Coastally restricted forests

There is a special interest in coastally restricted forests in which a few tree species are found growing in relatively narrow ribbons bordering an oceanic edge, usually within 250 km of a coast (Laderman, 1998). Many of these tree species have unusual morphological, physiological or ecological characteristics, which enable them to survive conditions - including toxic soils - in which others either do not flourish or fail altogether. What do these species have in common, and why are they not naturally found inland? The soils they grow in are often so damp as to cause problems with root aeration (see Section 2.5), but are never salt-laden like those in which mangroves establish along the coasts and estuaries subject to daily tidal inundation (Section 4.2.3).

Laurel forests form a transition between tropical and temperate regions where the climate is very humid with rainfall year round but with a distinct cool season. These specialized evergreen rain forests exist mainly on the eastern side of continents in southern China and southern Japan, South America, southern Australia and south-east Africa where they are influenced by trade and monsoon winds. In Japan, these forests consist of evergreen oaks and chinquapins Castanopsis in the beech family and, in the laurel family, Machilus spp. These ecosystems resemble those of Macaronesia (see the description of the Madeiran cloud forests in Box 1.2; Fig. 1.13) in being humid forests with typically laurel-like evergreen leaves, but are very different in other ways, usually possessing far more and different species of both animals and plants.

The southern tip of Florida and further west can also be included in this consideration of laurel forests. The southern forested wetlands of the USA, which extend through 12 states, have been more intensively studied than any others; descriptions of their economic importance, ecosystem properties, flora, fauna and possible future transition are given in Messina and Conner (1998). As with the Amazonian tropical rain forests, many of them have been converted for agriculture. Problems of root aeration, a particular difficulty here, are considered in Section 2.3.3. These areas are defined solely by the nature of the forest cover into three groups. Plots dominated by elm Ulmus spp., ash Fraxinus spp. and eastern cottonwood Populus deltoides, either singly or in combination, are designated as elm-ash-cottonwood forest. With oak-gum-cypress forests the trees present are tupelo Nyassa spp., sweetgum Liquidambar styraciflua, wetland oaks Quercus spp., and swamp cypresses Taxodium distichum and T. distichum var. nutans. Pond pine forests are dominated by pond pine Pinus serotina, either singly or in combination with other species.

The bald cypress-water tupelo Taxodium distichum-Nyassa aquatica association is important in the freshwater wetlands of southern USA including Mississippi, where it is associated with a number of other tree species. As in other cases the natural balance of individual communities is influenced by the degree of inundation and the type of soil present. There is a continuum from

Box 1.2 Ancient Laurel Forests of Madeira: a continuity with the Tertiary

The forests of the Island of Madeira were not subject to the ice ages which ravaged north-west Europe in the Quaternary: this has led to the survival of a laurel forest or Laurissylva that is almost identical to that existing in the Tertiary period (see Fig. 1.1). A considerable portion of this forest survives virtually intact, though its integrity is threatened by the introduction and spread of acacias, eucalypts, Douglas fir and other forest trees (Section 5.6). The present state of knowledge regarding the origin, history and dynamics of this remarkable rain forest is outlined in Packham (2004). The laurel forests form a mosaic, heavily influenced by relative humidity, which differ in their structure and floristic composition, as Costa Neves et al. (1996) demonstrated. Lianas are uncommon while ferns are abundant; hare's foot fern Davallia canariensis and Macaronesian polypody Polypodium macaronesicum are amongst those being epiphytic on trees. The rich damp soils support a wide variety of native herbs, including four of the five orchids native to Madeira. There is, however, a tendency for non-native species, such as cape sorrel Oxalis pes-caprae and greater periwinkle Vinca major to advance into the laurel forest.

Before fires started in 1420, when the island was first colonized, the laurel forests covered most of the higher parts of the island, but they are now mainly in the northern half where they grow on steeply sloping mountainsides. These humid forests flourish in the mists of the cloud zone and although only four of their species belong to the laurel family, almost all their trees are evergreen, frequently multi-shooted and with glossy coriaceous (leathery) leaves. Many produce new shoots directly from the roots and coppice well. Huge foetid laurels Ocotea foetens still occur in southern parts of the island, as do Canary Island laurels, many being remnants of the former southern forest. The forest vegetation, which still covers rather more than 20% of the island, is dominated by Canary Island laurel Laurus azorica that grows to a height of 20 m and often bears the fruiting bodies of the fungus Laurobasidium lauri which grows only on this species of tree. Madeira mahogany Persea indica, which has been exploited for its valuable reddish timber, and babusano Apollonia barbujana can both reach a height of 25 m, but mature foetid laurels are massive trees reaching 40 m which tower over their relatives.

