Development of old-growth structure and timber volume growth trends in maturing Douglas-fir stands

Transcription

1 Ž. Forest Ecology nd Mngement Development of old-growth structure nd timber volume growth trends in mturing Dougls-fir stnds S.A. Acker ), T.E. Sbin, L.M. Gnio, W.A. McKee Deprtment of Forest Science, Oregon Stte UniÕersity, CorÕllis, OR 97331, USA Received 24 April 1997; ccepted 22 August 1997 Abstrct Interest in the contributions to biologicl diversity of old-growth forests hs incresed in mny regions of the world. In the Pcific Northwest of the United Sttes, concern for the contributions hs led to proposls to extend the rottion between timber hrvests from the conventionl yers to 150 yers nd longer. However, the implictions of such chnge for both development of old-growth structure nd timber production re unknown. We exmined long-term records Žup to 82 yers. from permnent plots estblished in 20 stnds of Pseudotsug menziesii Ž Mirb.. Frnco in western Oregon nd Wshington tht re pproching these proposed rottion ges. Similrity to old-growth structure ws ssessed by compring the observed vlues of four structurl vribles to published men vlues for young nd old-growth forest. The ssessment of similrity to old-growth structure ws limited to chrcteristics of the live forest stnd, due to the lck of mesurements of sngs nd downed logs t initition of the permnent plots. Timber production ws ssessed by exmining trends in men nnul increment Ž MAI. of cubic volume. Development towrds old-growth structure ws rpid up to bout ge 80 yers, nd grdul therefter. About hlf of the trnsition from young to old-growth forest structure occurred by ge 100 yers. Stnds lest similr to old-growth in erly observtions hd reltively high tree densities nd reltively smll trees of uniform size. In lter observtions, stnds most similr to old-growth structure were those with higher densities of lrge Ž ) 100 cm DBH. P. menziesii. In generl, MAI declined grdully, verging bout 0.3% per yer. Thus, longer rottions my not result in lrge declines of timber growth while providing for forest structure similr to old-growth. Erly control of tree density my serve to hsten development of old-growth structure. Our pproch to understnding the development of old-growth forest structure could be pplied to long-term plot dt from forests in other regions, s long s there is dequte informtion on old-growth nd other forest stges. q 1998 Elsevier Science B.V. Keywords: Forest; Permnent plots; Pcific Northwest; Men nnul increment; Shde-tolernt species 1. Introduction There is incresing interest in old-growth forests s importnt components of biologicl diversity in mny regions of the world ŽFoster et l., 1996; Spies ) Corresponding uthor. Tel.: q ; fx: q ; e-mil: ckers@fsl.orst.edu. nd Frnklin, Among the fundmentl questions concerning the ecology of forests re how long old-growth structure tkes to develop nd whether some spects of this structure develop more quickly thn others. By understnding the processes underlying old-growth development in nturl forests, it my be possible to devise prctices tht will hsten such development in mnged forests ŽVor, 1994; Cole, r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž. PII S

2 266 ( ) S.A. Acker et l.rforest Ecology nd Mngement ; Dunwiddie et l., 1996; Foster et l., 1996; Spies nd Frnklin, Attempts to quntify the rte t which old-growth structure develops must be bsed on cler definition of the old-growth stte. Mny definitions hve been proposed nd used Žreviewed by Spies nd Frnklin, 1996., nd these vry between ecologicl regions Ž Burgmn, nd with differences in the objectives nd vlues of the people creting the definitions Ž Spies nd Frnklin, No single definition of old-growth structure is likely to encompss the myrid biologicl Ž nd esthetic. vlues ssocited with old-growth forests ŽSpies nd Frnklin, In ddition, ny precise definition of the old-growth stte requires detiled informtion on the biologicl nd ecologicl chrcteristics of oldgrowth nd other successionl stges of forests within prticulr region Ž Spies nd Frnklin, While mny regions lck such informtion Že.g., Burgmn, 1996; Dunwiddie et l., 1996., scientific studies of old-growth forests re beginning to fill mny of the informtion gps ŽFoster et l., 1996; Spies nd Frnklin, Some of the erliest ttempts to define old-growth were for Pseudotsug menziesii Ž Mirb.. Frnco Ž Dougls-fir. forests in the Pcific Northwest of the United Sttes Ž Burgmn, 1996., nd hence, the old-growth forests in this region my be the best known in the world ŽSpies nd Frnklin, Once the old-growth stte is defined for prticulr region, vriety of pproches my be tken to document development of old-growth structure, including pollen nlysis, stnd reconstruction, computer modeling, chronosequences, nd long-term permnent plots Ž Foster et l., Chronosequences hve been used to quntify the structurl differences between old-growth nd other forest successionl stges in the P. menziesii region of the Pcific Northwest Ž Spies nd Frnklin, The chronosequence method is lso referred to s spce-for-time Ž SFT. substitution, becuse stnds of different ges re compred to infer generl chnges over time Ž Pickett, The dvntge of SFT is obvious for phenomenon s potentilly slow s development of old-growth forest structure. However, SFT does not shed much light on the processes underlying the generl trends. Long-term observtion of permnent plots is much better suited to providing such criticl detils Ž Pickett, 1989., for exmple rtes of growth of individul trees or stnds, or rtes of development of old-growth structurl chrcteristics. Thus, direct observtion of development of nturl forest stnds complements chronosequence nd other studies, nd provides reference points for evluting new mngement methods. Vrious prctices hve been proposed to hsten development of old-growth structure fter timber hrvest Ž Cole, 1996; Spies nd Frnklin, These include leving lrge trees unhrvested Žeither scttered or in groups., thinning dense regenerting stnds, nd lengthening the rottion between hrvests Ž Vor, 1994; Spies nd Frnklin, In P. menziesii forests of the Pcific Northwest, for exmple, new plns for mngement of federl forests include lengthening the rottion from the conventionl yers to 150 yers nd longer ŽForest Ecosystem Mngement Assessment Tem, However, without direct, long-term observtions of the temporl ptterns of development of old-growth structure, it is difficult to predict the effects of these chnges in mngement. In mny cses, forests mnged for development of old-growth structure will lso be expected to produce timber Ž Vor, 1994; Cole, Thus, it is importnt to consider how mngement to promote old-growth structure might ffect timber yield. In P. menziesii forests in the Pcific Northwest, the rte of timber volume ccumultion peks between 70 nd 90 yers, nd then decreses monotoniclly ŽCurtis, However, direct field observtions re lcking for stnds ) 100 yers old; therefore, estimtes of tree volume growth rtes re specultive ŽCurtis nd Mrshll, The purpose of this pper is to explore trends in development of old-growth chrcteristics nd timber volume growth rtes from long-term observtions of permnent plots in mturing P. menziesii forests in western Oregon nd Wshington. When observtions begn, the stnds were between 45 nd 85 yers old, representing young Ž 40 to 80 yers. or mture Ž 80 to 195 yers. forest Žs defined by Spies nd Frnklin, The stnds hve been mesured periodiclly for between four nd eight decdes; ll were mture stnds t the most recent observtion. Thus, this set of long-term observtions of forest dynmics is pertinent to evluting chnges in forest

