taxodium ecology 1974

8/6/2019 Taxodium Ecology 1974

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Vege ta t io VoL 29 : t -10 , 1974

D I F F E R E N T I A T I O N I N H A B IT A T R E S P O N S E I N T A X O D I U M D I S T I C H U M , T A X O D I U M M U C R O N A T U M ,P L A T A N U S O C C I D E N T A L I S , A N D L I Q U I D A M B A R S T Y R A C I F L U A F R O ~ I T H E U N I T E D S T A T E S A N DM E X I C OC a l v i n M c M I L L A N *D e p a r t m e n t o f B o t a n y a n d P l a n t E c o l o g y R e s e a r c h L a b o r a t o r y , U n i v e r s i t y o f T e xa s , A u s t i n , T e x a sKeywordsA d a p t a t i o n , H a b i t a t r e s p o n se , I l li n o i s, M e x i c o , P h e n o l o g y , P h o t o p e r i o d , S t i p u l es , T e m p e r a t u r e , T e x a s

IntroductionA d a p t i v e d i f f e r e n t i a ti o n i n s w e e tg u m , Liquidambar styra-ciflua L . , w a s c o m p a r e d t o t h a t i n s y c a m o r e , Platanusoeeidentalis L . , a n d b a l d c y p r e s s , Taxodium distichum( L . ) R i c h . - T . mucronatum T e n . , t o d e t e r m i n e t h e m e c h -a n i s m s u n d e r l y i n g d i s t r i b u t i o n i n t o M e x i c o . T h e t r e e sh a v e b r o a d l a t i t u d i n a l d i s t r i b u t i o n s i n t h e U n i t e d S t a t e sb u t e a c h i s l o c a l i z e d i n i t s o c c u r r e n c e i n e a s t e r n M e x i c o .T h e r e s u l t s o f s t u d ie s i n Liquidambar p o p u l a t i o n s ( W i n -s t e a d 1 96 8, W i l l i a m s & M c M i l l a n 1 9 71 , u n p u b l . ) s u g -g e s t t h a t h a b i t a t r e s p o n s e s i n c l u d e l e s s s e n s it i v i ty t ot e m p e r a t u r e a n d p h o t o p e r i o d a s t h e l a t it u d e o f t h e lo c a lb i o t o p e d e c r e a s e s . T h e r o l e o f t h e s e p a r a m e t e r s i n e c o -t y p i c d i f f e r e n t i a t i o n i n Platanus a n d Taxodium w a se v a l u a t e d a n d c o m p a r e d w i t h a d a p t i v e v a lu e s in Liqui-dambar.

Liquidambar, Platanus a n d Taxodium o c c u r t o g e t h e ri n s o u t h e r n I l li n o i s a n d t h e i r s y m p a t r y e x t e n d s t o e a s t e r nT e x a s ( F o w e l l s 1 9 65 ) . O f t h e t h r e e Liquidambar d o e sn o t c o n t i n u e s o u t h a n d w e s t w a r d i n T e x a s , b u t i s c o n -f i n e d t o s a n d y , a c i d i c s o i l s w i t h Pinus. B o t h Taxodiuma n d Platanus h a v e w e s t w a r d e x t e n s i o n s o n t h e l im e s t o n eo f th e E d w a r d s P l a t e a u n e a r l y t o t h e R i o G r a n d e b u ta r e e x c l u d e d f r o m t h e b r o a d s o u t h e r n t i p o f T e xa s .Taxodium r e a p p e a r s a l o n g th e R i o G r a n d e n e a r t h es o u t h e r n t i p .

T h e t h r e e t re e s o c c u r o n t h e C o a s t a l P l a i n o f e a s te r nU n i t e d S t a te s . Taxodium r e a c h es i t s n o r t h e r n m o s t l i m i t* Res ea rch funds were p rov ided by a g ran t f rom the U . S .F o r e s t S e r v ic e a n d f r o m N a t i o n a l S c ie nc e F o u n d a t i o n G r a n tG B - 60 9 7. I a c k n o w l e d g e h e l p o f D a v i d S . M a y a n d G e o r g eJ . Wi l l i ams .

i n D e l a w a r e , Liquidambar i n C o n n e c t i c u t a n d Platanusi n n o r t h e r n N e w E n g l a n d . T h e y e a c h c o n t i n u e s o u t h -w a r d t o F l o r i d a ; Taxodium t o t h e t i p , Liquidambar t ot h e c e n t r a l p o r t i o n , a n d Ptatanus o n l y t o t h e n o r t h e r np o r t i o n .

I n M e x i c o t h e d i s t r i b u t i o n o f t h e t h r e e t r e e s is d i v e rs e .A l t h o u g h Platanus a n d Taxodium o c c u r t o g e t h e r i nc e r t a in s t r e a m s i d e h a b i t a ts i n n o r t h e a s t e r n M e x i c o ,Liquidambar i s c o n f i n e d t o c l o u d f o r e s t h a b i t a t s a l o n gS i e r r a M a d r e O r i e n t al e . P o p u l a t i o n s o f a l l th r e e t r e eso c c u r n e a r C i u d a d d e l M a i z , i n t h e m o u n t a i n s w e s t o fA n t i g u o M o r e l o s , T a m a u l i p a s , a n d a g a i n n e a r T a m a -z u n c h a l e , S a n L u i s P o t o s i . A t b o t h o f t h e s e si t es , Plata-nus a n d Taxodium o c c u r t o g e t h e r a t e l e v a t i o n s s l i g h t l yb e l o w t h a t o f Liquidambar i n t h e c l o u d f o r e s t s.

P o p u l a t i o n s o f t h e t h r e e t re e s in M e x i c o h a v e r e c e i v e dv a r i o u s ta x o n o m i c t r e a tm e n t . T h e b a l d c y p r e s s o f M e x i -c o ( a n d a l o n g t h e R i o G r a n d e i n s o u t h e r n T e x a s ) i su s u a l l y r e f e r r e d to T. mucronatum. T h e s y c a m o r e s o fn o r t h e a s t e r n M e x i c o h a v e b e e n r e f e r r e d t o P. occiden-talis, b u t t h e p o p u l a t i o n s o f C h i a p a s a n d O a x a c a i ns o u t h e r n M e x i c o h a v e b e e n r e f e r r e d t o o t h e r s p e c i e s .S w e e t g u m i n M e x i c o h a s b e e n r e f e r r e d to L. styraeifluav a r . mexieana ( O e r s t . ) N d z . I n t h e p r e s e n t i n v e s t i g a t io n ,t h e t a x o n o m i c s t at u s a n d t h e m o r p h o l o g i e s h a v e b e e n o fc o n c e r n , b u t t h e e m p h a s i s h a s b e e n p l a c e d u p o n t h eh a b i t a t r e s p o n s e s w h i c h p e r m i t d i s t r i b u t i o n o v e r ab r o a d l a t i t u d i n a l a r e a i n t h e U n i t e d S t a t e s a n d a c o n -t i n u a t i o n o f d i s t r i b u t io n i n M e x i c o.

