FOR OVER A CENTURY, many coniferous forests in the

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1 Effects of Vegettion Ptches on Soil Nutrient Pools nd Fluxes within Mixed-Conifer Forest Hether E. Erickson, Ptrici Soto, Dle W. Johnson, Brent Roth, nd Crolyn Hunsker Astrct: At sptilly heterogeneous mixed-conifer forest in the centrl Cliforni Sierrs, we quntified totl cron (C) nd nitrogen (N), inorgnic N, nd net N minerliztion in orgnic (O) nd surfce (0 15 cm) minerl soils, nd in situ fluxes of inorgnic N nd ortho-phosphte using resin lysimeters under three ptch types: closed cnopy conifer, sprsely vegetted open cnopy, nd Cenothus cordultus thickets. In O horizons, totl N nd C pools rnked: closed cnopy Cenothus open cnopy. In minerl soils, totl N pools under Cenothus exceeded those under closed cnopy ptches y out 30%. For 2 yers, nitrte concentrtions nd rtes of net N minerliztion in O horizons were 4 greter under Cenothus thn under the other ptch types. Similrly, mmonium concentrtions nd net N minerliztion in minerl horizons were more thn 80 nd 200% greter, respectively, under Cenothus thn under the other ptch types. In situ fluxes of nitrte were 4 greter eneth Cenothus for 1 yer, nd no differences were found mong the ptch types for nother yer. Despite the internnul vriility, our dt suggest tht Cenothus contriutes to greter proportion of totl N minerlized thn sed on cover lone, nd my e importnt for forest nutrition under current conditions nd fter forest urning. FOR. SCI. 51(3): Key Words: Soil nitrogen, soil cron, net nitrogen minerliztion, Cenothus, Sierr Nevd forests, Aies concolor. FOR OVER A CENTURY, mny coniferous forests in the western United Sttes were suject to fire suppression. These forests re now eing considered for thinning nd urning s mens to remove ccumulted fuels nd restore the structurl conditions tht chrcterized the forests efore suppression (North et l. 2002). How forest responds to these tretments my depend, in prt, on the soil conditions tht result from the tretments. For exmple, while forest urning my increse ville pools of nitrogen (N) (Nery et l. 1999), totl N pools my decrese (Cldwell et l. 2002, Smith et l. 2000) with potentilly negtive consequences for forest productivity over the long term (e.g., Monleon et l. 1997). However, these responses re not consistent cross ll forests (Wn et l. 2001). Moreover, N vilility hs een shown to ffect seedling performnce in estern mixed temperte forests (Ctovsky nd Bzzz 2002) nd western cedr-hemlock forests (Bennett et l. 2003), nd hs the potentil to lter successionl dynmics (Ctovsky et l. 2002). Thus, hving etter understnding of how urning nd thinning ffects N cycling nd dynmics would gretly improve our ility to predict future conditions. How forest responds to thinning nd urning my lso depend on soil conditions efore disturnce. Aville nd totl pools of N nd cron (C) stores re inherently vrile in forest soils (Homnn et l. 2001, Connt et l. 2003, Rothe et l. 2002). Within-stnd vrition in nutrient cycling nd soil properties hs een ttriuted to tree species composition (Zinke 1962, Boerner nd Koslowsky 1989, Finzi et l. 1998, Ventere et l. 2003), tree mortlity pockets (Mtson nd Boone 1984), nd physiogrphic fetures (Ventere et l. 2003). Whether pretretment soil vriility remins following forest conversion or restortive tretments is not well known, lrgely ecuse the cuses of the sptil vrition in forest soils re still poorly understood. Mixed-conifer forests rnge throughout the western United Sttes, including much of Cliforni. Structurlly, these forests exhiit striking degree of sptil heterogeneity. In ddition to mixture of severl species of conifers (hence the nme), mny mixed-conifer forests lso contin res of sustntilly reduced cnopy cover, either completely without woody vegettion or contining thickets of shrus. Although open cnopy res often hve shllow soil depths to edrock, mny hve soil depths cple of supporting permnent tree cover. Hether E. Erickson, School of Science nd Technology, Universidd Metropolitn, PO Box 21150, Sn Jun, Puerto Rico ericksonhether@yhoo.com. Current ddress: SE McGillivry Boulevrd, Vncouver, WA Ptrici Soto, School of Science nd Technology, Universidd Metropolitn, PO Box 21150, Sn Jun, Puerto Rico Dle W. Johnson, Deprtment of Environmentl nd Resource Science, University of Nevd, Reno, NV dwj@cnr.unr.edu. Brent Roth, USFS, Sierr Ntionl Forest, Clovis, CA Crolyn Hunsker, USFS, Pcific Southwest Reserch Sttion, Fresno, CA Acknowledgments: We thnk M. North nd the USDA Forest Service PSW Rnge nd Experiment Sttion for support for this reserch, the personnel t the PSW l in Fresno for giving us ccess to the preprtion l nd providing logisticl support, nd the chemistry l t the USDA Forest Service Interntionl Institute of Tropicl Forestry in Rio Piedrs, Puerto Rico for nlyzing soils nd soil extrcts. A Model Institution for Excellence grnt from the Ntionl Science Foundtion to Universidd Metropolitn (UMET) provided dditionl support to the first uthor, s well s support for the yerly cdre of enthusistic undergrdutes from UMET who helped in the field nd in the lortory. Support vi USDA NRI Strengthening Awrd to H. Erickson is lso gretly pprecited. Mnuscript received June 23, 2003, ccepted Septemer 14, 2004 Copyright 2005 y the Society of Americn Foresters Forest Science 51(3)

