Susceptibility of burned black spruce (Picea mariana) forests to non-native plant invasions in interior Alaska

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1 Biol Invsions (2014) 16: DOI /s ORIGINAL PAPER Susceptiility of urned lck spruce (Pice mrin) forests to non-ntive plnt invsions in interior Alsk Ktie V. Spellmn Christ P. H. Mulder Teres N. Hollingsworth Received: 8 Ferury 2013 / Accepted: 25 Decemer 2013 / Pulished online: 5 Jnury 2014 Ó Springer Science+Business Medi Dordrecht 2014 Astrct As climte rpidly wrms t high-ltitudes, the orel forest fces the simultneous threts of incresing invsive plnt undnces nd incresing re urned y wildfire. Highly flmmle nd widespred lck spruce (Pice mrin) forest represents orel hitt tht my e incresingly susceptile to non-ntive plnt invsion. This study ssess the role of urn severity, site moisture nd time elpsed since urning in determining the invisiility of lck spruce forests. We conducted field surveys for presence of non-ntive plnts t 99 urned lck spruce forest sites urned in 2004 in three regions of interior Alsk tht spnned grdient of urn severities nd site moisture levels, nd chronosequence of sites in single region tht hd urned in 1987, 1994, nd We lso conducted greenhouse experiment where we grew invsive plnts in vegettion nd soil cores tken from suset of these sites. In oth our field survey nd the greenhouse experiment, regionl differences in soils nd vegettion etween urn complexes outweighed locl urn severity or site moisture in determining the invsiility of urned lck spruce sites. In the greenhouse experiments using cores from the 2004 urns, we found tht the invsive focl species grew etter in cores with soil nd vegettion properties chrcteristic of low severity urns. Invsive plnt growth in the greenhouse ws greter in cores from the chronosequence urns with higher soil wter holding cpcity or lower ntive vsculr iomss. We concluded tht there re differences in susceptiility to non-ntive plnt invsions etween different regions of orel Alsk sed on ntive species regenertion. Reestlishment of ntive ground cover vegettion, including rpidly colonizing ryophytes, pper to offer urned res level of resistnce to invsive plnt estlishment. Keywords Borel forest Bromus inermis ssp. inermis Climte chnge Disturnce Fire Hiercium urnticum Melilotus lus Wildfire K. V. Spellmn (&) C. P. H. Mulder Deprtment of Biology nd Wildlife, Institute of Arctic Biology, University of Alsk Firnks, Firnks, AK 99775, USA e-mil: ktie.spellmn@lsk.edu T. N. Hollingsworth Pcific Northwest Reserch Sttion, Borel Ecology Coopertive Reserch Unit, US Forest Service, Firnks, AK 99775, USA Introduction While high ltitude ecosystems tend to hve fewer nonntive plnt species reltive to other plces, the introduction nd spred of non-ntive species is ccelerting in some rctic nd su-rctic ecosystems (Crlson nd Shephrd 2007). In Alsk, the numer of non-ntive plnt species incresed y 46 % etween

2 1880 K. V. Spellmn et l nd 2006 ccording to sttewide herrium nd plnt survey records (Crlson nd Shephrd 2007). Much of this increse in non-ntive plnt introduction nd spred my e ttriuted to increses in nthropogenic disturnces due to incresed militry ctivity nd resource extrction, nd the resulting increses in humn popultion, commerce nd rod construction, over the this time period (Wlker nd Wlker 1991; U.S. Census Bureu 2000; Crlson nd Shephrd 2007). Further, reserchers in Alsk hve egun to oserve nd document community nd ecosystem-level chnges to orel hitts where non-ntive plnts hve invded (Spellmn nd Wurtz 2011; Roon 2011). A rpidly chnging climte my increse the vulnerility of high ltitude systems to non-ntive plnt invsions directly y ffecting temperture nd growing seson, nd indirectly y ffecting the disturnce regimes. Wrmer winters (Chpmn nd Wlsh 1993; Serreze et l. 2000) nd longer growing sesons (Myneni et l. 1997) my fcilitte the survivl of greter diversity of non-ntive species in Alsk nd increse their undnce. Wrmer summer tempertures nd decresed precipittion in Alsk nd northern Cnd hve drmticlly incresed wildfire disturnces in the pst few decdes (Overpeck et l. 1997; Stocks et l. 2000; Gillett et l. 2004; Bchelet et l. 2005), which my provide further dvntges to invding non-ntive plnt species. During the 2004 fire seson 2.7 million hectres of orel forest urned, mking it the lrgest fire yer in Alsk s recorded fire history (Todd nd Jewkes 2006). Fire frequency, severity, nd extent re predicted to continue to increse cross orel North Americ in the future (Flnnign et l. 2001;Ksischke nd Turetsky 2006; Blshi et l. 2009). Blshi et l. (2009) predict douling of the re urned per decde y 2050 reltive to 2000, nd times more fires y the end of the century. Blck spruce (Pice mrin) forest comprises over 40 % of the forested lndscpe of Alsk s orel region (Vn Cleve et l. 1983) nd represents one orel hitt tht is potentilly vulnerle to non-ntive plnt invsions. Wildfire, primrily ignited y lightning strikes, is the most frequent disturnce in fire-dpted lck spruce forest (Vn Cleve et l. 1983). Wildfire cn increse the susceptiility of n re to non-ntive plnt invsions y incresing the vilility of resources such s light y reducing competition from ntive cnopy species (Dvis et l. 2000; D Antonio 2000). For n invsion to ctully occur, the incresed resource vilility or decresed competition fter fire must coincide with the vilility of propgules of non-ntive plnt le to compete with the ntive vegettion (Dvis et l. 2000). The increses in fire frequency nd sptil extent (Overpeck et l. 1997; Stocks et l. 2000; Bchelet et l. 2005) long with increses in propgule pressure long rod corridors in Alsk (Alsk Exotic Plnt Informtion Cleringhouse (AKEPIC) 2013) my further excerte the vulnerility of the orel forest to invsion y non-ntive plnts. The reltionship etween fire nd non-ntive plnt invsions in the northern orel forest, however, remins little studied (Snderson et l. 2012). Currently, Alsk s invsive plnts lrgely occur on rod corridors with few instnces of spred off the humn