3 ( ) S.A. Acker et l.rforest Ecology nd Mngement mngement now being debted nd, in some cses, implemented in the Pcific Northwest. Also, our method of quntifying the development of old-growth forest structure my be generlly pplicble to other regions. Rther thn focusing on prticulr function of old-growth forest Že.g., enhncement of biologicl diversity, criticl hbitt for prticulr species, crbon sequestrtion., our objective is to document development of old-growth conditions. Our pproch to quntifying old-growth conditions is bsed on set of descriptors of the structure of the live forest stnd Ž see below.. Thus, to drw inferences from this study to prticulr structurl, compositionl, or functionl spects of old-growth forest would require criticl exmintion of the reltionship between our generl mesure of old-growth structure nd the prticulr old-growth ttribute of interest. 2. Study res All 20 of the stnds exmined were originlly estblished s permnent plots to provide relible estimtes of timber volume growth of nturl, young, well-stocked stnds of pure P. menziesii ŽWillim- son, Reserchers from the Pcific Northwest Forest nd Rnge Experiment Sttion estblished the plots between 1910 nd 1940 in wht re now the Mt. Hood, Siuslw, Willmette, Gifford Pinchot, nd Olympic Ntionl Forests in Oregon nd Wshington Tble 1 Loction, ge, nd productivity of study stnds Group Ntionl forest, stterstnd Ltitude Longitude Elevtion Ž m. Yer estblished Initil stnd Site index ge Ž yers. Siuslw, OR CH1 CH CH2 CH CH Gifford Pinchot, WA GP1 GP GP GP GP2 GP GP Mt. Hood, OR MH MH MH MH Olympic, WA OL OL OL Willmette, OR WI WI WI WI Gifford Pinchot, WA WR1 WR WR WR2 WR WR Ž. 50-yer site index for Pseudotsug menziesii by King 1966, in feet.

4 268 ( ) S.A. Acker et l.rforest Ecology nd Mngement Ž Tble 1.. Plots tht re in close proximity nd hve identicl Ž or very similr. stnd ges were plced in the sme group for some nlyses Ž Tble 1.. The plots re ll 0.4 h Ž 1 horizontl cre., with the exceptions of plots WR04 Ž0.397 h Ž 0.98 cre.. nd WR05 Ž0.38 h Ž 0.94 cre... Trees were tgged t brest height Ž1.37 m Ž 4.5 feet..; dimeters t brest height Ž DBH. were mesured t the tg. Only trees with DBH G 6.6 cm Ž 2.6 in.. re included in this nlysis. When the permnent plots were estblished, stnd ges were determined by counting rings on increment cores tken from smple of the trees. At first mesurement, totl tree bsl re rnged from 40.8 to 80.3 m 2 h ; P. menziesii ccounted for 80% to 100% of bsl re Ž Tble 2.. Tsug heterophyll Ž Rf.. Srg. Ž western hemlock. ws present in hlf the stnds t first observtion nd ccounted for s much s 18% of tree bsl re. Stnds were remesured every five yers until the mid-1950s, when interest in the study slckened. Mny of the stnds were not mesured gin for 15 to 20 yers. Two of the stnds included in this nlysis were hrvested in the mid-1960s ŽWR02 nd WR09; Sollins, Since the erly 1980s, the remining 18 stnds hve been remesured every five yers. 3. Dt nlysis 3.1. Assessment of old-growth chrcteristics To evlute the degree of old-growth chrcter of ech stnd t ech mesurement, we used multivrite comprison of 196 P. menziesii stnds of known ges Ž rnging from 40 to 900 yers. from Wshington nd Oregon Ž Spies nd Frnklin, Although old-growth forests differ from younger forests in lrge number of ttributes ŽFrnklin et l., 1981., Spies nd Frnklin Ž identified four Tble 2 Bsl re of stnds when study ws initited Ž 2 Stnd Tree bsl re m h. Totl Pseudotsug menziesii Tsug heterophyll Other conifers Hrdwoods CH CH CH GP GP GP GP GP MH MH MH OL OL WI WI WI WR WR WR WR Abies mbilis, A. grndis, A. procer Rehd. Ž noble fir., Pinus monticol Dougl. Ž western white pine., Pice sitchensis Ž Bong.. Crr. Ž Sitk spruce., Thuj plict, Tsug mertensin. b Acer mcrophyllum Pursh Ž big-lef mple., Alnus rubr Bong. Ž red lder., Arbutus menziesii Pursh Ž Pcific mdrone., Cstnopsis chrysophyll Ž Dougl.. DC. Ž gint chinkpin., Cornus nuttlli Aud. Ž Pcific dogwood., Populus trichocrp T. nd G. Ž blck cottonwood., Prunus emrgint Ž Dougl.. Wlp. Ž bittercherry., Prunus sp., Rhmnus purshin DC. Ž cscr.. b

