ATOLL RESEARCH BULLETIN h ~ s t 15, 1955

ATOLL RESEARCH _BULLETIN

No.

43

The n a t u r a l vegetation of Canton Island, an e q u a t o r i a l P a c i f i c a t o l l

by

William H. Hatheway

Issued by

THE PACIFIC SCIENCE

BOARD

National Academy of Sciences--National Research Council Washington, D. C.

h ~ s t 15,

1955

The n a t u r a l vegetation of Canton Island,

. . a!i e q u a t o r i a l P a c i f i c a t o l I . by

W i l l i a m H. Hatheway

g

Canton 1sland ( k t . 20 ~ O I S . , Long. 1710 3 8 ' ~ ) , a British-American condominium s i t u a t e d i n t h e Pltoenix Group, has gained i n p o l i t i c a l and^

economic importance since 1939, l a r g e l y because Pan-American Airways i n ' t h a t year established a permanent r e f u e l i n g s t a t i o n t h e r e , building a h o t e l on t h e south s i d e of t h e i s l a n d . I n 1946 t h e United S t a t e s C i v i l Aeronautics Administration s e t u? a base on Canton. The i s l a n d was occupied by United S t a t e s rnilitary f o r c e s during t h e war gears 1942-1945, when

extensive construction and grading work was done. Before 1939 t h e i s l a n d was uninhabited, being.visi.ted only infrequently by occasional guano diggers (Bryan 1942).

-

A t t h e request of N r . Garrison Costar, of t h e C i v i l A e r o n a u t i c s Administration, a f l o r i s t i c . and v e g e t a t i o n a l survey of Canton Island was

conducted by D r . Gtto Degensr and t h e author from July 1 1 t o J u l y 19% 1950.

The i s l a n d was r e v i s i t e d by t h e author i n February 19.51 and by Dr. Degener i n April and May 1951. As a r e s u l t of t h e s e surveys, recormendations were made leading t o t h e introduction of rnany species of ornanental and other- wise u s e f u l plants. This paper i s a report on t h e s t r u c t u r e , d i s t r i b u t i o n : and ecology of t h e n a t u r a l vegetation of Canton Island.

PHYSICAL Ei\NIROiiIGNT

Canton Island i s a c o r a l a t o l l approximately 21 miles i n circumfe-.

rence (Fig. 1 ) . 'The land surface i s a t no place wider than 3200 f e e t , and maximum e l e v a t i o n i s aSout 20 f e e t abov$ mean sea l e v e l . Local r e l i e f i s s l i g h t , but a cpnsiderable proportion of t h e land surface i s of. s u f f i c i e n t - l y low e l e v a t i o i l t o be s u b j e c t a t high t i d e t o t h e influence o f ' a brackish water t a b l e . Such swales a r e occupied by a c h a r a c t e r i s t i c vegetation, a s i s discussed beloy,Along t h e ocean s i d e of t h e northern portion of t h e i s l a n d high dunes have formed, and t h e scanty vegetation of Triurrfetta

~rocumbens which occurs t h e r e has a d i s t i n c t l y x e r i c aspect.

The s o i l of Canton Island. i s derived from fragments of coraL rock and calcareous sand deposited by waves and wind on a reef f l a t . - I t s t e x t u r e i s mostly r a t h e r coarse. I n a l l places investigated a ' d a r k brown l a y e r , from

7

t o 1 5 inches t h i c k , , underlies t h e upper inch of sand. o r c o r a l rock, or occurs a t t h e ,surface of t h e ground. Thi:s dark l a y e r , which contains a l a r g e proportion of guano, i s believed t o be of considerable s i g n i f i c a n c e i n t h e development of t h e present vegetation of Canton, Island.

. ^. .

.