disturnce footprint (Conn et l. 2008; AKEPIC 2013). The low numer of invsions into orel forest hitts provides n opportunity for Alsk to prevent the spred of these plnts, ut lso mkes it more difficult to predict which res will e most vulnerle to invsive plnts in the future. Propgule pressure ppers to drive the loctions of the few sites where non-ntive plnt species hve moved eyond these rod corridors into urned forest (Cortés-Burns et l. 2007; Villno nd Mulder 2008). However, mny urned res with high propgule pressure from rodsides remin uninvded (Villno nd Mulder 2008). Understnding iotic nd iotic fctors within urned res tht contriute to invsive plnt seedling success my improve the ility of lnd mngers to trget monitoring or control efforts on infested rodwys ner prticulrly invsile urned res. Differences in urn severity, site moisture, or the mount of time elpsed since fire my influence the suitility of urned lck spruce site for non-ntive plnt coloniztion. Blck spruce forest in interior Alsk cn hve thick lyer of moss, lichen nd orgnic soil tht often overlys the minerl soil nd permfrost (Viereck et l. 1992). This lyer is referred to s duff, with prtilly decomposed orgnic (Oi/ Oe) soil horizons mking up the upper duff nd the well decomposed orgnic soil (O) horizon comprising the lower duff. This duff lyer is commonly up to 50 cm thick nd ccounts for lrge proportion of urnle iomss in lte-succession lck spruce forest in Alsk (Ksischke et l. 1995). As result, high severity urns re commonly defined in plnt

3 Susceptiility of urned lck spruce 1881 regenertion studies s res where little duff remins (defined in this pper s\7 cm of duff remining fter the fire; Johnstone et l. 2008). The totl depth of the duff lyer remining fter fire plys criticl role in determining the post-fire plnt community composition (Johnstone nd Chpin 2006; Johnstone et l. 2010). Among ntive orel species, plnts regenerting from seeds tend to hve higher estlishment rtes in high severity urn sites where little duff remins nd ptches of minerl soil re exposed (Schimel nd Grnstrom 1996; Johnstone et l. 2010; Mllik et l. 2010; Bernhrdt et l. 2011). In low severity urns, much of the chrred, dry upper duff remins, offering less hospitle surfce for germintion nd estlishment (Johnstone nd Chpin 2006; Veilleux-Nolin nd Pyette 2012). Currently nonntive plnts re regenerting from dispersed seeds rther thn clonlly or from seednks in orel Alsk (reviewed in Zouhr et l. 2008). We expect invsive plnts would fce the sme recruitment rriers s ntive plnts regenerting y seed in low severity urn sites. In ddition, the invisiility of Alsk s urned lck spruce forests should decrese with incresing time since fire due to the resurgence of ntive plnts nd cnopy closure. This trend hs een documented in mny North Americn coniferous forests, where non-ntive species undnce nd richness tend to decrese with incresing urn ge (Agee nd Huff 1987; Turner et l. 1997; Doyle et l. 1998; Klinger et l. 2006; reviewed in Zouhr et l. 2008). Within urn site of prticulr severity, moisture or ge clssifiction, the soil surfce exhiits some degree of microvrition due to smll-scle differences in soil moisture, vegettion, microtopogrphy, nd chnges in wether during the fire. For exmple, within single site, islnds of unurned ryophytes could remin longside ptches of high severity urn with minerl soil or rpidly estlishing ryophytes such s Certdon purpureus or Mrchnti polymorph (Johnstone et l. 2008; Bernhrdt et l. 2011). If we understnd how different invsive plnt species respond to smll-scle vrition within urns, we cn etter understnd which lrger-scle res might e most vulnerle to non-ntive plnt coloniztion sed on the proportions of the soil or vegettion chrcteristics present. In the experimentl prt of this study we focus on three non-ntive species with very different functionl trits nd regenertion strtegies: nitrogen-fixing for, Melilotus lus Medik. (Fcee); for, Hiercium urnticum L. (Astercee); nd grminoid, Bromus inermis ssp. inermis Leyss. (Pocee). Ech species is highly invsive in mny prts of the stte nd ll re from plnt fmilies tht occur disproportiontely mong invsive tx in Alsk (Crlson et l. 2008; AKEPIC 2013). Similrities in responses cross severl invder species would indicte prticulrly vulnerle (or invulnerle) hitt. We pose three primry questions to help identify urned res most susceptile to non-ntive plnt invsions: (1) Do urn severity, soil moisture, region nd urn ge of lck spruce sites influence the likelihood of non-ntive plnt estlishment? (2) Wht smll-scle soil nd vegettion chrcteristics within urns influence vulnerility to invsion? nd (3) Wht cross-tx trends cn e used to help inform invsive plnt mngement decisions in urned orel forest? Methods We used two complementry pproches to ddress these questions: field survey of urned res djcent to rodwys nd greenhouse experiment using soil nd vegettion cores tken from some of our field sites. The field survey provided n ssessment of current ptterns of non-ntive plnt movement into urned res. The greenhouse pproch controlled for propgule pressure ( confounding fctor in most field studies on invsiility). We used greenhouse rther thn field experiments to prevent ccidentl introductions of invsives into the lrgely uninvded urn sites. Our greenhouse pproch could not ccount for climtic vritions or iotic interctions eyond competition from smll plnts, fctors tht my further limit or promote invsive growth in the field. At the time of this study, no invsive plnts hd een recorded moving into undistured lck spruce stnds (AKEPIC 2013); therefore, our reserch effort focused on urned sites. Study re In July 2006, we smpled urned lck spruce stnds within 120,000 km 2 re locted etween the Alsk nd Brooks Mountin Rnges (Fig. 1). All sites were estlished prior to this study s prt of the Bonnz

