled on the pet field nd dt were collected dily to clculte totl precipittion for ech experimentl period. Polytrichum strictum s Seed Trp. Studies with other moss species hve shown tht ryophyte crpets my ct s seed trps nd reduce the predtion of lrge seeds (Vn Tooren 1988). Andoned pet fields re usully ner nturl petlnds, nd dispores dispersion from nturl sites to pet field is frequent (Cmpell et l. 3). The purpose of this experiment ws to determine if P. strictum crpets ct s fcilittor for immigrtion y trpping dispores s suggested y Prker et l. (1997), which we supposed should fvor recoloniztion of the site fterwrd. The experiment ws designed with the following ssumption: ll ground covers receive n equl mount of DECEMBER 7 Restortion Ecology 711

4 Polytrichum strictum s Nurse-Plnt seed rin. After given mount of time, more seeds will remin on some sustrtes thn others ecuse the sustrtes vry in their ility to retin the seeds. The suitility of P. strictum moss lyer s seed trp ws tested using rtificil seeds. Two different sustrtes were tested for seed retention: P. strictum crpet nd re pet. Artificil seeds were constructed from chicken fether dyed right red nd were used s dispore dispersion model. Rel seeds were very frgile nd difficult to mnipulte nd these rtificil seeds hd the dvntge of eing more durle nd visile thn rel seeds. Forty rtificil seeds were gently deposited (to simulte rrivl y wind) on the re pet nd on the crpet. Artificil seeds mesured round 3 mm long nd wide nd were comprle to rel seed dimensions. The experiment ws conducted on two different wind conditions: (1) strong wind on the 8 August (n ¼ 3) nd () light wind on August (n ¼ 6; for this second tril, 3 rtificil seeds per plot were used). The numer of rtificil seeds remining fter 1 nd hours for the strong-wind conditions nd fter hours for the light-wind conditions ws counted. Polytrichum strictum s Nurse-Plnt. Species tht enefit from the protection of vrious memers of the Polytrichum genus include lck spruce seedlings, white spruce seedlings, vrious woody plnts, nd Sphgnum mosses (Mrsh & Koener 197; Buttler et l. 1996; Filion & Morin 1996; Prker et l. 1997). We wnted to evlute if P. strictum fcilittes estlishment of reintroduced plnts, such s Sphgnum. As proxy for time, we used fir (Aies lsme) seedlings s iologicl model for plnt reintroduction in P. strictum crpet ecuse we knew they would respond more quickly thn witing for the formtion of Sphgnum cushions ( 5 yers to develop). Fir seedlings were chosen ecuse they were undnt on the site nd esy to trnsplnt. In ech experimentl unit, eight smll fir seedlings were plnted in the pet. Seedlings with only one shoot (no rnches) nd not more thn two rown needles were trnsplnted. The verge shoot length ws. ±.6 cm, the verge root length 9.9 ±.8 cm. Seedling helth on scle of 1 3 (1 ¼ ded, ¼ intermedite, nd 3 ¼ helthy) ws evluted two nd 11 months fter plnting, on August nd 9 My 1. A finl rting ws given 16 months fter the seedlings hd een plnted, on 16 Octoer 1. On this dte scle of 1 5 ws used, with 1 ¼ ded; ¼ lmost ded, only few living needles; 3 ¼ intermedite, ll leves chlorotic or mny leves ded; ¼ lmost helthy, only few chlorotic or ded leves; nd 5 ¼ perfectly helthy, ll leves right green. Seedlings tht were missing ut hd een noted s ded on prior smpling dte were considered ded; ll other missing seedlings were excluded from the nlysis. Sttisticl Anlyses For the field survey on the presence of Sphgnum, chisqure test ws used to determine if Sphgnum ws more likely to e found ssocited with P. strictum thn y chnce lone. All other sttisticl nlyses were performed using the generl liner models procedure of SAS (SAS Institute, Inc. 1988). The Microclimte in Polytrichum strictum Crpet. We used n nlysis of vrince (ANOVA) with smpling period s repeted fctor to evlute the effect of the four tretments on irrdince nd soil moisture. A protected lest significnt difference ws used to locte differences etween tretment mens once tretment effects were found significnt. For the irrdince, the percent photosyntheticlly ctive rdition (PAR) trnsmitted through ech cover ws clculted nd dt were trnsformed (log [x 1 1]) to normlize the distriution of the residuls. For the field experiment on wter content cuic trnsformtion ws necessry to reduce vrince heterogeneity. Five dt out of 1 were removed from the dtset ecuse the percent moisture of Sphgnum cme out negtive ecuse of n error in the initil weight of either the screw-top ottle or Sphgnum. To help interpret the vrition in wter content, we used totl precipittions mesured with the pluviometer instlled on the ndoned pet field nd dily temperture recorded with the dtloggers. We used records from dtloggers plced on the re pet for the microclimte experiment to clculte verge dily temperture etween six nd 18 hours for ech experimentl period. For the experimentl period from 18 July, temperture