During most of t h e year Canton Island i s hot and humid, although ,:

a c t u a l p r e c i p i t a t i o n i s scanty. Xean annual r a i n f a l l i s about 1 9 inches,

The ilockefeller Foundation, Xedellin, Colombia.

only t h e months i k r c h through August receiving over one inch of precipi- t a t i o n , on t h e average. Indeed, t h e months September through February

c o n s t i t u t e a w e l l defined dry season, which a t t a i n s i t s g r e a t e s t i n t e n s i t y , on t h e average, i n November. Zxtremely heavy p r e c i p i t a t i o n has been

reported i n two years. In 19110, 70.60 inches of r a i n f a l l were recorded, while i n t h e f i r s t e i g h t months of 1941, 9 . 8 0 inches were reported. The

records a r e t o he i n t w p r e t e d with some caution, f o r t h e r e a r e i n d i c a t i o n s t h a t i n a t l e a s t one case d a t a was i n c o r r e c t l y renorted. It i s undoubtedly t r u e , however, t h a t exceptionally heavy p r e c i p i t a t i o n was received i n those years, s i n c e Van Zwaluwenhurg ( 1 9 ~ ~ 3 ) has shown by photographs t h e u n ~ u s u d l y luxuriant vegetation t h a t dcveloned i n 1941, and s t a t e d t h a t pools of standing water were of s u f f i c i e n t duration t o allow i n s e c t s with aquatic l a r v a e t o complete t h e i r life-cycles. Additional evidence f o r t h e great v a r i a b i l i t y i n annual r a t n f a l l on Canton can be derived from r e l i a b l e records of only 8.46 inches t o t a l p r e c i p i t a t i o n i n 1947 and a s much a s

33.17

inches i n 1948. It should a l s o be emphasized t h a t exceedingly heavy downpours o f t e n a r e followed by weeks of drcught: t h u s t h e t o t a l precipi- t a t i o n of even a wet month may be received i n two o r t h r e e storms.

Very l i t t l e v a r i a t i o n i n temperature occurs throughout t h e year.

Mean annual temperature i s about 840F. The marimom recorded temperature f o r t h e t h r e e year period 1947-1949 was 98O~., t h e minimum 71'~. ixelative hwnidity i s constantly r a t h e r high, varying from a recorded minimum of 57 per cent t o a maximum of 85 per cent. Average r e l a t i v e humidity a t 1 p.m. i s about 62 per cent. A very constant strong e a s t e r l y wind i s an outstanding f e a t u r e of t h e Canton climate, and may a f f e c t t h e developnent of t h e n a t u r a l vegetation. Planted ornamental t r e e s and shrubs a r e cormonly severely wind-clinped. S a l t s p m y i s undoubtedly a f a c t o r contributing t o t h e s t u n t i n g of t h e vegetation i n some l c c a t i o n s on Canton.

FLORA

The n a t i v e f l o r a of Canton Island i s not r i c h , consisting of

14

species of vascular p l a n t s i n a s many genera. I n addition, approximately 150 species have been introduced a s ornamentals or adventive weeds. With only a few exceptions, t h e n a t i v e f l o r a i s composed of wide-ranging

Indo-Pacific strand p l a n t s , of which Scaevola frutescens m y be c i t e d a s a t y p i c a l example. The f r u i t s o r seeds of rnost of these s p e c i e s e x h i b i t s t r i k i n g adaptations f o r f l o a t i n g .

The n a t i v e vascular f l o r a i s l a r g e l y confined t o those p a r t s of Canton Island which have not been subject t o grading o r other c u l t u r a l operations. A weedy f l o r a best characterized by t h e troublesome but short-lived sandbur, Cenchrus ecninatus, dominates t h e semi-natural

vegetation i n t h e v i c i n i t y of t h e Pan-American Hotel, t h e Airlines Terminal, and t h e CAA Housing Areas during t h e wetter months of t h e year. It is a s t r i k i n g f a c t t h a t t h i s introduced flora, has had very l i t t l e tendency t o invade t h e vegetation of those p a r t s of t h e i s l a n d which have not been subject t o much human disturbance. Since t h i s paper i s concerned only with t h i s l l n a t u r a l n vegetation, no f u r t h e r reference t o t h e weed f l o r a w i l l be made.

The n a t u r a l vsgetation of Canton I s l a n d can be conveniently separated i ~ l t o t h r e e major types: conunbnitj.es dominated by t r e e s , shrubs or herbs.

A l l a r e r e l a t i v e l y s5.mple b composition and can e a s i l y be delineated on

a

map @ig. 1 ) . The r e s u l t s of planimeter measurements of t h e a r e a s domi- nated by t h e p l a n t .communities recognized on Canton Island a r e presented i n Table I.

