Foothill Oak Woodlands of the Interior Valleys of Southwestern Oregon

Gregg M. Riegel,' BradleyG. Smith,'Departmentof ForestScienceOregonStateUnversity, Corvalis Oregon97331 ano Lab, Corva s, Oregon97337 Jerry F. Franklin,'USDAForestServce,ForestryScrences

FoothillOakWoodlandsof the InteriorValleysof SouthwesternOregon Abstract X e d e s o i b e d r e l e g e t . t i o n o f t h e O r e g o n i v h i t e o a k w o o d h n d s f o u n d o n l o o t h i l l s i n t h r : i n t r : r i o rr a l L , y s o l * , u L h w , n . m O r i , s o n . Reconnaissanceplotsltrlurdlo5{mp|i]|hi]vCgi]la|ion.clushr!n.lgradnjnrana aLr:rornmrnity onrposition to th. .i nlironment. l ive .omn uDitv types $ere exannre relalionshits bel{cen @mmunit! Lypi,s"!nd recognized lrom a rlata sct ol 53 plots. l.rur oi Lhr:scrlnrnrunirl rrpcs uere ordded aloDg a precipitatnr gradient- rhile the fifth and sLrlrrL,k ol thcsc rrx lands harr hor disturbr:d br firt occuLred on r ge lines und rock ouLcrops. l'loristic rtpositnrn supPression,liIeslocksr!,ing.nr|ro(|UCtionol!|jcnspi:rn's.andfrr:uoodharrrlst-\t(iLhourchalgesillaldnralagcllrrt.lcoak \ L o o d l a n d sc h a r l d e r i s l i c o l s o u r h i On,aon will rlinrirrish.

lntroduction Low eler.ations vithin southwcstcrn Orcgon arc bounded by oak woodlands, which are transitional to savannas and grasslands. Thcsc rvoodlands arc the lor,er elevationlimit [of lorestvegetationin the region (Waring 1969, l'ranklin and Dvrncss '19731. (Jucft:ut ganlnna (Oreplonwhite oak). with a natural r|ngc from Vancouvcr Island to sorrLhem Calilbrnia (Criffin and Critchficld 1972), dominaLesLhe loothill and !alley bottom \rood'fhese lands. oak t'oodlands reflect u clirnatic transition bchrccn mesic lorvlandsof the Willamette Vallev anrl northwestern (hegon (r'here Q. ga4r, {rzar rcachcs oplirnal development), and xeric lot'lands within thc intcrior vallcvs of northern California. This unique environment supporls thc convcrgcncr of taxa common to both California and Pacilic Nortllvest lloristic pror.inces (Stcbbins and fi{ajor 1965, Ralen 1977). Many bottomland stlnds $crc clcarecl frrr agriculture. Slands on less lertile upland sites have oftcn bccn Lrtilizedfrrrlivestockgrazing.and more recentl,v for fucl Nood. Wc assume olrrstorv canopy structure and composition ofthese sampled stnnds are nedr pristinc but littlc is known about original underslorv species composition.Disturbance from livestock €irazing.firc supprcssion. and wood han'esLing have initiated the establishment 'l'resent address: LSDA Forest Serrice. Sil culrure Iabora ron. 102; Ntr Trenton lre. lo,rl. OR 97i01. 'Pr.rsenl!d,lr(ss: O k l n o s a n \ a L i r n a l l i r r c s L .1 2 1 0 S o u r h 2 r r l . Okanosan. 8 A 98810. 3Presenr &lilress: ColJegeol for.rsr Rrso!r(cs. AR-10. l,nivctr s i r ! o { W a s h j n s l o n . S c u r rc . W A 9 8 1 9 5 .

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of non-nativehcrbaceoLrs speciesthat cr)rnpetilivelv displace many of thc nativc taxa. The objectives of this studv vere to: 1) documcnl the major plant commonilv tvpesthat occupv these lbothill t'oodlands, determining their composition tnd slnrdurc; and 2) intcrprtt thc cnvironmental factors influencing their distribution.

StudyArea l l r e i r r t e l i orre € d o on f - o u l h r r e - l e r O n r e g o rir- e o m prised of the Llmpqua and Ro€ge Riveliratersheds (Figurc 1). Thc arca is a complcx mixtulc of vdll , 1 . . f . , o t h i l l s .-r r r J m u r r n t r i n s \. T : r i n r m , r u n t u i n rangesborder lhe region on Lheeast, souLh.and t'est. fhe northern edge of the Umpqua River t'atcrshid forms the northern boLlndary.Elevations typically range from 150 to more than 1300 m tithin the region. The near-Nlediterranean clirnate is char-aclerized by *intcr rains and dry, hot srrmmers(Figure 2). I'recipitation maps published lbr the area (l'rochlich er al. 1982. NlcNabbet al. l9B2) show a q)mple\ pattern of fainshadovieffects. but an overall trend oi decreasinplprecipitation from north to south and from t'est to edst. Roseburg (Figure 2) receivcs an annual mcan of 800 mm ofprccipitation, r'hilc Nledford reccires on averageonl,v400 mm. The dry season lypically lasts llre months (lIa,v-September). Occasiona] liosts can occur throughurm r t u , l r . , I t h , r , r r . . r l t h , , ' r grhn ,r n m i n i rnLlmtemperature drops belorv fieezing only at the Medford station. The region overlaps por-tionsof boLh the Klamath llTountain Province and the Western

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