4 1882 K. V. Spellmn et l. Fig. 1 Mp of study sites in interior Alsk (modified from Johnstone et l. 2009). Perimeters of res urned in 2004 re shown y gry polygons. The choronosequence of res which urned in 1999, 1994, nd 1987 ( chrono ) re indicted y white polygons with gry orders. Sites smpled for greenhouse experiments re indicted y white-filled squres (N = 18), nd Creek Borel Long Term Ecologicl Reserch Progrm (BNZ LTER). The sites occurred long rodwys within four distinct urn complexes throughout interior Alsk, including the Boundry Fire Complex long the Steese Highwy, the Tylor Fire Complex long the Tylor Highwy, the Dll City Fire Complex long the Dlton Highwy, nd the Delt Fire Chronosequence ( set of djcent res tht urned in 1999, 1994, nd 1987) t the junction of the Alsk nd Richrdson Highwys. Although these re considered mjor rodwys in interior Alsk, they differ from mjor rods in other prts of the U.S. in tht they re only one lne wide in ech direction, nd the Steese, Tylor nd Dlton re lrgely unpved. The oldest rod is the sites surveyed for non-ntive plnt species re indicted y lck squres (N = 99). Rods shown on this mp re the only rodwys in interior Alsk outside of towns nd villges. Becuse of the smll scle of the mp, symols overlp for some of our survey sites Richrdson Highwy (completed in 1910), followed y the Steese Highwy (1927), the Alsk Highwy (1942), the Tylor Highwy (1951), nd the Dlton Highwy (1974) (Vlenci 2007). According to the Alsk Deprtment of Trnsporttion fixed recorder sttions nerest to our field sites, the verge dily trffic counts the yer of our study were gretest on the Steese Highwy (1,433 vehicles per dy), followed y the Dlton Highwy (1,110 vehicles per dy), the Richrdson Highwy (987 vehicles per dy), nd the Alsk Highwy (526 vehicles per dy; Birkholz 2013). Trffic is not recorded on the Tylor Highwy. Our sites on the Tylor nd Steese Highwys occur within the Yukon-Tnn Uplnds ecoregion of

5 Susceptiility of urned lck spruce 1883 Alsk, which is underlin y the metsedimentry Yukon-Tnn terrne nd is typified y rocky soils, nd discontinuous permfrost on north-fcing slopes nd vlley ottoms (Nowcki et l. 2001). Our sites long the Dlton Highwy re within the Ry Mountins ecoregion, which is underlin y the metmorphic Ruy terrne (Nowcki et l. 2001). The Ry Mountins ecoregion typiclly hs shllow rocky soils nd discontinuous permfrost (Nowcki et l. 2001). Both the Yukon-Tnn Uplnds nd the Ry Mountins ecoregions were lrgely unglcited during the Pleistocene ice ges, ut the highest peks in these res supported smll lpine glciers (Foster et l. 1994; Nowcki et l. 2001). The sites in the Delt Choronsequence occur within the Tnn-Kuskokwim Lowlnds ecoregion, which is typified y fluvil nd glciofluvil sediments, overlin y vrying depths of eolin silts nd orgnic soils (Nowcki et l. 2001). Surfce moisture is undnt in the Tnn-Kuskokwim Lowlnds due to the reltively gentle topogrphy, poor soil dringe nd ptches of permfrost (Nowcki et l. 2001). Field survey To ssess current ptterns of invsion in urned hitts in interior Alsk, we surveyed for presence nd undnce of non-ntive plnt species t 99 loctions within our study re (Fig. 1). Survey loctions were selected sed on close proximity to or overlp with BNZ LTER sites to tke dvntge of lrge existing dtsets on iotic nd iotic vriles t ech urn site. Ninety-three of these sites hd ll urned in 2004 nd were pproximtely evenly distriuted etween the urn complexes long the Steese (33 sites), Tylor (29 sites) nd Dlton (31 sites) highwys, which we will refer to s regions. The remining six of these sites were chronosequence of sites long the Alsk nd Richrdson Highwys which urned in 1999 (7 yers old t the time of smpling), 1994 (12 yers old), nd 1987 (19 yers old). We did not include unurned hitt in our survey effort ecuse t the time of the study, invsive plnts were not documented in unurned lck spruce forest in our smple re (Cortés-Burns et l. 2007; AKEPIC 2013). We recorded the presence nd numer of ll nonntive plnt species within 30 m y 60 m elt trnsect extending from the edge of the rod shoulder into the urn. Percent cover of non-ntive plnts within urned hitt ws not used ecuse cover ws miniml (B1 % cover, nd only few individuls in most cses) t ll sites where they were present. Using the Alsk Nturl Heritge Project invsiveness rnkings (Crlson et l. 2008), we ctegorized the non-ntive plnt species present t ech site s ggressive (invsive, rnking [50) or s nonggressive (non-invsive, rnking\50). These rnkings re sed on literture records of species impcts, iologicl trits, dispersl ility, distriution, nd fesiility of control in Alsk nd similr hitts elsewhere (see Crlson et l. 2008). To provide n index of non-ntive plnt propgule pressure from rodsides t suset of 19 intensively studied sites, we surveyed 1 m m elt trnsect prllel to the rod 1 m from the edge of the urned forest in the rod right-of-wy. Within the elt trnsect, we visully estimted the cover of ll nonntive species in ten 1 m 9 10 m lengths. Becuse we found regionl differences in the presence of non-ntive plnts occurring within hitt tht urned in 2004, we conducted dditionl nlysis of iotic nd iotic vriles tht might help explin these differences. We used geogrphic vriles (ltitude, longitude, elevtion, slope, spect), surfce vegettion cover estimtes (percent live moss, ded moss, nd urned orgnics cover), residul orgnic soil remining, soil texture ctegoriztions, cnopy urn severity, nd minerl soil ph from pulicly ville BNZ LTER dtset (Johnstone 2013). For detiled informtion on how ech of these vriles were collected see Johnstone (2013). Experimentl design At the intensively studied sites, we conducted two seprte experimentl mnipultions: one which investigted site fctors nd regionl influences tht ffect invisiility of sites tht ll urned within single yer (2004) in 12 sites spred cross three regions, nd nother which investigted the influence of the time elpsed since urning on site invisiility in 6 sites within single region. Sites consisted of 30 m 9 30 m plots plced in reltively homogeneous res with respect to topogrphy nd regenerting vegettion. The sites smpled for the experiment were overlpping or djcent to (\150 m wy) the 30 m 9 60 m survey trnsects t those sites.