TABLE I

Approximate a r e a s included i n t h e p l a n t comnunities of Canton Island

' Veget,ation Ty?e :Per cont of :Fer cent of land :Approximate : t o t a l land a r e a t a r e a covered by :area i n a c r e s

-

: n a t i v e vege4;ation :

Cordia f o r e s t : 0.29 0.81 8.3

Messerschmidia f o r e s t : 1.05 2,92 70.0"

Scaevola scrub : 6.17 ^.

.

^{, 17.11 : 176.0}

Suriana-Sesuvium f l a t s : 88.58

.

23.80 : 2 U . 9

Portulaca herbaceous : 19.96 .55.37 : 569.6

cornunity ^{. .} :

Areas n a t u r a l l y bare : 23.37

---

^{: 667.0}

Disturbed a r e a s ^: 40.60 ^. :

---

^{: 1,15E,7}

T o t a l ^, . : 100.02 100.01 : 2,854.5

The Cordia Forest--"ell developed stands of -$ subcordata e x i s t today only on t h e south s i d e of t h e i s l a n d . l c s s than one mile from Nusick Light (Fig. 1 ) . There two groves of t r e e s f l o u r i s h , covering about 3500 and 2700 square f e e t respectively. Other m a l l groves of Cordia occur s c a t t e r e d around t h e i s l a n d , but i n those t h e t r e e s a r e nlostly l e a f l e s s and i n some cases a c t u a l l y dead.

The two groves of vigorous t r e e s 'consist of pure stands of Cordia averaging-about

19

f e e t i n haight; t h e t a l l e s t t r e e observed was 24 f e e t t a l l . Both f,orests a r e i n r e a l i t 7 n e a r l y impenetrable t h i c k e t s . Since

Cordia on banton .&land reproducSs'~hief1;r by ground layering, t h e numerous shoots ' a r i s i n g from,decumbent branches make walking through t h e grove i n a s t r a i g h t . l i n e v i r t u a l l y impossible.' Ch t h e b a s i s of a sample plot: o f : approximately 400 square f e e t , i n which 2 3 upright stems between 2 - a n d 9 inches i n diameter were .counted, bas@' a r e a was approximately 272 square f e e t and d e n s i t y about 2450 stems per,ac:re. Average height t o t h e f i r s t branch was only 29 inches, g r e a t e s t length. of c l e a r bole being 75 inches.

Light reaching t h e ground l e v e l i n Cordia ' f o r e s t i s r a t h a r d i f f u s e ;

i n t e n s i t i e s a s measured with a Weston photographic l i g h t meter i n s i d e t h e grove were only 5 t o 10 per cent cf those obtained outside. The ground i s l i t t e r e d with dead branches, f r u i t s , and a few leaves. Reproduction from seed within t h e grove i s p r a c t i c a l l y non-existent, but many

Cordia

seedlings which had recentlygerminated were observed near t h e periphery of t h e groves, most of them:severely wilted. A small p i t dug i n t h e s o i l

i n s i d e t h e l a r g e r grove revealed a very dark sandy loam extending from t h e ground snrfaca t o a depth of a b o ~ ~ t seven inches. Underlying t h e dark horizon was a l a y e r of coarse sand which graded i n t o fragments of c o r a l rock and sand a.t a depth of 10 t o 21 inches. The dark s u r f a c e l a y e r i s interpreted a s being composed c h i s f l y of a mixture of guano and sand with addition of humus derived from r o t t i n g 1e.aves and wood.

The Cordia groves a r e during t h e heat of t h e day a s h e l t e r f o r land h e r r i t c r a m e n o b i t a p e r l a t u s )

.

Large numbers of crabs may be found under f a l l e n logs znd branches and even climbing i n t h e t r e e s ; two hermit crabs were discove1:ed perched i n a t r e e 69 inches above t h e ground. The crabs seem t o do l i . t t l e damage t o t h e t r e e s . I n some cases, however, young shoots have been s t r i p p e d 3f bark and t h i s mischief possibly m y be

a t t r i b u t e d t o t h e crabs.