5 ( ) S.A. Acker et l.rforest Ecology nd Mngement structurl vribles tht successfully discriminted mong ge clsses of forest: 1. stndrd devition Ž SD. of tree DBH; 2. density of lrge Ž ) 100 cm DBH. P. menziesii Ž trees trees h.; 3. men tree DBH Ž cm.; nd Ž 4. density of ll trees )5 cm DBH trees h.. Men vlues of ech vrible reported by Spies nd Frnklin Ž for young nd old-growth forests re listed in Tble 3. Spies nd Frnklin Ž selected these four vribles from set of 22 descriptors of live forest structure, ll of demonstrted or hypothesized importnce for wildlife hbitt, ecosystem function, nd successionl development. Incresed vribility in tree size Ž i.e., SD of tree DBH. is ssocited with incresed diversity of microhbitts within forest stnds Ž Spies nd Frnklin, Among other ecologicl roles, lrge P. menziesii trees provide hbitt for lrge number of species of epiphytes Ž including nitrogen-fixing lichens., rthropods, birds, nd mmmls, nd re source of lrge sngs nd downed logs Ž Frnklin nd Spies, Men tree DBH nd density of trees re indictors of successionl development tht tend to hve reciprocl reltionship Ž Oliver nd Lrson, 1990., with men tree DBH incresing nd density of trees decresing over the first few centuries of development of P. menziesii stnds Ž Spies nd Frnklin, However, it is importnt to note tht in very old P. menziesii stnds Že.g., 1000-yer-old stnd in Fig. 6 of Frnklin et l., 1981., the density of trees my increse nd cuse men tree DBH to decline. These four structurl vribles were used s follows to compute n old-growth index Ž I. og for the stnds in this study: x i,obsyxi,young Iog s25si Ž 1. x yx i,old i,young where i s 1 to 4, representing ech of the four structurl vribles, x i, obs is the observed vlue of ith structurl vrible, x i, young is the men vlue of ith structurl vrible for young stnds Ž Tble 3., x i, old is the men vlue of ith structurl vrible for old-growth stnds Ž Tble 3.. If observed vlues of the structurl vribles re more extreme thn the men vlues for either young or old-growth forest, the vlues of Iog would be inppropritely lrge Že.g., observed vlues more extreme thn the men vlue for young stnds would mke positive contribution to I. og. To void this possibility, x i, obs ws constrined s follows: x, if x - x for is1 to 3 nd x s~ i,obs i,young i,obs i,young if x i,obs) xi,young for is4; x i,old, if x i,obs) xi,old for is1 to 3 nd if x - x for is4 i,obs i,old Thus, I rnges from 0 Ž og for typicl young stnd structure. to 100 Ž for typicl old-growth structure.. Iog is specil cse of the dissimilrity mesure known s the Gower metric Ž Greig-Smith, I og mesures dissimilrity to young stnd conditions nd Tble 3 Summry of structurl vribles nd old-growth index Ž I. og vlues Stnd SD of tree b Density of lrge Men tree Density of ll trees I og DBH Pseudotsug menziesii DBH Ž cm. Ž trees h. Ž trees h. c Young forest Study stnds d e Minimum Men f Mximum c Old-growth forest SDsstndrd devition; DBHsdimeter t brest height. b Lrge trees, )100 cm DBH. c Men vlues from Spies nd Frnklin Ž d From observtions when stnds were closest to ge 100 yers. e Mximum for density of ll trees. f Minimum for density of ll trees.

6 270 ( ) S.A. Acker et l.rforest Ecology nd Mngement is constructed such tht young nd old-growth stnds represent opposite ends of one-dimensionl continuum. To ssess net chnges over different time intervls, the differences between ending nd beginning vlues of Iog were computed for ech stnd. These vlues were divided by the intervl length to compute verge nnul chnge. T-tests were used to test the null hypothesis of no increse in similrity to old-growth structure Ž mesured s I. og over time. To determine which components of old-growth structure included in Iog were most strongly relted to differ- ences in Iog between the stnds, we computed corre- ltion coefficients between Iog nd ech of the struc- turl vribles s well s mong the structurl vribles. We computed the correltions t two different stnd ges: in the 6th nd 12th decdes Ži.e., 51 to 60 nd 111 to 120 yers old, respectively.. Pirwise sctterplots of Iog nd the structurl vribles ug- mented the correltion nlysis; we summrized trends in the pirwise reltionships with the robust, locl smoothing procedure, loess ŽClevelnd, 1979; Chmbers et l., Shde-tolernt coniferous trees re significnt component of the live structure of old-growth stnds ŽOld-Growth Definition Tsk Group, 1986; Frnklin nd Spies, 1991b; J. Frnklin, personl communiction; T. Spies, personl communiction., but re not included in I og. To document the development of this component of old-growth structure, we computed the density of shde-tolernt individuls ) 40 cm DBH Ž Frnklin nd Spies, 1991b. for ech stnd t ech observtion. Frnklin nd Spies Ž 1991b. found tht density of ten shde-tolernt trees h of this size distinguished old-growth from younger stnds. In decresing order of bundnce, shde-tolernt species in the dt-set were T. heterophyll, Thuj plict Donn. Ž western red cedr., Abies grndis Ž Dougl.. Forbes Ž grnd fir., Abies mbilis Ž Dougl.. Forbes Ž Pcific silver fir., nd T. mertensin Ž Bong.. Crr. Ž mountin hemlock Volume nd Õolume growth rte To ssess chnges in productivity of timber volume during stnd development, it ws necessry to estimte volume from mesured DBHs nd, in some cses, heights. Dt vilbility differed; therefore, seprte pproches were tken to estimte volume of Ž. 1 P. menziesii nd Ž. 2 ll other tree species. P. menziesii ws, by fr, the most bundnt species in the dt set Ž 83% of ll observtions., nd it ws the only species with numerous height mesurements. Given tht the reltionship between DBH nd height of this species Žnd hence DBH nd volume. might vry between the different stnds, we evluted the reltionship between DBH nd volume seprtely for ech stnd. We were concerned tht stndrd volume equtions in the literture for P. menziesii might underpredict volume for the older nd lrger trees in this study Ž i.e., up to 147 yers old nd 118 cm DBH.. For exmple, Browne Ž developed the following eqution from whole-tree hrvest dt for totl wood volume of costl P. menziesii up to 140 yers old: log10 Ž volume. s q log10 Ž DBH. q log10 Ž height. Ž 2. where volume is in m 3, DBH is in cm, nd height is in m; 99% of Browne s smple hd DBHF71 cm. To test the suitbility of Eq. Ž. 2, we used regression nlysis to compre observed volumes from published dt-set with volumes predicted by the eqution. The observed volumes were mesured with n opticl dendrometer over brod size-rnge of P. menziesii in the Pcific Northwest Ždt-set AND001 in Michener et l., We regressed the logrithm of observed volumes on the logrithm of predicted volumes nd tested the hypotheses tht the slopes1 nd the intercepts0 Ž Myer et l., The intercept ws not significntly different thn 0 Ž p s nd the slope ws not significntly different thn 1 Ž p s Eq. Ž 2. slightly underpredicted the observed dendrometer volumes of very lrge trees Že.g., predicted volume ws 6.8% less thn the observed volume of 100 m 3, which corresponded to DBH f 200 cm in the observed dendrometer dtset..