2 Despite the prominent oveground heterogeneity within mixed-conifer forests, few studies hve documented how elowground resources vry mong ptches. Exceptions exist for some woody shrus, which re cple of symioticlly fixing N 2. In the estern Sierr Nevd, Johnson (1995) found tht soils under Cenothus velutinus contined more totl C nd N thn soils under djcent ptches of Jeffrey pine. Okley et l. (2003) found greter inorgnic nd totl N in the litter lyers under Cenothus cordultus thn under nother shru, Arctostphylos ptul. This suggests tht N-fixing ssocited shrus, such s Cenothus sp., my e importnt in retining or reuilding N stocks fter restortion. Documenting heterogeneity of soil resources t the ptch scle is not only crucil for understnding how forest might respond to restortion or other mngement tretments, ut lso for ccurtely scling estimtes of soil C nd N storge to lndscpe nd regionl scles. In this study we sought to exmine the reltion etween vegettion ptch types nd soil nutrient pools nd processes t mixed-conifer forest locted within the Tekettle Experimentl Forest in centrl Cliforni. The forest consists of distinct ptches of conifers (primrily white fir (Aies concolor), incense cedr (Clocedrus decurrens), nd sugr pine (Pinus lmertin)), low-stture shrus (C. cordultus nd A. ptul), nd sprsely vegetted open res, ll locted on nerly consistent southern-fcing slope. We hypothesized tht res with low vegettive cover would contin lower pools of C nd N, lower inorgnic N concentrtions, nd lower fluxes of N nd phosphorus (P) in contrst to res with dense forest vegettion nd shru cover. We lso expected tht soils under C. cordultus would hve greter N vilility nd greter stores of totl C nd N thn the other ptches. To test these hypotheses, we conducted intensive studies for more thn 2 yers cross replicted ptches of the three mjor ptch types t Tekettle: closed cnopy mixed-conifer, C. cordultus thickets ( Cenothus ), nd open cnopy res lcking significnt vegettion nd without exposed rock. Methods The Tekettle Experimentl Forest The Tekettle Experimentl Forest (36 58 N, W) is 1,300-h old-growth forest reserve (men elevtion 2,200 m) locted on the western slope of the centrl Sierr Nevd Mountins in Cliforni. The climte is Mediterrnen with wrm dry summers nd cool winters. Most of the 1,000-mm nnul precipittion flls s snow. The mixedconifer forest t Tekettle is sptilly heterogeneous with discontinuous cnopy consisting of primrily conifer-dominted tree ptches contining comintions of white fir (Aies concolor [Gordon nd Glend.] Lindley), incense cedr (Clocedrus decurrens [Torrey] Florin), red fir (Aies mgnific A. Murr), sugr pine (Pinus lmertin Dougl.), or Jeffrey pine (Pinus jeffreyi Grev. nd Blf); shru ptches consisting of primrily mountin whitethorn (Cenothus cordultus Kellogg), chinqupin (Chrysolepis sempevirens [Kell.] Hjelmq.), or greenlef mnznit (Arctostphylos ptul E. Greene); nd open cnopy res lcking permnent woody vegettion (North et l. 2002). Open cnopy ptches cn hve diverse, ut sprse ( 0.2%, North et l. 2005) herceous cover (e.g., mountin pennyroyl [Monrdell odortissim Benth.], lupine [Lupinus dsurgens E. Drew], nd scrlet gili [Ipomopsis congest {Hook.} V. Grnt]), nd often contin llochthonous litter from djcent conifer ptches. Soils in the study re re mostly well-drined Xerochrepts, Xerumrepts (Inceptisols), nd Xeropsmments (Entisols) formed from decomposed grnite. Textures rnge from grvelly corse sndy loms to grvelly lomy corse snds of the Cnnell nd Cgwin series, respectively. Although severl lightening strikes hve occurred in recent decdes within Tekettle, the lst mjor fire to occur in the re ws 1943, ut no impct ws mde on the study res (North et l. 2002). As detiled in North et l. (2002), in 1998 series of 18 4-h plots were estlished t Tekettle to ssess the effects of urning nd thinning s tools for restortion. The 4-h size ws chosen to sufficiently cpture the sptil vriility inherent in vegettion in the mixed-conifer forest t Tekettle (North et l. 2002), in tht ll of the ptch types re included in ech of the plots. The plots rnge in elevtion from out 1,950 to 2,270 m. The spect is southerly with slope ngles rnging from 5 to 20. The 18 plots fll into one of six tretments s prt of the restortion experiment. We used eight of these plots to locte replicte ptches (see elow) for the current study, which ws completed efore the restortion experiment egn; thus the results we present descrie pretretment conditions. Ptch Description nd Selection The three dominnt ptch types, closed cnopy, open cnopy, nd Cenothus, occur throughout Tekettle. Cnopy cover rnges from 79% in closed cnopy ptches to 32% in open cnopy ptches (North et l. 2002; note cnopy cover ws mesured with moose horn which, ecuse of 30 view ngle from verticl, incorportes folige from djcent ptches). Although sizes nd densities of the ptches vry within nd mong the ptch types (for exmple, ptch sizes my rnge from 3 m 2 for Cenothus to 1,000 m 2 for conifer tree groups), ech 4-h plot contined sufficient numer of ech of the ptch types for replicted smpling. For this study, potentil ptch needed to hve consistent vegettion for t lest 10 m in ny one direction nd cover minimum re of 40 m 2. Ongoing reserch t Tekettle suggests tht some of these ptches my hve originted s recently s the 1970s, perhps linked to drier climtic conditions tht occurred during tht time (Dve Rizzo, personl communiction, University of Cliforni, Dvis, Mrch 2004). Furthermore, there is little evidence to suggest tht the ptches hve chnged since tht time. Bsed on cluster nlysis of vegettion dt otined from 402 regulrly spced grid points covering 72-h re within Tekettle, North et l. (2002) estimte tht closed cnopy conifer represents 68% of the re, shru thickets 212 Forest Science 51(3) 2005

