There is no real alternative to light for flowering plants unless it is parasitism on other species. The dependence of parasitic plants on their hosts ranges from hemi-parasitism in chlorophyllous species such as the louseworts (Pedicularis spp.) and mistletoes (Viscum spp.). An arctic example is Pedicularis dasyantha shown in the frontispiece to this chapter (Fig. 3.1). Achlorophyllous species are obligate parasites entirely dependent on soluble sugars from their hosts for their
energy supply. Such species remain entirely underground except when flowering. A particularly colourful example of this life strategy is the purple toothwort (Lathraea clandestina) which is a widespread parasite on poplars and willow and is even cultivated in gardens, as is the specimen flowering in March in the Botanic Garden at St Andrews, Scotland (Fig. 3.30).
Fortunately for many terrestrial flowering plants the upper soil layer is not the only place where water can be found. Dew, deep underground reserves, and parasitism are resources that specially adapted plants can exploit in the search for water. Hemi-parasitism is an immediate alternate source of water for adapted species that is exploited by many species. The genus Pedicularis is notable for having numerous species mainly in the northern hemisphere, and especially in drought-prone alpine and arctic habitats. In areas where water stress is frequent these plants transpire freely, largely at the expense of water obtained from host plants (Fig. 3.1).
Apart from parasitism there are two other main sources which can supply plants with water in the absence of precipitation. The first and commonest is ground water, or more exactly the phreatic zone (Greek phrear, a well) - the region of the soil or rock zone below the level of the water table and where all voids are saturated. The second is dew. For dew to be condensed in any quantity and provide water that can be taken up by plants it has to be protected from re-evaporation. Thus dew will contribute to the water needs of plants only when it condenses inside some protected system such as underneath leaf scales as in some bromeliad species or else trapped in the wells formed by leaf rosettes on many epiphytic species (Fig. 3.25). The forest canopy can also serve for trapping moisture as is commonly found in the trees that inhabit mountain cloud zones. A cryptic form of dew is provided by the upward movement of water vapour at night from the warmer water table to the cold surface soil in porous soils. This is particularly important for the grasses that colonize sand dunes (see below).
Plants that can access the phreatic zone are termed phreatophytes, and can reach water that is not available to shallower rooting species. The creosote bush (Larrea divaricata) has a root system that can extend many metres down to the phreatic zone. A study of the root systems of some Chihuahuan Desert plants found that the roots of 11 shrub or shrub-like species could be traced through various soil horizons to depths of 5 m (Gibbens & Lenz, 2001). Desert phreatophytes, and also some riparian species, combine the possession of long-lived water-seeking roots with more ephemeral surface root systems which supply the plant with nutrients as well as water when the soil is in a humid condition. Samples collected from the native Australian tree Banksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rainwater) contained within the upper soil layers, while taproots derived water from the underlying water table. The shoots therefore obtained a mixture of these two water sources. As the dry season approached, dependence on recent rainwater decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well-supplied with ground water taken up by the taproot. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4-6 month) dry season. Parallel studies of other native species and two plantation-grown species of Eucalyptus all demonstrated behaviour similar to that of Banksia prionotes in that shallower resources were accessed by lateral roots in the upper soil layers (Dawson & Pate, 1996).
This potential to tap an alternative source of water has contributed to the invasive success of several southern European species of the genus Tamarix spp. (saltcedars) which were first introduced to North America in the 1800s. Many of the species escaped from cultivation and by 1987 were estimated to have invaded at least 600 000 ha. The saltcedars benefit from a deep water supply but are only facultative phreatophytes as they are often able to survive under conditions where deep ground water is inaccessible. The high evapotranspiration rates of salt cedars can lower the water table in heavily infested areas and even alter the ancient composition of the underground water reserves. Mature plants are tolerant of a variety of stress conditions, including heat, cold, drought, flooding and high salinity. Saltcedars are not obligate halophytes but survive in areas where ground-water concentrations of dissolved solids can average 8000 ppm or higher. In addition, the leaves of saltcedars excrete salts that are deposited on the soil surface under the plants, inhibiting germination and growth of competing species (Di Tomaso, 1998).
Variation in the sources of water used by tree species has significant implications for forestry. Riparian forests in their association with river banks are further examples of marginal plant communities. In a study of tree transpiration in a riparian forest in southeastern Arizona containing Populus fremontii, Salix gooddingii and Prosopis velutina it was found that Salix gooddingii did not take up water in the upper soil layers during the summer rainy period, but instead used only ground water, even where the depth of the ground water exceeded 4 m. Populus fremontii, a dominant 'phreatophyte' in these semi-arid riparian ecosystems, used mainly ground water, and at an ephemeral stream site during the summer rainy season derived only 26-33% of its transpiration water from upper soil layers. By contrast, at another ephemeral stream site during the summer rainy period, Prosopis velutina derived a greater fraction ofits transpiration water from upper soil layers than at a perennial stream site where ground-water depth was less than 2 m (Snyder & Williams, 2000). These results show the flexible manner in which phreatophytes and riparian trees can use alternative sources of water depending on availability.
