Trees By The

Worldwide there are many areas where trees or even forests survive due to the combination of climatic amelioration, reduced competition and habitat protection from grazing and human disturbance that is found in many coastal habitats. Even in the exposed islands of the North Atlantic, sheltered gullies, protected from excessive sea spray, as on the upper areas of the cliffs, harbour a number of species that are more typical of woodlands, with large stands of the greater woodrush (Luzula sylvatica) and honeysuckle (Lonicera pericly-menum). Sometimes, as in pinnacles and cliffs, there can be found relict populations from former woodlands, with rowan (Sorbus aucuparia) and aspen (Populus tremula; Fig. 7.37). On drier cliffs, as on sandstone block cliffs, a heath-type vegetation can be found with heather (Calluna vulgaris), bell heather (Erica cinerea), crowberry (Empetrum nigrum), wood sage (Teucrium scorodonia) and golden rod (Solidago virgaurea). On shores of Scottish sea lochs and the steep sides of Norwegian fiords tree cover can be extensive. The mild wet conditions of the Atlantic coastline of Scotland if free from too much disturbance allow the development of extensive oak woods and associated species.

7.6.1 Mangrove swamps

In cool temperate climates tree establishment on salt marshes is an ecological impossibility. Yet throughout the tropics and subtropics, vigorous and productive salt- and flood-tolerant mangrove forests survive on tidal mudflats and are a source of wonder for this

Fig. 7.35 The opposite-leaved golden saxifrage (Chrysosplenium oppositifolium) together with the hart's tinge fern (Phyllitis scolopendrium) a highly shade tolerant fern inhabiting the rear wall ofthe Smoo Cave — for details ofmetabolic adaptation see text.
Fig. 7.36 Cliff-top maritime heath vegetation with a mixture of sedge and heath species on North Hill, Papa Westray, Orkney, after a period of wet and stormy weather.

remarkable physiological endurance of both flooding and high salt levels.

The term mangrove is commonly applied to the habitat as well as to approximately 40 species of trees that dominate these inter tidal tropical forests which lie mostly within 25° north and south of the Equator. However, to avoid confusion some authors use the Portuguese term Mangal to refer to just the habitat. The change in salt marsh vegetation that takes place when trees replace grasses as the dominant vegetation on salt marshes is a worldwide ecological boundary of considerable ecological significance. This change from herbaceous to woody vegetation on salt marshes becomes possible when frost is rare. Due to the movement of unusually warm waters in certain locations, mangroves can be found outside the 25° north and south general limits. The grey mangrove (Avi-cennia marina) can be found as far north as 27° N in the Red Sea (Fig. 7.38), while A. resinifera occurs at 38° S in New Zealand, where it succeeds in occupying tidal muds to a lower level than any other angiosperm other than Zostera and Spartina (Wardle, 1991). In North America, mangroves are found from the southern tip of Florida along the Gulf Coast to Texas.

Mangrove forests are usually in estuaries at the interface between salt and fresh water where impenetrable stands of woody vegetation develop on the mudflats. They are, however, very susceptible to frost.

Fig. 7.37 A clone of aspen (Populus tremula) clinging to a cliff at Waulkmill Bay, Orkney. In the Orkney Islands the only remnants of natural woodland are to be found in deep gullies or on cliff faces. Aspen has never been observed to produce seedlings in Orkney and DNA tests have shown that remnant stands in their various refuges consist of a limited number of clones.

Fig. 7.37 A clone of aspen (Populus tremula) clinging to a cliff at Waulkmill Bay, Orkney. In the Orkney Islands the only remnants of natural woodland are to be found in deep gullies or on cliff faces. Aspen has never been observed to produce seedlings in Orkney and DNA tests have shown that remnant stands in their various refuges consist of a limited number of clones.

Fig. 7.38 Mangroves approaching their northern limits. A colony of dwarf mangrove Avicennia marina at Bahrain (26° N) in the Persian Gulf.

As little as three to four nights of light frost can be sufficient to kill most mangrove species. Consequently mangroves are not found north of 32° N and 40° S (Fig. 7.38). Why only frost-sensitive woody plants are capable of surviving on salt marshes has long been a mystery.

A highly significant observation that the increasing water deficit caused frost-induced xylem failure in mangroves led to the discovery that frost-induced xylem embolism appears to set a latitudinal limit to the distribution of mangroves (Stuart et al., 2007). Further study has also revealed that the susceptibility of the mangroves to freeze-induced xylem embolism (blocking of a xylem vessel by an air bubble) is related to xylem diameter, the species with small vessel diameters being able to survive at higher latitudes than those with larger diameters (Table 7.1). A significant relationship was also found between vessel diameter and the frost-induced percentage loss in hydraulic conductivity (Fig. 7.39).

It would be simplistic to suggest that cold temperatures alone are sufficient to exclude plants from saline habitats given the extensive growth of herbaceous halophytes even north of the Arctic Circle. However, herbaceous plants do not have to maintain woody stems with meristematic tissues that are liable to desiccation injury during the cold season. It has been suggested that the above-ground tissues in areas of mangrove-salt marsh transition should therefore be likened to those in other areas that are treeless due to disturbance by fire or grazing.