dt were ville only for the first two dys. Polytrichum strictum s Seed Trp. Different ANOVAs were done for ech seed count (ecuse they included different totl numer of locks) on ech windy condition to compre percent seed remining etween P. strictum crpet nd re pet. Polytrichum strictum s Nurse-plnt. We used fctoril design to investigte the effect of P. strictum on plnt growth nd survivl. We considered helth index s quntittive vrile nd thus clculte men helth index (MHI) for every tretment. Where there ws no interction etween the min effects (Polytrichum cover, strw cover), Tukey multiple comprison test ws done on the mens of ech fctor verged over ll the levels of the other fctor (Sokl & Rohlf 1981). Where there ws n interction etween the min effects, the Tukey multiple comprison test ws limited to compring the mens of one fctor within ech level of the other fctor. Results Co-occurrence of Polytrichum strictum nd Sphgnum Polytrichum strictum ws present t 93 smpling points out of 11 on the ndoned pet fields. Where P. strictum ws spontneously revegetting, we found 31 smpling points where Sphgnum ws lso present. On the other 71 Restortion Ecology DECEMBER 7

5 Polytrichum strictum s Nurse-Plnt hnd, we found no point where Sphgnum grew lone (v ¼ 3.85, p ¼.5). When Sphgnum nd P. strictum were sent, wter tle ws reltively nerer the surfce (Tle ). Polytrichum strictum nd Sphgnum were ssocited where wter tle ws intermedite nd P. strictum ws present lone t drier smpling points (Tle ). Nursing Effect The Microclimte in Polytrichum strictum Crpet. Irrdince. The mount of PAR pssing through the P. strictum crpet ws 5%, through the P. strictum frgments %, nd 3% in the presence of the strw mulch (ANOVA Tretment : df ¼ 3, F ¼ , p <.1). Although the smpling period hd significnt effect on PAR s it ws mesured t different times of the seson, the difference etween the tretments ws similr for the two smpling periods (ANOVA Time : df ¼ 1, F ¼ 1., p ¼.; ANOV- A Time3Tretment : df ¼ 3, F ¼.7, p ¼.15). Averge photosynthetic photon flux density reching the surfce ws 38 for the crpet, 19 for the strw mulch, 39 for the frgments, nd 677 lmol m second 1 for the re pet. Air Temperture. The temperture t the surfce of the re pet nd eneth the P. strictum crpet, frgments, nd strw re presented in Figure 1. Temperture dt re presented s cumultive temperture curves, where the curve reflects the percent time when temperture is greter thn tht shown on the y-xis. The trends were similr for the three time periods; ove 1 19 C, tempertures under the covers re generlly cooler thn those oserved on re pet. Cumultive temperture difference curves etween the tretment nd the re pet is presented in Figure 1. Temperture reduction cused y the tretments is shown when the temperture difference is positive, nd temperture increse cused y the tretments is shown when the temperture difference is negtive. Below this sme rnge, tempertures re generlly wrmer thn those oserved on re pet. In generl, the gretest temperture reduction compred to re pet ws oserved t the highest tempertures (Fig. 1). In other words, mplitude of vrition is lessened y the presence of P. strictum tretments when compred to re pet surfce temperture. Tle. Assocition etween Polytrichum strictum nd Sphgnum on n ndoned pet field spontneously revegetting t Lc St-Jen, Quéec, nd the ssocite wter level.* n Men Wter Tle (cm) P. strictum present Sphgnum sent ±.6 Sphgnum present ± 1.7 P. strictum sent Sphgnum sent ±. Sphgnum present No occurrence Totl 11 *N ¼ numer of smpling points where the genus ws recorded, out of 11 points. Temperture ( C) Temperture ( C) Temperture ( C) Pet Strw Temperture difference ( C) Temperture difference ( C) Temperture difference ( C) Percent of the time where temperture > tht shown on the Y xis Crpet Frgment Figure 1. Temperture durtion curves for surfce temperture on n ndoned vcuum-hrvested og locted ner Rivière-du-Loup, Quéec, from 1 19 July upper grphs, 3 July to 3 August middle grphs, nd 8 August lower grphs. Temperture ws mesured on re pet, nd eneth Polytrichum strictum crpet, etween P. strictum frgments, nd strw (left), nd temperture difference etween the three covers nd re pet ws clculted (right). Wter Content of Sphgnum. The effect of P. strictum crpet, P. strictum frgments, nd strw on moisture vried from period to period (ANOVA Dte3Tretment : df ¼ 15, F ¼.66, p ¼.) nd ws influenced y the temperture nd precipittions (Fig. ). When experiencing dry nd wrm spells (see first two periods of July of Fig. ), Sphgnum stems in ll tretments were eqully dry, the verge wter content over oth periods for ll the tretments eing %. On the other hnd, when it is hot ut some moisture is ville from precipittion (see the two periods t end of July nd eginning of August of Fig. ), P. strictum or strw tretments ll hve firly similr effect in improving moisture condition t the ir pet interfce when compred to re pet: rnging from 35 to 53% compred to 1% in July nd from 95 to 165% compred to 31% in August. During moist periods (see the lst two oservtion periods of August of Fig. ), wter content in Sphgnum for ll tretments re higher (in the rnge of 7%), ut still ll tretments (P. strictum or strw dditions) tend to provide more moisture t the interfce ir pet DECEMBER 7 Restortion Ecology 713