S t u d y o f h a b i t a t r e s p o n s e i s o n e b a s i s o f u n d e r s t a n d -i n g c o m m u n i t y r e l a t i o n s h i p s o f s p e c i e s . A t l e a s t t h r e em a j o r a p p r o a c h e s t o h a b i t a t r e s p o n s e m a y b e d i s t i n -g u i s h e d : ( 1 ) T h e r a n g e o f o c c t ~ r e n c e o f t h e s p e c i e s i n


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r e l a t i o n t o e n v i r o n m e n t a l f a c t o r s m a y b e s t ud i e d , e i t h eri n t e rm s o f p re s e n c e a n d a b s e n c e o r o f p o p u l a t i o n d e n -s i t y . M u c h r e s e a r c h i n g r a d i e n t a n a l y s i s r e l a t e s s p e c i e sp o p u l a t i o n s t o e n v i r o n m e n t a l f a c to r s , a n d i n t h e a p -p r o a c h o f B r a u n - B l a n q u e t t h e h a b i t a t r e sp o n s e s o fs p e c ie s a r e t a k e n a d v a n t a g e o f to u s e t h e m a s d i a g n o s t ics p e c i e s ( W e s t h o f f & v a n d e r M a a r e l 1 9 73 ). ( 2 ) T h ep h y s i o l o g i c a l r e s p o n s e s o f s p e c ie s w i t h i n a g i v e n b i o t o p em a y b e a n a l y z e d . B y t h i s m e a n s t h e s p e ci e s' t o l e r a n c eo f t h e b i o t o p e m a y b e u n d e r s t o o d , i n r e l a t i o n t o t h ec o n c e p t o f t h e " o p e r a t i o n a l e n v i r o n m e n t " o f M a s o n &L a n g e n h e i m ( 1 9 57 ) . B y t h i s m e a n s a l s o d i ff e r e n ce sb e t w e e n s p e ci e s in r e s p o n s e t o a g i v e n b i o t o p e m a y b er e c o g n i z e d ; s u c h d i f f e r e n ce s m a y r e p r e s e n t p a r t o f t h en i c h e d i f f e r e n t i a t i o n t h a t m a k e s s p e c i e s c o - o c c u r r e n c e

p o s s i b le . W h e n a s p e c i es ' re s p o n s e i s d e s c r ib e d t h r o u g ha n a b s t r a c t " v o l u m e " o u t l i n e d b y i ts l i m i t s i n r e la t i o nt o i n t e ns i v e o r i n t r a -b i o t o p e f a c to r s o f e n v i r o n m e n t , t h er e s u lt i n g n i ch e h y p e r v o l u m e i s t h e " f u n d a m e n t a l n i c h e "o f H u t c h i n s o n ( 1 9 58 ) . ( 3 ) F i n a l l y , a s a n a p p r o a c h i ns o m e w a y s r e l a t i n g t h e o t h e r t w o , s p e c i e s ' a d a p t i v er e s p o n s e s t o b i o t o p e d i f f e r e n c e s a l o n g m a j o r e n v i r o n -m e n t a l g r a d i en t s m a y b e s tu d i e d. R a n g e o f o c c u r r e n c e( I ) m a y d e p e n d i n p a r t o n c h a n g e i n a s p e c i e s ' i n t r a -b i o t o p e r e s p o n s e s ( 2 ) w i t h c h a n g e i n b i o t o p e i w h e nt h e s e d i f f e r e n c e s i n r e s p o n s e w i t h i n t h e s p e c i e s a r eg e n e t i c a l l y b a s e d t h e y i m p l y e c o t y p i c d i f f e r e n t i a t i o n .T h i s a r t ic l e co n c e r n s a p p r o a c h ( 3) a n d d e a l s w i t h a d a p -t i v e re s p o n s e s o f t h r e e t r e e s p e c i es t o d i f f e r e n c e inc l i m a t e f r o m I l li n o i s t h r o u g h T e x a s t o M e x i c o .

T a b l e 1Co l l e c t io n d a ta *L o c a t i o n L a t i t u d e S y m b o l s

Platanus Taxodium** LiquidambarI l l i n o i sU n i o n C o u n t yK e n t u c k y

G r a v e s C o u n t yT e x a sAn g e l in a Co u n ty 31 3 1 ' E a t - I - -E a t - 2

T y le r Co u n ty 3 0 5 4 ' - - E a tH a r r i s C o u n t y 2 9 4 5 ' - - - -T r a v i s Co u n t y 3 0 1 8" Au t - 1 Au t - IA u t - 2A u t - 3H i d a l g o C o u n t y 2 6 2 0" - - D o t

M e x i c oM o n t e r r e y , N u e v o L e o n 2 5 4 0" M o m - I M o r nL in a r e s , Nu e v o L e o n 2 4 5 7 ' L i ra -1 ( L i r a )M e xA n t i g u o M o r e l o s , T a m a u l i p a s 2 2 3 5 ' - - ( A M )M e xT a m a z u n c h a le , S a n L u i s P o to s i 2 1 1 8 ' - - ( T Z )M e xI x miq u i lp a n , H id a lg o 2 0 2 9" - - l x m

37 30"

36 45 '

lll - I I I1 I ll-L31tl-2 I ll-L5K e n - l K e nK e n - 2

E a t - L 2E a t - L t 3

A M - L IA M - L 2T Z - L 1 3

* S e e d c o l l e c t io n s we r e ma d e b y D . T . F u n k ( I l li n o i s ) , J . E . W in s te a d ( Ke n tu c k y ) . E a c hc o l l e c t io n i s f r o m a s in g le t r e e .* * Co l l e c t io n s in Taxodium f r o m th r e e s it e s ( L ir a , AM , T Z ) w e r e c o m b in e d f o r th e s t a t is t i c a la n a ly s e s a s Me x .