7 ( ) S.A. Acker et l.rforest Ecology nd Mngement To determine whether cceptble stnd-level volume-dbh reltionships could be developed, we regressed the predicted tree volume on DBH for ech stnd s follows: log10 Ž volume. sb0 qb1log10ž DBH. Ž 3. Volume ws predicted from Eq. Ž. 2 for ech tree with height mesurement. For trees with multiple height mesurements, only the observtion with the lrgest dimeter ws used. All regressions were significnt Ž s0.01.; smple sizes rnged from 51 to 144; R 2 vlues were between 0.97 nd To determine whether stnd ge should be included in volume-estimtion models, we fit the following regression model to dt for ech stnd: log10 Ž volume. sb0 qb1log10ž DBH. qb2log10ž ge. Ž 4. Although stnd ge ws sttisticlly significnt for most stnds, we wished to determine if it ws bio- Tble 4 Smple sizes nd goodness-of-fit sttistics for nonliner regression models to estimte volume Ž including top nd stump. of Pseudotsug menziesii 2 Stnd n MSE R CH CH CH GP GP GP GP GP MH MH MH OL OL WI WI WI WR WR WR WR MSE, men squre error. The regression model tht predicted volume from DBH only ŽEq. Ž 5.. ws used for ll stnds except GP09, MH01, nd OL02, for which the regression model tht predicted volume from DBH nd stnd ge ŽEq. Ž 6.. ws used. Tble 5 Volume equtions from Mens et l. Ž used for tree species other thn Pseudotsug menziesii. For conifers, volume equtions re derived from the biomss equtions nd species-specific wood densities in Mens et l. Ž Ž. Species Eq. no. in Mens et l Abies mbilis 249 Abies grndis 378 Abies procer 252 b Acer mcrophyllum 655 Alnus rubr 423 b Arbutus menziesii 674 b Cstnopsis chrysophyll 664 c Cornus nuttlli 423 Pice sitchensis 309 d Pinus monticol 390 c Populus trichocrp 423 c Prunus emrgint 423 c Prunus sp. 423 c Rhmnus purshin 423 Thuj plict 399 e Tsug heterophyll 312 f Tsug heterophyll 258 Tsug mertensin 339 No eqution vilble; eqution for Abies concolor ŽGord. nd Glend.. Lindl. Ž while fir. used. b Volume estimted from DBH nd height; height estimted from DBH using species-specific eqution from Grmn et l. Ž c No eqution vilble; eqution for Alnus rubr used. d No eqution vilble; eqution for Pinus lmbertin Dougl. Ž sugr pine. used. e Used for stnds CH14, CH41, CH42. f Used for ll other stnds. logiclly importnt. To do so, we estblished the following criterion to ssess the precision of regression models: 95% of the volumes predicted by the regression model of interest must be within 10% of the corresponding volume predicted by Eq. Ž. 2. Chisqured tests Ž Freese, tht were performed to ssess the regression model without stnd ge ŽEq. Ž.. 3 indicted tht the criterion ws met in ll but three stnds. Further chi-squred tests to ssess the regression model with stnd ge ŽEq. Ž 4.. for these three stnds indicted tht two stnds met the criterion. The third stnd hd one unusul tree Ž smll tree tht ws reltively short nd long-lived for its DBH.. When this tree ws dropped, the criterion ws met. Therefore, we felt tht it ws resonble to use Eq. Ž. 4. However, this tree ws not excluded in computing the finl model.

8 272 ( ) S.A. Acker et l.rforest Ecology nd Mngement Fig. 1. Trends in old-growth index Ž I. versus stnd ge. Stnds in the sme pnel nd with the sme symbol type Ž filled or open. og belong to the sme group Ž see Tble 1.. Finlly, the equtions were computed seprtely for ech stnd using nonliner regression of the form: volumesb0ž DBH b 1. Ž 5. nd volumesb Ž DBH b 1. ge b 2 Ž 6. 0 Ž. We chose nonliner regression over bck-trnsforming Eqs. Ž. 3 nd Ž. 4 becuse bck-trnsformtion will estimte the medin, not the men response tht is required for stnd-level volume estimtion. Smple sizes nd goodness-of-fit sttistics for the resulting models re listed in Tble 4. For tree species other thn P. menziesii, we used regionl, species-specific equtions to estimte volume from DBH Ž Mens et l., Tble 5 lists the equtions by species. In order to compre timber volume growth nd to ssess the most pproprite intervl between hrvests, trends of timber volume over time re com- monly expressed s men nnul increment Ž MAI. Ž Husch et l., 1982; Curtis nd Mrshll, 1993.: live volume MAIs Ž 7. stnd ge We computed MAI for ech observtion of ech stnd. To ssess net chnges over different time intervls, the nnulized percent chnge in MAI Tble 6 T-tests of temporl trends in I og Intervl Ž decdes. n Annul chnge in Iog p Men SE Rnge Iog 40 t first observtion 7th to 12th to th to 9th to th to 12th to Iog f0 t first observtion 7th to 12th to th to 9th to th to 12th to

9 ( ) S.A. Acker et l.rforest Ecology nd Mngement Tble 7 Correltions between I nd the structurl vribles nd correltions mong the structurl vribles for stnds in the 6th decde Ž ns13. og. Vlues re Person s correltion coefficients; p-vlues re in prentheses Ž. Ž Vrible SD of tree DBH Men tree DBH cm Density of ll trees trees h. Men tree DBH Ž cm Ž Ž Density of ll trees trees h. y0.73 Ž y0.88 Ž I 0.70 Ž Ž y0.98 Ž og Ž. No lrge )100 cm DBH Pseudotsug menziesii trees were present in ny of the stnds in the 6th decde. Ž C. m ws computed for ech stnd s follows: Ž MAI 2yMAI1. rž MAI1. Cm s100 Ž 8. Ž ge yge. 2 1 where MAI1 is the MAI t beginning of intervl, MAI is the MAI t end of intervl, ge is the stnd 2 1 ge t beginning of intervl, ge2 is the stnd ge t end of intervl. T-tests were used to test the null hypothesis of no decrese in MAI over time. We summrized time trends in MAI for ech stnd using liner regression. To remove the effect of differences in productivity between stnds, MAI vl- Fig. 2. Pirwise sctterplots of old-growth index Ž I. og nd structurl vribles in the 6th nd 12th decdes. Trends were fit to the dt with the loess procedure Ž Clevelnd, 1979; Chmbers et l., Open circles re observtions in the sixth decde Ž stnd ges 51 to 60 yers.; filled circles re observtions in the 12th decde Ž stnd ges 111 to 120 yers..

