3.3.1 Water yield and quality
Water is of tremendous importance to all vegetation. On a global scale, small changes in precipitation produce major variations in the type of forest able to grow (see Section 1.6.1). Like the nutrient cycles described in Section 8.3, water moves in a global cycle, falling on the land, evaporating or running off back to the sea, before evaporating and falling back on the land again. This water cycle is important in the global picture of what precipitation falls where. On a smaller scale, foresters and forest ecologists are often more interested in what happens within a catchment area (or watershed), a hydrologically discrete area that feeds all its water into a main river system (Fig. 3.13). With forests the interest often lies in water yield and quality. Within a watershed the water cycle can be summarized as:
Where Q, the water yield reaching a stream, is determined by the amount of precipitation (P) minus the water that is lost from the catchment by evaporation (ET - evapotranspiration - described below) and the water temporarily
retained in the catchment, either stored in the soil (S) or as groundwater (G- i.e. in the water table).
The amount of precipitation falling in a catchment area is primarily determined by the prevailing climate and is largely unaffected by the vegetation. In the past many foresters held the view that planting forests increased rainfall in a watershed but in temperate areas at least it is likely to be insignificant and certainly < 5% (Golding, 1970). On the continental scale, forests help to increase rainfall in the sense that they repeatedly recycle the amount of atmospheric moisture passing from the oceans to the land. Due to this, in the Amazon basin, for example, where much of the daily rainfall is immediately evaporated to generate clouds for rainfall downwind, it is highly likely that continual clearance of the forest will reduce rainfall elsewhere in the region since much of the water will enter rivers and be lost to the system. Moreover, it is now becoming known that the effects of such tropical deforestation have far wider repercussions in mid and high latitudes through hydrometeor-ological teleconnections, large-scale links in the water cycle and weather. Avissar and Werth (2005) have shown that deforestation of Amazonia and Central Africa severely reduces rainfall in the US Midwest in the spring and summer, and in the upper US Midwest during the growing season. Deforestation of south-east Asia similarly affects China and the Balkan Peninsula.
Some extra water can be captured as dew (the condensation of water vapour on cold surfaces). As this can reach around 1 mm of water per night it can be significant in keeping small seedlings alive. In humid coastal areas and at high elevations that intercept clouds, significant extra precipitation can arrive in the form of fog-drip. With their large surface area, trees are particularly good at sieving small particles of water from cloud and fog, and can increase precipitation by 30-50%.
The vegetation has most influence on what happens to precipitation once it has fallen. All vegetation intercepts rainfall which evaporates before it reaches the ground, but this process is much more important in forests than in pasture because of the larger surface area available to hold water, known as the interception storage capacity, which may be anywhere between 0.25 and 7.5 mm of rain in forests. The proportion of rain lost by this interception obviously depends upon the rainfall event. In light showers all the water may be evaporated from tree foliage so none reaches the ground and in general interception is in the order of 50-90% in showers of less than 20 mm of rain and 10-30% in heavier downpours. This is why we shelter under trees in light showers but find they leak heavily when the canopy storage capacity is full. Wind will also make a difference since it will shake drops from the canopy before they can evaporate. Over a year, the approximate interception loss is 20-25% from deciduous trees and 15-40% from conifers (the latter have a higher surface area and in evergreen species this is kept year round). Interception losses are lower from more open forests but some of the saving is lost by increased evaporation from the soil. In addition to interception losses, the vegetation also loses 'internal' water by evaporation from inside the leaves and to a lesser extent through the bark. This transpiration is under the control of the plants but in forests may still approach 15-20% of precipitation. When all forms of water loss by evaporation are combined, the overall losses expected from this évapotranspiration are something in the order of 30-60% of precipitation in deciduous forests, 50-60% in tropical evergreen forests and 60-70% in coniferous forests, compared with around 20% in grasslands. The remaining water either soaks into the soil or runs over the surface as runoff, either way eventually reaching streams. The important difference is that runoff is more likely to cause erosion and it speeds the delivery of water to the stream (the importance of which is discussed below).
Given the high rates of evapotranspiration from a forest, it is not surprising that discussion of water yield from forests relative to that of pastures is often of considerable importance, especially in New Zealand where the subject has been investigated on a national basis (Maclaren, 1996; Davie and Fahey, 2005). Forested areas do indeed have water yields (runoff or total streamflow) that are 25-80% lower than pastures; the highest differences being in dry South Island sites. Much work has also been carried out into the effect of forest removal on water yields. Most show that regardless of the forest type in question, removal of less than 20% of the trees has an insignificant effect on water yield, presumably because of increased soil evaporation replacing interception loss and transpiration (Brown et al., 2005 provide a good summary). Bosch and Hewlett (1982) concluded that each extra removal of 10% of the forest led to an increase in stream yield of around 25 mm for deciduous hardwoods and around 40 mm for conifers and eucalypts - comparatively small amounts. Nevertheless, where water flow is low, afforestation is likely to reduce it even further, unless it assists aquifer recharge by superior infiltration (permeation of water into the soil rather than becoming runoff). Low flows can be increased by drainage of soils for forest planting.