6 1884 K. V. Spellmn et l. To understnd locl nd regionl fctors tht ffected the susceptiility of res urned in the yer 2004, low severity-high moisture, low severity-low moisture, high severity-high moisture, nd high severity-low moisture site ws selected in ech of the Steese, Tylor, nd Dlton regions, for totl of 12 sites. We used the site clssifictions determined y Johnstone et l. (2008) in 2005, the summer fter the fire. In rief, they ssigned urn severity clsses y visully estimting the percent minerl soil cover nd mesuring the remining orgnic soil (duff) depth t eleven rndom points. To determine site moisture level, they evluted topogrphic position, soil texture nd men percent moisture in minerl soil (s mesured over 3-week period in July 2006 using hnd-held moisture proe (Hydro-Sense, Cmpell Scientific, Edmonton, AB, Cnd) t eleven rndom points). High severity sites hd [5 % minerl soil exposed nd \7 cm men remining duff. Low severity sites hd \5 % minerl soil exposed nd [7 cm men remining duff. High moisture sites tended to hve fine soil texture, [30 % minerl soil moisture, nd were locted in low lying res. Low moisture sites tended to hve corse soil texture, \30 % minerl soil moisture, nd e locted on slopes or ridges. Further detils on the 2004 site clssifiction protocols re ville in Johnstone et l. (2008). We evluted the influence of time elpsed since fire on site susceptiility to non-ntive plnt coloniztion using the Delt Chronosequence sites. In these sites, we crossed the three urn ges with two levels of urn severity to identify six different site types. Site urn severity ws ssigned in the sme mnner s for the 2004 urn sites, nd severity clssifictions were estlished prior to this study y Ksischke nd Johnstone (2005). Due to similrities in topogrphy nd soil texture, site moisture ws not fctor in the chronosequence investigtion. Core collection nd mesurements Within ech intensively smpled urn site we estlished three rndomly selected 30 m trnsects in uniform compss direction with minimum distnce of 2 m etween trnsects. We took three cores (7 cm dimeter 9 22 cm depth) every 5 m long the trnsects using soil corer inserted t the vegettion surfce. Cores were inserted into cylindricl pots (Stuewe nd Sons D40L pots, Tngent, Oregon) of similr dimensions to ring ck to the greenhouse for experimenttion. The cores remined intct from the surfce vegettion down through the duff lyers nd minerl soil. Dwrf shru, herceous nd ryophyte vegettion remined live on the surfce of the core. We determined the ntive plnt richness of the plnts remining on the surfce of the core y counting the numer of vsculr nd non-vsculr species. Ntive non-vsculr nd vsculr iomss were determined for ech core y clipping live shoots t the soil surfce t the end of the greenhouse experiment. Inside the core holes, we mesured the horizon thicknesses of the different duff lyers: live moss, upper duff nd lower duff. Every 10 m long ech trnsect n dditionl core ws tken with soil corer inserted t the soil surfce to determine the wter holding cpcity of the upper duff, lower duff, nd minerl soil horizons. To clculte wter holding cpcity (WHC) of ech horizon, we divided fieldmoist cores into sections sed on the field mesurements of the horizon thickness. Ech horizon section ws homogenized in try, nd then su-smple ws plced in pre-weighed nd sturted filter pper inside funnel. We sturted the soil with deionized wter nd let it drip. When the soil cesed dripping, we weighed it, dried it to constnt weight, nd then reweighed it. WHC is expressed in g H 2 O per g dry soil. Invsive plnt growth mesurements in greenhouse To ssess the invsiility of urn soils independent of propgule pressure, nd to exmine the influence of edphic fctors nd smll-scle plnt interctions on three different invsive species, we trnsported the intct soil cores to greenhouse t the University of Alsk Firnks for experimenttion. We used two wtering tretments in the greenhouse. High moisture vilility ws mintined in cores from high moisture sites y wtering with n overhed oom every 2 3 dys nd y inserting the ottom 3 cm of the cylindricl pots in trys with wter. Cores from low moisture nd chronosequence sites were wtered on the sme schedule with the overhed oom only. High moisture nd low moisture tretments cores were stggered evenly long the greenhouse ench to ccount for potentil positionl effects. On ech of the three cores from ech smpling point, we sowed seeds of one of our focl species M.

7 Susceptiility of urned lck spruce 1885 lus, B. inermis (collected from outside the greenhouse), H. urnticum (Chiltern Seeds, UK). These focl species were chosen sed on the current undnces nd potentil thret to Alskn orel forest ecosystems (e.g. Crlson et l. 2008) rther thn their occurrence in our field sites. There were no visile estlished popultions of the three focl invsive species t our smpling loctions. We scrified ll M. lus seeds y hnd using sndpper to prepre them for germintion. We plced five seeds of M. lus or B. inermis, or seven seeds of H. urnticum (preliminry experiments indicted lower germintion rtes for this species) on core top for ll sites to ensure t lest one seedling per pot germinted. We scored germintion of M. lus fter 10 dys. Due to longer time requirements for germintion (s determined in pilot project), we scored B. inermis nd H. urnticum germintion fter 14 dys. After 30 dys, we rndomly thinned plnts to 1 invsive per pot for the reminder of the experiment. Seedlings were grown t men temperture of 22 C with 20 h of light per dy, typicl of the growing seson in interior Alsk. After 150 dys, we determined survivl, ssessed proportion of reproductive survivors (either flowering or reproducing sexully), nd hrvested oth shoots nd roots of 9 plnts per species per site. The 9 finl plnts per species per site used in the finl nlyses were from the sme smpling loctions tht the cores for dditionl soil nlysis were collected. Sttisticl nlyses All sttisticl nlyses were performed using SAS v.9.1 (SAS Institute, Cry, North Crolin). To nlyze field survey dt, we determined if the presence of nonntive plnts in urned field sites ws influenced y urn severity, site moisture, or region using Chi squre nlysis. We used multiple logistic