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M a t e r i a l s a n d M e t h o d s

In securing seed lots from the United States and north-eastern Mexico, an attempt was made to obtain collec-tions of all three tree species from a geographic site(Table 1). Emphasis was placed upon southern Illinois(and adjacent Kentucky), Texas, and Mexico. In Pla-tanus and Taxodium collections were obtained for bothcentral and easternTe xas and additional collectionswere studied from Florida, Louisiana and Mississippi.

In assembling seed lots of Taxodium, variation inseed size was noted (Table 2). Those from Illinois toeastern Texas contained the large seeds associated withT. distichum (Correll & Johnston 1970). Some of thelargest seeds were from Mississippi. Mexico and south-ern Texas (Rio Grande) collections were the smaller

Table 2Seed weights in 72~xodiumCollection site* Mean seed SD Average weightweight (mg) for duplicate(mg)** lots of 25 seeds

93.48 20.64 2179I00.28 29.20 238379.52 -- 1988148.76 49.57 330563.52 21.40 156914.24 6.27 31119.96 -- 49925.42 63616.36 5.47 40713.08 -- 327

5.96 2.85 1376.52 -- 1636.12 -- 153

Illinois-1Illinois-2KentuckyMississippi (M)FloridaTexasAustin-1

Austin-2Austin-3Austin-4Austin-5MexicoMonterrey- 1Monterrey-2Linares (M)AntiguoMorelos 10.64 2.93 279Tamazunchale (M) 17.36 4.08 435Ixmiquilpan- 1 6.76 1.85 157lxmiquilpan-2 8.74 -- 219* Single tree collections except those indicated with (M).Mississippi seed is from 3 trees in Sunflower Co., lat. 33 28".Linares and Tamazunchale seed is from 3 trees at each locality.Flor ida collection is from Alachua County, lat. 29 30'.** Mean seed weights and standard deviations were calcu-lated from individual weights for 25 seeds. A seed weightwithout standard deviation is an average for 50 seeds weightedtogether.

seeds of T. mucronatum. The seeds from central Texastrees were varied, hut most were only slightly largerthan seeds from Mexico,

Th e Platanus samples showed slight variation in seedsize among the various lots but the greatest differencewas in the time of shedding seed. In Mexic o in late No-vember there were no intact seed balls on the trees andmost seeds were on the ground. In contrast, seed fromTexas to Illinois was taken from trees that in most caseshad not begun to shed seed by late November. AlthoughU.S. trees usually shed their seed in spring, the Mexicotrees probably lose their seed in fail.

As has been noted in Liquidambar (Williams &McM ill an 1971a), seed weights differed amo ng thevarious lots. Those from more nothern lots generallycontained the heaviest seeds. Seeds were usually ob-tained in northeastern Mexico in late December toearly January. At that time, fruits were on the trees or onthe ground and contained many seeds. Seed dissemina-tion in the U.S. was earlier. The Mex ico collections con-tained the smallest seeds. The various lots were put intomoist trays of sand in mid-December at 4 C and weregerminated in mid-Feb ruary at 30 C day an d 20 Cnight temperatures under continuous light. The seed-lings were transplanted to individual quart containersfilled with a 3 : 1 mixture, a fine sandy loam-perlite.

Five seedlings of each seed lot were placed in each offour photoperiod-temperature conditions. These con-ditions were similar to those that had been used in aprevious phenological comparison of U.S. provenancesin L. styraciflua (Williams & McMillan 1971a). Thefour growth chambers were programmed as follows:two at temperatures of 30 C and 24 C night and twoat 24 C day and 15 C night; each temp erature p rogra mwith 15- and 12-h photoperiods. Seedlings were observedweekly for stem elongation. Records of the appearanceof dormant apical buds were taken over a period of 85days, and a plant was recorded as dormant if bud scaleformation was observed. Data on height were treatedstatistically to determine variance. For sycamore, leafblade area and stipule production also were treatedstatistically. The Student-Newman-Kuels multiple rangetest (Steel & Torrie 1960) was used to determine thesignificance of differences between means of variouscombinations of populations. The computations weremade using the Fortran Program MORSNK (Adams1969) with the University of Texas IBM 6603 Com pute r.

Following the 85-day test, a series of plants in T axo-dium and Platam4s were placed under other controlled


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c o n d i t i o n s f o r f o u r a d d i t i o n a l w e e k s t o o b s e r v e d o r m a n tb u d f o r m a t i o n . O n e s e r i e s w a s k e p t a t t h e i r o r i g i n a lt e m p e r a t u r e s b u t u n d e r t 0 - h d a y l e n g th s . A c o m p a r a b l ese r i e s was a t 15 -h day l eng ths and 30-24 C ,

A f t e r th e p h o t o p e r i o d - t e m p e r a t u r e c o m p a r i s o n s , t h eTaxodium a n d Platanus p l a n t s w e r e k e p t o u t - o f - d o o r s i nt r a y s f il le d w i t h w a t e r . T h e p a t t e r n s o f d o r m a n t b u df o r m a t i o n a n d b u d b u r s t i n g w e r e c o m p a r e d w i t h ph e -n o l o g i c a l e v e n t s i n Liquidambar p o p u l a t i o n s u n d e r c e n -t r a l T e x a s c o n d i t i o n s .

S e e d l i n g s f r o m e a s t e r n T e x a s s h o w e d s y m p t o m s o fi ron ch lo ros i s i n t he so i l (pH 8 .0 -8 .5 ) used i n t he s tudy .T h e sa n d y l o a m w a s t a k e n f r o m t h e C o l o r a d o R i v e rw h i c h f l o w s t h r o u g h t h e l i m e s t o n e s o f t h e E d w a r d sTable 3

P l a t ea u . T h e r e g u l a r n u t r i e n t a d d i t i o n s i n c l u d e d c h e l a te di ron t o co r rec t t he ch lo ros i s .Results

T h e m e a n h e i g h t s o f Platanus seed l ings showed l a t i -t u d i n a l t r e n d s ( T a b l e 3 ). U n d e r 1 5 -h 2 4 - 15 C c o n d i -t i o ns , t h e M e x i c o p la n t s ( M o r n - l , L i m - 1 ) w e r e th es h o r t e s t a n d d i f fe r ed f r o m a l l U . S . p l a n ts . U n d e r t h et 2 - h 2 4 - 1 5 C c o n d i ti o n s , t h e M e x i c o p l a n t s w e r e t h et a l l e s t (F ig . 1 ) bu t we re no t s i gn i f i c an t ly d i f f e ren t f rome a s t e r n T e x a s t r e e s . U n d e r t h e h i g h e r t e m p e r a t u r ep r o g r a m , 3 0 - 2 4 C , t h e M e x i c o p l a n ts w e r e a m o n g t h eshor t e s t a t e i t he r day l eng th , bu t t hey we re no t s i gn i -f i c an t ly d i f f e ren t f rom U.S . p l an t s (F ig . 2 ) .

