CHAPTER VTHE FIJIAN STRAND-FLORA
The inland extension of the beach plants.—The grouping of the coast plants.—Their modes of dispersal.—The zone of change.—Summary.
Havinglearned from the British flora the real significance of the buoyant seed or fruit in a littoral flora, we will now return to the Pacific and proceed to deal with the composition and general character of the strand-plants.
Speaking of the Malayan strand-plants, Professor Schimper remarks (pp. 11, 12) that both in outward appearance and in anatomical structure they are xerophilous in character, whether in the case of those of the mangrove-swamp or in those of the beach. Since the tropical shore-flora of the Pacific islands is essentially Malayan, the identity usually extending to the species, the same conclusion may be applied to its character. The xerophilous habit may show itself externally in a variety of ways, as in hairiness, leaf-structure, a leathery cuticle, succulency, &c.
From this xerophilous habit of the Pacific strand-flora we should expect to find that many of the plants stray far from the coast, wherever the suitable conditions for their type of organisation occur, whether in the inland plain or on the mountain-top. This is indeed the case; but in dealing with this subject it will be necessary to discuss in some general detail the littoral floras of the Fijian, Hawaiian, and Tahitian groups in succession.
Viewed from the old standpoint of “station,” where one would distinguish sharply between the coast and the inland plants, the Fijian strand-flora exhibits a number of inconsistencies, all at firstsight extremely puzzling. When, however, we regard their xerophilous character and reflect that this habit, and not mere fitness for growing at the coast, is the primary determining factor of their station, much that is strange appears normal and plain.
Let me refer in this connection to the impression that the distribution of the Fijian shore-plants made on Mr. Horne, the director of the Botanic Gardens of Mauritius, who spent a year in the botanical investigation of the group about a quarter of a century ago. In his account of the group (pp. 59, 60) he says that several of “what are known as sea-shore plants” are found far in the interior of the larger islands; and amongst others he names such characteristic beach plants as Cerbera Odollam, Hibiscus tiliaceus, Ipomœa pes capræ, and Pandanus odoratissimus. On the other hand, he remarks that several species of inland plants occur at the coast, and that several plants growing on the mountain-tops are found near the sea. This apparent confusion of station he seems to attribute to the circumstance that the mountains of Fiji are not high enough for the development of an alpine flora. But such a view could not be held now, since the effect of an alpine flora would be the introduction of further elements of confusion in the occasional occurrence of some of the alpine plants on the sea-coast, as we find in Hawaii.
Yet this apparent mingling of the littoral and inland floras in Fiji becomes intelligible when we perceive that the seeming confusion of station is mainly restricted to the xerophilous plants of the arid inland plains and of the bare mountain-tops. The rank humid forests that cover so much of the interior of the islands, and the luxuriant vegetation of the mountain-gorges, are not here concerned. Such a mingling occurs it is true under certain conditions; but in the general physiognomy of the flora the distinction between the shore and inland plants holds good. The same shore plants that are distributed far and wide over the Pacific here present themselves; and although some of them extend far inland, where the scantily-vegetated plains descend to the coast, this does not deprive them of the right of being still regarded as littoral plants.
Still, when we look at a fairly complete list of the shore-plants of Fiji, numbering in all about eighty, we perceive that about two-thirds of them also occur inland, either in Fiji or in some other tropical region; and if we reflect that many of the residue are plants of the mangroves that would not be found inland except under estuarine conditions, it becomes evident that with thisreservation there are very few littoral plants in Fiji that do not at times leave the coast.
Cæsalpinia Bonducella may be taken as a type of those shore-plants that stray far away from the coast, even into the interior of continents, since in India it reaches the Himalayas. Although Terminalia Katappa and Calophyllum Inophyllum often owe their existence inland in different parts of the tropics to man’s agency, this cannot be said of most others, as Cassytha filiformis, Casuarina equisetifolia, Cycas circinalis, Ipomœa pes capræ, Pandanus odoratissimus, Premna tahitensis, Tacca pinnatifida, Tephrosia piscatoria, Vitex trifolia, &c., when they occupy the extensive inland plains that slope to the coasts on the lee sides of the large islands of Fiji. Plants, like Hibiscus tiliaceus, are found in a Pacific island almost as frequently away from the beach as on the beach itself; and this is true of most other regions of the tropics where it occurs.