3 13%, open cnopy 11%, nd the reminder composed lrgely of exposed rock. Cenothus represents 75% of the shruy re (North et l. 2005). Fifty-four ptches, three replictes of the three dominnt ptch types (closed cnopy, open cnopy, nd Cenothus) within ech of six of the 4-h plots were selected in In 2000, six dditionl ptches of ech type from two dditionl plots were smpled for totl of 72 ptches (3 ptch types 3 replicte ptches 8 plots). Soil Smpling nd Anlyses In 1999, orgnic (Oi nd Oe) nd surfce minerl horizon soils (0 15 cm) were collected from the 54 ptches nd used to determine inorgnic nitrogen (N), net N minerliztion using eroic lortory incutions, totl pools of cron (C) nd N, ph, nd ulk density. In 2000, soils were collected from 18 new ptches s well s from 18 of the ptches previously smpled. Due to time limittions, not ll ptches could e smpled in Within ech ptch, mesuring tpe ws centered long the longest xis to select three evenly spced smpling points tht were t lest 3 m from the edge of ptch. At ech point, O (10 10 cm, smpled to depth) nd surfce minerl horizon (2 cm dimeter corer, 0 15 cm) soils were collected nd pooled y ptch. Depths of the O horizon were recorded to the nerest 0.5 cm; if depths less thn 1 cm were encountered no smple ws tken. Soils were collected July 6 8 in 1999 nd July 7 in 2000, kept cold in coolers nd rought to the USFS PSW Lortory in Fresno, CA within few dys for processing. Orgnic horizon soils were sieved (5 mm) to remove corser litter prticles nd lrge roots. Minerl soils were lso sieved (2 mm) to remove roots, ded uried wood, nd corse minerl frctions. For extrctle inorgnic N pools, sieved soils (5 nd 10 g for O nd minerl horizons, respectively) were extrcted with 100 ml 2 M KCl y shking for 30 min, then stored overnight. The extrct ws filtered (0.45 m), stored frozen, nd sent to the USDA Interntionl Institute of Tropicl Forestry (IITF) in Rio Piedrs, Puerto Rico for utomted colorimetric nlysis (Alpkem, Wilsonville, OR) of NO 3 nd NH 4. To estimte net N minerliztion, we used 10-dy eroic incutions t constnt moisture nd temperture (25 C); hence differences in N minerliztion should e primrily due to differences in sustrte qulity or fctors not relted to soil moisture. Becuse soil moisture ws often lredy low y erly July in oth O (rnging from 6.5 to 42.8% for open nd closed cnopy, respectively) nd minerl horizons (rnging from 6.4 to 9.8% for open nd closed cnopy, respectively), to void moisture limittions (Stnford nd Epstein 1974) incuting smples were wetted to out 55% wter holding cpcity (38% nd 12% grvimetric moisture contents for O nd A horizons, respectively). Net N minerliztion ws clculted s the chnge in NH 4 NO 3 over the 10 dys nd net nitrifiction s the chnge in NO 3. Additionl susmples were dried for grvimetric moisture contents (70 C for 48 hours) nd ground with mortr nd pestle for totl C nd N nlyses y dry comustion on LECO-CN nlyzer (LECO Corportion, St. Joseph, MI) t IITF or Universidd Metropolitn. Soil ph ws mesured on 5- nd 25-g field moist susmples, using soil:distilled wter rtios of 1:5 nd 1:1, for orgnic nd minerl smples, respectively. Bulk density ws mesured in July 1999 or 2000, depending on plot. For the O horizon, precisely mesured cm squre hole ws dug djcent to the center smpling point in ech ptch, nd crefully excvted of orgnic mteril. Wood greter thn 1 cm dimeter ws discrded. Depths were recorded from severl points on ech side to clculte volume. For the A horizon, 5 cm-dimeter soil corer ws used to remove soil from two depths, 0 7 cm nd 7 14 cm, from single loction lso djcent to the center smpling point. Minerl soils were sieved to remove frctions 2 mm; corse frctions were weighed nd converted to volume using rock density of 2.16 g/cm 3, otined y mesuring msses nd volumes y wter displcement on five 70-g corse frction smples. For the mss estimtes, orgnic nd minerl soils were dried for 48 h t 70 C nd 101 C, respectively, efore weighing. Volumes of the corse frctions were sutrcted from core volumes to clculte minerl horizon ulk densities, nd the verges of the two depths were used for susequent clcultions. Soil depths nd rock content djusted ulk densities were used to clculte totl pools of C nd N nd net N minerliztion on n re sis (Boone et l. 1999). Rock frctions re expressed on mss sis. Inorgnic N nd P Fluxes Soil solution fluxes were mesured with resin lysimeter collectors (Susflk nd Johnson 2002). In the utumn of 1999 nd 2000, two lysimeters contining nion-ction exchnge resins were inserted into the soil in two-thirds (i.e., 54) of the ptches where N minerliztion ws mesured. The resin lysimeters consisted of 5.5-cm long, 4-cm inside-dimeter (12.57 cm 2 re) PVC pipe within which 10 g of oven-dried Rexyn I-300 (H-OH) resin were trpped etween