10 274 ( ) S.A. Acker et l.rforest Ecology nd Mngement Tble 8 Correltions between I nd the structurl vribles nd correltions mong the structurl vribles for stnds in the 12th decde Ž ns17. og. Vlues re Person s correltion coefficients; p-vlues re in prentheses Vrible SD Density of Men tree Density of of tree lrge Ž ) 100 cm DBH. DBH Ž cm. ll trees DBH Pseudotsug menziesii Ž trees h. Ž trees h. Ž. Ž. Density of lrge P. menziesii trees h Men tree DBH Ž cm. y0.16 Ž Ž Ž Density of ll trees trees h Ž y0.06 Ž y0.92 Ž I 0.63 Ž Ž Ž y0.51 Ž og ues were converted to percent difference reltive to the initil vlue for ech stnd prior to regression: MAI tymai1 Dts100 Ž 9 ž. MAI 1 / where Dt is the percent difference in MAI t time t reltive to first observed vlue of MAI for the stnd, MAI t is the MAI vlue for stnd t time t, MAI1 is the MAI vlue for stnd t first observtion. We performed T-test to evlute the null hypothesis tht, on verge, the regression slopes were greter thn 0. We lso performed one-wy nlysis of vrince Ž ANOVA. to test the null hypothesis of no difference in slope between the groups of stnds. For pirwise comprisons between groups, we used pirwise T-tests when ANOVA detected significnt differences Ž SAS Institute Inc., For ll sttisticl tests pertining to old-growth chrcteristics nd volume growth, the Type I error rte ws ge of old-growth Ži.e., 200 yers, Frnklin et l., The stnds exhibited two types of temporl trends over the period of observtion. Fourteen of the 20 stnds hd lredy undergone considerble development towrd old-growth structure by the first observtion. During the study period, this set of stnds displyed slow nd stedy Ž rther thn rpid. devel- 4. Results 4.1. DeÕelopment of old-growth chrcteristics The overll trend mong ll the stnds ws rpid increse in I Ž og i.e., rpid development of old-growth chrcteristics. up to bout ge 60 to 80, followed by more grdul increse in I Ž og i.e., slower develop- ment of old-growth chrcteristics.ž Fig. 1.. At the mesurement closest to 100 yers of ge, observed I vlues rnged from 43 to 67 Ž Tble 3. og. Thus, in terms of I og, the stnds completed roughly hlf the trnsition to old-growth structure in hlf the nominl Fig. 3. Trends in density of shde-tolernt coniferous trees )40 cm DBH versus stnd ge. Stnds in the sme pnel, with the sme symbol type Ž filled or open. nd identified by the sme letters belong to the sme group Ž see Tble 1.. Species include Tsug heterophyll, Thuj plict, nd Abies grndis. Stnds without shde-tolernt species were excluded Žstnds MH01, MH02, WI01, WI03, GP01, GP03, GP05, GP09, WR02, WR09..

11 ( ) S.A. Acker et l.rforest Ecology nd Mngement Fig. 4. Trends in men nnul increment Ž MAI. versus stnd ge. Stnds in the sme pnel nd with the sme symbol type Ž filled or open. belong to the sme group Ž see Tble 1.. opment of old-growth chrcteristics. The other six stnds were indistinguishble from typicl young stnds t the first observtion Ž i.e., I fo. og. This set of stnds did disply period of rpid development of old-growth chrcteristics. For the most prt, these stnds continued to lg behind the other set in the degree of similrity to typicl old-growth structure. In light of these two temporl trends, we tested chnges in old-growth structure seprtely for the two sets of stnds. Iog incresed for both sets of stnds nd for ll intervls exmined Ž Tble 6.. The nnul chnges in Iog were lrger for the stnds initilly lcking old- growth chrcteristics nd for the erlier rther thn the lter intervl. The nnul chnge in Iog rnged from 0.24 Žstnds with old-growth chrcteristics t first observtion, 7th to 12th decde. to 1.05 Žstnds initilly without old-growth chrcteristics, 6th to 9th decde.. In ll cses, the increse in Iog ws signifi- cnt Ž Tble 6.. The structurl vribles tht were most responsible for differences in I chnged over time. For the og 6th decde, Iog ws significntly correlted with density of ll trees, men tree DBH, nd SD of tree DBH Ž in decresing order of bsolute vlue of R. Ž Tble 7, Fig. 2.. None of the stnds contined lrge P. menziesii t tht time. The correltion with density of ll trees ws negtive; correltions with the other structurl vribles were positive. There were lso significnt negtive correltions between density of ll trees nd men tree DBH, s well s between density of ll trees nd SD of tree DBH ŽTble 7, Fig. 2.. By the 12th decde, Iog ws significntly corre- lted with density of lrge P. menziesii, SD of tree DBH, men tree DBH, nd density of ll trees Žin decresing order of bsolute vlue of R.ŽTble 8, Fig. 2.. The correltion with density of ll trees ws negtive; correltions with the other structurl vribles were positive. There were lso significnt correltions between density of ll trees nd men tree DBH Ž negtive. s well s between density of lrge P. menziesii nd SD of tree DBH Ž positive. ŽTble 8, Fig. 2..

12 276 ( ) S.A. Acker et l.rforest Ecology nd Mngement The four structurl vribles contributing to I og hd different ptterns of temporl chnge. At the observtion closest to 100 yers of ge, for exmple, ll the stnds hd men tree DBH equl to or greter thn the old-growth vlue, nd most of the stnds hd the sme or lower density of ll trees s did typicl old-growth Ž Tble 3.. For most of the stnds, however, SD of tree DBH ws closer to the men vlue for young forests thn to the vlue for oldgrowth Ž Tble 3.. Only five of the stnds contined ny lrge P. menziesii. Density of shde-tolernt conifers tended to increse with time, lthough not consistently cross ll stnds Ž Fig. 3.. Between the 7th nd 12th decdes, the density of shde-tolernt conifers ) 40 cm DBH incresed by 3.2 trees h on verge ŽSE s 1.7; ns12; rnge y3.0 to 18.0; ps0.046 for T-test of the null hypothesis of no increse in density of shde-tolernts.. Ten of the 20 stnds hd no lrge shde-tolernt conifers throughout the study period. Two of the stnds lredy hd more thn 10 lrge shde-tolernt trees h t first observtion. On only two of the other stnds did the density of lrge shde-tolernts surpss 10 trees h during the study period Trends in men nnul increment Overll, there ws grdul decline in MAI over the study period Ž Fig. 4.. The rte of decline ws rther consistent for different decde intervls: C m f 0.3% per yer Ž Tble 9.. In ll cses, the declines in MAI were sttisticlly significnt Ž Tble 9.. Bsed on regression of Dt on stnd ge for the entire study period for ll stnds, the verge slope ws y0.36% per yer ŽSEs0.042; ns20; rnge y0.71 to In T-test, the null hypothesis tht Tble 9 T-tests of temporl trends in C m, nnulized percent chnge in MAI Ž. Ž. m Intervl decdes n C % yer p Men SE Rnge 7th to 12th 12 y y0.48 to th to 9th 13 y y0.67 to th to 12th 17 y y0.86 to MAI, men nnul increment. Tble 10 Pirwise comprisons of slopes of percent chnge per yer of men nnul increment for groups of stnds defined in Tble 1. Vlues followed by the sme letter were not significntly different t s0.05 Ž pirwise T-tests. Group GP2 OL WI GP1 CH2 WR1 MH CH1 WR2 Men slope y0.04 y0.20 b y0.26 b y0.40 bc y0.39 bcd y0.42 bcd y0.42 bcd y0.60 cd y0.66 d Groups GP1 nd CH2 re switched with respect to ordering by men slope becuse of the comprison of these two groups with WR2. The lrger smple size for GP1 resulted in smller stndrd error of men slope thn for CH2. Thus the difference between GP1 nd WR2 is significnt while difference between CH2 nd WR2 is not. the verge slope ws greter thn or equl to 0 ws rejected Ž p s One-wy ANOVA of the slopes bsed on the groups of stnds ws significnt Ž Fs4.46; ps0.0125; dfs8,11; MSEs There were severl significnt pirwise comprisons between groups. Most pprent ws tht group GP2 Ž stnds GP07 nd GP09. hd significntly slower rte of decline of MAI thn did most other groups of stnds Ž Tble Discussion 5.1. Trends in old-growth deõelopment Although ll the stnds were developing oldgrowth structure over the entire course of the study, two different ptterns were pprent. Most of the stnds lredy hd considerble old-growth structure t the first observtion nd continued to develop slowly nd stedily towrds old-growth therefter. A smller number of stnds hd very little in common with old-growth t the first observtion. These stnds hd n initil period of rpid chnge towrd oldgrowth, followed by period of slower development of old-growth fetures. Throughout the study, these

