Variation in microclimate associated with dispersed-retention harvests in coniferous forests of western Washington

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1 Forest Ecology nd Mngement 226 (06) Vrition in microclimte ssocited with dispersed-retention hrvests in coniferous forests of western Wshington Troy D. Heithecker*, Chrles B. Hlpern College of Forest Resources, Box , University of Wshington, Settle Wshington , USA Abstrct Green-tree or structurl retention is becoming incresingly common s method of regenertion hrvest in the Pcific Northwest. Ameliortion of microclimtic stress is ssumed to be one mechnism by which overstory retention enhnces the survivl of forest orgnisms nd the potentil for ecosystem recovery following timber hrvest. We exmined ptterns of trnsmitted light (photosynthetic photon flux density, PPFD), ir nd soil temperture, nd soil moisture cross brod grdient of dispersed retention in mture, coniferous forests t three loctions in western Wshington. Tretment mens nd within-tretment vrition (coefficients of vrition mong smple points within tretments) were compred for wrm, sunny dys in 6- to 7-yer-old experimentl hrvest units representing 0, 15, 40, nd 100% retention of originl bsl re. Multiple liner regression ws used to explore reltionships between microclimte nd plot-scle mesures of forest structure (including overstory ttributes, understory vegettion, nd logging slsh). PPFD nd men nd mximum ir nd soil tempertures decresed with level of retention. PPFD showed the strongest response, but did not differ between 40 nd 100% retention. Men nd mximum ir tempertures were significntly greter t 0 nd 15% retention thn t 100%. Among hrvest tretments (0, 15, nd 40%), men ir temperture ws significntly lower t 40 thn t 0%, but mximum ir temperture did not differ mong tretments. Men nd mximum soil tempertures differed only between 0 nd 100% retention. Minimum ir nd soil tempertures nd lte-summer soil moisture did not differ mong tretments. Within-tretment vribility (CV) did not differ significntly with level of retention for ny of the vribles smpled, but CVs for soil temperture showed consistent increse with decresing retention. In combintion, topogrphy, residul forest structure, nd understory vribles were good predictors of PPFD nd men nd mximum tempertures (R 2 of in multiple regression models), but were poorer predictors of minimum tempertures nd soil moisture (R 2 of ). Cnopy cover ppered most frequently in the models nd cover of understory vegettion ws significnt predictor in models of soil temperture. Trends in microclimte mong experimentl tretments were consistent, in lrge prt, with the erly responses of bryophyte, herbceous, nd fungl communities t these sites. Our results suggest tht 15% retention, the current minimum stndrd on federl forests within the rnge of the northern spotted owl, does little to meliorte microclimtic conditions reltive to trditionl clercut logging. Keywords: DEMO, Forest mngement; Forest microclimte; Light; Pcific Northwest; Structurl retention; Temperture; Vrible-retention hrvest *Corresponding uthor. Present ddress: USDA Forest Service, Pcific Northwest Reserch Sttion, 2770 Sherwood Lne, Suite 2A, Juneu, AK 99801, USA. Tel.: ; fx E-mil ddress: theithecker@fs.fed.us (T. D. Heithecker), chlpern@u.wshington.edu (C. B. Hlpern) 1. Introduction In the Pcific Northwest, vrible-retention hrvests tht retin elements of old forest structure (lrge live trees, sngs, nd logs) hve replced clercut logging on federl forests within the rnge of the northern spotted owl (Frnklin et l, 1997; (Frnklin et l., 1997; Aubry et l., 1999; Beese et l., 03). Current

2 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 1 federl stndrds require tht live trees re retined cross minimum of 15% of ech hrvest unit (USDA nd USDI 1994) to moderte loss of biologicl diversity nd to fcilitte recovery of the regenerting forest. Although there re vrious mechnisms by which overstory retention cn minimize species loss nd fcilitte ecosystem recovery, it is generlly ssumed tht meliortion of environmentl stress (excess solr rdition, extremes in temperture, or soil moisture deficit) plys criticl role (Chen et l., 1992; Chen et l., 1995; Frnklin et l., 1997; Brg nd Edmonds, 1999). However, few studies hve exmined the reltionships between microclimte nd forest structure in the context of vrible-retention systems (but see Brg nd Edmonds, 1999; Chen et l., 1999; Zheng et l., 00). Some spects of microclimte show strong nd predictble reltionships with forest structure. For exmple, solr rdition t the forest floor is directly relted to the mount nd sptil distribution of overstory cover (Drever nd Lertzmn, 03). Other elements of microclimte re less predictble from forest structure. For exmple, soil nd ground-surfce tempertures re ffected by incoming (shortwve) nd outgoing (long-wve) rdition, which re determined, in prt, by the full verticl profile of vegettion cover (Yoshino, 1975; Aussenc, 00; Prevost nd Pothier, 03). Removl of cnopy cover increses solr rdition which should elevte dytime tempertures; however, this should lso result in greter loss of long-wve rdition, thus lowering nighttime tempertures nd incresing potentil for frost (Groot nd Crlson, 1996). Cnopy removl cn lso fcilitte growth of understory vegettion, thereby reducing het exchnge with the soil nd mitigting loss of overstory cover. Effects of forest structure on soil moisture my lso be difficult to predict: reductions in cnopy cover my led to more evportion from the soil surfce (Morecroft et l., 1998; Chen et l., 1999), but less trnspirtionl loss (e.g., Bred et l., 1995). Dispersed retention of trees should serve to moderte forest-floor microclimte nd thus benefit orgnisms sensitive to excess solr rdition or extremes in temperture. Logiclly, these benefits should increse with the mount of retention. However, little reserch hs been devoted to understnding the nture of this reltionship (e.g., the existence of thresholds), or to identifying the fetures of residul forest structure tht most influence microclimtic vrition (Brg nd Edmonds, 1999; Drever nd Lertzmn, 03). Reltive to clercut logging, dispersed retention should lso ffect the sptil vribility of microclimte in the forest understory. Ptchy shding by residul trees, locl ccumultions of logging slsh, nd differentil survivl nd growth of ground vegettion should increse the sptil heterogeneity of light, temperture, nd soil moisture, nd thus sptil vribility in the survivl of forest orgnisms tht re sensitive to vrition in these environmentl fctors (Hungerford nd Bbbitt, 1987; McInnis nd Roberts, 1995; Gry nd Spies, 1997; Grimmond et l., 00; Mrtens et l., 00). However, to dte, studies of forest microclimte hve emphsized the verge conditions of hrvest tretments, not the mgnitude or sources of vrition within them (but see Chen et l., 1999; Zheng et l., 00; Drever nd Lertzmn, 03) In this study we exmine ptterns of microclimtic vrition mong hrvest tretments tht represent brod grdient in overstory retention in mture, coniferous forests of western Wshington. The tretments re prt of the Demonstrtion of Ecosystem Mngement Options (DEMO) Study, regionl experiment in vrible-retention hrvest tht evlutes the roles of level nd pttern of retention in the persistence nd recovery of orgnisms ssocited with lte-serl forests (Aubry et l., 1999; Hlpern et l., 05). We ssess vrition in light, ir nd soil temperture, nd soil moisture mong nd within hrvest tretments nd identify the components of residul forest structure (including overstory chrcteristics, understory vegettion, nd logging slsh) tht best explin this vrition. We ddress the following questions: (1) How do tretment-scle ptterns of light, ir nd soil temperture, nd soil moisture vry with level of retention? (2) Is the sptil heterogeneity (within-tretment vrition) of light, temperture, or soil moisture greter t intermedite levels of retention,