two Nitex nylon sheets. Ech end of the lysimeter outside of the resin section ws filled with 20 g lyer of moist, wshed silic snd. To keep the snd in the tue until instlltion, the ottom of ech PVC tue ws covered with cheesecloth held in plce with ruer nd. The resin collectors were removed nd replced nnully. The resins were extrcted with 2 M KCl nd nlyzed for NH 4 -N, NO 3 -N, nd ortho-p y utomted colorimetric nlysis y the Wter Anlysis Lortory t the Desert Reserch Institute in Reno, NV. Fluxes were clculted from the mount of nutrient in the resin minus lnks divided y the surfce re of the resin lysimeter. Sttisticl Anlyses Differences mong ptch type nd yer for most vriles were nlyzed using rndomized lock design with repliction using Generl Liner Models (SAS Institute, Forest Science 51(3)

4 kg C/m 2 kg N/m Orgnic lyer 0-15 cm ) Soil C pools CC CECO OC ) Soil N pools Inc.). The effect of ptch ws tested s Men Squre (MS) Ptch/MS Ptch Plot, with the interction term treted s rndom effect (Underwood 1997). For dt collected over 2 yers, the effect of yer ws exmined using dt from ptches tht were smpled in oth yers. If yer ws significnt, seprte nlyses were done y yer using ll of the dt for tht yer. If yer ws nonsignificnt, dt were pooled y ptch cross yers efore testing for the effect of ptch. The effect of yer ws not exmined for totl pools, ph, or ulk density. In most cses, the effect of plot, treted s lock in the model, ws not significnt (dt not presented). A Tukey s multiple comprisons test ws used to indicte significnt differences for mens cross ptch types. Typiclly, dt were log-trnsformed to stilize vrinces. We lso explored possile reltionships etween rtes of N cycling (e.g., net N minerliztion nd nitrifiction) nd vrious soil properties. Prmetric correltion nlyses were used fter verifying tht model ssumptions were not seriously violted CC CECO OC c) Soil C:N rtio CC CECO OC Figure 1. Totl cron (), nitrogen (), nd C:N rtios (c) for O nd surfce (0-15 cm) minerl horizons for the three ptch types. Codes for ptch types re listed in Tle 1. Different letters indicte significntly different mens y ptch type nd within horizon. Error rs indicte 1 SE, n 24. c c Results Nutrient Pools nd Dynmics in Ptches Closed cnopy orgnic (O) horizons were less cidic y hlf ph unit compred with the O horizons in the other two ptch types (Tle 1). Minerl soil ph vried y only 0.2 ph units mong the ptch types (Tle 1), with soils under open cnopy slightly more cidic thn the others. Bulk density of the O horizons ws highly vrile nd there were no differences mong the ptch types (Tle 1). However, in surfce minerl horizons ulk density ws lmost 20% greter in open cnopy ptches thn in Cenothus or closed cnopy ptches. Open cnopy minerl soils lso contined greter proportion of rock frctions thn soils in closed cnopy ptches (Tle 1). Totl pools of C nd N in the O horizons, excluding those in woody deris, differed significntly mong ptch types nd rnked closed cnopy Cenothus open cnopy (P 0.001, Figure 1) nd corresponded with the rnking of the mss of O horizon (Tle 1). Totl C pools in the O horizon under closed cnopy were two times greter thn C pools in Cenothus ptches nd, not surprisingly, over five times greter thn O horizon C pools in open res (Figure 1). Similrly, totl N pools in the O Tle 1. Selected chrcteristics of orgnic (O) nd 0-15-cm minerl soils (men, 1 stndrd error (SE)) for the three dominnt ptch types t the Tekettle Experiment Forest. Ptch type O horizon mss (kg/m 2 ) Depth of the O horizon (cm) Soil ph (H 2 O) Bulk density (g/cm 3 ) Rock frction (% of mss) O horizon 0-15 cm O horizon 0-15 cm 0-15 cm OC 1.1 (0.4) c * 1.0 (0.3) c 5.0 (0.15) 5.7 (0.06) (0.02) 1.12 (0.03) 22.6 (1.54) CECO 3.3 (0.4) 3.0 (0.4) 5.1 (0.08) 5.9 (0.09) (0.01) 0.91 (0.03) 19.5 (1.25) CC 6.8 (0.6) 5.5 (0.4) 5.5 (0.9) 5.9 (0.06) (0.01) 0.95 (0.03) 16.2 (1.30) Codes for ptch type re: OC, open cnopy res; CECO, Cenothus cordultus; CC, closed cnopy conifer. For O mss, O depth, minerl horizon ulk density, nd rock frction, n 24. For O horizon ulk density, n 24 except for OC, where n 13 (see text). For ph, n 18 except for OC O horizon, where n 13. * Different superscripts within columns indicte significnt effects (P 0.05) using Tukey s multiple comprison. 214 Forest Science 51(3) 2005

5 Figure 2. Extrctle NH 4 () nd NO 3 () for O horizon soils nd extrctle NH 4 (c) for surfce minerl horizon y ptch type. For ll vriles, the effect of ptch type ws significnt. Where results re shown for 2 yers, the effect of yer ws lso significnt. Different letters indicte significntly different mens y ptch type or y ptch type within yer if dt re for 2 yers. Error rs indicte 1 SE. Where results re presented for 2 yers, n 18 nd 14 for 1999 nd 2000, respectively; otherwise, n 24. For O horizons under OC, n 8 nd 6 for 1999 nd 2000, respectively. horizon under closed cnopy exceeded those under Cenothus nd open res. Across ll ptches (n 71), O horizon depth ws highly correlted with O horizon totl C (Person s r 0.900, P ) nd N pools (Person s r 0.817, P 0.0,001). Totl C pools in the upper 0 15-cm minerl horizons (men 3.5 kg/m 2 ) did not differ mong the three ptch types (P 0.063) (Figure 1); however, ptch type did hve significnt effect on N pools in this horizon (P 0.019). Here, totl N pools under Figure 3. Net N minerliztion, nd net nitrifiction for O horizon (, ) nd upper (0-15 cm) minerl (c, d) soils y ptch type, respectively. For ll vriles, the effect of ptch type ws significnt. Where results re shown for 2 yers, the effect of yer ws significnt. Different letters indicte significntly different mens y ptch type or y ptch type within yer if dt re for 2 yers. Error rs indicte 1 SE, smple sizes re explined in Figure 2. Cenothus significntly exceeded those under closed cnopy y out 34% (Figure 1). Cron:nitrogen rtios for oth O nd upper minerl horizons were significntly lower under Cenothus thn under closed cnopy or open cnopy (Figure 1). Inorgnic N, often considered n instntneous mesure Forest Science 51(3)