A much more s e r i o u s enemy of i s t h e moth Achaea .ianata (L.1 C a t e r p i l l a r s were abundant i n both groves i n J u l y 1950, and it was

observed t h a t aany shoots had been completely d e f o l i a t e d by t h e s e i n s e c t s ; n e a r l y every l e a f showedsome damage. Abcut 8 months l a t e r t h e author

observed t h a t t h e smaller of t h e two groves had 5een completely defoliated.

Because of t h e depredations of t h i s i n s e c t , it agpears unlikely t h a t Cordia subcordata w i l l extend i t s range on Canton Island i n t h e absence of control measures. This i s unfortunate, s i n c e t h i s t r e e i s perhaps t h e most d e s i r a b l e spccies now knmn f o r purposes of a f f o r e s t a t i o n . It i s r e l a t i v e l y l a r g e , has at'cractive flowers and f o l i a g e , and appears t o be well adapted t o t h e physical environment of Canton Island.

The Nesserschmidia Forest--3nly one healthy, well developed stand of 'lesserschmidia argcr,tea e x i s t s on Canton I s l a n d , This grove i s s i t u a t e d . .

a few hundred yards west of t h e p r i n c i p a l a i r s t r i p , on t h e north s i d e of t h e island. Another grove consisting of dead o r dying t r e e s was found southeast of t h e " f i g h t e r a i r s t r i p , " and .many i s o l a t e d t r e e s grow scat- t e r e d about t h e i s l a n d .

The healthy grove was studied i n some d e t a i l . It was 158 f e e t long and from 35 t o 5 1 f e e t wide; t o t a l a r e a was about 7000 square f e e t , o r 0.6 a c r e . The average height of t h e t r e e s was about 15 f e e t , t h e l a r g e s t t r e e i n t h e grove being 19 f e e t t a l l . Like t h e Cordia f o r e s t , t h e

Messerschmidia grove was i n most places a t a n g l e through which one proceed- ed only with d i f f i c u l t y . The t r e e s branched a t an average of 1 8 inches above t h e ground, and reproduction within t h e grove was apparently e n t i r e l y by root suckers. A sample p l o t of 675 square f e e t was s e t out i n s i d e t h e grove, v i t h i n which 1 8 l i v i n g upright stems between two and e i g h t inches i n diameter were counted. The calculated d e n s i t y was there- f o r e about 1160 stems and t h e b a s a l a r e a approximately 132 square f e e t per acre. Thus both i n d e n s i t y of stems and b a s a l a r e a t h e f i g u r e s f o r l~!esserschmidia f o r e s t were about one half those obtained i n Cordia f o r e s t . 'The l i g h t i n t e n s i t y a t ground l e v e l was correspondingly g r e a t e r than i n Cordia f o r e s t , being about 25 per cent o f t h a t outside t h e grove.

Possibly because of t h e r e l a t i v e l y high l i g h t i n t e n s i t y , a s c a t t e r i n g of p l a n t s of Portulaca l;ltea, Boerhavia t e t r a n d r a , S e t a r i a v e r t i c i l l a t a and Lepturus

-,

a l l shade-intolerant plants, were established i n t h e understory of t n e i~iesserschmldia grove. S o i l p r o f i l e s t u d i e s revealed a

p a t t e r n similai. t o %!>at i n t h e ^-. Cwdia

-

^{grove. A} dark sandy loam, presunably derived c h i e f l y from c o r a l sand, guano and l e a f and branch l i t t e r overlay a light-colored sand, which contained fragments of c o r a l rcck.

. .

. .

Like t h e h S & g : ' o v e , t h e M e s s e r s c h i d i a f o r e s t i s a f a v o r i t e s h e l t e r durinr: t h e warm daylight hours f o r l a r g e numbers of hermit crabs.

NO damage a t t r i b u t a 5 l e t o t h e crabs or t o i n s e c t s may he observed i n t h e grove. It i s believed t h a t t h e crabs. forage c h i e f l y by night, but feeding h a b i t s a r e imperfectly known. Cine crab was observed t o devour a flower of Portulaca K e & , however, and it i s suspected t h a t damage done t o pianted New Zealand sp'inach (Tetragonia t e t r a p o n i o i d e s ) may be a t t r i b u t e d t o t h e crabs.