3 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 2 reflecting the greter dispersion of trees? (3) Which elements of residul forest structure, including overstory chrcteristics, understory vegettion, nd logging slsh explin locl vrition in microclimte? We conclude by exmining whether microclimte trends mong tretments re consistent with the biologicl responses tht hve been observed in compnion studies on these sites. 2. Methods 2.1. Study res This study ws conducted t three of the six experimentl blocks tht comprise the DEMO study Butte (BU), Little White Slmon (LWS), nd Prdise Hills (PH). All re locted in the southern Cscde Rnge of Wshington (Aubry et l., 1999). The climte of this region is chrcterized by reltively wrm, dry summers nd cool, wet winters with most precipittion flling between October nd April (Frnklin nd Dyrness, 1988). However, locl climtic conditions vry both mong nd within the experimentl blocks, reflecting vrition in ltitude, elevtion, nd spect (Tble 1) (see lso Hlpern et l., 1999; Hlpern et l., 05). Soils re modertely deep nd well-drined loms to lomy snds derived from ndesite, bslt, or brecci prent mterils, or from eril deposits of pumice (Wde et l., 1992). Three forest zones re represented, defined by the climx tree species: Tsug heterophyll (BU), Abies grndis (LWS), nd Abies mbilis (PH). At the time of hrvest, forests were dominted by Pseudotsug menziesii with no previous history of mngement. Forest ge nd structure vried gretly mong blocks, nd to degree, mong tretment units within blocks (Tble 1). BU (70-80 yers) nd PH ( yers) were reltively dense forests (~1000 trees h -1 ); LWS ( yers) ws chrcterized by lrge, widely spced trees (~2 trees h -1 ) (Tble 1). Understory development lso vried mrkedly mong blocks: herb nd shrub cover were much higher t LWS (43 nd 69%, respectively) thn t BU (27 nd %) or PH (19 nd 13%) (Hlpern et l., 05) Experimentl tretments The DEMO experimentl design consists of six, 13-h tretments rndomly ssigned to experimentl units within ech block. Tretments differ in level of retention (percentge of originl bsl re) nd/or the sptil pttern in which trees re retined (dispersed vs. ggregted) (detils cn be found in Aubry et l., 1999). For this study, four of these tretments were selected to represent grdient of dispersed overstory retention (Fig. 1): %: control, no hrvest % dispersed (40%D): residul trees re dominnts or co-dominnts evenly dispersed through the hrvest unit % dispersed (15%D): residul trees re dominnts or co-dominnts evenly dispersed through the hrvest unit. 4. 0%: represented by the hrvested portions of the 15% ggregted-retention tretment (15%A) within which ll merchntble trees (>18 cm dbh) were removed. Smller trees were retined t BU, were felled t PH, nd were lrgely bsent t LWS. Becuse the initil density nd bsl re of trees vried widely mong blocks, tretments t common level of retention often exhibited wide vrition in residul density nd bsl re (Tble 1). Yrding ws conducted with helicopters t BU nd LWS, nd with ground-bsed mchinery t PH. Hrvest opertions were completed in fll 1997 t BU nd PH, nd in fll 1998 t LWS (for detils see Hlpern nd McKenzie, 01; Hlpern et l., 05) Smpling design Microclimtic mesurements were tken during summer 04, 6-7 yers fter hrvest. Within ech experimentl unit, we rndomly selected (in one cse 21) from pool of permnent tree plots (0.04 h; 11.3 m rdius) spced 40 m prt on systemtic grid of 7 x 9 or 8 x 8 points (Hlpern et l., 05). To represent the 0% retention tretment, only plots within the hrvested portion of 15%A were considered (Fig. 1). Within ech plot microclimtic sttion ws estblished in