Comp arison of mean heights (cm) for Platanus under four photoper iod- tempera ture programs for 85 days*t5 h Ken-1 Eat-2 Il i-1 Eat-1 K en-2 Aut-2 Lim-1 Aut-1 Ill-2 Mom -130-2 4 C 5 9 . 6 - - 51.8---------5 0.9 _50.5 1 43.8 ~ 43 .0 42.2--_ 39.8 32.7 23.12 h Eat-13 0 -2 4 C 4 6 . ! ~

Eat-2 Aut- 2 Aut-1 Ken-1 Lira-1 Ken-2 Mom-1 IIl-1 Il l-23 2 .9 -- ~ 2 .1 ~ _-.~.5 22.2 ~.d : - - .L23~ ~ 22 .5 1-5.0 8 .615 h Ken-1 Ill-1 Eat-1 Au t-2 111-2 Aut -1 Eat- 2 K en-2 Lim-1 Morn-124- 15 C 43.3 36.5 3~ 2 33~5 39.6 31.5 ~1 a 29 .8 19.0--'-----'----16.512 h Lim-1 Morn-1 Eat-1 Eat-2 Aut-2 Aut-1 Ken-1 Il l-2 II1-1 Ken-224 -15 C 14.7-- 1-4.3- ! 9 n 8.5 4.9 - 4.5 3.2 2.9 t.8 ........ 1.7* Each progeny w i thin each provenance o f I l linois 0H-l , -2) , Kentucky (Ken- l , -2) , eastern Texas (Eat - l , -2) , Aust in , cent ra lTexas (A nt- l , -2) and Mexico (Lira- l , M orn-l ) was represented by f ive plants in each of the four photoper iod- tempera turecondi t ions. Student-Newman-Kuels mul t iple range test i s indica ted by bars . Numbers connected by bars a re not s igni f icant lydifferent (0.05 > P). Num ber s no t connected are significantly different (0.05 > P).

Figs. 1-2. Com parison of seedlings under two photo per iod- tempera ture condi t ions. F ig . 1. Und er t 5 h 30-2 4 C, le f t to r ight :Platanus, I l l inois (L) , Mexico (M); Liquidambar; Taxodium. Fig. 2. Un de r 12 h 24-15 C , left to right: as in fig. 1.


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T a b l e 4C o m p a r i s o n o f m e a n h e i g h t s ( c m ) fo r Taxodium u n d e r f o u r p h o t o p e r i o d - t e m p e r a t u r e p r o g r a m s f o r 8 5 d a ys *15 h E a t Aut -1 Au t -2 Ixm 111 NIom Ken Me x Aut -3 Do t30- 24 C 54.8 50.3 48.9 45.9 44.6 43.5 42.8 42.3 40.1 38.41 2 h E a t M e x M o r n A u t - 2 I x m A u t - 1 D o t I l l A u t - 3 K e n30- 24 C 52.7 47.8 45.4 44.1 43.3 38.8 37.5 31.6 3t.1 27.91 5 h M o r n D o t I x m M e x E a t A u t - 1 A u t - 2 A u t - 3 I l l K e n24-1 5 C 31.4 30.4 29.0 25.8 22.5 16.6 13.9 13.4 12.8 5.61 2 h D o t M o r n M e x I x m E a t A u t - 3 A u t - I K e n 111 A u t - 224--15 C 22.6 22.5 21.2 19.8 5.8 4.6 4.4 2.2 1.2 1.1* E ach p roge ny wi th in each p rovenance o f I l l ino i s ( I l l ) , Ken tucky (Ken) , e a s te rn T exas (E a t ) , Aus t in , c en t ra l T exas (Aut -1 ,-2 , -3 ) , Do nna , s ou th T exas (D ot ) , and M exico (Mom , Mex , Ixm) was rep re s en ted by f ive p lan t s in each o f the four pho to pe r iod -temp era tu re cond i t ions . See foo tno te to t ab le 3 conce rn ing s ign if icance .

T h e t r ee s f r o m I l li n o i s ( Il l - l , I l l- 2 ) a n d K e n t u c k y( K e n - l , K e n - 2 ) w e r e t h e s h o r t es t p l a n t s u n d e r 1 2 -h2 4 - t 5 C c o n d i t i o n s , b u t t h e r a n g e o f v a r i a t i o n a m o n gt h e se p l a n t s o v e r l a p p e d t h e h e i g h t s o f T e x a s p l a n t s( A u t - 1 , A u t - 2 , E a t - l , E a t - 2 ) . O n e p r o g e n y f r o m K e n -t u c k y h a d t h e g r e a te s t m e a n h e i g h t u n d e r 1 5 -h d a yl e n g t h s a t b o t h t e m p e r a t u r e p r o g r a m s . T h e g r o w t ht r e n d s s u g g e s te d t h a t t h e n o r t h e r n t re e s h a d e a r l i e rc e s s a t io n o f s t e m e l o n g a t i o n u n d e r t h e s h o r t e r d a yl e n g t h s .