The Scacvola Ccrub--Cn Canto;? 1slC&d S c a e v o k frutescens becomes a shrub

.---

t e n f e e t t z l l ar.4 i n sone places nearly 50 f e e t i n diameter. Reproduction proceeds r a d i a l l y from t h e oldest arid t a l l e s t pla,n.l; in t h e center of a

c l u ~ p , s o t h a t whaCI may appear t o be a s i n g l e enormous bush may a c t u a l l y consist of %any i n d i v i d u a l s belonging t o s e v e r a l generations. Scaevola occurred i n extensive, n e a r l y pure stands a t t h e southeast corner of Canton Island; other species occurring occasionally i n t h e Scaevola scrub

a r e Kesserschrnidia argentea, ^lacs l u t e a . f a l l a x , B0erha.vi.a t e t r a n d r a and

--

--

^. _{. .}

I n &d& t o determine t h e appro:;imte densi-ty of t h e ScaevoLa scrub, a . sample. p l o t of 625 square f e e t was l a i d out i n a randomly s e l e c t e d a r e a within t h e , vegetation type. 1 2 Scaevola bushes were counted, t h e t a l l . e s t e i g h t f e e t high. This i s a . d e n s i t y of one bush per 52 square' f e e t . Along a 1000-foot l i n e t r a n s e c t , 115 hushes were counted, or one bush per 6.7 f e e t . The surface hcr5.zon of t h e s o i l i s a stony sandy loam,

containing considerable ainoufits o f guano. . . .. . .

. .

An i n t e r e s t i n g f e a t u r e of t h i s t r a c t of Scaevola sc&b i s t h e enormous number of s e a b i r d s roosting and nesting i n t h e shrubs. F r i g a t e b i r d s (Freaata. paherstoni.) and red-footed boobies (Sula s u l a

rcb-)predominate. I n J u l y many adults; young and eggs of both species

-

were observed, but by February t h e boobies ,had ' l a r g e l y disappeared. To estimate t h e number of b i r d s roosting i n a s i n g l e b u s h , c o u n t s were !mde on t h e night of J u l y 1 6 i n t h e Scaevola scrub i n t h e southeast corner of t h e island. Five bushes, each abvdt LO f e e t t a l l and ranging i n a r e a from 300 t o 2000 square f e e t each w e r e s t u d i e d . 115 many asL!.+.5..birds were count- ed i n a s i n g l e bush; t h e average pop,ulation"for"the f i v e bushes studied was 34 b i r d s per bush, t h a t i s rougillybne b i r d per 30 square f e e t of bush.

It a p p e a r s n o t u n l i k e l y t h a t t h e p r i n c i p a l f a c t o r l i m i t i n g t h e s i s e of th&,Canton I d a n d b i r d population i s t h e absence of more such roosting and nesting s i t e s . Indeed, a t tiinus t h e bushes a r e s o crowded t h a t f r i g a t e b i r d s a r e forckd t o r o o s t on t h e jida f a l l a x p l a n t s : these a r e spindly dwarf shrubs only about 10 t o 12 inches high, and form a v e v i n s e c u r e perch f o r a l a r g e b i r d . It i s t h e opinion of t h e w t h o r t h a t t h e l m g e populations of roosting sea b i r d s , which tend t o congregate i n r e l a t i v e l y small areas, a r e of g r e a t s i g n i f i c a n c e i n t h e development of t h e present n a t u r a l vegetation of Cant or, I s l a n d .

The Svriana-Sesuvim Flats--8ordering extensive areas of t i d a l mudflats

---- ---

or covering small pockets of land infiuenced by t h e d i u r n a l vari.ation i n t h e depth of t h e ground-water t a b l e i s a plant cornunity dominated by S e s u v i u ~ portu1acastrv.n and Suriana maritimna. The former i s a f l e s h y

--- --

decumbent herb, which occurs i n s t r i k i n a bri'trht trreen mats about 20 t o 50 square f e e . t i n s i z e . It u s u a l l y o c c u i i e s t h e c e n t e r s of swales, o r occurs near t h e high-tide l e v e l of mudflats of t h e sheltered lagoon s i d e of t h e island. Suriaiia i s a woody shrub, comnonly about four - f e e t t a l l , with f l e s h y yellow-green leaves. It c h a r a c t e r i s t i c a l l y borders t h e Sesuvim mats on t h e g e n t l y sloping edges of mudflats and swales. Large numbers of f r i g a t e b i r d s ape commonly observed i n bushes of Suriana ma.ritim.