13 ( ) S.A. Acker et l.rforest Ecology nd Mngement stnds were less similr to old-growth thn ws the other subset. Correltions between Iog nd the structurl vri- bles s well s mong the structurl vribles indicte which components of old-growth structure contribute most to these ptterns. In the 6th decde, density of ll trees ws the component most strongly Ž negtively. correlted to I og. This vrible ws lso significntly Ž negtively. correlted with men tree DBH nd SD of tree DBH. The stnds lest similr to old-growth in the erly observtions hd reltively high tree density nd, correspondingly, reltively smll nd uniform trees. Thus it is possible tht limiting tree density in the first severl decdes of the development of mnged stnds my speed development of old-growth structure. This pproch to hstening development of old-growth structure hs been suggested in severl recent reviews ŽSpies et l., 1991; Cole, 1996; Spies nd Frnklin, In the 12th decde, density of lrge P. menziesii ws the structurl vrible most strongly correlted with I og. Also, correltions between density of lrge P. menziesii nd SD of tree DBH were significnt nd positive. This suggests tht in mture stnds, mngement ctivities tht promote growth of lrge individuls nd ccentute heterogeneity of tree sizes my be the most importnt steps for speeding development of old-growth structure. For ll the stnds, some spects of old-growth structure were slower to develop thn others. Men tree DBH nd density of ll trees pproched oldgrowth vlues reltively quickly. This hd lrgely occurred by 100 yers. SD of tree DBH nd density of lrge P. menziesii developed more slowly. Agin, this suggests tht lter in stnd development, mngement tht promotes heterogeneity of tree sizes nd enhnces the growth of vigorous dominnt trees could hsten development of old-growth structure. This concept hs been demonstrted in limited number of cses Ž Newton nd Cole, Although density of shde-tolernt conifers did not emerge s discrimintor between old-growth nd other forest ge clsses in the nlysis of Spies nd Frnklin Ž 1991., they re notble spect of old-growth forest structure ŽOld-Growth Definition Tsk Group, 1986; Frnklin nd Spies, 1991b.. On verge, the density of shde-tolernt conifers ) 40 cm DBH incresed between the 7th nd 12th decdes. However, hlf of the stnds hd no shde-tolernt conifers of this size throughout the study. Thus, if the presence of lrge shde-tolernt conifers is mngement gol, it my be necessry in mny cses to plnt trees or promote growth of smller individuls lredy present. Old-growth forest differs from erlier stges of succession in number of structurl, compositionl, nd functionl ttributes Ž Frnklin nd Spies, In ddition to the components of live stnd structure included in I og, other workers hve frequently sug- gested including in definitions of old-growth Ž. 1 mesures of sngs nd downed logs; Ž. 2 density of trees of shde-tolernt species; nd Ž. 3 the degree to which forest cnopies re multilyered ŽFrnklin et l., 1981; Old-Growth Definition Tsk Group, 1986; Spies nd Frnklin, 1988; Frnklin nd Spies, 1991,b; Spies nd Frnklin, 1991; Burgmn, These mesures were not included in Iog either becuse mesurements were not tken over the entire period of record of the permnent plots Že.g., sngs, logs, number of cnopy lyers., ndror becuse the vribles did not emerge in the nlysis of Spies nd Frnklin Ž s the best discrimintors between forest ge-clsses Ž density of shde-tolernts.. However, these ttributes hve considerble functionl significnce. For exmple, sngs provide hbitt for vriety of species, nd logs both provide hbitt nd re source of energy nd nutrients for oldgrowth ecosystems Ž Frnklin nd Spies, High vlues of Iog my in mny cses be ssocited with high vlues of these other chrcteristics of oldgrowth structure Že.g., old-growth stnds in the dtset of Spies nd Frnklin Ž However, future efforts to monitor the development of old-growth structure should include mesurements of s mny importnt structure fetures s possible, rther thn ssuming tht selected subset of mesures Že.g., the live stnd vribles included in I. og dequtely rep- resent other spects of stnd structure Že.g., bundnce of sngs nd logs.. A more inclusive pproch to quntifying old-growth structure my be especilly criticl in studies of mnged stnds. Wheres in the set of nturlly regenerted stnds studied by Spies nd Frnklin Ž 1991., young nd old-growth forest hd similrly high volumes of sngs nd logs, plnttion mngement in the Pcific Northwest typiclly hs included removl of sngs nd logs ŽSpies