The ability ofphreatophytes to access deep water also has consequences for the water status of the upper layers of the soil due to a phenomenon described as hydraulic lift. During the night when transpiration is less, the upward movement of water in the roots of the phreatophytes nevertheless continues. As supply can exceed demand, the deeper-rooted species release some of the water they are transporting upwards into the top layers of the soil, where it becomes available to shallow-rooting species (Williams et al., 1993). Shallow-rooted species can then profit from the presence of deep-rooted plants by the provision of indirect access to the ground-water reserves that would otherwise be beyond their reach. The deep-rooted plants in turn benefit from the growth of the shallower rooted plants as many of them can fix atmospheric nitrogen (bird's foot trefoil, clover, medick) and thus contribute to the nutritional status of the entire plant community.
In addition to the phenomenon termed 'hydraulic lift', where water is redistributed from a depth to dry topsoil, recent studies have detected a process of hydraulic redistribution which includes downward transfer of water when the surface layers of soils with low permeability become wet after rainfall. A comparison of sap flow in vertical and lateral roots of Grevillea robusta trees growing without access to ground water at a semiarid site in Kenya showed that a reversal of sap flow occurred when root systems crossed gradients in soil water potential. Reverse flow in roots descending vertically from the base of the tree occurred, while uptake by lateral roots continued, when the top of the soil profile was wetter than the subsoil. This transfer of water downwards by root systems, from high to low soil water potential, was termed downward siphoning and represents the reverse of hydraulic lift. This downward siphoning was induced by the first rain at the end of the dry season and by irrigation of the soil surface during a dry period. It has been suggested that by transferring water beyond the reach of shallow-rooted neighbours, downward siphoning may enhance the competitiveness of deep-rooted perennials (Smith et al., 1999).
Dew is another major source of water that can have profound ecological consequences. In times past there was much discussion as to whether 'dew descended or rose', causing the remark that dew is 'nowhere until it is formed'. Meteorologically this may be correct, but from an ecological point of view it is nevertheless possible to indicate the source of the water that contributes to the dew that can be used as an alternative source of water by vegetation. Dew is formed by the distillation of water either upwards or downwards and provides another potential source of moisture which in certain circumstances can benefit vegetation. Cool surfaces will condense dew from the atmosphere as well as from water vapour in the soil profile. Many species do not have roots capable of reaching the phreatic zone or even that region about 40 cm above the water table from which water may be expected to rise through capillary action.
In sand dunes dew can distil upwards from the deeper layer as the temperature there is relatively constant and at night remains warmer than the upper dune layers which cool and therefore condense water vapour coming from the phreatic zone. Sand dunes have a low retention of water and grasses that inhabit dunes such as marram grass (Ammophila arenaria) are not phreatophytes and cannot reach the depths where these water reserves lie. Because of these daily oscillations of temperature in sand dunes the upper layers are able nevertheless to have their water content replenished on a daily basis, even during periods of drought, provided the phreatic zone is not too far below the surface of the dune. Unfortunately, in many places the local needs for farming and golf courses can lower the water table to such a depth that it renders the internal circulation of dew ineffective. In such situations dunes erode rapidly, as has been found in the Netherlands where coastal dune systems have been extensively exploited as a source of fresh water (Maarel, 1979).
Distillation of dew in semi-desert soils is a phenomenon that has been recognized since antiquity. In the Negev Desert there are stone mounds called teleilat el einab, literally the hillocks for grapevines, which are believed to have been constructed for the cultivation of vines or other small fruits by the Nabatean communities in the first and second centuries AD (Glueck, 1959). Even larger structures up to 10 m high and 24-30 m in diameter are to be found at Theodosia in the Crimea (Hitier, 1925). These may be expected to condense up to 300 litres of water as dew in one night (Monteith, 1963).
This trapping of dew with stones is generally described as lithic mulching and is an agricultural strategy that has been practised for more than one thousand years both in the Old and the New World. The earliest lithic-mulch plots are associated with ancient Nabatean sites in the Negev of southern Israel mentioned above. Roman viticulturalists and olive growers in Italy and nearby Mediterranean regions used stone mounds between 100 BC and AD 400 and perhaps a century earlier or later (White, 1970).
In the Atacama Desert site of Chilca (Peru) approximately 1500 stone-lined pits were built in the lomas or fog oases (see below) for the retention of subsurface moisture and possibly the collection of dew from fog moisture may have allowed both maize and potatoes to be grown in these pits (Lightfoot, 1994). The ecological importance of fog (which is just a large body of water-saturated air at the dew-point temperature where water droplets condense), is also an alternative source of water for plants and deserves serious discussion. The coastal redwood forests of California are one example, and the pine forests of the Canary Islands are another, of remarkable plant communities that are sustained by fog (Figs. 3.31-3.32). In a study of the heavily fog-inundated coastal redwood (Sequoia sempervirens) forests of northern California it was found on average that 34% of the annual hydrological input was from fog-drip from the redwood trees themselves. When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influenced the magnitude of fog-water input to the ecosystem. Stable-isotope studies were used to characterize the water sources used by the plants. In summer, when fog was most frequent, 19% of the water within S. sempervirens, and 66% of the water within the understorey plants, came from fog after it had dripped from tree foliage into the soil. For the redwood trees this fog water input comprised 13-45% of their annual transpiration (Dawson, 1998).