Other plants that appear to be altogether confined to the sandy beach in Fiji, break away on rare occasions from their usual station and appear on the bare rocky summits of hills near the coast, even though the hill-slopes are densely wooded. On such bare hilltops in Vanua Levu, varying from 500 to 1,100 feet in elevation, one is surprised at times to find shore creepers and climbers like Canavalia obtusifolia and Derris uliginosa associated with other beach-plants more frequently found inland, such as Tephrosia piscatoria and Vitex trifolia, and in the company of climbing species of Morinda and of small trees of Fagræa Berteriana. When the “talasinga” (sun-burnt) districts, as the Fijians term the plains on the north sides of the islands, extend a long distance from the coast into the heart of the island, they carry with them their peculiar vegetation and the intruding beach-plants up to considerable elevations above the sea. We then find familiar beach-plants like Cerbera Odollam and Ipomœa pes capræ growing far inland at heights of 1,000 feet and over above the sea. (See Notes20and21.)
One is never quite sure of the behaviour of shore-plants in Fiji when the “talasinga” plains lie behind the beach, since even Scævola Kœnigii, usually a steadfast beach-plant, occurs at times some miles inland. (See Notes20and55.) There are, however, a few that never came under my notice inland, such as Pemphis acidula, Triumfetta procumbens, and Tournefortia argentea. The extension of sea-coast plants for any distance inland depends a good deal on the occurrence of scantily-vegetated plains, or ofscrub-covered, rolling country at the back of the beaches; and doubtless that which I have described in the case of Fiji is to be found in other tropical coast-regions. Professor Schimper informed me by letter that he had noticed a similar inland extension of the shore-plants in the Seychelles.... I have only here touched on this subject. In Notes20and21the reader will find further details of the inland extension of the beach-plants, and inNote 22is given a general account of the “talasinga” plains, in which the wandering beach-plants mingle with the peculiar vegetation of the plains themselves. Covered with reeds and bracken, and dotted over with clumps of Casuarinas and Acacias, with the Cycad and Pandanus distributed irregularly over their surfaces, such level districts possess, as remarked by Seemann, a South Australian look.
The littoral plants readily divide themselves into three principal groups as concerning their station, namely:
(a) The “beach-formation,” typically exhibited on the whitish calcareous beaches of reef-bound coasts.
(b) The “mangrove-formation,” found at intervals all along the coasts, but most fully developed at the estuaries, and for the most part occupying flats regularly overflown by the tide.
(c) The “intermediate formation,” comprising the plants of the tracts between the beach and the mangrove-swamp and at the borders of the swamps.
This grouping does not differ materially from that adopted by Professor Schimper in the instance of the Indo-Malayan strand-flora. (SeeNote 23.)
To the beach-formation belong, amongst the trees and shrubs, Barringtonia speciosa, Calophyllum Inophyllum, Guettarda speciosa, Pemphis acidula, Scævola Kœnigii, Tournefortia argentea, &c., and amongst the creepers and procumbent plants, Canavalia obtusifolia, Ipomœa pes capræ, Triumfetta procumbens, &c. To the mangrove-formation belong the Asiatic and the American species of Rhizophora, and species of Bruguiera, Carapa, Lumnitzera, &c. Amongst the trees that gather around the borders of the mangrove-swamp, constituting the intermediate formation, occur Barringtonia racemosa, Excæcaria Agallocha, Heritiera littoralis, Hibiscus tiliaceus, and several other species, all of them being equally at home on the sandy beach, at the borderof a mangrove-swamp, and on the banks of an estuary. The climbers, such as Entada scandens, Mucuna gigantea, Derris uliginosa, &c., belong more to the mangrove and to the intermediate formations than they do to that of the beach. Referring the reader to the more complete lists given inNote 24, I may remark that it is not always possible to distinguish sharply between the three formations, since some of the plants belong to two, and a tree like Cerbera Odollam may, in different localities, be referred to all three formations. The general distinction, however, prevails in the physiognomy of the coast-flora.
The mangrove-formation comprises, it may be pointed out, many plants other than mangroves, plants that find a home in the mangrove-swamps of Fiji, either within their limits or at their borders. It presents, indeed, a world in itself. When the mangroves establish themselves in a new locality they carry along with them a host of hangers-on, both plants and animals, that only find a home under the favourable conditions of a mangrove-swamp. Thus, the absence of the mangrove-formation from a Pacific island deprives its littoral flora of many very striking features. For this reason the Tahitian shore-flora must seem to a botanist coming from Fiji comparatively tame and monotonous; whilst that of Hawaii, for this and for other reasons to be subsequently mentioned, is still less interesting, and scarcely even gives a character to the coasts.