4 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 3 rndom direction 1.5 m from the plot center. At ech point we mesured slope, spect (trnsformed to southwestness [cos (spect 225 )]), nd four microclimtic vribles: light, ir temperture, soil temperture, nd soil moisture, s described below Microclimtic mesurements Light An index of light vilbility ws obtined from hemisphericl photogrph of the forest cnopy. A Nikon Coolpix 990 digitl cmer with Nikon FC-E8 fisheye converter ws leveled on monopod t height of 2 m from the ground surfce (bove understory vegettion except t LWS where vine mple ws occsionlly tller), with the top of the cmer oriented north. Photogrphs were tken under overcst sky conditions between June nd November 04. Imges were nlyzed with the softwre Gp Light Anlyzer 2.0 (GLA; Frzer et l., 1999), employing the stndrd overcst sky model (UOC). Totl trnsmitted light, or photosynthetic photon flux density (PPFD; mol m -2 dy -1 ), ws clculted for the growing seson (June through September) (Frzer et l., 1999; Drever nd Lertzmn, 03) Air nd soil temperture Air nd soil temperture were mesured using temperture dt loggers (Model DS1921G, ibutton Thermochron, Mxim/Dlls Semiconductor Corp., Dlls, Texs). Two loggers were plced t ech point: the first on wooden stke 1 m bove the ground surfce (ir), the second t 15 cm beneth the soil surfce (soil). For mesurements of ir temperture, loggers were plced on the inside of one-hlf of smll (10 cm long) plstic continer shielded with luminum foil to prevent direct rdition, nd perforted to llow irflow nd minimize het ccumultion. Plstic continers were ttched to wooden rm extending perpendiculr from the top of ech stke. Temperture ws recorded hourly t ech point over 2-3 week period between mid July nd lte September 04 to smple the most stressful portion of the growing seson. Mesurements were tken synchronously within ech block, but smpling ws stggered in time mong blocks (LWS: 19 July - 5 August, BU: August, nd PH: 1-23 September) Soil moisture Volumetric soil moisture ws mesured using time domin reflectometry (TDR; Gry nd Spies, 1995). Stinless steel probes, 30 cm long, were inserted t n ngle of 30 from the soil surfce to smple the upper 15 cm of soil; probes remined in plce for the entire smpling period. Multiple mesurements were tken over the growing seson. At ech mesurement, ll points within block were smpled over 1-2 dy period of dry wether (no precipittion in the previous 48 hr) nd ll blocks were visited within the sme 1-week period. Probes were ttched to TDR monitor with lligtor clips soldered to coxil wire; dt were recorded on plmtop computer. Volumetric soil moisture ws clculted using the clibrtion curves of Gry nd Spies (1995) Overstory structure nd understory cover Within ech tree plot, ll stems 5 cm in dimeter t brest height (dbh) were mesured for dimeter. Heights of ll trees were estimted from species- nd tretment-specific height:dimeter equtions (D. Mguire, unpublished dt). Four predictors of overstory structure were then generted for ech plot: totl tree density, totl bsl re, simple stnd-density index ([density * bsl re] 1/2 ), nd totl tree height (summed height of ll trees; Drever nd Lertzmn, 03). In ddition, overstory cnopy cover (%) ws clculted from the hemisphericl photogrph tken t the center of ech plot (GLA; Frzer et l., 1999). To quntify the potentil shding effects on soil of understory vegettion nd logging slsh, we mde two dditionl estimtes. Using 1-m 2 frme centered on ech microclimtic sttion, we visully estimted the cover (nerest 1%) of vegettion <1.5 m tll nd logging slsh (fine brnches nd other woody debris resulting from hrvest opertions) Dt reduction From the continuous mesurements of ir nd soil temperture, we grouped dys s either

5 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 4 wrm/sunny or cool/cloudy (Fig. 2). Given the emphsis of this study on meliortion of microclimtic stress, we rndomly selected 5 dys from the pool of wrm/sunny dys t ech block. Bsed on hourly redings t ech smple point, we clculted men dytime temperture for ir (06:00 to :00 hr) nd soil (09:00 to 23:00 hr, displced 3 hr to cpture the heting lg between ir nd soil). We lso identified the minimum nd mximum tempertures t ech point for ech dy. For ech of these (men, minimum, nd mximum), we then computed mens of the 5 dys t ech point. From these 5- dy, point-scle mens we generted men nd coefficient of vrition (CV) for ech tretment unit. These yielded totl of 12 response vribles for ir nd soil temperture. For nlysis of soil moisture, one mesurement ws selected for ech block the driest during the growing seson. Although minimum soil moisture cn occur during erly fll in Pcific Northwest forests (Gry nd Spies, 1997), severl extended periods of precipittion precluded use of September smples. Insted, for ech block, we used mesurement from the period 4-12 August 04. As with ir nd soil temperture, men nd coefficient of vrition were computed for ech tretment unit. In six of the 12 tretment units, mesurements of temperture or soil moisture from one or two smple points were deleted from the nlysis becuse dt loggers or soil moisture probes were dmged or disturbed; finl smple sizes per tretment unit rnged from 18 to Sttisticl nlyses Anlysis of vrince (ANOVA) ws used to confirm tht residul forest structure differed significntly mong hrvest tretments. A rndomized block ANOVA model ws run for ech mesure of forest structure: tree density, bsl re, stnd density index, totl tree height, nd overstory cnopy cover (with degrees of freedom of 2 [block], 3 [tretment], nd 6 [error]). Tretment effects were judged to be significnt t α Individul tretment mens were then compred with Tukey HSD test (Zr, 1999). Tree density nd totl tree height were log trnsformed prior to nlysis to correct for heterogeneity of vrince. Rndomized block ANOVA ws lso used to compre microclimtic vribles mong tretments, both for men responses (Question 1) nd within-tretment vribility (CVs) (Question 2). Vrition ttributble to geogrphic loction nd to the stggered smpling of blocks through time ws subsumed in the block term. Dignostic tests reveled miniml deprtures from normlity nd homogeneity of vrince mong tretments, thus microclimtic dt were not trnsformed. For ANOVA models in which there ws significnt min effect, tretment mens were compred with Tukey HSD test. We tested for dditionl vrition in microclimte ttributble to topogrphy nd residul forest structure with nlysis of covrince (ANCOVA). Covrites included tretment-level mens for slope, southwestness (spect), overstory cnopy cover, nd the four predictors of overstory structure (see Section 2.5). None of the covrites were significnt in these models; consequently, only the results of ANOVA re presented. Multiple liner regression ws used to explore reltionships between microclimte nd mesures of locl (plot-scle) forest structure, including overstory nd understory chrcteristics (Question 3). Becuse climte vried significntly mong blocks, seprte models were developed for ech block (n = smple points per block derived from ll tretments). From the full set of predictors, stepwise selection (Zr, 1999) ws used to dd those vribles to the model with the lowest probbility of F t ech step; vribles lredy present were dropped if their probbility of F exceeded Stndrd dignostics were used to test the ssumptions of normlity nd constnt vrince of residuls. As result, tree density nd totl tree height were log trnsformed. Severl models were bsed on reduced set of predictors. For PPFD, the predictors slope, spect, nd overstory cnopy cover were not considered becuse they re used implicitly in the clcultion of light vilbility. For PPFD nd men, mximum nd minimum ir tempertures, cover of understory vegettion nd slsh were not considered.