14 278 ( ) S.A. Acker et l.rforest Ecology nd Mngement nd Frnklin, Thus, use of Iog to study devel- opment of old-growth structure in mnged stnds would run gret risk of overlooking essentil spects of old-growth structure. In generl, whether pplying the results of this study, or employing I og in other studies, it must be borne in mind tht Iog is bsed on limited set of mesures of live stnd structure. Thus, in ny prticulr cse, high vlues of Iog my or my not correspond to the presence or bsence of other spects of old-growth structure, composition, or function Chnges in Õolume growth This study confirms tht, on verge, timber volume growth rte declines s P. menziesii stnds exceed common rottion lengths Ži.e., 40 to 80 yers; Scott, However, for these stnds, the decline ws neither precipitous Žverging round 0.3% to 0.4% per yer reltive to vlues observed between ges 45 nd 85 yers. nor universl. For exmple, two of the 20 stnds hd no chnge or n increse in volume growth rte over the entire study period. Although grdul decline in MAI for mturing stnds hs been inferred from stnd simultion model studies, prior to this study direct observtions to confirm or refute those inferences hve been mostly lcking Ž Curtis nd Mrshll, 1993; Curtis, The rte of chnge in volume growth Žmesured s chnge in MAI over time. vried significntly between groups of stnds. Within groups, stnds occupied very similr environments nd experienced similr histories Že.g., density of tree estblishment, extreme wether events, pthogens; Willimson, Thus, differences in volume growth rtes between groups of stnds could be due to differences in environment, stnd history, or both. For exmple, in simultion studies, MAI peks lter nd declines more slowly on less productive sites Ž Curtis, Simultion studies lso suggest tht, under some circumstnces, thinning my dely the pek in MAI Ž Curtis, Thus, mortlity episodes in nturl stnds my hve n effect on the chnges in MAI over time. Further investigtion of these stnds is wrrnted, s it my suggest detils of stnd mngement tht could enhnce timber yields under longer rottions or indicte the types of sites tht re more or less suited to long rottions. The stnds included in this study regenerted nturlly nd for the most prt hve not been directly mnged, wheres the vst mjority of younger P. menziesii stnds in the Pcific Northwest now re mnged. However, with incresing emphsis on forest mngement for ecologicl objectives, mnged stnds my come to resemble nturl stnds more closely Ž Swnson nd Frnklin, For nturl stnds such s those in this study, it ppers tht the decline in timber volume growth rte over longer rottions is smll nd tht, with interventions such s density control nd underplnting, resonble fcsimile to old-growth structure my be chievble within the longer intervls between hrvest now under discussion Že.g., 150 to 180 yers; Forest Ecosystem Mngement Assessment Tem, Determining the degree to which these conclusions pply to mnged stnds depends on continuing experiments such s those described by Curtis nd Mrshll Ž Appliction to old-growth reserch in other regions This study demonstrtes gin tht direct, longterm observtion of forest succession complements chronosequences nd other methods Ž Pickett, Although chrcteristics of old-growth forests my be better understood in the Pcific Northwest of the United Sttes thn in mny other regions ŽBurgmn, 1996; Spies nd Frnklin, 1996., our pproch my prove vluble for quntifying old-growth development elsewhere. Our pproch requires Ž. 1 long-term records from forest plots, nd Ž. 2 quntittive definitions of old-growth nd other stges of forest development pproprite to the region. Fr from being limited to the Pcific Northwest, long-term plots re used to study forests throughout the world, nd interest in them is growing, especilly in reltion to biologicl diversity Ž McBryde, Quntittive informtion to distinguish old-growth from other forest stges my be more serious limittion outside of the Pcific Northwest ŽSpies nd Frnklin, However, reserch to ddress this need is underwy in mny regions Že.g., Burgmn, 1996; Foster et l., 1996; McCrthy nd Biley, 1996; Timoney nd Robinson, 1996; Woodgte et l., Thus it is likely tht our

15 ( ) S.A. Acker et l.rforest Ecology nd Mngement pproch of documenting old-growth development with long-term observtions, nd identifying which fcets of old-growth structure develop most quickly, could be useful outside of the Pcific Northwest. Acknowledgements This study would not hve been possible without the foresight in the erly 20th century of silviculturl reserchers P.A. Brieglieb, E.J. Hnzlik, J.V. Hofmnn, R.E. McArdle, W.H. Meyer, T.T. Munger, nd W. Peterson of the Pcific Northwest Forest nd Rnge Experiment Sttion Ž PNW. of the USDA Forest Service. Reserchers from the Olympi Žespe- cilly Robert O. Curtis. nd Corvllis ŽJerry Frnklin nd collegues. lbortories of PNW hve been instrumentl in mintining interest in the permnent plots nd sfegurding the dt. We thnk Dick Brinerd nd Gody Spycher for help with dt mngement; Den Berg, Dve Shw, nd Ken Bible for help with fieldwork; nd Robert O. Curtis, Steve Grmn, Mrk Hrmon, Fred Swnson, nd two nonymous reviewers for criticl redings of the mnuscript. Also, numerous Ntionl Forest personnel hve cooperted in mintining the permnent plots s reserch sites. This reserch hs been supported by grnts DEB , DEB , DEB , BSR , BSR , nd BSR from the Ntionl Science Foundtion to the H.J. Andrews Experimentl Forest Long- Term Ecologicl Reserch Progrm nd by coopertive greements between Oregon Stte University nd PNW. This is pper 3114 of the Forest Reserch Lbortory, Oregon Stte University, Corvllis. References Browne, J.E., Stndrd cubic-foot volume tbles for commercil tree species of British Columbi. British Columbi Forest Service, Victori, BC, Cnd, 107 pp. Burgmn, M.A., Chrcteristion nd delinetion of the euclypt old-growth forest estte in Austrli: review. For. Ecol. Mnge. 83, Chmbers, J.M., Clevelnd, W.S., Kleiner, B., Tukey, P.A., Grphicl Methods for Dt Anlysis. Wdsworth, Belmont, CA. Clevelnd, W.S., Robust loclly weighted regression nd smoothing sctterplots. J. Am. Stt. Assoc. 74, Cole, E.C., Mnging for mture hbitt in production forests of western Oregon nd Wshington. Weed Technol. 10, Curtis, R.O., A new look t n old question Dougls-fir culmintion ge. West. J. Appl. For. 7, Curtis, R.O., Some simultion estimtes of men nnul increment of Dougls-fir: results, limittions, nd implictions for mngement. USDA For. Serv., Res. Pp. PNW-RP-471, Pc. Northwest Res. Stn., Portlnd, OR, 27 pp. Curtis, R.O., Mrshll, D.D., Dougls-fir rottions time for repprisl?. West. J. Appl. For. 8, Dunwiddie, P., Foster, D., Leopold, D., Leverett, R.T., Old-growth forests of southern New Englnd, New York, nd Pennsylvni. In: Dvis, M.B. Ž Ed.., Estern Old-growth Forests. Islnd Press, Wshington, DC, pp Forest Ecosystem Mngement Assessment Tem Ž FEMAT., Forest ecosystem mngement: n ecologicl, economic, nd socil ssessment. USDA; USDI wnd others x, Portlnd, OR. Foster, D.R., Orwig, D.A., McLchln, J.S., Ecologicl nd conservtion insights from reconstructive studies of temperte old-growth forests. Trends Ecol. Evol. 11, Frnklin, J.F., Spies, T.A., Composition, function, nd structure of old-growth Dougls-fir forests. In: Ruggiero, L. Ž Ed.., Wildlife nd Vegettion of Unmnged Dougls-fir Forests. USDA For. Serv. Gen. Tech. Rep. PNW-GTR-285, Pc. Northwest Res. Stn., Portlnd, OR, pp Frnklin, J.F., Spies, T.A., 1991b. Ecologicl definitions of oldgrowth Dougls-fir forests. In: Ruggiero, L. Ž Ed.., Wildlife nd Vegettion of Unmnged Dougls-fir Forests. USDA For. Serv. Gen. Tech. Rep. PNW-GTR-285, Pc. Northwest Res. Stn., Portlnd, OR, pp Frnklin, J.F., Cromck, K., Jr., Denison, W., McKee, A., Mser, C., Sedell, J., Swnson, F., Judy, G., Ecologicl chrcteristics of old-growth Dougls-fir forests. USDA For. Serv. Gen. Tech. Rep. PNW-118, Pc. Northwest For. Rnge Exp. Stn., Portlnd, OR, 48 pp. Freese, F., Testing ccurcy. For. Sci. 6, Grmn, S.L., Acker, S.A., Ohmnn, J.L., Spies, T.A., Asymptotic height-dimeter equtions for twenty-four tree species in western Oregon. Oreg. Stte Univ. For. Res. Lb. Res. Contrib. 10, Corvllis, OR, 22 pp. Greig-Smith, P., Quntittive Plnt Ecology. 3rd edn. Univ. of Cliforni Press, Berkeley, CA, 359 pp. Husch, B., Miller, C.I., Beers, T.W., Forest Mensurtion, 3rd edn., Wiley, New York, 402 pp. King, J.E., Site index curves for Dougls-fir in the Pc. Northwest. Weyerheuser Forestry Pper No. 8., Weyerheuser Forestry Reserch Center, Centrli, WA, 49 pp. McBryde, O.H. Ž Ed.., Mesuring nd monitoring forest biologicl diversity: the interntionl network of biodiversity plots, symposium progrm nd bstrcts. Smithsonin InstitutionrMn nd the Biosphere Biodiversity Progrm, Wshington, DC. Myer, D.G., Sturt, M.A., Swin, A.J., Regression of rel-world dt on model output: n pproprite overll test of vlidity. Agric. Syst. 45, McCrthy, B.C., Biley, D.R., Composition, structure, nd