6 Polytrichum strictum s Nurse-Plnt to 17 July 18 to July 5 to 8 July c c 1 to August 8 to 11 August 5 to 7 August Temperture ( C) or Precipittion (mm) Crpet Strw Pet Frgment Dytime temperture Precipittion Figure. Averge dily temperture (from 6 18 hours), totl precipittion, nd men wter content (±SE) of stem sections of Sphgnum fuscum plced in Polytrichum strictum crpet, etween P. strictum frgments, nd in strw, or on the re pet surfce of n ndoned vcuum-hrvested og locted ner Rivière-du-Loup, Quéec, during six mesurement periods. Identicl letters identify men wter content tht did not differ sttisticlly etween tretments (p.5) for specific time intervl. Polytrichum strictum s Seed Trp. For the seed retention test during strong-wind condition, fter one hour significntly more seeds were retined on the crpet (9%) thn on the re pet (5%) (ANOVA Tretment : df ¼ 1, F ¼ 8.8, p ¼.; Fig. 3). After two hours, the difference ws even greter, with 9% retention on the crpet nd 3% retention on the re pet (ANOVA Tretment : df ¼ 1, F ¼ 1., p ¼.1). When the tril ws repeted during light-wind conditions, more seeds were retined on the crpet (9%) thn on re pet (86%), ut this difference ws not significnt (ANOVA Tretment : df ¼ 1, F ¼.1, p ¼.1; Fig. 3). Polytrichum strictum s Nurse-plnt. Two months fter plnting, seedlings growing in the P. strictum crpet (men helth index ¼.85) were significntly less helthy thn seedlings growing in the re pet (MHI ¼ 3.), regrdless of the presence of strw (ANOVA Polytrichum3Strw : df ¼, F ¼.91, p ¼.; ANOVA Polytrichum : df ¼, F ¼ 3.55, p ¼.), ut still, the effect of P. strictum is minim (Fig. ). The helth of seedlings plnted in the frgments ws intermedite (.95) nd did not differ significntly from the two others. Strw hd no significnt effect on fir helth (ANOVA Strw : df ¼ 1, F ¼.73, p ¼.; strw: MHI ¼.97; no-strw MHI ¼.9). Eleven months fter they were plnted, seedling helth nd survivl were ffected only y the presence of strw (ANOVA Strw : df ¼ 1, F ¼ 9., p ¼.6; Fig. ). Seedlings were significntly helthier nd less likely to die when plnted in strw (MHI ¼.63) compred to when there ws no strw independently of the presence of P. strictum crpet (MHI ¼.; ANOVA Polytrichum3Strw : df ¼, F ¼ 1.7, p ¼.; ANOVA Polytrichum : df ¼, F ¼.15, p ¼.1). Sixteen months fter they were plnted, seedling helth nd survivl ws significntly ffected y the ground cover; the survivl of seedlings differed depending on the level of P. strictum nd the presence of strw (ANOVA Polytrichum3Strw : df ¼, F ¼.89, p ¼.; ANOVA Polytrichum : df ¼, F ¼ 1., p ¼.3; ANOVA Strw : df ¼ 1, F ¼ 5.85, p ¼.; Fig. c). Seedlings plnted in the P. strictum crpet were helthy regrdless of the presence of strw (crpet MHI ¼ 3.; crpet 1 strw MHI ¼.9). On the other hnd, seedlings plnted in frgments fred etter when lso covered with strw (frgment MHI ¼.7; frgments 1 strw MHI ¼ 3.7), s did seedlings plnted in re pet (re pet MHI ¼.; strw MHI ¼ 3.). When compring the three levels of P. strictum in sence of strw, seedlings were helthier when plnted in P. strictum crpet (Fig. c). Discussion Nursing Effect We found tht Polytrichum strictum is le to spontneously recolonize ndoned pet fields. On the other hnd, Sphgnum seems to only colonize microsites where P. strictum is present. We conclude tht when 71 Restortion Ecology DECEMBER 7

7 Polytrichum strictum s Nurse-Plnt () Windy dy () Fir seedling helth months Crpet Frgment Asent () Light wind Initil Initil Crpet 1 hour hours hours Pet Figure 3. Men percent rtificil seeds remining (±SE) on Polytrichum strictum crpet nd on the re pet surfce of n ndoned vcuum-hrvested og locted ner Rivière-du-Loup, Quéec, during () windy dy on 8 August nd () light wind dy on August. Identicl letters identify men percent seeds remining tht did not differ sttisticlly etween tretments (p.5). Anlyses were performed seprtely for ech time intervl. P. strictum Sphgnum ssocition is found, Sphgnum enefits from the presence of P. strictum. In generl, nurse-plnts offer mny enefits to the eneficiries, including reduction of temperture extremes, meliortion of light conditions, improved moisture vilility, soil stiliztion, incresed nutrient vilility, nd predtor protection (Ltheef & Ortiz 198; Gill & Mrks 1991; Vliente-Bnuet & Ezcurr 1991; Cllwy 199; Belsky 199; Suzán et l. 1996; Mrtinez & Moreno-Cssol 1998; Rffele & Velen 1998). () Fir seedling helth Fir seedling helth (c) 11 months months Crpet Frgment Asent Crpet No strw * * Frgment strw Asent Figure. MHI (±SE) of fir seedlings plnted on n ndoned vcuum-hrvested og ner Rivière-du-Loup, Quéec. () Two months fter plnttion, Tukey min effects for Polytrichum: crpet ¼, frgments ¼, sent ¼. Tukey min effect for strw: sent ¼, present ¼. () Eleven months fter plnttion, Tukey min effects for Polytrichum:crpet¼, frgments ¼, sent ¼. Tukey min effect for strw: sent ¼, present ¼. (c) Sixteen months fter plnttion, ANOVA Polytrichum3Strw : p ¼.. Asterisk (*) indictes significnt difference etween levels of strw. Helth indexes for () nd () 1 ¼ ded, ¼ intermedite, 3 ¼ helthy nd for (c) 1 ¼ ded, ¼ lmost ded, 3 ¼ intermedite, ¼ lmost helthy, 5 ¼ helthy. The Microclimte in Polytrichum strictum Crpet. Irrdince. The mount of PAR trnsmitted within the P. strictum crpet, frgments, nd strw ws reduced compred to the re pet. This reduction in light could explin, in prt, the ssocition found etween Sphgnum nd P. strictum in the field. In fct, reduction in light, rnging from 38 to 8% hs een found to fvor Sphgnum growth (Sonesson et l. 198; Murry et l. 1989; Rochefort & Bstien 1998). Although Sphgnum requires certin minimum of light to grow, exposure to high light levels cn reduce the photosynthetic cpcity (Hrley et l. 1989; Murry et l. 1993). As long s the light compenstion point is reched, Sphgnum will e le to grow in the shde. On the re pet, PAR will often e ove light sturtion levels, cusing photoinhiition. Thus, in context of DECEMBER 7 Restortion Ecology 715

8 Polytrichum strictum s Nurse-Plnt Sphgnum estlishment during petlnd restortion, microclimte with shde could e importnt ecuse the whole Sphgnum individul is exposed to light compred to nturl colony were only the cpitulum is exposed. Air Temperture. Our results demonstrte tht the presence of P. strictum is le to crete cooler microclimte for Sphgnum. Knowing tht high tempertures my limit the growth of Sphgnum y incresing evportion nd respirtion, nd reducing photosynthesis nd destroying plnt cells, P. strictum cn ply n importnt role s compnion to Sphgnum y keeping it cooler (Hrley et l. 1989; Blgurov et l. 1996; Sgot & Rochefort 1996; Riis & Snd-Jensen 1997). Mny studies hve shown tht nurse-plnts cn promote the estlishment of eneficiry species y reducing soil tempertures. For exmple, on highly distured re pit heps, Richrdson (1958) descries how lyer of moss nd lichens nursed grminoid y decresing the soil temperture up to 1 C. This effect hs lso een documented for mosses nd vsculr plnts in pet ogs, the Sonorn Desert, nd snd dunes (Frnco & Noel 1988, 1989; Noel 1989; Suzán et l. 1996; Mrtinez & Moreno-Cssol 1998; Boudreu & Rochefort 1999; Mrcoux ). The optiml temperture for Sphgnum photosynthesis vries y species nd climtic conditions. The optiml temperture for S. squrrosum, S. ngustifolium, nd S. wrnstorfii is ner C; etween pproximtely 13 nd 3 C, t lest 75% of mximl photosynthesis ws mintined (Hrley et l. 1989). According to Kurets et l. (1993), ove 3 C, the cron lnce for S. fuscum ecme negtive. In this study, the P. strictum crpet reduced mximum tempertures during the dy. Potentil Sphgnum mosses growing in P. strictum crpets would spend more time in the idel temperture window thn Sphgnum growing on re pet. At night, tempertures were on verge higher eneth the P. strictum crpet thn on re pet; mximl nightly increse eing 3.8 C. The crpets increse the nighttime tempertures y reducing rditive nd convective het loss. The degree of dytime temperture reduction depended on climtic conditions. In mid-august, the dytime temperture on re pet rnged etween 3 nd C; it ws 6.3 C cooler on verge in the P. strictum crpet thn on the re pet. We hypothesize tht the undnt rinfll permitted P. strictum to remin with open leves (Byfield 1973), thus reducing dytime tempertures compred to re pet. The opposite sitution occurred in the end of July, when it ws very hot nd dry. During this time, it ws on verge. C cooler under the strw thn on the re pet. On the other hnd, it ws.7 C hotter in the crpet thn on the re pet. Polytrichum strictum likely folded its leves up to reduce wter loss, thus incresing the exposure of Sphgnum, or in this cse, the temperture sensor, to elevted tempertures. Wter Content of Sphgnum. Sphgnum plnts must remin hydrted to photosynthesize. In s little s dys, plnts my die following complete desicction. Plnts tht survive my regin photosynthetic function ut t much reduced rte (Skre & Oechel 1981; Schipperges & Rydin 1998). Our results confirm the existence of moister microclimte in the P. strictum crpet, frgments, nd strw tretments. The presence nd mplitude of this microclimte depends on the dytime temperture nd precipittion. A minimum of precipittion ws necessry for the different covers to crete n environment moister thn the re pet. In the lmost complete sence of precipittions with high tempertures (13 nd 18 July ), none of the covers offered moister environment in comprison to re