6 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 5 3. Results 3.1. Residul stnd structure ANOVA models confirmed tht most mesures of residul forest structure vried significntly with level of retention (Fig. 3). However, for severl vribles bsl re, density, nd totl height one or more pirs of neighboring tretments did not differ significntly in post-hoc comprisons. Nevertheless, for ll mesures of residul forest structure, tretment mens showed monotonic increse with level of retention (Fig. 3) Generl microclimtic trends Air nd soil tempertures vried significntly mong blocks (Fig. 4), reflecting differences in geogrphic loction, elevtion, nd time of smpling. Blocks differed both in the men nd rnge of dily tempertures. Trends over the course of the dy were generlly similr mong tretments within ech block except t LWS where minimum nd mximum tempertures occurred c. 2 hr erlier in the control (100% retention), reflecting its distinct esterly spect (Fig. 4; Tble 1) Men responses Trnsmitted light (PPFD) nd men dytime nd mximum ir nd soil tempertures decresed significntly with level of retention (Question 1) (Fig. 5). PPFD (Fig. 5) showed the strongest trend, but vlues did not differ between 40 nd 100% retention. Men ir temperture ws significntly greter t 0% thn t 40 or 100% retention; however, mens did not differ between neighboring levels of retention (Fig. 5c). Mximum ir temperture did not differ mong 0, 15, nd 40% retention or between 40 nd 100% retention, but it ws significntly greter t 0 nd 15% retention thn in the control (Fig. 5c). Men nd mximum soil tempertures (Fig. 5e) showed similr trends, differing only between 0 nd 100% retention. Minimum ir nd soil tempertures (dt not shown) nd men soil moisture (Fig. 5g) did not vry significntly with level of retention Within-tretment vribility There were no significnt differences in within-tretment (plot-to-plot) vribility in microclimte (Question 2), but two trends were evident. Vribility in PPFD exhibited mrginlly significnt increse (Fig. 5b), nd vribility in soil temperture, mrginlly significnt decrese with incresing retention (Fig. 5f). Vribility in ir temperture nd soil moisture showed no discernble trends mong tretments. CVs for ir temperture were considerbly lower (<5%) thn those for the other microclimtic vribles Forest structure nd understory conditions s predictors of microclimte In combintion, topogrphy, residul forest structure, nd ground-surfce vribles were generlly good predictors of microclimte in multiple regression models (Question 3) (Tble 2). Coefficients of determintion (R 2 ) rnged from 0.63 to 0.84 for PPFD, from 0.55 to 0.85 for men/mximum ir temperture, nd from 0.25 to 0.61 for men/mximum soil temperture. Models for minimum temperture explined less vrition, but were comprble for ir nd soil (R 2 of 0.22 to 0.46 nd 0.10 to 0.51, respectively). Models for soil moisture were consistently poor (R 2 of 0.11 to 0.28). Among blocks, models were consistently weker for LWS thn for BU or PH. For light nd ir temperture, spect nd one or two mesures of overstory structure (cnopy cover, SDI, bsl re, or totl tree height) yielded highly significnt models (Tble 2). SDI ws selected in ll models of PPFD (cnopy cover ws not considered; see Section 2.7). Cnopy cover ws the most frequent predictor of ir temperture (i.e., seven of nine models nd ll models of men nd mximum temperture). In contrst, cover of understory vegettion (nd slsh t BU) were consistently selected in models of soil temperture (Tble 2). Neither overstory cnopy cover, nor vegettion cover were consistently included in models of soil moisture. 4. Discussion 4.1. Men responses to level of retention

7 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 6 We expected tht with increses in overstory retention, light vilbility nd men nd mximum tempertures would decline, nd minimum tempertures nd soil moisture would increse. Trends for trnsmitted light (PPFD) nd men nd mximum tempertures were consistent with these expecttions, lthough the bsolute differences in temperture mong most tretments were surprisingly smll nd nonsignificnt. PPFD showed the strongest response to level of retention, declining more thn three-fold cross the tretment grdient. Nevertheless, light vilbility did not differ sttisticlly between 40% retention nd the control. This result is due, in lrge prt, to trends t BU. Here, the combintion of more esterly spect nd shding by non-merchntble trees resulted in reltively smll difference (<30%) in PPFD between 40 nd 100% retention (in contrst, the differences between tretments t LW nd PH were >130%). This underscores the fct tht light penetrtion to the understory cn vry significntly t given level of overstory retention depending on topogrphy, initil forest structure, nd tretment of subcnopy trees during logging opertions (Lieffers et l., 1999). In contrst to light, differences in ir nd soil temperture mong tretments were more difficult to detect. Even on wrm sunny dys, mximum ir tempertures 1 m bove the ground surfce were comprble mong hrvest tretments (0-40%) nd men tempertures did not differ between 0 nd 15 or 15 nd 40% retention. Although these results do not point to cler threshold, they do suggest tht retention in excess of 15% is required to reduce verge dytime tempertures from those in clercut environments. These ptterns re generlly consistent with pst work in the Pcific Northwest. In 60- to 70-yer-old coniferous forests in western Wshington, Brg nd Edmonds (1999) documented comprble men nd mximum summer tempertures in clercut nd dispersed-retention hrvest units (~30% of originl bsl re), s did Chen et l. (1999). However, the implictions of trends observed t higher levels of retention in our study re less cler. The bsence of differences between 40 nd 100% retention suggest tht 60% of originl bsl re cn be removed without ffecting men or mximum ir tempertures in the understory. With reltively low repliction of tretments, however, this result my lso be n rtifct of topogrphic vrition t BU (Tble 1): the 40% hrvest unit fces estwrd nd lies 0 m higher in elevtion thn the control (which fces southestwrd) resulting in noticebly cooler tempertures. This points to the broder chllenge of detecting tretment effects in lrgescle experiments in lndscpes in which complex topogrphy nd vrition in forest structure cn interct with experimentl responses. Not surprisingly, tempertures 15 cm below the soil surfce differed less mong tretments thn did ir tempertures, which verged ~5 C greter. Although men tempertures consistently declined with level of retention, significnt differences were observed only between 0 nd 100% retention. Yet, it is possible tht greter differences existed t shllower depths nd t the soil surfce, prticulrly in res of exposed soil. It is lso likely tht differences in temperture were greter immeditely fter hrvest when minerl soils were first exposed nd understory plnt cover ws mrkedly reduced by logging disturbnce (Hlpern nd McKenzie, 01; Hlpern et l., 05). By contrst, regrowth of the understory ws considerble fter 6-7 yers nd plnt cover ws ctully greter in 0% thn in control plots (Tble 1), likely tempering the extreme differences in overstory shding between these tretments. Level of retention hd no detectble effect on minimum ir or soil tempertures. This result is consistent with observtions of Brg nd Edmonds (1999) nd with their conclusion tht prtil cnopy retention reduces loss of longwve rdition to greter degree thn it limits input of short-wve rdition. Tretment effects on minimum tempertures my be stronger in topogrphic settings where cold ir hs greter potentil to ccumulte (Willimson nd Minore, 1978; Groot nd Crlson, 1996), nd in spring or fll when the potentil for frost is greter. Consistent with temporl trends for this region (Gry nd Spies, 1997), volumetric soil moisture (0-15 cm) ws generlly low in mid-