6 Tle 2. Net N minerliztion nd net nitrifiction expressed on n re sis (mg N/m 2 of ptch for 10 dys) (men, SE) pooled over 2 yers for the three dominnt ptch types. Codes for ptch types re defined in Tle 1, n 24. O horizon 0-15 cm Ptch type Net N minerliztion Net nitrifiction Net N minerliztion Net nitrifiction OC 27.8 (27.5) * 3.3 (1.5) 135 (29.2) 182 (32.9) CECO 142 (30.3) 23.4 (5.5) 261 (38.4) 176 (4.5) CC 133 (84.9) 23.4 (3.1) 86.8 (32.1) 64 (16.8) * Different superscripts within columns indicte significntly different rtes (P 0.05) using Tukey s multiple comprison. of N vilility, differed mong ptch types nd for extrctle NH 4 in O horizons, lso depended on yer. In 1999, extrctle NH 4 in O horizons ws twice s high under closed cnopy s under the other two ptch types, while in 2000, extrctle NH 4 ws eqully high under Cenothus nd closed cnopy (Figure 2). Extrctle NO 3 ws nerly n order of mgnitude less thn extrctle NH 4 nd ws consistently higher under Cenothus thn under closed cnopy or open cnopy (Figure 2). Ammonium pools (Figure 2) in the upper minerl horizon were significntly greter in Cenothus ptches thn in the other ptch types. Nitrte ws not detected in minerl soils (detection limit 0.8 g/g) under closed cnopy nd ws only occsionlly present under open cnopy nd Cenothus, nd ws highly vrile (dt not shown). Differences mong ptches were pprent for N dynmics, nd often depended on yer. In 1999, O horizons under closed cnopy showed net immoiliztion rther thn net minerliztion (mss sis, Figure 3), wheres in 2000 net N minerliztion ws positive under closed cnopy nd did not differ significntly from net N minerliztion in Cenothus ptches. On verge, rtes of net nitrifiction in the O horizon rnged from 19 to 31% of net N minerliztion. Net nitrifiction in the O horizon ws not influenced y yer (P 0.05), ut s with the other vriles, ws influenced y ptch type; net nitrifiction ws greter under Cenothus compred to the others (Figure 3). Rtes of net nitrifiction on mss sis in minerl horizons were much lower thn rtes in O horizons nd differences mong ptches lso depended on yer; in 1999 nitrifiction in closed cnopy ws 40% less thn in the other ptch types, nd in 2000 there were no significnt differences mong ptch types (Figure 3). Rtes of net N minerliztion in the upper minerl horizon were nerly three times greter in Cenothus ptches compred with the other ptch types (Figure 3). Converting rtes of N minerliztion to n re sis shows the importnce of the minerl horizon in contriuting to N turnover for ll ptch types (Tle 2). Wheres net N minerliztion expressed on mss sis is roughly n order of mgnitude less for minerl horizons thn for O horizons (Figure 3), on n re sis rtes in the minerl horizon re similr to or greter thn those in the O horizon (Tle 2). Similr to results expressed on mss sis, net N minerliztion on n re sis in surfce minerl horizons for Cenothus ptches were more thn two times greter thn for closed nd open cnopy ptches. Nutrient fluxes were mesured in 2000 nd In 2000, nitrte, mmonium, nd ortho-phosphte fluxes were highly vrile nd there ws no effect of ptch type (Tle 3). In 2001, only nitrte fluxes differed significntly mong the ptches; here, nitrte ws more thn four times greter in Cenothus thn the other ptch types (Tle 3). Nutrient fluxes were much greter in 2000 thn in 2001 (Tle 3). No correltions were found etween nutrient fluxes nd KClextrctle N or rtes of net N minerliztion (P 0.05). Discussion Differences mong Ptches Soil properties nd elowground nitrogen dynmics pper to reflect the highly heterogeneous oveground ecosystem of the mixed-conifer forest t Tekettle. The three dominnt ptch types, closed cnopy, Cenothus, nd open cnopy, hd distinct signtures for numer of soil properties nd N cycling processes. For exmple, extrctle nitrte in the O horizon, vrile showing lrge differences mong ptch types, ws 8.5 nd 3.0 times greter in Cenothus versus closed nd open cnopy ptches, respectively. Similrly, net N minerliztion in the upper minerl soil ws 3.8 nd 2.3 times greter in Cenothus versus closed nd open cnopy ptches, respectively. M et l. (2004) lso found lrge differences in soil respirtion rtes mong ptch types; here, rtes were 90 nd Tle 3. Inorgnic N nd P fluxes (kg/h) (men, SE) s mesured y resin lysimeters in surfce soils of the three dominnt ptch types. Yer Ptch Type NO 3 -N NH 4 -N PO 4 -P 2000 OC 1.76 (1.49) * 0.64 (0.35) 0.41 (0.16) CECO 2.42 (1.08) 1.64 (0.84) 1.95 (1.25) CC 2.45 (1.22) 1.71 (0.71) 0.68 (0.24) 2001 OC 0.19 (0.04) 0.48 (0.07) 0.07 (0.02) CECO 2.21 (1.43) 1.28 (0.53) 0.42 (0.21) CC 0.24 (0.07) 0.68 (0.09) 0.13 (0.04) For 2000, n 12 ptches per type; for 2001, n 8 ptches per type. Codes for ptch types re defined in Tle 1. * Different superscripts indicte significntly different fluxes (P 0.05) within given yer using Tukey s multiple comprison. 216 Forest Science 51(3) 2005