The Portulaca IIerk,ceous Community--Portulaca

&tea,

t o g e t h e r with Lepturus

-- ----

r w e n s Boei.ha~<z ketrandra and S;.da f a l l a x dominates by f a r t h e l a r g e s t

--* - --

a r e a of n a t u r a l vegetation on Canton Island. Por.i;ulaca

lutes

on Canton averages

6.5

inches i n height and about 1 2 i n c h e ~ i n l a t e r a l spread. It has a very f l e s h y taproot and main stem, up t o one inch i n diameter, and many branches r a d i a t i n g f r o m t h e stem, each about 0.3 inches i n diameter.

Lepturus repenv i s a caespitose g r a s s up t o 19 inches i n height (but averaging about 11.5 inches). It propagztes i t s e l f a t t h e periphery of

clumps by ground-layering. The mats t h u s formed a r e occasionally q u i t e extensive, t h e l a r g e s t measured being 17.6 . : f e e t long and 9.3 f e e t wide, but covering only about 100 square f e e t . ~ e n ' e r a l l ~ , however, t h e clumps cover only about 1.25 square f e e t , t h e average diameter of t h e clumps being about 15 inches. Boerhavia t e t r a n d r a i s a t r a i l i n g v i n e w i t h a l a r g e f l e s h y taproot up t o 1.5 inches i n d i a z e t e r .

Sida

f a l l a x , ?,;,dwarf shrub seldom over 1 2 inches t a l l on Canton Island, i s a very co:nmon member of t h e community.

The Portulaca herbaceous cormmnity was studiede q u a n t i t a t i v e l y a t two s t a t i o n s about nine miles a p a r t . A s i s suggested by t h e d a t a i n Tables 11 and 111, Portulaca i s a c o n s i s t e n t dominant i n t h e commnity, while t h e frequency and cover degree of i t s a s s o c i a t e s a r e somewhat v a r i a b l e . I n general it appears t h a t t h e abundance of

Sida

and B o e r h a v i a i s corre- l a t e d with a somewhat stony s o i l phase, while Lepturus i s more. prominent i n a r e a s of even-textured sand.

TABLE I1

Frequency and Foliage Cover i n t h e Portulaca Herbaceous Community. Data from f i v e meter-square p l o t s . S t a t i o n I, about 10 miles by road from

"Turning Basin.

"

Species Frequency Foliage Cover (Per cent)

Portulaca l u t e a Lepturus repens Sida f a l l a x

Boerhavia t e t r a n d r a

Total 52

TABLE IIL

Frequency and Foliage Cover i n t h e Portulaca Herbaceous Community. Data from 10 square-meter p l o t s . S t a t i o n 11, about 19.5 miles by road from ttTurning Basin. ^1'

Species Freauency F o l - i a ~ e Cover (Per cent)

Portulaca l u t e a Sida f a l l a x

Boerhavia t e t r a n d r a

T o t a l 55

Beaches, Dunes and other n a t u r a l l y Bare Areas--About 23 per cent of t h e land surface of Canton Island has l i t t l e o r no n a t u r a l vegetation cover.

Such a r e a s include beaches and o t h e r places subject t o ocean wave action, and high dune. Triumfetta procumbens, a creeping perennial v i n e occurs occasionally on dunes on t h e north s i d e of t h e i s l a n d 'out i t s cover degree i s very low. Ipomoea pes-caprae, which often i s predominant i n such

s i t u a t i o n s i n t h e P a c i f i c i s very r a r e on Canton Island. Only a s i n g l e p l a n t of t h i s species was found i n 1951, growing among rocks near f o r t i - f i c a t i n n s erected i n t h e war. It may have been planted by a s o l d i e r . Lepturus repens, reported by Christophersen (1937) t o be c h a r a c t e r i s t i c of beach c r e s t s and s h i f t i n g dunes on other P a c i f i c e q u a t o r i a l i s l a n d s , was not found i n these s i t e s on Canton.