pet. Estimtes of the compenstion wter content of mosses, elow which the net cron exchnge is negtive, vry from 6 to 5% (Skre & Oechel 1981; Schipperges & Rydin 1998), depending on oth species nd environmentl conditions. In mid-july the wter content ws well elow 6%, the lowest moisture compenstion point cited in literture. Buttler et l. (1998) showed in n experimentl setup tht microclimtic conditions under plstic mulch my compenste for low wter tle for the growth of Sphgnum. However, during severe drought conditions like we experienced in mid-july, mulch or nurse-plnts y themselves without ny hydrologicl remedition cnnot supply sufficient moisture to Sphgnum. With lower tempertures nd higher precipittion towrd the end of July nd eginning of August, wter content of Sphgnum mosses ws significntly higher in the crpet, strw, nd frgments tretments compred to re pet. Murry et l. (1989) found tht the optiml rnge for photosynthesis in Sphgnum moss is 6 1,%. Other uthors hve found optiml rnges of,5% for vrious species (Schipperges & Rydin 1998), 75% for S. susecundum (Skre & Oechel 1981), nd 6 1,% for S. fuscum (Silvol & Altonen 198). In this study, Sphgnum ws within its optiml rnge of wter content for photosynthesis more often in the P. strictum crpet, frgments, nd strw tretments, thn on re pet. In fct, for most of August, the wter content of Sphgnum in the crpet ws ove %, well within the moisture rnge required for Sphgnum photosynthesis. Nurse-plnts hve een found to increse soil moisture in mny environments including hrvested petlnds, shru lnds, deserts, nd distured re pit heps (Richrdson 1958; Vliente-Bnuet & Ezcurr 1991; Slonen 199; Rffele & Velen 1998). Mny studies hve shown tht the more humid conditions eneth vegettion or other protective covers were fvorle to the growth of Sphgnum (Slonen 199; Buttler et l. 1996; Price et l. 1998; Rochefort & Bstien 1998; Boudreu & Rochefort 1999). Polytrichum strictum s Seed Trp. Dispores dispersion from nturl sites to ndoned pet fields is n importnt source of moss frgments or seeds (Cmpell 716 Restortion Ecology DECEMBER 7

9 Polytrichum strictum s Nurse-Plnt et l. 3). Prker et l. (1997) suggest tht P. commune my help other plnts estlish y cpturing their seeds nd llowing them to germinte. In this study we present evidence tht P. strictum crpet cptures more rtificil seeds thn re soil. This is similr to Vn Tooren (1988) who determined tht the ryophyte lyer in chlk grsslnd cts s seed trp, ccumulting lrge numers of seeds. Mllik et l. (198) lso showed tht ryophyte lyers in hethlnds cn contin lrge numers of vile seeds. Other nurse-plnts with flt, cushion growth form hve een noted s seed trps for wind-dispersed seeds (Griggs 1956; Kikvidze 1993). Welden (1985) explins how nurse-plnts reduce wind velocity, thus llowing the deposition of fine wind-orn mterils, which potentilly could include seeds. Once the dispores hve een trpped y nurse-plnt, they re offered protected environment in which to germinte nd grow. However, not ll plnts grow etter in ryophyte crpet. Polytrichum commune hs een found to inhiit the growth of other hethlnd plnts (Corrdini & Clément 1999). Polytrichum formosum hs een shown to reduce soil minerliztion, proly cused y n llelopthic effect (Rozé 1987). This spect needs to e further investigted in reltion to Sphgnum frgment or spore germintion lthough the co-occurrence of Sphgnum mosses nd P. strictum in spontneously revegetting ndoned pet fields points to the contrry. Polytrichum strictum s Nurse-plnt. Once dispores hve een cptured y P. strictum crpet, they must survive nd grow. The third ojective of this study ws to evlute growth nd survivl within P. strictum crpets using fir seedlings s plnt reintroduction model. Two months fter plnting, seedlings growing in P. strictum crpet were less helthy thn the ones growing on re pet. This ws likely due to the trnsplnttion shock of seedlings in the P. strictum crpet. Indeed, the seedlings were inserted deeper within the crpet to rech firm soil nd tht method could hve een more stressful thn just plnting on re pet. Sixteen months fter plnting, we found tht seedlings growing in P. strictum crpets or under strw mulch were helthier thn the ones growing on re pet. Thus, we propose tht P. strictum crpets crete more fvorle microenvironment for og plnt growth. Nutrient competition could potentilly reduce the growth of plnts tking shelter in the P. strictum crpet. Phosphorus is the element most likely to e limiting ecuse it is scrce in posthrvested ogs (Wind-Mulder et l. 1996). However, study y Chpin et l. (1987) reveled tht Sphgnum moss, which my e nursed y Polytrichum, hd much higher rte of phosphte sorption thn Polytrichum, which helped it survive when developing within Polytrichum-dominted environment. In other cses, nurse-plnts cn ctully increse the mount of nutrients ville to the eneficiry (Wlker & Chpin 1987; Belsky 199). For