8 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 7 August, yet there ws little vrition mong tretments (rnge of soil moisture of 14-17%). Brg nd Edmonds (1999) were lso unble to detect differences in soil moisture in lte summer mong clercut, dispersed retention, nd uncut forests. Two processes with opposing effects my contribute to the smll differences in soil moisture mong hrvest units with contrsting overstory structure. At lower levels of retention, greter heting of the soil surfce should led to greter evportion; however, trnspirtion by trees should lso be reduced due to lower tree densities. Rtes of evportion nd trnspirtion re lso likely to be ffected by understory vegettion through vrition in folir cover, root system development, nd wter-use of plnt species (Joffre nd Rmbl, 1993; Breshers et l., 1998; Xu et l., 02). A clerer picture of soil moisture dynmics would require more complete understnding of these fctors nd their interctions Within-tretment vrition in microclimte We expected tht greter vribility in overstory structure within dispersed tretments (15% nd 40%) would led to greter vribility in understory microclimte thn in clercut or undisturbed forests. However, we were unble to detect significnt effect of retention level on coefficients of vrition for ny of the vribles considered. For ir temperture, rpid mixing of ir msses (Chen nd Frnklin, 1997) is likely explntion for the smll vrition (CVs <5%) mong hrvest tretments. Although not sttisticlly significnt, CVs for soil temperture showed n interesting nd potentilly relevnt trend when considered together with tretmentscle differences. CVs for men nd mximum soil temperture incresed with decresing retention; thus, not only were verge tempertures of tretment units greter t lower retention, but within-tretment vribility ws higher, incresing the potentil for unusully high tempertures t prticulr loctions. These effects could hve been ccentuted t shllower soil depths (or t the surfce), prticulrly fter hrvest when residul plnt cover ws low. It is possible tht the generl bsence of tretment effects on microclimtic vrition within hrvest units reflects the sptil scle of smpling. The distnces between smple points (40 to >100 m) my be too lrge to cpture the vrition ssocited with overstory structure, prticulrly t higher levels of retention. Greter vribility my insted be detected t finer sptil scles, e.g., within 1-5 m ssocited with individul tree cnopies (but see Brg nd Edmonds, 1999) Predicting microclimte from ttributes of forest structure To wht extent cn vrition in locl microclimte be predicted by residul forest structure? Multiple regression models indicted tht simple mesures of overstory structure explined much of the vrition in light vilbility nd ir temperture. Stnd density index, which incorportes both the number nd bsl re of trees, emerged s the strongest predictor of light in ll blocks, suggesting tht both the density nd sizes of trees contribute to light ttenution in the understory. This result is not prticulrly surprising, s light hs been modeled with similr plot-scle mesures of structure (e.g., bsl re, stem density, or the summed dimeters or heights of trees) in both coniferous nd brodlef forests (e.g., Plik et l., 1997; Comeu nd Heinemn, 03; Drever nd Lertzmn, 03). However, ttempts to predict locl vrition in other chrcteristics of forest microclimte (e.g., ir or soil temperture) re less common in the literture (but see Kng et l., 00). Our results suggest tht men nd mximum ir temperture (t lest for wrm summer dys) cn be predicted from forest structure nd spect. Cnopy cover (estimted from hemisphericl photogrphs) ws significnt predictor in ll blocks, reflecting the strong reltionships mong cnopy cover, solr rdition, nd energy blnce t the forest floor (Yoshino, 1975; Aussenc, 00). In contrst, we could explin considerbly less vrition in soil temperture nd very little vrition in soil moisture. Models for soil temperture included not only overstory ttributes (cnopy cover or SDI), but cover of understory plnts, s shding by herbceous nd woody vegettion cn contribute significntly to modertion of soil tempertures (Pierson nd Wight, 1991; Breshers et l., 1998; Buckley et l., 1998; Xu