The most s t r i k i n g f e a t u r e s of t h e n a t u r a l vegetation of Canton Island a r e t h e a r e a l predominance of t h e Portulaca herbaceous community and t h e paucity of f o r e s t . This i s perplexing i n view of t h e f a c t t h a t two n a t i v e t r e e s , Cordia s1dbcordata and Messerschnidia argentea, a r e known i n c e r t a i n instances a t l e a s t , t o grow vigorously and t o reproduce from seed. The f o r e s t communities t h u s formed tend t o exclude t h e shade- i n t o l e r a n t herbaceous species of t h e Portulaca community. Xoreover, t h e l a r g e shrub, Scaevola frutescezs, i s apparently invading t h e Portulaca herbaceous community about 9.5 miles by road f r o m t h e "Turnina Basin,"

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on t h e north s i d e of t h e i s l a n d and i n o t h e r places. A r e l a t i - v e l y open stand of Scaevola might form a nurse crop favorable f o r t h e

establishment of occasional seedlings of Cordia and Xesserschmidia.

Seedlings of Cordia soon w i l t a f t e r ferminating i n t h e open herbaceous communities. It i s q u i t e possible t h a t e x i s t i n g groves of Cordia and Messerschmidia may be traced t o t h e successful establishment of s i n g l e

seedlings which by vegetative propagation have come t o occupy t h e i r present areas. Thus it would appear t h a t i n t h e absence of d i s t u r b i n g f a c t o r s t h e development of t h e n a t u r a l vegetation of Canton I s l a n d might t r e n d toward g r e a t e r a r e a l extent of f o r e s t and scrub, a t t h e expense of t h e hearbaceous communities.

It i s evident, however, t h a t such development i s i n process i n only a few l o c a l i t i e s . Indeed, i n inost places t h e trend 'appears t o be taking a d i f f e r e n t d i r e c t i o n . Patches of n a t i v e f o r e s t and scrub which a r e

s i t u a t e d more thau one mile f r o n humn h a b i t a t i o n a r e generally dead o r dying. Extensive areas of

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i4esserschridi. andCordia f o r e s t about 10 miles by road from t h e "Turning &sin" a r e l e a f l e s s and armarentlv dead. Counts _A

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of Scaevola bushes 16.5

Lies

from t h e l ~ T u r n ~ r g Basin" revealed i n a

loco-

foot s t r i p 35 vigorous p l a n t s , 40 dying p l a n t s and 40 dead ones. I n a 625 square-foot p l o t , seven dead and f i v e l i v i n g bushes were counted.

Since hsalthy groves of Cordia and Xesserschmidia e x i s t on Canton Island possible c l i m t i c d e t e r i o r a t i o n i s not s u f f i c i e n t t o explain t h e dying condition of most stands. Noevidence has been found t h a t t h e

healthy s t a n d s owe t h e i r vigor t o any possible c u l t u r a l operations c a r r i e d on within them. Instead, t h e r e i s much evidence t h a t a causal connection e x i s t s between t h e dead or dying condition of n a t i v e f o r e s t and scrub i n areas not; affected by human a c t i v i t y and t h e enorinous populations of f i s h -

e a t i n g seabirds xhich roost and n e s t i n t h e . t r e e s and shrubs. 'The argiments favoring -such an hypothesis a r e l i s t e d b r i e f l y b e l o w : ^{. .}

. . .

1. Patches of nai;ive f o r e s t and .scrub heavily populated with f r i g a t e b i r d s and boobies invariably show high proportions .of. dying or dead t r e e s and shrubs

.

2. Patches of n a t i v e forest and i n d i v i d u a l volunteer shrubs of Scievola ^: near-hunan habitation and not v i s i t e d by f r i g a t e birds and boobies a r e vigorous except when damaged by i n s e c t s . Only

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Cordia exhibited,noteworthy.

i n s e c t i n j u r y . . . - .'! .