exmple, study y Grtner et l. (1986) showed tht Eriophorum seedlings growing in the protection of moss mts hd higher nitrogen nd phosphorus concentrtions thn seedlings growing in frost oils. Becuse light phosphorus fertilizer ws pplied t the eginning of our experiment, nutrient competition ws unlikely. Mny uthors hve recently suggested tht nurse-plnts my hsten the reestlishment of typicl petlnd flor (Buttler et l. 1996; Boudreu & Rochefort 1999; Roert et l. 1999; Mrcoux ; Tuitill et l. ). In this pper we demonstrted tht (1) Sphgnum is present only where P. strictum ws spontneously recolonizing ndoned re pet surfces; () P. strictum crpet cn generte microclimtic conditions fvorle for the prolifertion of Sphgnum; nd (3) P. strictum my ply role s dispore trp nd nurse-plnt y improving plnt estlishment. As discussed in Groeneveld nd Rochefort (5) on P. strictum nd pet stility, strw mulch used in restortion my e the est option for protecting Sphgnum ecuse it does not compete for wter nd my relese nutrients s it decomposes. However, it degrdes rpidly, for most prt within three yers, nd on windy sites it my low wy. On sites where pet instility is prolem nd longer-term protection of Sphgnum is needed, fcilittion y P. strictum crpets is n interesting id to restortion. Implictions for Prctice d The use of Polytrichum strictum s nurse-plnt in orel forest or petlnd restortion is recommended for sites prone to frost heving nd with hrsh microclimtic conditions. d Estlishing P. strictum crpet is simple. Simply spreding moss frgments collected from og where P. strictum is present, nturlly occurring mong the Sphgnum lyer, lightly fertilizing with phosphorus, nd covering with strw mulch is sufficient to ensure continuous P. strictum crpet within yer. d To the est of our knowledge, P. strictum mosses should e reintroduced t the sme time s Sphgnum mosses when restoring cutover ogs s wrrnty ginst potentil dverse climtic conditions. Acknowledgments This study ws funded y the Nturl Sciences nd Engineering Reserch Council of Cnd nd y industril prtners who re memers of the Cndin Sphgnum Pet Moss Assocition. We extend our deepest thnks to the reserch ssistnts who prticipted in dt collection during the study. We re lso thnkful to Stéphnie Boudreu, Monique Poulin, nd mny grdute collegues for their helpful comments on the mnuscript. DECEMBER 7 Restortion Ecology 717

10 Polytrichum strictum s Nurse-Plnt LITERATURE CITED Anderson, P Fire dmge on lnket mires. Pges 16 8 in J. H. Tllis, R. Mede, nd P. D. Hulme, editors. Blnket mire degrdtion: cuses, consequences nd chllenges. The Mculy Lnd Use Reserch Institute, Aerdeen, United Kingdom. Blgurov, N., S. Drozdov, nd S. Grovik Cold nd het resistnce of five species of Sphgnum. Annles Botnici Fennici 33: Byfield, N. G Notes on the wter reltions of Polytrichum commune Hedw. Journl of Bryology 7: Belsky, A. J Influence of trees on svnn productivity: tests of shde, nutrients, nd tree-grss competition. Ecology 75:9 93. Birse, E. M., S. Y. Lndserg, nd C. H. Giminghm The effects of uril y snd on dune mosses. Trnsctions of the British Bryologicl Society 3: Boudreu, S., nd L. Rochefort Étlissement de sphignes réintroduites sous diverses communutés végétles recolonisnt les tourières près l exploittion. Écologie 3:53 6. Buttler, A., P. Grosvernier, nd Y. Mtthey Development of Sphgnum fllx dispores on re pet with implictions for the restortion of cut-over ogs. Journl of Applied Ecology 35: Buttler, A., B. Wrner, P. Grosvernier, nd Y. Mtthey Verticl ptterns of testte moee (Protozo: Rhizopod) nd petforming vegettion on cutover ogs in the Jur, Switzerlnd. New Phytologist 13: Cllghn, T. V., N. J. Collins, nd C. H. Cllghn Photosynthesis, growth nd reproduction of Hylocomnium splendens nd Polytrichum commune in Swedish Lplnd-Strtegies of growth nd popultion dynmics of tundr plnts.. Oikos 31: Cllwy, R. M Effect of shrus on recruitment of Quercus douglsii nd Quercus lot in Cliforni. Ecology 73: Cmpell, D. R., C. Lvoie, nd L. Rochefort.. Wind erosion nd surfce stility in ndoned milled petlnds. Cndin Journl of Soil Science 8: Cmpell, D. R., L. Rochefort, nd C. Lvoie. 3. Determining the immigrtion potentil of plnts colonizing distured environments: the cse of milled petlnds in Quéec. Journl of Applied Ecology : Chpin, F. S. III., W. C. Oechel, K. Vn Cleve, nd W. Lwrence The role of mosses in the phosphorus cycling of n Alskn (USA) lck spruce forest. Oecologi 7: Collins, N. J The effects of volcnic ctivity on the vegettion of Deception Islnd. British Antrctic Survey Bulletin 1:79 9. Corrdini, P., nd B. Clément Growth pttern nd modulr reitertion of hrdy coloniser Polytrichum commune Hedw. Plnt Ecology 13: Fuert, P., nd L. Rochefort.. Response of petlnd mosses to uril y wind-dispersed pet. The Bryologist 15: Fenton, J. H. C., nd R. I. Lewis Smith Distriution, composition nd generl chrcteristics of the moss nks of