9 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 8 et l., 02). Interestingly, cover of logging slsh ws significnt predictor of soil temperture t BU 7 yers fter tretment. This suggests tht its meliorting effect ws likely to hve been stronger immeditely fter hrvest when slsh cover nd depth were greter (Hlpern nd McKenzie, 01). In fct, moderte levels of slsh were positively correlted to initil survivl of shde-tolernt herbs in these sites (Nelson nd Hlpern, 05). Clerly, however, fctors other thn overstory structure nd understory cover contribute to locl vrition in soil microclimte. Models for soil temperture t LWS nd models for soil moisture t ll blocks were unble to ccount for most of this vrition. Fctors not smpled in this study my exert stronger controls on soil moisture; these include microtopogrphy, soil texture, nd orgnic mtter content, which cn vry considerbly t smll sptil scles (Gry nd Spies, 1997; Messin et l., 1997; Breshers et l., 1998) Correspondence of microclimtic nd biologicl responses Are trends in microclimte consistent with the biologicl responses documented in other studies on these sites? Studies of vsculr plnts, bryophytes, nd fungl sporocrps, groups tht should be sensitive to chnges in light nd temperture (Renhorn et l., 1997; Jones et l., 03; Fenton nd Frego, 05), reveled initil (1-3 yer) responses tht were lrgely consistent with ptterns of light vilbility, nd to some extent, ir nd soil temperture. For exmple, declines in cover of forest herbs were greter t lower levels of retention, nd plnts typiclly ssocited with lte-serl forests were more frequently lost from clercut plots (0% retention) thn from those with residul trees (15 or 40% retention) (Hlpern et l., 05). For forest-floor bryophytes, however, incresing levels of retention did not mitigte loss of cover (C. Hlpern, unpublished dt) suggesting tht declines were either induced by other fctors (e.g., physicl disturbnce) or by environmentl stresses tht were not mesured (Sunders et l., 1991; Renhorn et l., 1997; Fenton nd Frego, 05). In studies of ectomycorrhizl fungi, sporocrp (mushroom nd truffle) production ws virtully eliminted in clercut res (0% retention) nd ws significntly reduced t 15% retention (Luom et l., 04). At 40% retention, however, production of sporocrps ws generlly comprble to tht in controls, consistent with trends in light nd temperture. Despite the mny consistencies between microclimtic nd biologicl responses, fctors other thn environmentl chnges cn shpe biologicl responses to overstory removl. For exmple, production of fungl sporocrps requires crbon subsidies from ssocited trees; greter retention my simply increse ccess to these subsidies. Vrition in disturbnce intensity lso cn ply criticl role in survivl of understory plnts (Hlpern, 1989; Heussler et l., 02; Roberts nd Zhu, 02; Fenton nd Frego, 05). Unfortuntely, it is difficult to differentite between the effects of disturbnce nd those resulting from physiologicl stress following timber hrvest becuse they typiclly co-vry with level of retention (Hlpern nd McKenzie, 01; Hlpern et l., 05) Mngement implictions Structurl retention is now stndrd prctice in hrvest of mture forests on federl lnds within the rnge of the northern spotted owl. Current stndrds require mngers to retin t lest 15% of the originl stnd within ech hrvest unit, with 70% of this retention in ggregtes of h (USDA nd USDI, 1994). Although this prctice hs been widely dopted, few dt exist to evlute whether this minimum retention stndrd is sufficient to chieve its intended gols. One mechnism by which overstory retention hs been hypothesized to fcilitte species persistence nd recovery is by moderting climte t the forest floor (Frnklin et l., 1997). Our results provide direct evidence tht t 15% dispersed retention, the potentil for meliorting ir or soil tempertures in hrvest res is very limited. Although verge levels of light re reduced, ir nd soil tempertures re not, resulting in men nd mxim tht re no different from those found in clercut environments. In opertionl pplictions of this minimum stndrd, where 70% of the tree cover must be ggregted, light

10 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 9 nd temperture cross most of the hrvest unit re likely to be even greter. Studies of understory response (Luom et l., 04; Hlpern et l., 05; Nelson nd Hlpern, 05, b) nd susceptibility of trees to wind-induced mortlity (C. Hlpern, unpublished dt) further suggest tht there my be few short-term benefits ssocited with this minimum stndrd. Yet, it is not cler t wht point increses in retention bove this minimum provide microclimtic benefits. This my depend, in prt, on the microclimtic vribles of interest nd how they medite biologicl responses. For exmple, men ir tempertures were significntly cooler thn the clercut t 40, but not 15% retention, wheres mxim were similr mong ll hrvest tretments. Thus, biologicl processes medited by extremes in temperture would suggest different retention threshold thn those shped by verge conditions. On the other hnd, chnges in light vilbility t lower levels of retention indicte tht smll increses in cnopy cover cn yield lrge reductions in light. If sensitivity to excess solr rdition dicttes biologicl responses (Svenning, 00; Coxson et l., 03; Fenton nd Frego, 05), smll chnges in cnopy retention could yield lrge effects. The results of this study nd of compnion studies of biologicl response point to importnt reltionships tht wrrnt further investigtion. For now, however, forest mngers must continue to implement vrible-retention designs with incomplete knowledge of their ecologicl consequences. Our study begins to fill some of these gps in knowledge: it provides strong evidence tht current minimum stndrds for retention do not substntilly moderte the effects of cnopy removl on understory microclimte. Acknowledgements We wish to thnk Michel Olsen nd Timothy Erickson for ssistnce in the field. Don McKenzie nd Andy Gry provided input on smpling design nd sttisticl nlyses, nd criticl comments on n erlier version of the mnuscript. We lso pprecite the comments of two nonymous reviewers. This is product of the Demonstrtion of Ecosystem Mngement Options (DEMO) Study ( wshington.edu/reserch.demo/), joint effort of the USDA Forest Service Region 6 nd Pcific Northwest Reserch Sttion. Reserch prtners include the University of Wshington, Oregon Stte University, University of Oregon, Gifford Pinchot nd Umpqu Ntionl Forests, nd the Wshington Stte Deprtment of Nturl Resources. Funds were provided by the USDA Forest Service, PNW Reserch Sttion (PNW CA). References Aubry, K.B., Amrnthus, M.P., Hlpern, C.B., White, J.D., Woodrd, B.L., Peterson, C.E., Lgoudkis, C.A., Horton, A.J., Evluting the effects of vrying levels nd ptterns of green-tree retention: experimentl design of the DEMO study. Northw. Sci. 73, 12-26, (specil issue). Aussenc, G., 00. Interctions between forest stnds nd microclimte: ecophysiologicl spects nd consequences for silviculture. Ann. For. Sci. 57, Brg, A.K., Edmonds, R.L., Influence of prtil cutting on site microclimte, soil nitrogen dynmics, nd microbil biomss in Dougls-fir stnds in western Wshington. Cn. J. For. Res. 29, Beese, W.J., Dunsworth, B.G., Zielke, K., Bncroft, B., 03. Mintining ttributes of old-growth forests in costl B.C. through vrible retention. For. Chron. 79, Bred, N., Grnier, A., Aussenc, G., Effects of thinning on soil wter blnce nd tree wter reltions, trnspirtion nd growth in ok forest (Quercus petre (Mtt) Liebl.). Tree Physiol. 15, Breshers, D.D., Nyhn, J.W., Heil, C.E., Wilcox, B.P., Effects of woody plnts on microclimte in semirid woodlnd: soil temperture nd evportion in cnopy nd intercnopy ptches. Int. J. Plnt Sci. 159, Buckley, D.S., Shrik, T.L., Isebrnds, J.G., Regenertion of northern red ok:

11 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 10 positive nd negtive effects of competitor removl. Ecology 79, Chen, J., Frnklin, J.F., Growing-seson microclimte vribility within n oldgrowth Dougls-fir forest. Climte Res. 8, Chen, J., Frnklin, J.F., Spies, T.A., Vegettion responses to edge environments in old-growth Dougls-fir forests. Ecol. Appl. 2, Chen, J., Frnklin, J.F., Spies, T.A., Growing-seson microclimte grdients from clercut edges into old-growth Dougls-fir forests. Ecol. Appl. 5, Chen, J., Sunders, S.C., Crow, T.R., Nimn, R.J., Brosofske, K.D., Mroz, G.D., Brookshire, B.L., Frnklin, J.F., Microclimte in forest ecosystem nd lndscpe ecology. Bioscience 49, Comeu, P.G., Heinemn, J.L., 03. Predicting understory light microclimte from stnd prmeters in young pper birch stnds. For. Ecol. Mnge. 180, Coxson, D., Stevenson, S., Cmpbell, J., 03. Short-term impcts of prtil cutting on lichen retention nd cnopy microclimte in n Engelmnn spruce - sublpine fir forest in north-centrl British Columbi. Cn. J. For. Res. 33, Drever, R.C., Lertzmn, K.P., 03. Effects of wide grdient of retined tree structure on understory light in costl Dougls-fir forests. Cn. J. For. Res. 33, Fenton, N.J., Frego, K.A., 05. Bryophyte (moss nd liverwort) conservtion under remnnt cnopy in mnged forests. Biol. Conserv. 122, Frnklin, J.F., Dyrness, C.T., Nturl Vegettion of Oregon nd Wshington. Oregon Stte University Press, Corvllis, OR, p Frnklin, J.F., Berg, D.R., Thornburgh, D.A., Tppeiner, J.C., Alterntive silviculturl pproches to timber hrvesting: vrible retention hrvest systems. In: Kohm, K.A., Frnklin, J.F. (Eds.), Creting Forestry for the 21 st Century. Islnd Press, Wshington DC, pp Frzer, G.W., Cnhm, C.D., nd Lertzmn, K.P Gp light nlyzer (GLA), version 2.0: imging softwre to extrct cnopy structure nd gp light indices from true-colour fisheye photogrphs. Simon Frser University, Burnby, B.C., nd the Institute of Ecosystem Studies, Millbrook, N.Y. Gry, A.N., Spies, T.A., Wter content mesurement in forest soils nd decyed wood using time domin reflectometry. Cn. J. For. Res. 25, Gry, A.N., Spies, T.A., Microsite controls on tree seedling estblishment in conifer forest cnopy gps. Ecology 78, Grimmond, C.S.B., Robeson, S.M., Schoof, J.T., 00. Sptil vribility of micro-climtic conditions within mid-ltitude deciduous forest. Climte Res. 15, Groot, A., Crlson, D.W., Influence of shelter on night tempertures, frost dmge, nd bud brek of white spruce seedlings. Cn. J. For. Res. 26, Heussler, S., Bedford, L., Leduc, A., Bergeron, Y., Krnbetter, J.M., 02. Silviculturl disturbnce severity nd plnt communities of the southern Cndin borel forest. Silv Fenn. 36, Hlpern, C.B., Erly successionl ptterns of forest species: interction of life history trits nd disturbnce. Ecology 70, Hlpern, C.B., McKenzie, D., 01. Disturbnce nd post-hrvest ground conditions in structurl retention experiment. For. Ecol. Mnge. 154, Hlpern, C.B., McKenzie, D., Evns, S.A., Mguire, D.A., 05. Initil responses of forest understories to vrying levels nd ptterns of green-tree retention. Ecol. Appl. 15, Hlpern, C.B., Evns, S.A., Nelson, C.R., McKenzie, D., Liguori, D.A., Hibbs, D.E., Hlj, M.G., Response of forest vegettion to vrying levels nd ptterns of green-tree retention: n overview of longterm experiment. Northw. Sci. 73, (specil issue). Hungerford, R.D., Bbbitt, R.E Overstory removl nd residue tretments ffect soil surfce, ir, nd soil temperture: implictions for seedling survivl. USDA Forest Service Reserch Pper INT-377,