T h e m e a n h e i g h t s o f Taxodium s e e d l in g s a l s o s h o w e dl a t i t u d i n a l t r e n d s ( T a b l e 4 , F i g . 1, 2 ) . U n d e r t h e 1 2 -h2 4 - 1 5 C p r o g r a m , t h e t h r e e M e x i c o c o ll e c ti o n s ( M o m ,M e x , I x m ) a n d t h e R i o G r a n d e , T e x a s c o l l e ct i o n ( D o t )w e r e s i m i l a r t o e a c h o t h e r a n d d i f f e re n t f r o m a l l o t h e rTaxodium c o l l e c t io n s . U n d e r 1 5 - h 2 4 - 1 5 C , h o w e v e r ,t h e M e x i c o a n d R i o G r a n d e c o l l ec t io n s w e r e n o t s ig n i -f i c a n t l y d i f f e r e n t f r o m p l a n t s o f e a s t e r n T e x a s ( E a t ) .U n d e r b o t h 1 5 a n d 1 2 h a t 2 4 - 1 5 C a n d 1 2 - h a t3 0 - 2 4 C , K e n t u c k y a n d I l l i n o i s s e e d l i n g s w e r e e i t h e rt h e s h o r t e s t p l a n ts o r a m o n g t h e t h r e e s h o r t e st c o l l ec -t i o n s . T h e n o r t h e r n t r e e s sh o w e d g r e a t e r s e n s i ti v i t y t od a y l e n g t h a n d t e m p e r a t u r e t h a n d i d t r e es o f t h e lo w e rl a t i t u d e s .C o m p a r i s o n o f mean h e i g h t s o f s p e c i f ic Platanus c o l -l e c t i o n s i n t h e g r o w t h c o n d i t i o n s s h o w e d s i g n i f i c a n td i f f e re n c e s t h a t v a r i e d l a t i t u d i n a l l y ( T a b l e 5 ) . P r o g e n i e so f I l l in o i s a n d K e n t u c k y r e p r e s e n t e d o n e e x t r e m e i nw h i c h t h e r e w a s a s i g n i f i c a n t r e d u c t i o n i n m e a n h e i g h tu n d e r e a c h o f th e f o u r p h o t o p e r i o d - t e m p e r a t u r e c o n -d i t i o n s . T h e o t h e r e x t r e m e w a s r e p r e s e n t e d b y t h ep o p u l a t i o n f ro m M o n t e r r e y , M e x ic o ( M o m ) . I n t h e

l a t t e r g r o u p t h e r e w a s n o d i ff e re n c e a m o n g t h e f o u rt r e a t m e n t s . T h e n o r t h e r n p r o g e n i e s s h o w e d g r e a t e rm e a n h e i g h t s u n d e r 1 5 -h a t e i th e r t e m p e r a t u r e p r o g r a m ,b u t m o s t T e x a s a n d M e x i c o p r o g e n i e s h a d g r e a t er m e a nh e i g h t s u n d e r b o t h l i g h t p e r i o d s a t 3 0 - 2 4 C .

T h e m e a n h e i g h t s o f s p e c i f i c Taxodium c o l l e c t i o n sa l s o s h o w e d d i f fe r en c e s a m o n g t h e p h o t o p e r i o d - t e m -p e r a t u r e p r o g r a m s t h a t v a r i e d l a t i t u d i n a l l y ( T a b l e 6 ).T h e m e a n h e i g h t s o f t h e n o r t h e r n U . S . p o p u l a t i o n s w e r ed i f fe r en t f r o m e a c h o t h e r u n d e r t h e t w o d a y l e n g t h s a tt h e h i g h e r t e m p e r a t u r e p r o g r a m . I n m o s t o f t h e T e x a sa n d M e x i c o p o p u l a t i o n s t h e m e a n h e i g h ts w e r e d if f e re n tf r o m e a c h o t h e r a t th e l o w e r te m p e r a t u r e p r o g r a m . T h ep r i m a r y d i f fe r en c e b e t w e e n t h e T e x a s a n d M e x i c o p l a n t sw a s i n m e a n s t e m h e i g h t . T h e T e x a s p l a n t s t e n d e d t oh a v e v a l u e s t h a t w e r e i n t e r m e d i a t e b e tw e e n t h o s e o f t h en o r t h e r n p o p u l a t i o n s a n d t h o s e o f t h e M e x i c o p o p u -l a t i o n s .

T h e m e a n h e i g h t s fo r t h e Liquidambar p o p u l a t i o n s i nt h i s 8 5 - d a y c o m p a r i s o n w e r e n o t a n a l y z e d s t a t i s t ic a l l y .A c o m p a r i s o n o f p o p u l a t i o n s f r o m T e x a s a n d I l l i n o i sw a s i n c l u d e d i n a 1 0 0 - da y s t u d y ( W i l li a m s & M c M i l l a n1 9 7 1 a) . A s i n th e e a r l i e r s t u d y , t h e U . S . p o p u l a t i o n ss h o w e d c e s s a ti o n o f g r o w t h e a r l i e r u n d e r t h e c o o l e rt e m p e r a t u r e p r o g r a m u n d e r b o t h d a y le n g t h s. I ll i n o i sp l a n t s h a d l e ss m e a n h e i g h t t h a n T e x a s p l a n t s u n d e re a c h o f t h e f o u r p h o t o p e r i o d - t e m p e r a t u r e c o n d i ti o n s .T h e M e x i c o p l a n t s c o n t i n u e d t o e l o n g a t e s t em s a n ds h o w e d l e ss s e n si t iv i t y t o d a y l e n g t h s a n d t e m p e r a t u r ea s s h o w n b y W i n s t e a d ( t 9 6 8 ) a n d M c M i l l a n ( u n p u b l . ).

T h e p a t t e r n o f d o r m a n t b u d f o r m a t i o n d i ff e re d a m o n gt h e t h r e e t r e e s ( T a b l e 7 ) . I n Platanus d o r m a n t b u d s


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Table 5Comp aris on of mean heights (cm) for 85-day Plamnus within each pr ovena nce of illinois (111-1, -2), Kentu cky (Ken -l, -2), Austin,Texas (Aut- 1, -3), eastern Te xas (Eat- 1, -2), Linares, N uev o Leon, Mexico (Lira- 1), Monterre y, Nuev o Leon, Mexico (Morn- 1)*.Northern samples111-1 111-2 Ken - 1 Ke n -2a. 50.9 b. 15.0 a. 32.7 b. 8.6 a. 59.6 b.32.3 a. 43.8 ,b.c. 36.5 d. 1.8 c. 32.6 d. 2.9 c. 43.3 d.3.2 c. 2 9 . 8 / d .Texas samplesAut-1 Aut-3 Eat- 1 Eat-2a. 39.8 .b. 32.5 a. 43.0 ,b. 32.8 a. 50.5 b. 46,1 a. 51.8 ,b.c. 31 .5 /d . 4.5 c. 33. 5,/ d. 4.9 c. 35.2 d. 12.0 c. 31 .0 /d .Mexican samples

Lim-I Morn-1a, 42.2 .b. 25.4 a. 23A b, 22,5c. 19.0/d.-- 14.7 c. 16.5 d . 14.3

23.61.7

32.98.5

* Unde r four cond itions : a. 15 h 30-24 C; b. 12 h 30-24 C; c. 15 h 24-15 C; d. 12 h 24-15 C. Stude nt- Newm an-K uel s mul-tiple range test is indicated by bars. Num ber s connected by bar s are not significantly different (0.05 > P). Numb ers no t con-nected are significantly different (0.05 > P).