the mritime Antrctic. British Antrctic Survey Bulletin 51: Filion, J., nd H. Morin Sptil distriution of lck spruce regenertion 8 yers fter fire in Queec orel forest. Cndin Journl of Forest Reserch 6: Frnco, A. C., nd P. S. Noel Interctions etween seedlings of Agve deserti nd the nurse-plnt Hilri rigid. Ecology 69: Frnco, A. C., nd P. S. Noel Effect of nurse-plnts on the microhitt nd growth of ccti. Journl of Ecology 77: Grtner, B. L., F. S. Chpin III, nd G. R. Shver Reproduction of Eriophorum vgintum y seed in Alskn tussock tundr. Journl of Ecology 7:1 18. Gill, D. S., nd P. L. Mrks Tree nd shru seedling coloniztion of old fields in centrl New York. Ecologicl Monogrphs 61: Griggs, R. F Competition nd succession on Rocky Mountin Fellfield. Ecology 37:8. Groeneveld, E. V. G., nd L. Rochefort.. Nursing plnts in petlnd restortion: on their potentil use to llevite frost heving prolems. Suo 53: Groeneveld, E. V. G., nd L. Rochefort. 5. Polytrichum strictum s solution to frost heving in distured ecosystems: cse study with milled petlnds. Restortion Ecology 13:7 8. Grosvernier, P., Y. Mtthey, nd A. Buttler Microclimte nd physicl properties of pet: new clues to the understnding of og restortion processes. Pges 35 9 in B. D. Wheeler, S. C. Shw, W. J. Fojt, nd R. A. Roertson, editors. Restortion of temperte wetlnds. John Wiley & Sons Ltd., Chichester, United Kingdom. Hrley, P. C., J. D. Tenhunen, K. J. Murry, nd J. Beyers Irrdince nd temperture effects on photosynthesis of tussock tundr Sphgnum mosses from the foothills of the Philip Smith Mountins, Alsk. Oecologi 79: Huoplinen, M., E. S. Tuittl, J. Line, nd H. Vsnder Seed nd spore nk in cut-wy pets twenty yers fter ndonment. Interntionl Pet Journl 8: 51. Johnson, E. A Vegettion orgniztion nd dynmics of lichen woodlnd communities in Northwest Territories, Cnd. Ecology 6: 15. Joosten, J. H. J Time to regenerte: long-term perspectives of rised og restortion with specil emphsis on pleoecologicl studies. Pges 379 in B. D. Wheeler, S. C. Shw, W. J. Fojt, nd R. A. Roertson, editors. Restortion of temperte wetlnds. John Wiley & Sons Ltd., Chichester, United Kingdom. Keizer, P. J., B. F. Vn Tooren, nd H. J. During Effects of ryophytes on seedling emergence nd estlishment of short-lived fors in chlk grsslnd. Journl of Ecology 73:93 5. Kikvidze, Z Plnt species ssocitions in lpine-sunivl vegettion ptches in the Centrl Cucsus. Journl of Vegettion Science :7 3. Kurets, V. K., A. V. Tlnov, E. G. Popov, nd S. N. Drozdov Light-temperture dependencies of pprent photosynthesis nd drk respirtion in Sphgnum. Russin Journl of Plnt Physiology :7 78. Ltheef, M. A., nd J. H. Ortiz Influence of compnion hers on Phyllotret crucifere (Coleopter: Chrysomelide) on collrd plnts. Journl of Economic Entomology 77:8 8. Lvoie, C., K. Mrcoux, A. Sint-Louis, nd J. S. Price. 5. The dynmics of cotton-grss (Eriophorum vgintum L.) cover expnsion in vcuum-mined petlnd, southern Quéec, Cnd. Wetlnds 5:6 75. Lvoie, C., A. Sint-Louis, nd D. Lchnce. 5. Vegettion dynmics on n ndoned vcuum-mined petlnd: five yers of monitoring. Wetlnds Ecology nd Mngement 13: Lech, W On the importnce of some mosses s pioneers on unstle soils. Journl of Ecology 19:98 1. Mllik, A. U., R. J. Hos, nd C. J. Legg Seed dynmics in Cllun-Arctostphylos heth in North-Ester Scotlnd. Journl of Ecology 7: Mlty, E., C. J. Legg, nd M. C. F. Proctor The ecology of severe moorlnd fire on the North York Moors: effects of the 1976 fires, nd susequent surfce nd vegettion development. Journl of Ecology 78: Mrcoux, K.. Les invsions de l linigrette (Eriophorum vgintum L.): ide ou frein à l resturtion des tourières? M. Sc. thesis. Université Lvl, Sinte-Foy, Quéec, Cnd. Mrsh, W. M., nd J. M. Koerner Role of moss in slope formtion. Ecology 53: Mrtinez, M. L., nd C. P. Moreno-Cssol The iologicl flor of costl dunes nd wetlnds: Chmecrist chmecristoides (Colldon) I. nd B. Journl of Costl Reserch 1: Restortion Ecology DECEMBER 7

11 Polytrichum strictum s Nurse-Plnt Morneu, C., nd S. Pyette Postfire lichen spruce woodlnd recovery t the limit of the orel forest in northern Quéec. Cndin Journl of Botny 67: Murry, K. J., P. C. Hrley, J. Beyers, H. Wlz, nd J. D. Tenhunen Wter content effects on photosynthetic response of Sphgnum from the foothills of the Philip Smith Mountins, Alsk. Oecologi 79: 5. Murry, K. J., J. D. Tenhunen, nd J. Kummerow Limittions on Sphgnum growth nd net primry production in the foothills of the Philip Smith Mountins, Alsk. Oecologi 8:56 6. Murry, K. J., J. D. Tenhunen, nd R. S. Nowk Photoinhiition s control on photosynthesis nd production of Sphgnum mosses. Oecologi 96: 7. Noel, P. S Temperture, wter vilility, nd nutrient level t vrious soil depths consequences for shllow-rooted desert succulents, including nurse-plnt effects. Americn Journl of Botny 76: Prker, W. C., S. R. Wtson, nd D. W. Cirns The role of hir-cp mosses (Polytrichum spp.) in nturl regenertion of white spruce (Pice gluc (Moench) Voss). Forest Ecology nd Mngement 9:19 8. Poschold, P Dispore rin nd dispore nk in rised ogs nd implictions for the restortion of pet-mined sites. Pges 71 9 in B. D. Wheeler, S. C. Shw, W. J. Fojt, nd R. A. Roertson, editors. Restortion of temperte wetlnds. John Wiley & Sons Ltd., Chichester, United Kingdom. Potter, E., J. Wood, nd C. Nicholl SunScn Cnopy Anlysis System. User mnul. SS1-UM-1.5 (version 1.5). Delt-T Devices Ltd., Cmridge, United Kingdom. Poulin, M., L. Rochefort, F. Quinty, nd C. Lvoie. 