12 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 11 Intermountin Forest nd Rnge Experiment Sttion, Ogden, UT, p. 19. Joffre, R., Rmbl, S., How tree cover influences the wter blnce of Mediterrnen rngelnds. Ecology 74, Jones, M.D., Durll, D.M., Cirney, J.W.G., 03. Ectomycorrhizl fungl communities in young forest stnds regenerting fter clercut logging. New Phytol. 157, Kng, S., Kim, S., Oh, S., Lee, D., 00. Predicting sptil nd temporl ptterns of soil temperture bsed on topogrphy, surfce cover nd ir temperture. For. Ecol. Mnge. 136, Lieffers, V.J., Messier, C., Gendron, F., Comeu, P.G., Predicting nd mnging light in the understory of borel forests. Cn. J. For. Res. 29, Luom, D.L., Eberhrt, J.L., Molin, R., Amrnthus, M.P., 04. Response of ectomycorrhizl fungus sporocrp production to vrying levels nd ptterns of green-tree retention. For. Ecol. Mnge. 2, Mrtens, S.N., Breshers, D.D., Meyer, C.W., 00. Sptil distributions of understory light long the grsslnd/forest continuum: effects of cover, height, nd sptil pttern of tree cnopies. Ecol. Model. 126, McInnis, B.G., Roberts, M.R., Seedling microenvironment in full-tree nd treelength logging slsh. Cn. J. For. Res. 25, Messin, M.G., Schoenholtz, S.H., Lowe, M.W., Wng, Z., Gunter, D.K., Londo, A.J., Initil responses of woody vegettion, wter qulity, nd soils to hrvesting intensity in Texs bottomlnd hrdwood ecosystem. For. Ecol. Mnge. 90, Morecroft, M.D., Tylor, M.E., Oliver, H.R., Air nd soil microclimtes of deciduous woodlnd compred to n open site. Agr. For. Meteorol. 90, Nelson, C.R., Hlpern, C.B., 05. Short-term effects of timber hrvest nd forest edges on ground-lyer mosses nd liverworts. Cn. J. Bot. 83, Nelson, C.R., Hlpern, C.B., 05b. Edgerelted responses of understory species to ggregted retention hrvest in the Pcific Northwest. Ecol. Appl. 15, Plik, B.J., Mitchell, R.J., Housel, G., Pedersen, N., Effects of cnopy structure on resource vilbility nd seedling responses in longlef pine ecosystem. Cn. J. For. Res. 27, Pierson, F., Wight, J.R., Vribility of ner-surfce soil temperture on sgebrush rngelnd. J. Rnge Mnge. 44, Prevost, M., Pothier, D., 03. Prtil cuts in trembling spen - conifer stnd: effects on microenvironmentl conditions nd regenertion dynmics. Cn. J. For. Res. 33, Renhorn, K.-E., Esseen, P.-A., Plmqvist, K., Sundberg, B., Growth nd vitlity of epiphytic lichens I. Responses to microclimte long forest edge-interior grdient. Oecologi 109, 1-9. Roberts, M.R., Zhu, L., 02. Erly response of the herbceous lyer to hrvesting in mixed coniferous-deciduous forest in New Brunswick, Cnd. For. Ecol. Mnge. 155, Sunders, D.A., Hobbs, R.J., Mrgules, C.R., Biologicl consequences of ecosystem frgmenttion: A review. Conserv. Biol. 5, Svenning, J.-C., 00. Smll cnopy gps influence plnt distributions in the rin forest understory. Biotropic 32, USDA, USDI Finl supplementl environmentl impct sttement on mngement of hbitt for lte-successionl nd old-growth relted species within the rnge of the northern spotted owl. USDA Forest Service, Portlnd, OR, Wde, J.L., Hermn, T.H., Couche, D Soil resource inventory, Gifford Pinchot Ntionl Forest. Gifford Pinchot Ntionl Forest, USDA Forest Service, Pcific Northwest Region, Portlnd, Oregon, Willimson, D.M., Minore, D Survivl nd growth of plnted conifers on the Ded Indin Plteu est of Ashlnd, Oregon. USDA Forest Service Reserch Pper PNW-242, Pcific Northwest Forest nd Rnge Experiment Sttion, Portlnd, OR, p. 15.

13 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 12 Xu, M., Chen, J., Qi, Y., 02. Growing-seson temperture nd soil moisture long 10 km trnsect cross forested lndscpe. Climte Res. 22, Yoshino, M.M., Climte in Smll Are. University of Tokyo Press, Tokyo, p Zr, J.H., Biosttisticl Anlysis. Prentice- Hll, Upper Sddle River, NJ, p Zheng, D., Chen, J., Song, B., Xu, M., Sneed, P., Jensen, R., 00. Effects of silviculturl tretments on summer forest microclimte in southestern Missouri Ozrks. Climte Res. 15,

14 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 13 Tble 1 Environmentl ttributes, post-hrvest forest structure, nd ground conditions in the four tretment units in ech block Block Level of retention (%) Lt., long. (deg) Stnd ge (yr) Elevtion (m) Slope (deg) Aspect b (deg) Bsl re c (m 2 h -1 ) Tree density c (no. h -1 ) Cnopy cover d (%) SDI e Butte N, W Little N, White W Slmon Prdise N, Hills W All vlues (except for ltitude longitude, stnd ge, nd elevtion) re bsed on mens of 18- smple points per tretment Age t time of hrvest b Derived from men southwestness: cos (spect ) c Trees 5 cm dbh d Overstory cnopy cover estimted from hemisphericl photogrphs using GLA softwre (Frzer et l. 1999). e Stnd density index: (bsl re * tree density) 1/2 f Cover of understory vegettion <1.5 m tll (mximum 100%) Veg cover f (%) Slsh cover (%)

15 T.D. Heithecker, C. B. Hlpern/Forest Ecology nd Mngement (06) In press 14 Tble 2 Signs (+ nd -) nd p vlues of coefficients for significnt predictors in multiple regression models of light (PPFD), temperture, nd soil moisture Model/ Block Slope (deg) SWness b Tree density (no. h -1 ) c Bsl re (m 2 h -1 ) SDI Overstory cnopy cover (%) Totl tree height (m) c Vegettion cover (%) Slsh cover (%) R 2 PPFD (mols m -2 dy -1 ) BU nc d nc - / <0.001 nc nc nc 0.84 LWS nc nc - / <0.001 nc nc nc 0.63 PH nc nc - / <0.001 nc - / nc nc 0.82 Air temperture ( C) Men BU + / / <0.001 nc nc 0.76 LWS - / <0.001 nc nc 0.69 PH + / / < / <0.001 nc nc 0.85 Mximum BU + / < / / <0.001 nc nc 0.78 LWS + / < / <0.001 nc nc 0.55 PH + / / < / / nc nc 0.83 Minimum BU + / <0.001 nc nc 0.35 LWS + / < / <0.001 nc nc 0.22 PH + / <0.001 nc nc 0.46 Soil temperture ( C) Men BU + / / < / / LWS - / < / PH + / / / / Mximum BU + / / < / / LWS + / / < / PH - / / / Minimum BU + / / < / < / LWS - / / PH + / / < /