regression to investigte geogrphic nd environmentl vriles tht might help predict presence of non-ntive plnts within the urned re. We used ANOVA tukey tests to look for differences in rodside non-ntive plnt cover etween site types nd regions. We evluted the dt from greenhouse experiments t two sptil scles: cross our sites exmining the effects of lrge-scle fire chrcteristics (fire severity, moisture, ge nd region), nd within-sites looking t the smll scle soil nd vegettion vriles tht might explin differences in invsive plnt responses etween our sites. To determine the influence of urn severity, moisture, region, nd ge on invsive plnt growth in the greenhouse, we conducted ANOVA using sitelevel mens (2004 urns experiment N = 12, chronosequence experiment N = 6) of response vriles for ech of the three focl species. The explntory vriles for 2004 urn sites were urn severity, site moisture, nd region; for chronosequence urn sites the explntory vriles were urn ge nd urn severity. Response vriles individully tested for ech species included germintion (% per pot), survivl, finl iomss nd reproduction (numer of tillers, stolons, or flowers). Interction terms were not significnt, so our finl models exclude the interction terms to increse the model degrees of freedom. To evlute smll-scle mechnisms tht might explin differences in invsive plnt responses etween these sites, we took dvntge of the high microvriility in soil nd vegettion chrcteristics within urns. We used core-level dt to determine which soil nd ntive vegettion vriles est explined the finl iomss of the three focl species in oth the 2004 urn experiment nd the chronosequence experiment. We rn multiple liner regressions for ll possile models using the nine soil nd vegettion covrites. We then rnked the models using Akike s Informtion Criterion djusted for smll smple size (AIC c ; Burnhm nd Anderson 2002). We clculted the verge prmeter estimtes for ech covrite in the set of est-supported models (those within 3 AIC c units of the model with the lowest AICc score) to ssess the direction of the response of ech invsive plnt species to the prmeter. We lso clculted cumultive AIC c weights (0 P x i 1), or importnce vlues, to evlute strength of evidence for ech soil or vegettion vrile s contriuting fctor to invsive plnt finl iomss (Burnhm nd Anderson 2002; Arnold 2010). We considered importnce vlues [0.5 s indiction of well-supported prmeters. In the 2004 urn experiment we tested for differences in soil properties nd ntive vegettion on the soil cores etween site types using ANOVA on sitelevel mens. Tukey tests were used to distinguish differences etween regions nd etween urn severity nd site moisture clsses. In the chronosequence

8 1886 K. V. Spellmn et l. experiment there ws only one site for ech urn ge nd urn severity comintion, so to nlyze differences in soil properties, ntive vegettion, nd surfce cover on the soil cores etween site types we conducted ANOVA on soil core level dt, followed y Tukey tests to distinguish differences etween the sites. Initil grphing of the chronosequence experiment dt suggested tht there ws difference etween sites, ut tht the reltionship etween ge nd invsive plnt growth ws not liner, therefore, we chose n ANOVA pproch over liner regression pproch. In ll nlyses, finl invsive plnt iomss dt were log-trnsformed to meet model ssumptions. Results Field survey We found eleven non-ntive plnt species estlished nd reproducing long rodsides djcent to our urn sites: B. inermis, Chenopodium lum L., Crepistecto- rum L.,Elymus repens(l.) Gould, Lepidium densiflorum Schrd., Mtricri discoide DC., M. lus, Plntgo mjor L., Polygonum viculre L., Trxcum officinle F. H. Wigg. ssp. officinle, nd Trifolium hyridum L. Men non-ntive plnt cover on the rodsides djcent to our intensively smpled sites ws highest long the Dlton Highwy, followed y the Tylor Highwy nd the Steese Highwy (Fig. 2). Rodside cover of nonntive plnts ner the Delt Chronosequence urns ws very low (Fig. 2), ut this ws due to intentionl weed control efforts y oth the Deprtment of Trnsporttion (L. Johnson, pers. comm.) nd the nery Ft. Greeley Army Bse (J. Mson, pers. comm.). Five of the non-ntive species present on rodsides were found within urned forest: C. lum, C. tectorum, E. repens, M. lus, nd T. officinle ssp. officinle. Of these, C. tectorum, ndm. lus re considered ggressive (Crlson et l. 2008). Across ll 2004 urn sites surveyed (n = 93), 12 sites hd non-ntive plnts occurring within urned res. Non-ntive plnt presence within urn sites ws significntly influenced y the 2 region in which it ws locted (V (3) = 14.52, P = 0.002). Burns in the Dlton region were more frequently invded thn urns in the Steese nd Tylor regions nd were the only 2004 urns with ggressive % urn sites with non-ntives present Aggressive species Non-ggressive species % cover on rodside Steese Tylor Dlton Chrono % non-ntive cover on rodside Fig. 2 Percent urn sites with non-ntive plnts present nd percent cover of non-ntive plnts on rodsides of different regions in interior Alsk, July Brs indicte percent of urn sites surveyed where non-ntive plnts occurred within the urns on the Steese (n = 33), Tylor (n = 29) nd Dlton (n = 31) regions, nd in the Delt Chronosequence urns ( Chrono ; n = 6). Striped r segments indicte percent of sites where only non-ggressive non-ntive species were present, while solid r segments indicte only ggressive non-ntive species present or ggressive nd non-ggressive species co-occurring. Open dots indicte men percent nonntive plnt cover (±SE) from suset of sites where we conducted rodside surveys in ech region (Steese N = 4, Tylor N = 4, Dlton N = 4, Chrono N = 5) species (Fig. 2). The Dlton urns lso hd higher undnces nd richness of non-ntive plnts estlished in urned hitt thn in the Tylor or Steese sites (Dlton: 4 species, pprox. 150 totl individuls; Tylor: 1 species, 4 totl individuls; Steese: 0 individuls). Four of the sites with non-ntive plnts growing were high severity urns nd eight of the sites were low severity, however there ws not significnt ssocition etween urn severity nd presence of non-ntive plnts. When investigting which geogrphic nd environmentl vriles tht helped predict non-ntive plnt presence within the 2004 urns, the only significnt vriles (p \ 0.05) were the slope of the urn site ( =-0.38, p = 0.04) nd the minerl soil ph ( = 1.78, p = 0.01). The Delt Chronosequence sites (n = 6) hd oth ggressive nd non-ggressive non-ntive plnts occurring in urned hitt nd showed the gretest non-ntive undnce nd richness of ll res surveyed. The highest undnce nd richness of non-ntive plnts within chronosequence site occurred in the 1994 urn (1994 high severity site: 4 species, [500 individuls; 1994 low severity site: 3 species, pprox. 150 individuls)