In the Canary Islands, the Canary pine (Pinus canariensis) maintains a relatively high timberline at 1900-2000 m due to its ability to intercept moisture from the north-west trade winds on the enormous surface area of the pine needles in the forest canopy (Fig. 3.31). The needles of the Canary pine are borne in threes on densely crowded clusters up to 270 mm in length (Fig. 3.32). This great needle length increases the surface area of foliage available for the condensation of dew. Rain gauges placed within the forest can record annual precipitation levels of as much as 2030 mm while those on deforested parts of the mountain record only 510 mm. Quantitative studies of the interception of fog water in relation to tree density have revealed the initially surprising result that nine years after a light thinning in a 48-year-old Pinus canariensis plantation the throughfall of water in the forest increased to about twice that measured in the unthinned stand. The long period that elapsed between thinning and re-estimation of throughfall had resulted in an increase in the leaf area index (LAI, the total leaf surface exposed to incoming light expressed in relation to the ground surface beneath the plant) oflightly thinned plots, and this together with surface roughness of the forest canopy (tree height variability) aided interception of the fog and enhanced the amount of throughfall. Continuous severe thinning, however, had the reverse effect, and led to a gradual decline in throughfall values (Aboal et al., 2000).
The Japanese cedar (Cryptomeria japonica) resembles the giant redwood (Sequoiadendron giganteum) both in its foliage and being a dominant tree in mountain cloud forests. Like the giant redwood the Cryptomeria forests enhance their rainfall through intercepting moisture from fog. Unfortunately, in the modern polluted world, fog now transports to mountain forests increased amounts of ammonium, nitrates and sulphates above the level that would be supplied by rain. In a study of the Cryptomeria forests at Mt Rokko, Kobe, Japan, it was found that in addition to increasing precipitation, fog also increased the deposition of the atmospheric pollutants SO42~, NO3~, H+, and NH4+ to six to twelve times the corresponding deposition via rain. These values are equal to, or exceed, the maximum deposition reported for Appalachian forests in the eastern United States (Kobayashi et al., 2001). Deposition of mineral nutrients to forests is, however, probably neither a new nor entirely unnatural phenomenon. At a remote site in southern Chile it has been noted that organic nitrogen
Fig. 3.31 View looking down on a forest of Canary pine (Pinus canariensis) emerging through the cloud zone at 1800 m on Tenerife, Canary Islands.
was the dominant component of the mineral deposition from cloud water and was being deposited at a greater rate than in more human-impacted sites in north-eastern USA at up to 9 kg ha-1 and contributing substantially to the N-economy of Chilean coastal forests. It was suggested that it was the adjacent ocean, where biological productivity was high, that may have been the major source of the nitrogen in Chilean cloud water. This proposed marine-terrestrial flux via cloud deposition had not previously been identified as a significant source of nutrients but nevertheless could be taken as an example of the ocean feeding the forest (Weathers et al., 2000).
One of the most spectacular examples of a fog-supported vegetation is to be found in the Atacama
Desert in Chile and Peru. The desert here is one of the driest in the world. At Antofagasta measurable rainfall can sometimes be detected in less than 6 years out of 20, and even then the precipitation does not exceed 4-6 mm per annum. However, from May to November a dense cold mist, the Garua (Peru) or Camancha (Chile), rolls in with the sea breeze from the cold Humboldt Current. It is particularly dense at night but is sufficiently thick by day to obscure the tropical sun for days at a time. In the month of August some areas exposed to this mist have only 36 hours of sunshine and an average temperature of 13 °C, while less than 800 m above, on the neighbouring hills, the temperature rises sharply to 24 °C. This mist supports a lichen-dominated vegetation over extensive areas of the fog-shrouded hills facing the
sea. In certain favoured spots there even exists the unique forest vegetation, the lomas (see also Section 2.6). The precipitation under the trees can be eight times that which is condensed in the open. Typical tree species are Carica candens, various species of Eugenia, Caesalpina tinctoria and Acacia macrocantha (Hueck, 1966). The lomas communities have a high proportion of endemic species, both trees and ground flora, and were probably much more widespread in the past. Fog-dependent ecosystems are very fragile. The damage that has been done by cutting for fuel and grazing by goats has reduced this vegetation to a few isolated pockets, but some restoration success has been achieved by planting imported Eucalyptus and Casuarina, which has led to a partial recovery of the local flora (Section 2.6).
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