16 280 ( ) S.A. Acker et l.rforest Ecology nd Mngement disturbnce history of Crbtree Woods: n old-growth forest of western Mrylnd. Bull. Torrey Bot. Club 123, Mens, J.E., Hnsen, H.A., Koerper, G.J., Albck, P.B., Klopsch, M.W., Softwre for computing plnt biomss BIOPAK users guide. USDA For. Serv. Gen. Tech. Rep. PNW-GTR-340, Pc. Northwest Res. Stn., Portlnd, OR, 184 pp. Michener, W.K., Miller, A.B., Nottrott, R. Ž Eds.., Long- Term Ecologicl Reserch Network Core Dt Set Ctlog. Belle W. Bruch Institute for Mrine Biology nd Costl Reserch, Univ. of South Crolin, Columbi, SC, 322 pp. Newton, M., Cole, E.C., A sustined-yield scheme for old-growth Dougls-fir. West. J. Appl. For. 2, Old-Growth Definition Tsk Group, Interim definitions for old-growth Dougls-fir nd mixed-conifer forests in the Pc. Northwest nd Cliforni. USDA For. Serv. Res. Note PNW- 447, Pc. Northwest Res. Stn., Portlnd, OR, 7 pp. Oliver, C.D., B.C. Lrson, Forest Stnd Dynmics. Mc- Grw-Hill, Sn Frncisco, CA, 467 pp. Pickett, S.T.A., Spce-for-time substitution s n lterntive to long-term studies. In: Likens, G.E. Ž Ed.., Long-term Studies in Ecology. Springer-Verlg, New York, pp SAS Institute, SASrSTAT w User s Guide, Version 6, 4th edn., Vol. 2. SAS Institute, Cry, NC. Scott, D.R.M The Pcific Northwest region. In: Brrett, J.W. Ž Ed.., Regionl Silviculture of the United Sttes, 2nd edn., Wiley, New York, pp Sollins, P., Input nd decy of corse woody debris in coniferous stnds in western Oregon nd Wshington. Cn. J. For. Res. 12, Spies, T.A., Frnklin, J.F., Old growth nd forest dynmics in the Dougls-fir region of western Oregon nd Wshington. Nt. Ares J. 8, Spies, T.A., Frnklin, J.F., The structure of nturl young, mture, nd old-growth Dougls-fir forests in Oregon nd Wshington. In: Ruggiero, L. Ž Ed.., Wildlife nd Vegettion of Unmnged Dougls-fir Forests. USDA For. Serv. Gen. Tech. Rep. PNW-GTR-285, Pc. Northwest Res. Stn., Portlnd, OR, pp Spies, T.A., Frnklin, J.F., The diversity nd mintennce of old-growth forests. In: Szro, R.C., Johnston, D.W. Ž Eds.., Biodiversity in Mnged Lndscpes: Theory nd Prctice. Oxford Univ. Press, New York, pp Spies, T.A., Tppeiner, J., Pojr, J., Cotes, D., Trends in Ecosystem Mngement t the Stnd Level. In: Trns. 56th N.A. Wildl. nd Nt. Resour. Conf. Wildlife Mngement Institute, Wshington, DC, pp Swnson, F.J., Frnklin, J.F., New forestry principles from ecosystem nlysis of Pcific Northwest forests. Ecol. Appli. 2, Timoney, K.P., Robinson, A.L., Old-growth white spruce nd blsm poplr forests of the Pece River Lowlnds, Wood Bufflo Ntionl Prk, Cnd: development, structure nd diversity. For. Ecol. Mnge. 81, Vor, R.S., Integrting old-growth forest dynmics into mnged lndscpes: northern Gret Lkes perspective. Nt. Ares J. 14, Willimson, R.L., Growth nd yield records from wellstocked stnds of Dougls-fir. USDA For. Serv. Res. Pp. PNW-4, Pc. Northwest For. Rnge Exp. Stn., Portlnd, OR, 24 pp. Woodgte, P.W., Peel, B.D., Corm, J.E., Frrell, S.J., Ritmn, K.T., Lewis, A., Old-growth forest studies in Austrli: concepts nd principles. For. Ecol. Mnge. 85,