9 Susceptiility of urned lck spruce 1887 Greenhouse experiment 2004 Burns: urn severity, moisture, nd region Corse urn severity nd moisture levels did not significntly influence the growth or fitness-relted responses of ny of our focl species (dt nlyzed using mens per site: Tle 1, Fig. 3). The region urn occurred in tended to outweigh the effect of sitelevel urn severity or moisture on our focl species growth responses. M. lus survivl ws influenced y region (Tle 1), with gretest percent survivl in soils from the Dlton region. Shoot iomss of H. urnticum ws significntly influenced y region (Tle 1), with lower iomss in soils from the Steese region thn in soils from the Dlton or Tylor regions. B. inermis grown in Steese region soils hd hd pproximtely 77 % fewer tillers thn when grown in Dlton nd Tylor region soils (Tle 1). Region explined pproximtely 7 times more vrition in M. lus survivl thn did severity or moisture levels comined (region R 2 = 0.64, severity? moisture R 2 = 0.09). Similrly, region explined 4 times more vrition in H. urnticum shoot iomss (region R 2 = 0.54, severity? moisture R 2 = 0.13), nd 2.2 times more vrition in B. inermis tiller production (region R 2 = 0.45, severity? moisture R 2 = 0.20), thn did site severity nd moisture levels comined Burns: smll-scle mechnisms At the smll-scle, within-site level, oth M. lus nd B. inermis iomss were positively correlted with lower duff thickness, which hd the highest vrile importnce vlues of ll the possile covrites (Tle 2). Non-vsculr plnt iomss hd negtive reltionship with H. urnticum nd B. inermis finl iomss, nd hd the highest nd second highest importnce vlues of ll the soil nd vegettion covrites, respectively (Tle 2). Between regions, we found severl differences in the soil nd vegettion vriles we mesured (Tle 3). When compred to cores from sites in the Dlton or Tylor regions, cores from the Steese region hd significntly greter minerl soil wter holding cpcity, non-vsculr iomss nd ntive plnt richness (Tle 3). Soil cores from the Dlton region tended to hve lower vlues for upper duff wter holding cpcity, upper duff ulk density, nd Tle 1 ANOVA F vlues for ech explntory vrile (source of vrition) in the 2004 urn experiment nd chronosequence experiment Experiment source df M. lus H. urnticum B. inermis Tillers Totl mss Root mss Stolons Germ Surv Shoot mss Totl mss Root mss Germ Surv Shoot mss Totl mss Root mss Germ Surv Shoot mss F F F F F F F F F F F F F F F F F 2004 Burns Region ** * * Severity Moisture error df = 7 Chronosequence Burns Age Severity error df = 2 Ech invsive species ws modeled seprtely with percent germintion, shoot iomss, root iomss, totl iomss, or reproductive cpcity (H. urnticum- # of stolons; B. inermis- # tillers) s the response vrile. No F vlues could e produced for B. inermis survivl from chronosequence urns ecuse of invrint (100 %) survivl rtes * p \ 0.05, ** p \ 0.01

10 1888 K. V. Spellmn et l. A Low Severity High Severity B 0.50 M. M. lus l M. lus M. l log 10 totl iomss (g) H. urnticum 0.60 H. urnticum B. inermis B. inermis High Low High Low Burn Site Moisture Level Burn Yer Fig. 3 Men log 10 totl iomss (± 1SE) of invsive plnts grown for 150 dys in soil cores tken from sites urned in 2004 () nd from sites in the Delt Chronosequence (). Low severity urn types re indicted y striped rs, nd high severity urn types re indicted y solid rs. The 2004 vlues represent the men iomss cross the three smpling regions. Different letters indicte significnt differences (p \ 0.05) in totl iomss etween severity-moisture ctegories or geseverity ctegories mximum ntive vsculr plnt heights compred to Tylor or Steese cores (Tle 3). Compring 2004 severity-moisture site types cross the three regions, low severity sites tended to hve greter lower duff thickness compred to high severity sites (Tle 4). High severity sites, prticulrly high severity-high moisture sites, tended to hve the greter non-vsculr plnt iomss thn low severity sites (Tle 4).

11 Susceptiility of urned lck spruce 1889 Tle 2 Averge Akike s Informtion Criterion prmeter estimtes () nd cumultive prmeter weights ( P x i ) for cndidte vriles explining totl iomss of three invsive plnt species fter 150 dys of growth in soil cores from the 2004 urns nd the chronosequence urns Vrile M. lus H. urnticum B. inermis P xi P xi P xi 2004 Experiment Moss lyer depth (cm) Upper duff lyer depth (cm) Lower duff lyer depth (cm) WHC upper duff (g H 2 O/g soil) WHC lower duff (g H 2 O/g soil) WHC minerl (g H 2 O/g soil) Ntive plnt richness (# spp.) Non-vsculr iomss (g) Ntive vsculr iomss (g) Chronosequence experiment Moss lyer depth (cm) Upper duff lyer depth (cm) Lower duff lyer depth (cm) WHC upper duff (g H 2 O/g soil) WHC lower duff (g H 2 O/g soil) WHC minerl (g H 2 O/g soil) Ntive plnt richness (# spp.) Non-vsculr iomss (g) Ntive vsculr iomss (g) Averge prmeter vlues were tken over models with difference in AICc \ 3. Soil wter holding cpcity is revited s WHC. Bold vlues indicte well-supported prmeters ( P x i [ 0.5) Chronosequence urns: urn ge nd severity Burn ge nd severity did not significntly influence the germintion, survivl, shoot iomss, root iomss, or reproduction for ny of the three focl invsive species in our chronosequence greenhouse experiment (dt nlyzed using mens per site; Tle 1). For ll three species there ws generl pttern towrd the gretest invsive plnt iomss in soil cores from the 1994 low severity urn site, second gretest iomss in the 1987 low severity soils nd lowest iomss in cores from the 1999 high severity urn (Fig. 3). Chronosequence urns: smll-scle mechnisms At the within-site level in the chronosequence experiment, ntive vsculr iomss nd ntive plnt richness hd high cumultive prmeter weights nd were negtively correlted with finl iomss of ll three species (Tle 2). Wter vilility my lso ply n importnt role in determining invsive plnt estlishment in the chronosequence urn sites. Wter holding cpcity of the minerl soil hd high importnce vlue nd positively influenced M. lus finl iomss (Tle 2). Similrly, the wter holding cpcity of the lower duff ws positively correlted with the finl iomss of oth H. urnticum nd B. inermis iomss nd hd high importnce vlues for oth species (Tle 2). Soils tken from the 1994 low severity urn chronosequence urns tended to hve greter remining lower duff depth nd greter soil wter holding cpcity for ll three soil lyers thn ll the other urn sites, mong other soil differences (Tle 5). While ntive vsculr nd non-vsculr iomss