The laurel forests which developed on Madeira after its formation in Late Tertiary times differed markedly from those which covered much of Europe and are well known from fossil deposits, particularly those in Bohemia and Austria. All four members of the laurel family mentioned above as being present now in the Laurissylva of Madeira, along with other of its components, have been found as fossils aged about 5.3 million years in the south of France, but many heavy-fruited tree species never reached the islands of Macaronesia. Madeira itself did not erupt above the surface of the sea as a volcanic island until 5.2 million years ago and the whole of its biota built up subsequently as a result of immigration by air or sea.

Oaks, sweet gums, maples, magnolias, walnuts and palms have reached Madeira only in the last 600 years. The natural fauna is also very restricted; rodents, deer and elephants did not exert the significant influences they did in Europe while the bird population was also more limited.

Metres

Metres

Rain The City
Figure 1.13 The trade-wind system and vegetation zones of Madeira; the southern end of the transect terminates in the city of Funchal. (Adapted from Szeimer, 2000. From Packham 2004. Arboricultural Journal, 28).

pure bald cypress to pure water tupelo. When the cypress is clearcut it is often succeeded by pure stands of tupelo.

Western seaboards in temporal zones have similar moist conifer forests, often termed temperate rain forests. In the southern hemisphere, southern Chile bears forests containing a number of conifers (Patagonian cypress Fitzroya cupressoides, Chilean incense cedar Austrocedrus chilensis, monkey puzzle Araucaria araucana) and many species of southern beech (Nothofagus). The Eucalyptus-Nothofagus forests of eastern Australia and Tasmania, and the kauri pine Agathis australis forest of New Zealand are also in this category (although the kauri forests are verging on subtropical).

The forests of the western seaboard of North America have been particularly well studied (e.g. Laderman, 1998). A high proportion of the species involved are evergreen needle-leaved conifers that have poor competitive ability in their early years. In view of the high economic and ecological value of these forests it is clearly essential to understand the successional processes which operate within them and to treat them accordingly. The seven species of false cypress Chamaecyparis are prominent in the coastally restricted forests of the Pacific Northwest and south-eastern USA, Japan and Taiwan. Yellow cedar (also called nootka cypress) C. nootkatensis is a most valuable timber tree in south-east Alaska, where it is also found as a low-growing prostrate form with upright stems a metre high which may be 100 years old. Such plants reproduce asexually by layering, while seedlings develop on better-drained sites. In Alaska, yellow cedar is currently suffering a forest decline which began around a century ago; this has been intensively investigated but its causes - biotic or abiotic - are still unclear (Hennon et al., 1998). Further south, yellow cedar grows under moist, humid conditions on neutral to acidic soils (including ferro-humic podzols) from sea level to altitudes of 2000 m in British Columbia. The poor reproductive success of this tree is a problem in the maintenance and expansion of its forests (many of which are at high altitude). The tree is tolerant of shade, frost and high soil-moisture and in nature its longevity and allocation ofresources to defensive compounds also promotes its long-term survival. It is not, however, a good competitor in its younger years and when mixed yellow cedar-hemlock Tsuga heterophylla forests are felled they often regenerate to pure hemlock.

Port Orford cedar (or Lawson's cypress) Chamaecyparis lawsoniana is a highly adaptable species, which thrives on diverse soil types adjacent to the Pacific coast and whose wood commands premium prices; its susceptibility to root disease is considered in Section 5.4.4. Atlantic white cedar (or white cypress) C. thyoides is an obligate wetland species with a natural range that extended along the Atlantic and Gulf coasts from southern Maine to northern Florida and through Mississippi. The failure of natural regeneration following intensive logging in swampy areas, which were considered worthless after felling, has led to a loss of as much as 90% of the area which this species occupied in Northern Carolina two centuries ago. Efforts are now being made to re-establish this large and splendid tree using both seedlings and rooted cuttings (Phillips et al., 1998). Two other false cypresses are prominent in the coastally restricted forests of Japan. These are the hinoki cypress C. obtusa and the sawara cypress C. pisifera, both of which are prominent in Shikoku Island (Yamamoto, 1998).

Although one of the most distinctive and tallest trees in the world (up to 115 m), coastal redwood Sequoia sempervirens grows only along a narrow portion of the Pacific coast of California and Oregon. The tree lives for several thousand years, grows very rapidly and can form pure stands in which no other tree species can compete successfully. Its tapered trunk is protected by spongy bark up to 30 cm thick and the tree, while lacking a tap root, forms an extensive fan of horizontal roots just below the soil surface. This species rarely occurs along the edge of the coast, being intolerant of strong, salt-laden maritime winds, and requires deep moist soils.

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