7 165% higher in closed cnopy res nd Cenothus ptches, respectively, thn in open cnopy res. In generl, the effect of species composition on withinstnd soil properties hs een exmined in more detil thn effects of oveground ptches on soil properties. For exmple, in closed cnopy estern temperte forest, C nd N pools nd net N minerliztion differed under vrious tree species (Finzi et l. 1998). Similrly, Boerner nd Koslowsky (1989) found tree species explined much of the vrition in N minerliztion nd C concentrtions in n estern mixed species forest. Zinke (1962) first reported differences in soil N concentrtions mong severl Cliforni conifer species. Though tree species nd site were often confounded in this clssic study, the findings suggest tht some of the vrition found within the closed cnopy ptches t Tekettle my e due to vritions in species composition. For exmple, N concentrtions were 20% greter under incense cedr thn under P. ponderos (Zinke 1962). More reserch is necessry t Tekettle nd in mixed-conifer in generl to ssess tree species level effects on soil properties. Furthermore, forest pthogens, which my lso chnge rtes of net N minerliztion (c.f. Mtson nd Boone 1984), re nother potentil source of vrition in the N cycle within the closed cnopy ptches t Tekettle. As documented in North et l. (2002) severl root disese/rk eetle/dwrf mistletoe complexes re found in Tekettle. While recognizing the importnce of these nd other fctors in potentilly contriuting to the vrition in soil properties within the closed cnopy ptches t Tekettle, we found tht the gretest mount of vrition for most soil properties ws etween, not within, the different ptch types. Low or negtive rtes of net N minerliztion re expected in most coniferous forest soils (Dvidson et l. 1992, Hrt et l. 1994), s well s low concentrtions of nd short-term net turnover of NO 3. The dominnt ptch type, closed cnopy, ws consistent with this expecttion in tht N minerliztion in the O horizon ws negtive one yer nd NO 3 concentrtions were low (O horizon) or not detectle (minerl horizon). In mny conifer forests in the Sierr Nevd, wter vilility is thought to limit tree distriutions nd net primry production (Miller nd Urn 1999), lthough it is likely tht semi-rid ecosystems re lso co-limited y nitrogen (Hooper nd Johnson 1999, Powers nd Reynolds 1999). Sierrn forest soils often show high microil N immoiliztion (Johnson et l. 2000, Dvidson et l. 1992) nd hve thick forest floors (Johnson et l. 2000). Tht reltive rtes of nitrifiction were low cross ll ptch types in this study supports the ide of low N vilility nd N limited processes in mixed conifer forests. Cenothus ptches hve the lrgest totl N pools in surfce minerl soils, lowest soil C:N rtios, highest rtes of net N minerliztion, net nitrifiction nd NO 3 -N concentrtions in the O horizon, nd the highest rtes of net N minerliztion nd NH 4 -N concentrtions in the minerl horizon. Johnson (1995) lso found greter rtes of N minerliztion under C. velutinus in contrst to under djcent pines t n estern Sierr site. Presumly, mny of these differences re due to symiotic N fixtion y Cenothus. While severl studies hve estimted in situ N fixtion rtes for C. velutinus (e.g., Busse 2000), common shru in northern Cliforni nd southern Oregon, we know of no work quntifying fixtion rtes for C. cordultus. Results from pilot study using the del 15 N nturl undnce method of estimting symiotic N 2 fixtion suggest tht C. cordultus otins significnt (ut unknown) portion of its nnul N requirement from fixtion (Erickson 2000). The incresed totl N under Cenothus reltive to the conifer ptches is consistent with other studies showing greter soil N pools under N-fixing trees nd shrus (c.f. Binkley nd Sollins 1990, Johnson 1995). Perhps s Johnson (1995) proposed, stiliztion of orgnic mtter might lso ply role in mintining high pools of C nd N under Cenothus. Tken together, our results suggest tht C. cordultus mkes significnt contriution to the N economy of the mixed-conifer forests t Tekettle. The sums of net N minerliztion for the O nd upper minerl horizons (Tle 2) for Cenothus nd closed cnopy ptches re 403 mg N/m 2 nd 220 mg N/m 2 per re of ptch type, respectively. Multiplying these rtes y the proportion of the forest re covered y ech ptch type (10 nd 68% for Cenothus nd closed cnopy, respectively), we clculte forestwide contriution to net N minerliztion of 40 mg/m 2 for Cenothus nd mg/m 2 for closed cnopy. Thus Cenothus, lthough covering only 10% of the forest re, potentilly contriutes to over 25% of the N minerlized in the forest. (Note: This estimte is sed on lortory incutions only nd ssumes field rtes would follow similr pttern.) Whether neighoring conifers re le to use the dditionl N is lrgely unknown. Although conifer regenertion is reltively low in Cenothus ptches (H. Erickson, personl oservtion), fruiting odies of ectomycorrhizl fungi hve een found in Cenothus ptches (M. North, personl communiction, June 2003), suggesting tht the ectomycorrhizl conifers my e enefiting