12 1890 K. V. Spellmn et l. Tle 3 Soil nd ntive vegettion chrcteristics for cores from 2004 urn sites in the Steese, Tylor, nd Dlton regions Vrile Steese Tylor Dlton Moss lyer depth (cm) Upper duff lyer depth (cm) Lower duff lyer depth (cm) WHC upper duff (g H 2 O/g soil) WHC lower duff (g H 2 O/g soil) WHC minerl (g H 2 O/g soil) Ntive plnt richness (# spp.) Non-vsculr iomss (g) Ntive vsculr iomss (g) 0.5 (0.1) 0.5 (0.3) 0.4 (0.2) 3.0 (1.0) 4.7 (0.8) 3.0 (0.8) 3.6 (0.3) 5.2 (0.4) 4.2 (0.8) 5.04 (0.57) 4.25 (0.42) 2.59 (0.75) 2.83 (0.28) 2.18 (0.09) 2.24 (0.38) 0.87 (0.05) 0.68 (0.05) 0.65 (0.14) 3.4 (0.5) 1.9 (0.6) 1.9 (0.4) 1.62 (0.13) 0.85 (0.36) 1.04 (0.32) 0.37 (0.08) 0.41 (0.14) 0.25 (0.08) All vlues re site-level mens with stndrd error in prentheses. Significnt differences (p \ 0.05) etween regions (columns) for ech vrile re denoted y different letters. Soil wter holding cpcity (WHC) is revited vried little etween sites, ntive plnt richness ws significntly higher in cores from the 1999 high severity site thn in cores from ny other site, nd tended to hve greter ntive vsculr plnt cover (Tle 5). Discussion Lrge scle ptterns Our field survey nd greenhouse dt suggest tht regionl effects outweigh the influence of urn site chrcteristics on invsive plnt growth, survivl nd reproduction in urned res (Fig. 2; Tle 1). In the 2004 urns, greter trffic nd propgule pressure long the Dlton Highwy could ccount in prt for the higher numers of invsives (Fig. 2), ut our greenhouse study indicted tht there were dditionl fctors eyond propgule pressure: ll three of our focl species showed greter (though not significnt for ll species) survivl nd iomss in soils from the Dlton region thn from other regions. Biotic nd iotic vriles tht we mesured in the greenhouse cnnot explin the regionl differences we found (Tle 3). Soil ph, however, significntly differs etween regions t these sites (Hollingsworth et l. 2013) nd ws n importnt predictor of nonntive plnt presence within urns. The higher minerl soil ph long the Dlton Highwy compred to the Steese nd the Tylor Highwys my ply role in the greter field undnces of non-ntive plnts. These results re consistent with the results of study y Rose nd Hermnutz (2004), who found incresing undnces of invsive plnts in orel forest sites with higher ph. The comined effects of higher propgule pressure, lower non-vsculr plnt cover, nd higher minerl soil ph my hve mde urns in Tle 4 Soil nd ntive vegettion chrcteristics for cores from n unurned control site nd 2004 urn sites of different severity nd moisture levels Vrile Unurned Low severity High severity High moist Low moist High moist Low moist Moss lyer depth (cm) (0.0) 0.3 (0.2) 1.0 (0.2)c 0.4 (0.1) Upper duff lyer depth (cm) (1.2) 4.6 (1.5)c 1.5 (0.6)d 0.6 (0.5)d Lower duff lyer depth (cm) (0.8) 5.6 (0.5) 3.0 (0.2) 2.6 (1.2) WHC upper duff (g H 2 O/g soil) (0.49) 4.89 (0.42) 3.07 (1.62)c 1.64 (0.89)c WHC lower duff (g H 2 O/g soil) (0.46) 2.76 (0.25) 2.06 (0.73)c 1.49 (0.29)c WHC minerl (g H 2 O/g soil) (0.12) 0.84 (0.12) 0.74 (0.10) 0.70 (0.14) Ntive plnt richness (# spp.) (0.4) 1.6 (0.6) 3.4 (0.5) 2.7 (1.0) Non-vsculr iomss (g) (0.36) 0.76 (0.48) 1.74 (0.17) 1.35 (0.11) Ntive vsculr iomss (g) (0.07) 0.21 (0.09) 0.44 (0.14) 0.22 (0.10) All vlues re site-level mens with stndrd error in prentheses. Significnt differences (p \ 0.05) etween urn types (columns) for ech vrile re denoted y different letters. Soil wter holding cpcity (WHC) is revited

13 Susceptiility of urned lck spruce 1891 Tle 5 Soil nd ntive vegettion chrcteristics for cores from the Delt Chronosequence high nd low severity sites urned in 1999, 1994, nd 1987 Vrile Low severity High severity Moss lyer depth (cm) 1.0 (0.2) 0.9 (0.1) 2.0 (0.4)c 0.6 (0.1) 1.3 (0.2)c 1.5 (0.2)c Upper duff lyer depth (cm) 5.5 (0.5) 3.4 (0.6)c 6.6 (0.5) 0.5 (0.2)d 0.1 (0.1)d 1.3 (0.3)c Lower duff lyer depth (cm) 4.1 (0.5) 7.3 (0.5) 4.2 (0.6)c 2.6 (0.3)c 2.6 (0.3)c 3.2 (0.4)c WHC upper duff (g H 2 O/g soil) 3.66 (0.21) 4.95 (0.30) 3.53 (0.29)c 3.28 (0.53)d 2.54 (0.00)e 2.71 (0.30)f WHC lower duff (g H 2 O/g soil) 2.14 (0.12) 3.25 (0.23) 2.08 (0.16)c 1.54 (0.17)d 1.54 (0.08)d 2.04 (0.15)e WHC minerl soil (g H 2 O/g soil) 0.84 (0.17) 1.70 (0.28) 0.77 (0.05) 0.82 (0.07) 0.73 (0.04) 0.77 (0.06) Ntive plnt richness (# spp.) 3.0 (0.2) 3.3 (0.2) 3.2 (0.2) 4.5 (0.1) 3.0 (0.2) 3.0 (0.1) Non-vsculr iomss (g) 1.57 (0.14) 1.32 (0.14) 1.85 (0.25) 2.10 (0.21) 1.64 (0.10) 1.10 (0.23) Ntive vsculr iomss (g) 0.53 (0.10) 0.59 (0.11) 0.51 (0.12) 0.50 (0.07) 0.76 (0.12) 0.61 (0.09) All vlues re core-level mens with stndrd error in prentheses. Significnt differences (p \ 0.05) etween urn types (columns) for ech vrile, s determined y Tukey tests, re denoted y different letters. Soil wter holding cpcity (WHC) is revited the Dlton region more susceptile to invsion thn urns in the Steese nd Tylor regions. Both the Steese nd Tylor sites occurred within the Yukon-Tnn Uplnd ecoregion while Dlton sites occurred within the Ry Mountin ecoregion (Nowcki et l. 2001). These ecoregions differ primrily in soils nd lithogrphy (Nowcki et l. 2001), which my correlte with differences in soil ph nd vegettion composition. Overll, the urned sites within the Delt Chronosequence showed the highest levels of non-ntive plnt invsion of ll the field sites we surveyed (Fig. 2). In the greenhouse study, invsive plnts grown in soils from the Delt Chronosequence sites hd, on verge, greter finl iomss