As has been found at the cost of many human lives in Third World countries, forests are significant in flood control, which they help to reduce in small catchments and particularly in areas close to streams. In larger catchments they can help by reducing sedimentation. Unwise heavy logging of tropical rain forests growing over deep layers of unconsolidated sediments can lead to disastrous mud slides with heavy losses of human life. The main role of forests in flood control, however, is in quickly evaporating water back into the atmosphere, reducing the burden on streams after rain but particularly in aiding water infiltration at the expense of runoff. Since the pathways through the soil are small and tortuous, infiltration slows the passage of water to the stream (the sponge effect) thus dampening the sudden influx of water into streams from rain events. Infiltration in forests is aided by the porous surface of leaf litter and by the vegetation protecting the soil surface from the high-energy impact of rain drops which can break apart soil aggregates and wash fine material into soil pores, blocking them for further infiltration. Water dripping from the canopy (through-fall in Fig. 3.13) can be equally damaging because the drops are large but this is usually concentrated in a small area around the perimeter of the canopy. Least damaging of all is stem flow, water running along branches and then the trunk to reach the ground.
The quality of water, which can vary diurnally, seasonally or with the flow of water, must be considered in relation to its intended use so various water quality indices are employed for uses such as drinking, bathing, fish spawning and general use. In sparsely developed areas of many countries water is of very high quality, while in lowland reaches of agriculturally developed catchments nitrate levels in shallow groundwaters are sometimes too high. Many small lakes are eutrophic (over-enriched with dissolved inorganic nitrogen and phosphorus), and these often contain aquatic weeds including aggressive exotics such as Egeria densa and oxygen weed Elodea canadensis, as well as blooms of blue-green algae. Faecal contamination and the presence of disease-causing organisms affects water from areas of intensive dairying. In contrast, levels of nitrogen and phosphorus in waters draining from indigenous or exotic forests are almost always much lower, as is surface runoff and sedimentation. The latter is highest at times of road-making and logging. In agricultural areas riparian buffer strips or forest zones, narrow zones of trees bordering lakes and waterways, are of particular value in reducing erosion and sedimentation as well as maintaining the integrity of the aquatic habitats, and protecting water quality by moderating shade and water temperature. Forestry agencies in the USA usually recommend that riparian strips are 10-30 m in width; the lower end of this scale is useful for protecting the physical and chemical characteristics of the streams while wider strips will do more to maintain the complete ecological integrity of the aquatic habitats (Broadmeadow and Nisbet, 2004).
3.3.2 Swamp forests and peatlands
Marshy forest known as taiga spreads across sub-arctic North America and Eurasia, with tundra to the north of it and steppe to the south (see
Section 1.6.1). Besides their own intrinsic interest, such peatlands preserve a temporal archive of community development. This makes it possible to reconstruct the history of the swamp forests through the analysis of long core samples of the sediments, and by developing detailed chronologies of succes-sional change. Peatlands form when bodies of water fill with sediments and peat (terrestrialization), or areas of dry land are converted to peatland by flooding (paludification). Figure 3.14 is based on the study of three similar c. 10-ha peatlands, all with pronounced hummock-hollow topography and close to the border between New Hampshire and Massachusetts, eastern USA. The trees of Black Gum Swamp, Rindge Bog and Ellinwood Bog are dominated by red spruce Picea rubens, eastern hemlock Tsuga canadensis and red maple Acer rubrum. In all three sites peatlands initially formed as a result of terrestrialization and there are abundant gyttja (lake deposits) and fossils associated with them. Following this there was a sharp transition to shrub peat which represents an expansion of a mat of low woody vegetation over shallow water; it contains ericaceous wood fragments within a matrix of decomposed Sphagnum and fern rhizomes. The much more gradual change from shrub peat to wood peat, as trees colonized from the margins, is indicative of successional processes involving a range of shrub and forest communities.
It is, however, apparent that paludification also occurred later, as all sites have areas with up to a metre or more of wood peat deposited directly on mineral soil or till without a layer of intermediate gyttja. Moreover, radiocarbon dates indicate progressively younger basal samples away from the lake basins. Although successional sequences were similar in all three sites they were not synchronous. Rindge Bog, the largest and deepest basin, had floating vegetation 2000 years after Black Gum Swamp. In this region of New England, and possibly elsewhere, autogenic processes and local topography exert greater control over peatland development than allogenic (large-scale, externally imposed) factors such as climate change (Anderson et al., 2003).
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