We are now, therefore, prepared to learn that a large number of the plants other than true mangroves, that thrive in or around the Fijian mangrove-swamp, are not to be found in those Polynesian islands where true mangroves do not exist; and that a law of association here exists. Many of the plants of the intermediate formation are so closely bound up with the mangroves in their life-conditions that they are not to be found where the mangroves are absent, even though their seeds or fruits are pre-eminently fitted for dispersal by the currents. The influence of “station” here rules supreme. This matter will be treated more in detail when discussing the Tahitian and Hawaiian strand-floras in ChaptersVI.andVII.
The predominant influence of the currents having been already established, there remains for consideration the distribution of the floating capacity of the seeds or fruits among the different formations.One can say that almost without exception the seeds or fruits or seedlings of the mangrove and intermediate formations float for long periods. In the case of some of the true mangroves, as in Rhizophora and in Bruguiera, where germination takes place on the tree, it is the seedling that floats, whilst in others, as in Carapa and Lumnitzera, it is the seedvessel that floats. The plants with non-buoyant seeds or fruits that belong to the littoral flora are all confined to the beach formation, but they do not form more than a sixth of the total. Almost all the “good floaters” of the beach-plants are widely spread over the shores of the Pacific and of much of the tropics, and include such familiar species as Barringtonia speciosa, Cæsalpinia Bonducella, Terminalia Katappa, and many others mentioned in the lists of Notes2and24.
When, however, we come to the dozen or so of beach-plants that possess seeds or fruits with little or no floating power, we find that several of them have a limited distribution in the Pacific, such as Acacia laurifolia, Drymispermum Burnettianum, Eugenia Richii, &c., whilst others, such as Casuarina equisetifolia, Tephrosia piscatoria, Triumfetta procumbens, and Wikstrœmia fœtida, are widely spread. This small non-buoyant group of the beach-plants has a nondescript appearance, and it is here that the inland flora is most likely to make its influence felt by additions to the number. It is here indeed that the littoral floras of the tropics mostly differ, the accessions from the inland flora varying in each region. It is in fact the zone of change.
A number of these plants, such as the species of Drymispermum, Eugenia, and Wikstrœmia, have probably been dispersed by frugivorous birds; whilst others, like Triumfetta procumbens, possess fruits that might have been transported in birds’ plumage. From the frequency with which Tephrosia piscatoria is associated on hilltops in Fiji with Fagræa Berteriana and climbing species of Morinda that are well suited for dispersal by frugivorous birds, it seems likely that it is also distributed by birds fond of a drier diet. It is possible that the Polynesians, who much value the wood of Casuarina equisetifolia, have often assisted in dispersing the tree.
The following is a summary of the contents of the chapter.
(1) The extension inland of the Fijian strand-flora is to be attributed to the xerophilous organisation of the plants, and to the exceptionally favourable conditions that are offered to such plants on the plains, and in other scantily vegetated localities, lying usually on the drier sides of the larger islands.
(2) Excluding the mangroves and the plants associated with them in the coast-swamps, there are few littoral plants of the islands of the tropical Pacific that do not extend inland in one region or another.
(3) The Fijian shore-plants can be rudely arranged in three groups, those of the mangrove-swamp, those of the sandy beach, and those of the intermediate districts, the last including those plants that occur typically at the borders of a mangrove-swamp, though some of them can thrive equally well on a beach.
(4) There is a law of association connecting many plants with a mangrove-swamp in such a manner that when the true mangroves are not represented in a Polynesian group, as in Tahiti or in Hawaii, the plants in question are also absent, notwithstanding that in many cases, such as those of Clerodendron inerme and Heritiera littoralis, they possess seeds or seedvessels of great floating power.
(5) The fruits or seeds or seedlings, as the case may be, of the plants of the mangrove-swamp and of the bordering districts float almost without exception for long periods. This is true also of five-sixths of the beach-plants, whilst the remainder owe their dispersal chiefly to birds.
(6) The small non-buoyant group of the beach-plants represents that portion of the strand-flora that is most likely to be recruited from the inland flora. It is here that exists the zone of change; and it is in this respect that the littoral floras of the tropics differ principally amongst each other, the recruits from inland varying naturally with the floras of different regions.
Though it does not come within my plan to discuss the littoral floras of the adjacent smaller groups of Tonga and Samoa, it may be remarked that they reflect most of the principal features of the strand-flora of Fiji. In particular it may be observed that they possess the mangrove-formation, but to a more limited extent. Both own the mangrove genera Rhizophora and Bruguiera, whilst Carapa is also found in Tonga. The intermediate formation is represented in Tonga by Clerodendron inerme, Excæcaria Agallocha, and Heritiera littoralis; whilst in Samoa we find, besides the first-named species, Barringtonia racemosa and Scirpodendron costatum. In both the beach-formation is well represented.