thn the verge iomss for plnts grown in 2004 urn soils (Fig. 3). This result my lso e driven y lrge-scle differences in soils nd geology etween ecoregions. These sites occurred within the Tnn-Kuskokwim Lowlnds ecoregion, which hs plenty of surfce moisture nd thick orgnic soils compred to the rocky, welldrined soils of the Yukon-Tnn Uplnds nd Ry Mountins ecoregions (Nowcki et l. 2001). Both of these chrcteristics ppered to positively correlte with invsive plnt growth in some species when urn ge ws held constnt (Tle 2). The differences in the susceptiility of the different regions in interior Alsk suggest tht ecoregion delinetions (i.e. Nowcki et l. 2001) my e n importnt tool for prioritizing post-fire invsive plnt mngement, nd further reserch regrding correltions etween invsions nd ecoregions is wrrnted. Contrry to the findings of other studies (Agee nd Huff 1987; Turner et l. 1997; Doyle et l. 1998; Keeley et l. 2003; Hunter et l. 2006; Klinger et l. 2006), our focl species did not show significnt response to overrching severity, moisture nd ge ctegories t the site level (Tle 1). We my hve filed to detect ny response due to lck of sttisticl power, s the numer of intensively smpled sites (2004 urn experiment: N = 12; chronosequence urn experiment: N = 6) ws limited y the mount of spce ville in the greenhouse to grow the plnts nd the logisticl chllenges of ccessing sites spred throughout lrge geogrphic re. Our multiple liner regression nlysis, however, suggests tht the iomss of ll three of our focl species ws influenced y smllscle vritions in soil nd vegettion chrcteristics within sites tht reflect the complex interctions etween urn severity, site moisture nd time elpsed since urning. These interctions will e discussed in the following section. Smll-scle mechnisms Burns experiment The thickness of the lower duff lyer ws highly importnt in explining oth M. lus nd B. inermis totl iomss (Tle 2). Lower duff thickness reflects the mount of time etween the most recent fire nd

14 1892 K. V. Spellmn et l. the previous fire, the pre-fire species composition, decomposition rtes, environmentl conditions, nd the post-fire site chrcteristics (with greter thickness of this lyer remining in low severity, moist sites) (Johnstone et l. 2010). Sites where fire consumes nerly ll of the moss nd upper duff lyer ut sustntil mount of the lower duff lyer intct my provide excellent growth sustrte for M. lus nd B. inermis. Indeed, the survey site with the highest numers of M. lus (30 reproductive individuls in the urn) fits exctly this description. In ddition, non-vsculr plnt iomss ws one of the most importnt vriles explining the iomss of H. urnitcum nd B. inermis (Tle 2). The proportion of ryophyte-free re ws much greter in our low severity sites thn in high severity sites (Tle 4; Johnstone et l. 2008; Bernhrdt et l. 2011). Quickly estlishing liverworts (Mrchnti polymorph) nd mosses (Certdon purpureus nd Polytrichum spp.) my present uffer to H. urnticum invsion in high-severity sites. A similr pttern ws documented in rctic tundr, where the undnt mosses decresed germintion nd estlishment of diverse rry of ntive tundr plnts (Gough 2006). Non-vsculr plnt undnce my represent n importnt rrier to invsive plnt coloniztion nd spred in lck spruce forest of interior Alsk, nd further study is wrrnted. Chronosequence urn experiment Vsculr plnt species iomss nd ntive plnt richness were importnt covrites for totl invsive plnt iomss in our chronosequence sites (Tle 2). Vsculr plnt cover nd richness tended to e highest in the 1999 high severity urn site (Tle 5). We my hve seen reduced iomss (Fig. 3) of our three focl invsive plnt species in the soil cores with higher ntive vsculr plnt richness nd undnce due to higher levels of resource competition nd reduced vcnt niche spce. In other smll-scle invsiility studies, sites with greter ntive plnt richness nd undnce tend to e more resistnt to non-ntive plnt species introductions (Tilmn 1997; Neem et l. 2000; Kennedy et l. 2002). The higher undnce of ntive vsculr plnt iomss in the 1999 high severity urn site compred to the older high severity urn sites my reflect the influence of cnopy closure on ntive plnt cover through time. Reduced light vilility ssocited with higher cnopy closure likely cused the reduced undnce of vsculr plnts in the soil cores from the 1994 nd 1987 high severity urns. Other studies in this chronosequence hve shown tht, s urn ge incresed, oth lef re index nd ove ground iomss incresed s tree species ecme lrger (Zhung et l. 2003; Liu et l. 2005). This cnopy closure should lso presumly reduce invsive plnt growth nd field undnces, s most invsive plnt species in Alsk re erly successionl species with limited shde tolernce (Crlson et l. 2008). In our greenhouse experiment, the reduced competition from understory plnts comined with the sence of shde from dominnt deciduous trees nd shrus likely llowed invsive plnt iomss nd reproduction to e greter thn it would e in the field in the 1994 nd 1987 sites. The length of time etween fire nd cnopy closure hs een cited s n importnt prmeter for site invsiility (Keeley et l. 2003). While high severity urns in lck spruce forest tend to develop into dense, closed cnopy deciduous forest reltively quickly (if seed source is ville), low severity urns tend to slowly regenerte into more open cnopy lck spruce forest (Johnstone nd Chpin 2006). This greter light vilility in low severity urns, even s time elpses, my provide mple opportunity for invsive coloniztion nd seed nk estlishment. We found some evidence of this in our field survey, where ggressive non-ntive plnts occurred more frequently in low severity chronosequence urn sites thn in high severity chronosequence sites. Minerl soil or lower duff soil wter holding cpcity lso plyed n importnt role in explining the vrition in finl iomss of ll three invsive plnt species grown in cores from the chronosequence urns (Tle 2). The higher wter holding cpcity of soils in the 1994 low severity urn site (Tle 5), my explin why the three focl species tended to hve the gretest iomss in cores from this site (Fig. 3). Implictions for mngement Comining the cross-tx trends in our greenhouse study with our field survey results, we cn provide