5. Spontneous revegettion of mined petlnds in estern Cnd. Cndin Journl of Botny 83: Price, J. S., A. L. Hethwite, nd A. J. Bird. 3. Hydrologicl processes in ndoned nd restored petlnds: n overview of mngement pproches. Wetlnds Ecology nd Mngement 11: Price, J. S., L. Rochefort, nd F. Quinty Energy nd moisture considertions on cutover petlnds: surfce microtopogrphy, mulch cover nd Sphgnum regenertion. Ecologicl Engineering 1: Price, J. S., nd G. S. Whitehed. 1. Developing hydrologic thresholds for Sphgnum recoloniztion on n ndoned cutover og. Wetlnds 1:3. Rffele, E., nd T. T. Velen, Fcilittion y nurse shrus of resprouting ehvior in post-fire shrulnd in northern Ptgoni, Argentin. Journl of Vegettion Science 9: Richrdson, J. A The effect of temperture on the growth of plnts on pit heps. Journl of Ecology 6: Rietveld, W. J., nd L. J. Heidmnn Direct seeding of Ponderos Pine on recent urns in Arizon. Reserch note RM-31. U.S.D.A. Forest Service, Wshington, D.C. Riis, T., nd K. Snd-Jensen Growth reconstruction nd photosynthesis of qutic mosses: influence of light, temperture nd cron dioxide t depth. Journl of Ecology 85: Roert, E., L. Rochefort, nd M. Grneu Nturl revegettion of two lock-cut mined petlnds in estern Cnd. Cndin Journl of Botny 77:1 13. Rochefort, L.. Sphgnum keystone genus in hitt restortion. The Bryologist 13: Rochefort, L., nd D. F. Bstien Réintroduction de sphignes dns une tourière exploitée: évlution de divers moyens de protection contre l dessicction. Ecoscience 5: Rochefort, L., F. Quinty, S. Cmpeu, K. W. Johnson, nd T. J. Mlterer. 3. North Americn pproch to the restortion of Sphgnum dominted petlnds. Wetlnds Ecology nd Mngement 11: 3. Rozé, F Effets comprés de différentes litières sur l minérlistion et l nitrifiction nettes de l zote dns un sol de lnde hute. Revue d écologie et de Biologie du Sol :73 8. Sgot, C., nd L. Rochefort Tolérnce des sphignes à l dessicction. Cryptogmie: Bryologie et Lichénologie 17: Slonen, V Reltionship etween the seed rin nd the estlishment of vegettion in two res ndoned fter pet hrvesting. Holrctic Ecology 1: Slonen, V Effects of rtificil plnt cover on plnt coloniztion of re pet surfce. Journl of Vegettion Science 3: SAS Institute, Inc SAS Proprietry Softwre Relese 6.1 TS. SAS Institute, Inc., Cry, North Crolin. Schipperges, B., nd H. Rydin Response of photosynthesis of Sphgnum species from contrsting microhitts to tissue wter content nd repeted desicction. New Phytologist 1: Schouwenrs, J. M Hydrologicl differences etween ogs nd og-relicts nd consequences for og restortion. Hydroiologi 65:17. Silvol, J Moisture dependence of cron dioxide exchnge nd its recovery fter drying in certin orel forest nd pet mosses. Lindergi 17:5 1. Silvol, J., nd H. Altonen Wter content nd photosynthesis in the pet mosses Sphgnum fuscum nd S. ngustifolium. Annles Botnici Fennici 1:1 6. Skre, O., nd W. C. Oechel Moss functioning in different tig ecosystems in interior Alsk. Oecologi 8:5 59. Sokl, R. R., nd F. J. Rohlf Biometry. nd edition. W.H. Freemn nd Compny, New York. Sonesson, M., S. Persson, K. Bsilier, nd T. A. Stenström Growth of Sphgnum riprium Ångst. in reltion to some environmentl fctors in the Stordlen mire. Ecology of surctic mire. Ecologicl Bulletin 3: Suzán, H., G. P. Nhn, nd D. T. Ptten The importnce of Olney tesot s nurse-plnt in the Sonorn Desert. Journl of Vegettion Science 7: Tuittil, E.-S., H. Rit, H. Vsnder, nd J. Line.. Vegettion ptterns round Eriophorum vgintum L. tussocks in cut-wy petlnd in southern Finlnd. Cndin Journl of Botny 78: Vliente-Bnuet, A., nd E. Ezcurr Shde s cuse of the ssocition etween the cctus Neouxumi tetetzo nd the nurse-plnt Mimos luisn in the Tehucn Vlley, Mexico. Journl of Ecology 79: Vn Breemen, N How Sphgnum ogs down other plnts. Tree 1:7 75. Vn Tooren, B. F The fte of seeds fter dispersl in chlk grsslnd: the role of the ryophyte lyer. Oikos 53:1 8. Wlker, L. R., nd F. S. Chpin III Physiologicl controls over seedling growth in primry succession on n Alskn floodplin. Ecology 67: Welden, C Structurl pttern in lpine tundr vegettion. Americn Journl of Botny 7:1 13. Wind-Mulder, H., L. Rochefort, nd D. H. Vitt Wter nd pet chemistry comprisons of nturl nd post-hrvested petlnds cross Cnd nd their relevnce to petlnd restortion. Ecologicl Engineering 7: DECEMBER 7 Restortion Ecology 719