from lterl trnsfer of nutrients. N my cycle fster under Cenothus, in prt ecuse of the greter N often found in litter produced y N fixers. Fresh lef litter from C. cordultus t Tekettle contins 1.27% N nd hs men C:N rtio of 42, in contrst to needle litter from the mjor Aies nd Pinus sp. t Tekettle, with verge N concentrtions nd C:N rtios of 0.62% nd 82, respectively (H. Erickson, unpulished dt). Litter C:N (or lignin:nitrogen) rtios hve een shown to relte inversely to rtes of net nitrogen minerliztion in surfce soils for numer of temperte (Pstor et l. 1984, Scott nd Binkley 1997) nd tropicl forest ecosystems (Erickson et l. 2001). We lso found inverse reltionships for soil C:N rtios nd rtes of net nitrogen minerliztion in O (Person s r 0.349, P 0.007, n 59) nd minerl horizons (Person s r 0.352, P 0.002, n 72), suggesting tht soil orgnic mtter qulity, expressed s the C:N rtio, lso ffects the rte of N minerliztion. Open cnopy res re mostly devoid of vegettion, ut hve thin (verge 1.0 cm) ccumultions of llochthonous Forest Science 51(3)

8 litter from djcent ptches. The high C:N rtios of these O horizons suggest low rtes of decomposition: the litter lyer ppers reltively undecomposed nd firic nd spric lyers re lcking. These high C:N rtios re lso consistent with the low rtes of net N minerliztion. In minerl soils, the intermedite rtes of N cycling nd intermedite N pools sizes found under open cnopies likely reflect, in prt, plnt species compositions within the ptches. Severl Lupinus sp. re lso found growing in open res. Mny lupines re known to symioticlly fix N nd could e dding significnt ut unknown contriution to the N pools in open res. Also, higher concentrtions of nitrte in open versus closed cnopy res my reflect reduced plnt uptke in those ptches. Allochthonous litter (i.e., from conifers) lso flls into Cenothus ptches (H. Erickson, personl oservtion), presumly modifying soil properties in those ptches resulting solely from input of Cenothus litter. Nonetheless, O horizons re shllower under Cenothus ptches compred with closed cnopy. This difference could e due to either greter litter input or slower decomposition in the closed cnopy ptches compred to the Cenothus ptches, or comintion of the two processes. Bsed on C:N rtios lone, Cenothus litter would e expected to decompose more rpidly thn conifer needle litter. Oviously litter inputs, llochthonous or otherwise, re not s lrge in open ptches s in the ptches with woody vegettion, ut pprently wht gets there remins. Decomposition is influenced y litter qulity nd y environmentl fctors. We mesured extremely low July soil moisture contents in open cnopy res (verging 6.4% for minerl horizons over 2 yers). M et l. (2004) report the highest soil tempertures t 10 cm depth (men 20.0 C) for open cnopy res t Tekettle. Cooler soil tempertures nd higher moisture contents within Cenothus nd closed cnopy ptches re more conducive to decy thn the low moisture nd high tempertures of open res which likely limit microil ctivity nd rtes of decomposition, t lest during extended droughty periods. Mck nd D Antonio (2003) experimentlly showed strong microclimte control of net N minerliztion during the dry seson in Hwiin woodlnd. The Hwiin soils in the dry seson were three times moister thn the open cnopy soils t Tekettle, suggesting strong potentil for soil moisture limittion of soil processes within the ptches t Tekettle. Collectively, our dt suggest tht the mjor differences in soil properties mong the ptch types t Tekettle re due to comintion of the quntity nd qulity of orgnic mtter inputs nd the steep differences in microclimte reported y M et l. (2004). Yer-to-Yer Vritions Inorgnic N pools re often highly vrile within nd mong yers, especilly in highly sesonl ecosystems (St. John nd Rundel 1976, Cin et l. 1999). Inorgnic N my ccumulte during periods of little rinfll (St. John nd Rundel 1976, Dvidson et l. 1992, Jensen et l. 2001). We ttempted to collect soils within few weeks fter snowmelt; the lck of ny difference in soil moisture etween yers (P 0.05; dt not shown) suggests tht the differences we found in mmonium concentrtions etween yers were not driven y differences in soil moisture. We lso found lrge differences in net N minerliztion within closed cnopy ptches; one yer showed net N immoiliztion nd the other yer net N relese. While smpling soils in 1999, n unusul erly July rinfll moistened the dry forest floor nd my hve stimulted microil immoiliztion (Pullemn nd Tietem 1999). However, ecuse the soils were incuted t 55% wter holding cpcity, other fctors (e.g., microil popultions, sustrte vilility) likely influenced the shifting pttern of immoiliztion nd relese etween the two yers. The smller soil fluxes of inorgnic N nd P for the second yer my hve een due to differences in the mount of soil wter flux, concentrtions in soil wter, or oth. Totl rinfll ws nerly 30% lower the second yer, ut unfortuntely, the resin dt lone do not llow us to differentite etween the two fctors. The yer-to-yer vrition in mmonium vilility, net N minerliztion, nd nutrient fluxes results in yer-to-yer vrition in ptch effects s well. For exmple, in 2001 nitrte flux ws significntly greter in Cenothus ptches compred to the other two ptch types, ut not in 2000 (Tle 3). This underscores the importnce of