3. S o i l p r o f i l e s t u d i e s i n t h e Portulaca herbaceous comnunity showbeneath t h e surface inch of bleach sand a dark brown sandy loam t o a depth of

1 5 inches. This dark l a y e r i s i n d i s t i n g u i s h a b l e i n , t h e f i e l d f r o m t h e guano ' s o i l a t present develop,ing under dying M e s s e r s c : - m u , Cordia. and Scaevola. The f r i g a t e b i r d s and red-footed boobies . a t present v i s i t only ^, . . t h e f o r e s t and scrub, and other, . s i n a l l e r , b i r d s were observgd only occa- . .

s i o n a l l y i n t h e Portulaca community. It ts::suggested, t h e r e f ore,.. t h a t . .

those a r e a s i n t h e Portulaca community e x h i b i t i n g guano s o i l p r o f i l e s , ^. . were once covered with f o r e s t o r scrub supporting l a r g e populations of roosting +nd:nesting seabirds, a t which time t h e guano was formed..

. .

. The mechanism by which b i r d s might. damage or destroy t h e n a t i v e f o r e s t and scrub i s not c l e a r , although it appears l i k e l y t h a t highly concentrated b i r d droppings might b& as.damaging t o t h e t r e e s and shrubs a s would be excessive doses of n i t r a t e ' and phosphate f e r t i l i z e r . Stewart

(1933) reported t h a t r o o s t i n g blackbirds. s e v e r e l y injured a plantation' of White Pine

(w

^{strobus L.,} and demonstrated t h a t t h e concentration of s o l u b l e n i t r a t e s i n t h e a r e a of dying t r e e s was enormously g r e a t e r j:

than i n t h e unaffected area. The present author has conducted no experi- ments on t h e nature and p r o p e r t i e s of Canton Island guano s o i l s . It i s , t h e experience of CAA personnel s t a t i o n e d on Canton, howeverj , t h a t .

ornamental plants potted i n guano s o i l obtained from Canton Island f o r e s t s grow much l e s s vigorously than those potted i n c o r a l sand from t h e beach;

t h e s e observations have been confirmed by t h e w r i t e r .

1. Canton Island i s a c o r a l a t o l l sj.tuated a t ht

.

^{2O S,} Long. 171° ltJJ i n t h e Phoenix Group, P a c i f i c Ocean. The t o t a l land a r e a of t h e i s l a n d i s about 2,850 acres.

2. The climate i s warm and humid, but r a i n f a l l i s scanty and seasonal.

3, The n a t i v e f l o r a i s scanty c o n s i s t i n g c h i e l f y of a few wide-ranging Indo-Pacific strand p l a n t s .

4+

Natural vegetation c o n s i s t s of f o r e s t s of Cordia subcordata and

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t h e most extensive type of n a t u r a l vegetatior, or. Canton k l a n d .

5. 14ost n a t i v e f o r e s t and scrub i s i n a dead o r dying condition. It i s suggested t h a t t h e agents c h i e f l y responsible f o r t h e death of n a t i v e woody species a r e f r i g a t e birds, red-footed boobies and other fish-eating seabirds, which roost i n enormom numbers i n t h e f o r e s t and scrub.

Observations a r e presented suggesting t h a t c e r t a i n a r e a s p r e s e n t l y covered by Portulaca and i t s a s s o c i a t e s f o r n e r l y may have been dominated by f o r e s t o r scrub. It i s believed t h a t i n t h e absence of l a r g e populations of b i r d s , n a t i v e f o r e s t and scrub would tend t o extend t h e i r present a r e a s a t t h e expense of herbaceous cownunitii's.

Bryan, E. H., J r .

Americar. Polynesia and t h e Hawaiian Chain.

1-253, Honolulu, 1942.

Christophersen, E.

Vegetation of P a c i f i c Equatorial I s l a n d s .

Bishop Musem B u l l e t i n

44:

1-79, pl. I - V I I , 1927.

Stewart, G. R.

Forest p l a n t a t i o n s injured by r o o s t i n g birds.

Jour. Forestry 31: 423-423, 1933.

Van Zwalu~ienburg, R. H.

The i n s e c t s of Canton Island.

Proc. Hawaiian Ent. Soc. l l ( 3 ) : 300-312, 1943.

Valuable manuscript c r i t i c i s m s have been received from 0. Degener,

F.

E. Egler, and F. R. Fosberg. The author i s indebted t o 'Ir. Edwin

Gillaspy, S t a t i o n Manager, C i v i l Aeronautics Administration, Canton Island, and Mr. William Cowdon, Pan-American Airways, Canton Island, f o r many

courtesies extended during h i s v i s i t s t o Canton Island.