Table 6Comparison of mean heights (cm) for 85-day Taxodium within each pr ovena nce of Illinois (111); Kent ucky (Ken); Austin, Texa s(Aut-1, -2); Rio Gran de at Donn a, Texas (Dot), Monterrey, Nu evo Leon, Mexico (M om) Ixmiqu ilpan, Hidalgo, Mexico (Ixm),under four conditions as in table 5.*

Northern samplestll Ken

a. 44.6 b. 31.6 a. 42.8 b. 27.9c. 12.8 d. 1.2 e. 5.6 3-- d. 2.2

Texas samplesAut-1 Aut-1 Aut-3a. 50.3 b. 38.8 a. 48. 9-- b. 44.1 a. 44. 7-- b, 38.8c. 16.6 d. 4,4 c. 13.9 d. 1.1 c. 15.9 d, 3,3

Texas and Mexican samplesDot Mona Ixma. 38 .4 ~b. 37.5 a. 43.5- -b. 45.4 a. 45.9--b , 43,3c. 30.4 / d. 22.6 c. 31.4 d. 22,5 c. 29.0 d. 19,9

Ea ta. 54.8 b.--52.7c. 22.5 d. 5.8

Mexa. 42.3--b. 47.8c. 25.8--d. 21.2

* Refer to footn ote fo r table 5 concerni ng significance.Table 7Dormant bud formation under four controlled conditions*Seed origin** Platanus Taxodium Liquidambar

A B C D A B C D A B C DIllinois -- ED -- ED -- ED ED ED -- -- ED EDTexas . . . . . D -- D D D -- -- D DMexico . . . . . . . . . . . . . . . .* Plants were under four conditions: A. 15 h, 30-24 C; B. 12 h, 30-24 C; C. 15 h, 24-15 C; D. 12 h,24-15 C. for 85 days. The appearance of dor mant buds in 50 ~ or more o f the plants is indicated by D.The earliest appearance of dorm ant buds in the series is indicated by ED.** Illinois and Kent ucky plants responded similarly. Texas plants do n ot include the Taxodium popula-tion from the Rio Grande at Donna which responded similarly to Mexico plants.


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appeared under the 12-h day lengths in both tempera-tures. In Taxodium dormant buds appeared in threetreatments, but were not developed under 15-h 30-24 Cconditions. The pattern in Liquidambar included dor-mant buds unde r 24-15 C under both day lengths.These patterns suggested that the control of bud for-matio n is more stro ngly influenced by day length inPlatanus, by temperature in Liquidarnbar and by day-length and temperature in Taxodium. This pattern de-veloped in the 85-day test but it might have been alteredby a longer exposure to these conditions .

The appearance of dormant buds showed latitudinalcorrelation in each tree (Table 7). I f dorma nt bud s wereproduced, they appeared earlier in Illinois (and Ken-tucky) plants tha n in Texas plants. The plants of Mexicodid not produce dormant buds under the four photo-period-temperature conditions.

The patterns of bud dormancy for plants (Taxodium,Platanus) from central a nd eastern Texas were similarto each other, but those from the Rio Grande (Taxo-dium) were similar to plants of Mexico. Ad ditio nal seedlots for Mississippi and Louisiana (Taxodium) and forFlorida (Liquidambar, Platanus)also had bud d ormancypatterns similar to central and eastern Texas plants.Altho ugh there were differences in time of dor mant budformation among the populations, those from the north-ern areas and those from Mexico showed the extremes.

Dormant buds were formed by some of the Taxodiumand Platanus that were placed under 10-h day lengthsafter their initial test under t2 a nd 15-h. Durin g theadditional 4 weeks at 10-h conditions, all the Illinoisand Texas plants that had not already produced dor-mant buds did so. The Mexico plants did not producedormant buds during this additional 4-week test, butthe plants under the cooler temperatures showed onlyslight stem elongation.

The series of Taxodium and Platanus that were movedfrom the four photoperiod-temperature conditions toa s tandar d one of 15-h 30-24 C showed some resump-tion of growth among plants with dormant buds. TheMexico plants from all four treatments showed addi-tional height increases as did the Texas and Illinoisplants from conditions in which they had no t formeddormant buds. In Platanus, plants from Illinois, Texasand Mexico that had been conti nuou sly under 15-h30-24 C conditi ons continued to show height increases.

Taxodium, Platanusand Liquidambarplants that werekept out-of-doors in central Texas showed deciduouscharacteristics that were latit udinally correlated. D urin geach of three years, the Illinois plants were leafless be-fore the Texas and Mexico plants. The Mexico plants inall three types of trees mai ntain ed leaves until they weredamaged by low temperatures during the early part ofJanuary. In two of the three years the Mexico (and Dot,Rio Grande, Texas) Taxodium maintained leavesthroughout the winter.

Bud bursting occurred in out-of-doors conditionswith latitudi nal correlation. Mexico trees of Taxodium,Platanus and Liquidambar had bud activity during thelatter half of February each year and usually had fullyexpande d leaves prior to bud swelling in the U.S. trees.Texas trees showed bud activity from late February tomid-March and Illinois trees in mid-March. Taxodiumplants of Mexico an d Texas were more similar in timingof bud activity and Platanus and Liquidambarplants ofTexas and Illinois were more similar in this phenologi-cal event.

The production of stipules in Platanus showed lati-tudinal correlation (Table 8). The Illinois and Kentuckyplants had little or no stipule production under thecooler temperatures at either day length. The Mexicoplants produced stipules under each of the four con-

Table 8Stipule production in Platanusunder four photoperiod-temperature conditions*15 h Eat-1 Eat-2 Lira-1 Ken-1 Ken-1 Ill-1 Aut-2 Ill-2 Morn-1 Aut-130-24 C 1.04 1.00 0.80 0.80 0.76 0.74 0.58 0.48 0.46 0.3212 h Ken-2 Aut-1 Eat-I Ken-1 Eat-2 Aut-2 II1-1 Morn-1 Lira-1 I11-230-24 C I. 14 1.06 1.02 1.00 0.98 0.96 0.88 0.72 0.72 0.6215 h Li ra -1 M o r n - 1 Eat-1 Eat-2 Aut-1 Aut-2 Ken-2 Ken-I 111-1 111-224--I 5 C 0.74 0.50 0,42 0.34 0.32 0.28 0.06 0.00 0.00 0.0012 h Mom-I Lira-1 Eat-1 Eat-2 Aut-1 Aut-2 Ken-2 Ken-1 I11-1 111-224-15 C 0.98 0.58 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00* See footnote to table 3 for key to progenies and provenances and significances.