multiyer smpling for determining how dynmic vriles differ mong the different ptches. Impliction for Restortion Our key finding of significnt differences mong ptch types for pools of C nd N nd rtes of N trnsformtions hs n importnt impliction for restortion. Burning, with or without forest thinning, is likely to ecome key restortion tool in the Sierr Nevd. Becuse much of the urning-relted losses of C nd N from forests in the Sierr re due to comustion of the O horizon (Cldwell et l. 2002), legcies my remin in pockets of minerl soils tht could ffect future regenertion nd survivl. The different verticl distriutions of C nd N mong the ptch types suggest tht their losses would e gretest in closed cnopy ptches, where lrger mount of totl C nd N pools is stored in the O horizon (Figure 1). And, ecuse lrger proportion (nerly 50%) of totl pools from the orgnic nd minerl horizons is stored in oveground litter in closed cnopy ptches, if fire consumed most of the orgnic horizons, Cenothus nd open cnopy res would retin more of their originl pools thn the conifer ptches. Assuming minerl pools remined unchnged with consumption of the O horizon, there would no longer e ny differences mong the ptch types for pools of C. However, the lrge mount of N stored in surfce minerl horizons under Cenothus suggests tht even with intense urns, elowground ptches of high N my remin under former Cenothus ptches. We lso found lower C:N rtios in soils under Cenothus; the inverse correltion etween soil C:N 218 Forest Science 51(3) 2005

9 rtio nd net N minerliztion suggests tht rtes of N turnover would lso e fster under former Cenothus ptches. For exmple, Okley et l. (2003) found tht inorgnic N pools were greter under Cenothus 6 months fter urning compred with djcent A. ptul ptches. These high N ptches my e importnt if nturl regenertion is N limited (c.f. Frnsen et l. 2001). Furthermore, ecuse N immoiliztion my increse fter fire on N limited sites (Bell nd Binkley 1989), such s within the conifer ptches, the differences in N vilility etween the high N former Cenothus ptches nd the likely N limited conifer ptches my widen. Our findings strongly suggest tht Cenothus ptches nd perhps open cnopy res, which tended to hve greter minerl horizon N pools thn closed cnopy res, my e importnt for future forest regenertion nd productivity. However, other potentilly competing processes could diminish this effect. For exmple, open cnopy soils, with greter rock frgment volumes nd higher ulk density thn the other soils, my not retin sufficient moisture for seedling estlishment fter fire. Also, resprouting of Cenothus is often common nd rpid fter fire, suggesting tht Cenothus my gin dominte N-rich sites fter disturnce. Ongoing studies t Tekettle re monitoring the response of Cenothus nd conifer seedling estlishment to urning nd thinning tretments. The chrcteristics of the N cycle tht we mesured for the conifer ptches t Tekettle, however typicl of mixedconifer forest, my reflect the history of fire suppression. White fir, shde-tolernt conifer, hs incresed in undnce in mny mixed-conifer forests in the region ecuse of the lck of nturl fires (e.g., Stephenson 1999). Within the conifer ptches t Tekettle, white-fir regenertion is undnt (North et l. 2002) presumly lso due to lck of fire. However, ecuse we did not exmine how shifts in conifer species composition my hve ltered N cycling, we hve no wy of knowing whether the current ptterns within the closed cnopy ptches re n rtifct of fire suppression. Literture Cited BELL, R.L., AND D. BINKLEY Soil nitrogen minerliztion nd immoiliztion in response to periodic prescried fire in lololly pine plnttion. Cn. J. For. Res. 19: BENNETT, J.N., L.L. BLEVINS, J.E. BARKER, D.P. BLEVINS, AND C.E. PRESCOTT Increses in tree growth nd nutrient supply still pprent 10 to 13 yers following fertiliztion nd vegettion control of sll-dominted cedr-hemlock stnds on Vncouver Islnd. Cn. J. For. Res. 33: BINKLEY, D., AND P. SOLLINS Fctors determining differences in soil ph in djcent conifer nd lder-conifer stnds. Soil Sci. Soc. Am. J. 54: BOERNER, R.E.J., AND S.D. KOSLOWSKY Microsite vritions in soil chemistry nd nitrogen minerliztion in eechmple forest. Soil Biol. nd Biochem. 21(6): BOONE, R.D., D.F. GRIGAL, P. SOLLINS, J. AHRENS, AND D.E. ARMSTRONG Soil smpling, preprtion, rchiving, nd qulity control. P in Stndrd Soil Methods for Long- Term Ecologicl Reserch, Roertson, G.P., D.C. Colemn, C.S. Bledsoe, nd P. Sollins (eds.). Oxford University Press, New York. BUSSE, M.D Suitility nd use of the 15 N isotope dilution method to estimte nitrogen fixtion y ctinorhizl shrus. For. Ecol. nd Mnge. 36:85. CAIN, M.L., S. SUBLER, J.P. EVANS, AND M.J. FORTIN Smpling sptil nd temporl vrition in soil nitrogen vilility. Oecologi 118: CALDWELL, T.J., D.W. JOHNSON, W.W. MILLER, AND R.G. QUALLS Forest floor cron nd nitrogen losses due to prescription fire. Soil Sci. Soc. Am. J. 66: CATOVSKY, S., AND F.A. BAZZAZ Nitrogen vilility influences regenertion of temperte tree species in the understory seedling nk. Ecol. Applic. 12: CATOVSKY, S., R.K. KOBE, AND F.A. BAZZAZ Nitrogen-induced chnges in seedling regenertion nd dynmics of mixed conifer-rod-leved forests. Ecol. Applic. 12: CONANT, R.T., G.R. SMITH, AND K. 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