The alternative question also arises as to why mangroves have to have such large vessels that they expose themselves to the risk of developing embolisms. Living as they do in the interface between saline and fresh water it may be that the shoot tissues require a very large supply of water to avoid osmotic injury and that having large vessels is the optimal solution, provided there is no risk of freezing. Although mangroves can reduce their transpiration rate when salinity levels are high they nevertheless under favourable conditions have transpiration rates that match those of lowland dipterocarp and tropical heath forests with a similar climate in north Borneo (Becker et al., 1997). Other factors that are important for the survival of mangrove forests are an adequate supply of silt and nutrients and a high tidal range. It is this latter factor that has probably the greatest importance for their survival as it reduces the increasing salinity coming from high evapotranspiration rates, supplies the silt necessary for physical support, as well as the nutrients, and the intervals of exposure to air which provide a diurnal relief from the dangers of anoxia imposed by flooding at high temperatures. Mangroves with their silt and peat accumulations protect tropical coastlines against hurricanes and storm surges. They are, however, susceptible to mass

Table 7.1. Vessel diameters for five mangrove species from the northern and southern limits of the worldwide mangrove distribution species

Latitude^

Aegiceras corniculatum Avicennia marina Avicennia germinans Rhizophora mangle Rhizophora stylosa

35° 42"30"S 35° 42"30"S 29° 40"08"N 29° 4"35"N 27° 46"41"S

b Hydraulically weighted mean vessel diameters, calculated as Dh = RD5/RD4 c Collection latitude.

Reproduced with permission from Stuart et al. (2007).

Fig. 7.39 Freeze-induced percentage loss in stem hydraulic conductivity (PLC) at native xylem tensions as a function of hydraulically weighted vessel diameter (Dh) in five mangrove species. Hydraulically weighted vessel diameters (Dh = XD5/ED4) account for the disproportionate contribution of larger vessels to conductivity. Regression line is y = 3.961x — 76.08, with r2 = 0.72. Bars denote standard error of the mean, with n = 5-13 stems for PLC and n = 5 stems for vessel diameters. Two-letter abbreviations indicate genus and species: Ac, Aegiceras corniculatum; Ag, Avicennia germinans; Am, Avicennia marina; Rm, Rhizophora mangle; Rs, Rhizophora stylosa. (Reproduced with permission from Stuart et al., 2007.)

Fig. 7.39 Freeze-induced percentage loss in stem hydraulic conductivity (PLC) at native xylem tensions as a function of hydraulically weighted vessel diameter (Dh) in five mangrove species. Hydraulically weighted vessel diameters (Dh = XD5/ED4) account for the disproportionate contribution of larger vessels to conductivity. Regression line is y = 3.961x — 76.08, with r2 = 0.72. Bars denote standard error of the mean, with n = 5-13 stems for PLC and n = 5 stems for vessel diameters. Two-letter abbreviations indicate genus and species: Ac, Aegiceras corniculatum; Ag, Avicennia germinans; Am, Avicennia marina; Rm, Rhizophora mangle; Rs, Rhizophora stylosa. (Reproduced with permission from Stuart et al., 2007.)

mortality as a result of storms, excessive salinity and in the Bay of Bengal, that the pneumatophores that the deposition of large amounts of silt that exceed their thrust upwards from the shallow rooting system are ability to grow and raise their anchoring organs to frequently laterally flattened with the broader side match the rate of salt deposition. Mangrove develop- facing the oncoming current. Such an orientation ment is principally cyclical, with older stands dying implies an active role in slowing down the current and either through erosion or decay and new colonies accumulating silt.

advancing onto fresh silt deposits. It is the generally The largest mangrove forests are found in the Bay accepted view that mangroves are not land builders of Bengal (Fig. 7.40). Here in the world's largest delta gradually encroaching on the sea but that they follow formed by the rivers Ganges, Brahmaputra and silt deposits and play a passive role in sediment accu- Meghna between India and Bangladesh are the Sun-

mulation. It can, however, be seen, as in the mangroves derbans, a stretch of impenetrable mangrove forest of

Fig. 7.40 The Sunderban marshes in the Bay of Bengal. This is a densely forested region with at least 26 species of mangroves.

great size and biodiversity and a UNESCO World Heritage Site. The lower delta plain west of the modern river mouths has been accumulated in three phases over the past 5000 years (Allison et al., 2003). Their present configuration is related to natural factors, such as eastward tilting of the delta, rapid sediment accumulation (to 0.7 cm yr—!), marked land subsidence (to 0.5 cm yr—!), and increasing anthropogenic influences, including large-scale land reclamation and decreased river flow influx (Stanley & Hait, 2000). A close network of rivers, channels and creeks intersect the entire Sunderbans forest area, which comprises hundreds of islands which get either partially or fully inundated during the diurnal high tides. The existing large rivers running from north to south are the remnants of the old courses of the Ganges.

The main current of the Ganges has gradually shifted eastwards over the last few centuries. A major tectonic movement in the sixteenth century appears to have caused a lifting of the upper crust towards the west, thus forcing the Ganges to drain mostly through Bangladesh, causing the sources of all the rivers in the western part of the Sunderbans to be progressively silted up, thus disconnecting the inflow of fresh water into the mangrove delta. This has increased the salinity of the river waters as well as making them shallower. Consequently, during the ebb tides the receding water level causes scouring and creates an innumerable number of small creeks, which normally originate from the centres of the islands. The receding water, while draining into the Bay of Bengal, carries away large volumes of silt. This silt is deposited along the banks of the rivers and creeks during high tide, increasing the height of the banks as compared with the interior of the islands. As a result, high tide cannot normally reach the interior of the islands (Blasco, 1977).

This environmental change from west to east in the Sunderbans highlights the conditions that mangrove forest requires for survival in relation to the quantity of silt together with optimal salinity and depth of the floodwaters. As already noted, there has been much discussion as to whether mangroves follow silting or cause silting. The answer is most probably a question of the degree in relation to the rapidity of the silting. In recent times the silt load in the great rivers of India has increased with the extensive erosion that is taking place in the Himalaya. This is likely to become even more severe as the mountain glaciers retreat and the glacial outwash is transported downstream. More sediment may enhance the cyclical processes in mangrove development in some places by providing fresh opportunities for colonization, while in others too much sediment may bury the mangroves at a rate at which they cannot survive.

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