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T a b l e 9L e a f a r e a ( l en g t h x w i d t h ) i n Platanus u n d e r f o u r p h o t o p e r i o d - t e m p e r a t u r e c o n d i ti o n s *15 h Ken-1 Eat-2 111-1 Eat-1 Ken -2 111-2 Au t-2 Aut-1 Lira-1 Mo m-130 -24 C 290.8 271.6 252.6 239.3 235.3 178.5 173.4 154.1 145.1 116.412 h Eat-1 Ea t-2 Au t-2 A ut-1 Ke n-1 Lira-1 111-1 Morn-1 K en- 2 111-230 -2 4 C 188.5 186.1 174.4 162.7 137.6 131.9 129.3 124.1 107.5 78.01 5 h Ke n-1 111-I Eat-1 111-2 Ke n-2 Au t-2 Eat -2 A ut-1 Lirn-1 Morn-124 -1 5 C 241.2 216.4 201.1 197.2 187.0 176.3 170.5 166.8 113.9 109.512 h Mom -1 L im-1 E a t -2 E a t -1 Aut -1 Ken-1 I l i -2 Aut -2 Ken-2 I l l-124 -15 C 97.7 81.8 49.0 45.4 30.6 21.8 18.6 17.9 8.3 7.8* See t ab le 3 fo r key to p rogen ie s and p rovenances an d s ign if icance .

d i t io n s . T h e T e x a s p l a n t s h a d n o s t ip u l e s u n d e r t h e1 2 -h 2 4 - 1 5 C c o n d i t i o n s b u t p r o d u c e d t h e m u n d e re a c h o f th e o t h e r t h r e e c o n d i t i o n s . T h e s i ze o f t h e s t i p u -l es a m o n g t h e M e x i c o p l a n t s s h o w e d t h e l e a s t v a r i a t i o nu n d e r t h e d i ff e r en t c o m b i n a t i o n s o f te m p e r a t u r e a n dp h o t o p e r i o d .

T h e s i z e o f l e a f b l a d e s i n Platanus a l s o s h o w e d l a t i -t u d i n a l c o r r e l a t i o n ( T a b l e 9 ) . U n d e r 1 2 -h 2 4 - 1 5 Cc o n d i t i o n s , th e l e n g th x w i d t h v a lu e s f o r M e x i c ol e a v e s w e r e l a r g e r a n d s i g n i f ic a n t l y d i f f e r e n t f r o m t h eU . S . v a l u e s . U n d e r 1 5 - h d a y l e n g t h s a t e i t h e r o f t h et e m p e r a t u r e p r o g r a m s , t h e M e x i c o l e a v e s ( M o r n - l ,L i r a - l ) w e r e t h e s m a l l e s t b u t th e5 , w e r e n o t s i g n i f i c a n t l yd i ff e re n t f r o m T e x a s p l a n t s a n d s o m e o f t h e I l l i n o is a n dK e n t u c k y p l a n t s .

T h e d a t a f o r s t i p u l e p r o d u c t i o n a n d l e a f a r e a ( T a b l e s8 , 9 ) s u g g e s t t h a t t h e y a r e a f f e c t e d d i f f e r e n t ly b y t e m -p e r a t u r e a n d d a y l e n g th . T h e l o w e r t e m p e r a t u r e s i n h i b i t -e d t h e p r o d u c t i o n o f s t ip u l e s i n t h e n o r t h e r n p l a n t su n d e r b o t h d a y l e n g th s , b u t t h is t e m p e r a t u r e c o n d i t i o n sy i e l d e d l o w l e a f a r e a v a l u e s o n l y u n d e r t h e s h o r t e r d a yl e n g t h . T h e v a l u e s f o r s t i p u l e p r o d u c t i o n a n d l e a f a r e af o r t h e M e x i c o p l a n t s w e r e o n l y s l i g h t l y d i f f e re n t u n d e rt h e t w o d a y l e n g t h s a t 2 4 - 1 5 C .

D i s c u s s i o n

T h e h a b i t a t r e l a t i o n s o f d i v e rs e p o p u l a t i o n s i n Platanus,T ax od ium a n d Liquidambar p e r m i t e a c h o f t h e s e t r e e st o h a v e a b r o a d d i s t r i b u t i o n f r o m n o r t h e r n U n i t e dS t a t e s t o M e x i c o . H a b i t a t r e s p o n s e s i n v o l v i n g s e n s i t i -v i t y t o p h o t o p e r i o d a n d t e m p e r a t u r e g i v e s e le c ti v ea d v a n t a g e i n t h e n o r t h e r n e c o s y s t e m s . T h e s e l e c ti v e a d -

v a n t a g e i n M e x i c o i n c l u d e s l e ss s e n s i ti v i t y t o p h o t o -p e r i o d a n d t e m p e r a t u r e . T h e l o n g g r o w i n g s e a s o n a n db r i e f c o l d p e r i o d o f c l o u d f o r e s t h a b i t a ts i n n o r t h -e a s t e r n M e x i c o h a v e f a v o r e d p o p u l a t i o n s t o l e r a n t o f ab r o a d r a n g e o f e n v i r o n m e n t a l c o n d i ti o n s .

F e w s p e c i e s o f d e c i d u o u s t r e e s t h a t a r e b r o a d l y d i s -t r i b u t e d i n t h e U n i t e d S t a t e s a p p e a r s o u t h o f t h e R i oG r a n d e , a n d m o s t a r e c o n f i n ed t o t h e r e g i o n n o r t h o ft h e G u a d a l u p e R i v e r i n V i c t o r i a C o . , T e x a s ( F o w e l l s1 9 6 5 ) . S u c h w i d e s p r e a d t r e e s a s Quercus macrocarpaM i c h x . , Ulmus americana L. , Acer negundo L . a n dFrax inus pennsy lvanica M a r s h . e x t e n d f r o m s o u t h e r nC a n a d a t o n e a r th e G u a d a t u p e R i v er , b u t h a v e n o p o -p u l a t i o n s i n M e x i c o . A l t h o u g h e a c h o f t h e a b o v e f o u rs p e c i e s i s w i d e l y d i s t r i b u t e d a s a r e s u l t o f d i v e r s i t y o fh a b i t a t r e sp o n s e s a m o n g i t s c o m p o n e n t p o p u l a t i o n s( V a a r t a j a 1 9 5 9 , 1 9 6 1 , D e m o s e t a l . t 9 7 3 , M e u l i &S h i r l e y 19 37 , M c M i U a n u n p u b l . ) , t h e i r h a b i t a t w i d t h sd o n o t p r e s e n t l y i n c l u d e t h e b i o t o p e s o c c u p i e d b y t h eM e x i c o p o p u l a t i o n s o f T ax od ium, P la tanus and L i -quidambar.

T h e u s u a l t a x o n o m i c t r e a t m e n t o f th e M e x i c o p l a n t sa s d i s t i n c t f r o m t h e U . S . p o p u l a t i o n s i n d i c a t e s t h a tm o r p h o l o g i c a l d if f e r e n ti a t io n h a s a c c o m p a n i e d t h er e d u c t i o n i n s e n s i t i v it y t o e n v i r o n m e n t a l c u e s . A s p o i n t -e d o u t a b o v e , h o w e v e r , T ax od ium p o p u l a t i o n s i n c e n t ra lT e x a s p r o d u c e s e e d s t h a t a r e i n t e r m e d i a t e i n s i z e b e -t w e e n t h o s e o f I l l i n o is a n d t h o s e o f M e x i c o , s u g g e s t i n gt h a t m o r p h o l o g i c a l g r a d ie n t s m a y p a r a l l e l p h y s i o l o g i c a le x p r e s s i o n s o f e c o t y p i c d i f f e r e n t i a t i o n f r o m I l l i n o i s t oM e x i c o .

P o p u l a t i o n s w i t h i n e a c h o f t h e t h r e e t r e e sp e c i e sd i f f er i n f r o s t t o l e r a n c e . P r i e s t ( 1 9 7 1 ) e x a m i n e d e x p e r i -m e n t a l p l a n t s o f t h es e s p ec i es in t h e s a m e p h o t o p e r i o d -


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temperature comparison as in the present study (seeTables 3 and 4), after they had been kept out-of-doorsfor 2 years. In early February the populations fromIllinois (and Kentucky) had greater frost tolerance thanthe trees of Texas and Mexico. He also showed, usingthe triphenyl tetrazolium chloride test developed bySteponkus & Lanphear (1967), that the populations ofMexico had the least frost hardiness. Liquidambarpopulations that were examined by Williams & McMil-lan (1971b) using the amino acid release test (Yemm &Cocking 1955) also differed latitudinally in their fros ttolerance. Other physiological and morphological para-meters that were tested in Liquidambar also showed la-titudinal correlat ions (Williams 197ta, 1971b, Winstead1971, 1972).

The seedlings that were studied for 85 days in con-trolled conditions continue to demonstrate differencesin their habitat responses under central Texas condi-tions. The Mexico trees maintain leaves later in the fallthan do U.S. trees. They also burst buds earlier eachyear. These endogenously controlled behavioral respons-es can be shifted slightly by the sequence of environ-mental cues presented from year-to-year, but they showno progressive tendency to become like Texas plants intheir response patterns. Regularly the question is asked:"How long will it take for these plants from Mexicoand Illinois to become like Texas plants"? In otherwords, "How many years does it take a plant to becomeacclimatized to Texas?" While we cannot at presentanswer this question for the three types of trees underinvestigation, we can present data from Prosopis andcertain grasses that have been observed for 10 years.For example, grass clones in Andropogon scopariusMichx. transplanted from near the sites of Liquidarnbar,Taxodiurn and Platanuswest of Antiguo Morelos, Mexi-co (McMillan 1965) had pollen anthesis initially eachyear in November. The latest flowering clones fromTexas have finished shedding pollen in mid- to lateOctober. There has been no tendency for the Mexicoclones to become progressively earlier in their floweringunder the conditions of central Texas. In Prosopisplantings o f diverse localities, the time of becomingdeciduous and the time of bursting buds is modifiablefrom one year to another, but the tendency for trees ofmore southern origin to maintain leaves Iater in the falland to open buds earlier in the spring has been a con-tinuous phenological feature under central Texas con-ditions (McMillan & Peacock 1964). PIants from dif-ferent provenances continue to respond to environmen-

tal cues on the basis of endogenous, genetically deter-mined controls.

The present investigation suggests that populationsin diverse groups of deciduous trees have been selectedfor similar habitat responses in a given biotope. Al-though it would be tempting to suggest that each forestecosystem is composed of a unique series of populations,the present investigation has been directed towardthree sites that are widely separated from each other. Ifseed collections had been included for many interme-diate sites between Illinois and Texas, the combina-tions of responses might show ecotypic gradients, orecoclines; however, the gap in the distribution in thesouthern part of Texas implies occurrence of tworather distinct patterns in the southern portion of thedistribution.

Although studies of forest ecosystems have notfocused upon the ecotypic status of component popu-lations, studies in grasslands have demonstrated geneticdifferentiation when species are studied along climaticgradients within "the same" major community-type(McMillan 1959, 1965). While the implications of thegrassland studies are patent (Kormondy 1969) , thedemonstration in forest populations is conducted withgreater difficulty. Mature grass plants can be easilytransplanted and can be observed for numerous phe-nological and morphological attributes. The trees areusually studied from seedling populations and are sub-ject to pre-conditioning interpretations such as presentedby Rowe (1964). Despite the difficulties of tree studies,the present investigation supports the idea that a localforest ecosystem is composed of a unique combinationof genetically distinctive populations. The results thusparallel those in grasslands in revealing the geneticdifferentiation that underlies species habitat responsesand ranges, and therefore underlies also communityrelations to environmental gradients. Selection producesboth adaptive divergence in behavior of a given speciesin different biotopes, and adaptive convergence in someof the behavior of different species in the same biotope.

SummaryHabitat specialization in populations of three broadlydistributed trees includes adaptive differentiation to daylength and temperature. Low sensitivity to environmen-tal cues is the adaptive strategy of the southernmostpopulations (from northeastern Mexico). Early cessa-


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taxodium ecology 1974

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