*See text for details.
*See text for details.
these ecosystems, NPP is often more closely related with the length of wet or dry seasons than with annual rainfall per se (House & Hall, 2001).
The timing of the rainy seasons is also important. Ecosystems with winter rain (Mediterranean) and summer rain (monsoonal) differ in NPP and community composition, for example, in the distribution of plants with C4 and C3 photosynthesis metabolism. As C4 plants are favoured by drought and high temperatures during the growing season, the mixture of C3 and C4 species can be achieved in one of two ways: a temporal separation, with C3 grasses active in winter-spring and C4 grasses active in summer, or by growth-form separation as in the monsoonal system with C4 grasses and C3 woody vegetation (Ehleringer & Cerling, 2001). The Mediterranean type of ecosystems, which have an active winter-spring C3 herbaceous component, do not have a native group of C4 plants because the summer is too dry, even though C4 crops (such as maize) thrive there when irrigated.
C4 crops have an intrinsic transpiration efficiency that is roughly twice that of C3 crops, due to lower stomatal conductance and higher photosynthetic capacity. In rainfed crops, however, actual transpiration efficiency under the usual climatic conditions for the different photosynthetic types is rather conservative. This is because WUEt is also determined by the prevailing vapour pressure deficit, and so for temperate zone C3 crops a less efficient photosynthetic pathway is compensated for by a more humid atmosphere (Rockstrom, 2003).
In many arid and semi-arid environments, rainfall pulses are a major feature of the climate and the ecosystem goes through repeated cycles of drying and rewetting (Schwinning et al., 2004). During wet periods plant production may occur and reserves are stored for the continuation of ecosystem functioning between rain events (Reynolds etal., 2004). However, some plant groups (e.g. trees) may obtain resources from different depths in the soil (Walter, 1973), behaving in partial independence from specific rainfall events. Plant responses may be (1) increase in LAI due to germination of annuals and sprouting of perennials, (2) beginning of photosynthesis in perennials as plant water status improves and (3) mineralisation of soil organic matter and improvement of nutrient availability. But not all rainfall events trigger the same responses. The rain thresholds will vary with plant functional group and response type. For example, the amount of water delivered by a given 'rainfall pulse' may not be enough to allow the increase in grass LAI, but permit the mineralisation of soil organic matter. The biological meaning of rainfall pulses will be different for each component of the ecosystem (Reynolds et al., 2004).
Plant responses also depend upon the 'memory' of the system, i.e., the time between rain events will modify the response, and there is often a decoupling between resource availability and their use. For example, as soils dry during the prolonged rainless season, their biological activity declines. When soils are subsequently rewetted by small rain events, there is a sudden 'burst' of decomposition, nutrient mineralisation and CO2 release - the Birch effect (Cui & Caldwell, 1997; Austin et al., 2004; Jarvis et al., in press) - but not plant activity (Pereira et al., 2003). At this time the herbaceous plants may not be there to utilise the released nutrients, and the deep-rooted perennials cannot use current rainfall until water is enough to reach deeper soil horizons. In these circumstances, the loss of carbon and nitrogen from the soil is inevitable (Pereira et al., 2003; Schwinning & Sala, 2004; Jarvis et al., in press), and so summer rains often have no effect on plant growth. Climate changes towards greater aridity may decrease water and nutrient availability due to enhanced temporal heterogeneity and increased asynchrony of water availability and the growing season (Austin et al., 2004). Rain falling when plant cover is scarce leads to a decrease in the proportion of water that is used by the plants [T/(T + E)] and lower WUEe.
Severe droughts may have long-lasting effects on ecosystems. For example, during the severe drought of 1994 in Spain there was high mortality of Quercus ilex trees and other woody species (Penuelas et al., 2001). Similar results have been reported for other regions as shown by tree-ring analyses, which allow a precise dating of tree deaths over decades. Episodes of massive tree mortality occurred in northern Patagonia and coincided with exceptionally dry springs and summers during the years 1910s, 1942-1943 and the 1950s (Villalba & Veblen, 1998). Different species may exhibit different sensitivities to drought. Those species that normally reach subsoil water, as Q. ilex ssp. rotundifolia (David et al., 2004), showed less variability in wood-ring patterns with climate than species that depend more on the use of current precipitation, e.g., Pinus halepensis (Ferrio et al., 2003).
In many cases there is not a simple short-term relationship between tree death and annual rainfall. Jenkins and Pallardy (1995) studied the effects of drought on growth and death of trees of the red oak group in Missouri Ozark Mountains and found that trees that were dead at the time of sampling had in all cases been severely affected by drought in the past. Likewise, ring variation could be used to predict the likelihood of tree death following a severe drought in Pinus edulis in arid northern Arizona (Ogle et al., 2000). In northeastern Spain Lloret et al. (2004) found that the response of Q. ilex to the 1994 drought was influenced by the effects of a drought 10 years earlier: plants that resprouted weakly after the previous drought were more likely to die in response to the recent event than the more vigorous plants. How vigorously a given plant recovers from stress will influence its hierarchy in the community and chances of survival. The resilience of ecosystems subjected to recurrent extreme droughts may be seriously affected by the loss of vigour and increasing difficulty of regeneration of surviving trees (Lloret et al., 2004).
Spatial variability in water resources may have a large effect on the landscape. In addition to micro-environment patterns the spatial variability in water can be affected by the plants themselves. Plant foliage intercepts rain before it reaches the soil, leading to evaporative losses and to the rearrangement of water input into the soil; roots and litter enhance water infiltration and reduce run-off, whereas roots may promote redistribution of moisture (Ryel et al., 2004). Work in evergreen oak Mediterranean savannas showed that more water was stored and was available in soils underneath tree crowns than in the open. This may result from better soil properties (e.g. more organic matter; Joffre & Rambal, 1993) and increased rain capture by canopy interception and throughfall (David et al., 2005) as well as from hydraulic redistribution through roots (Ludwig et al., 2003).
An increasing number of studies have reiterated the crucial role of deep rooting for plant survival during the drought season (but see Section 6.3.3). In tropical and temperate zone savannas, the long dry seasons tend to select either for deep-rooting woody perennials that may use subsoil water (Schenk & Jackson, 2005) and/or for herbaceous plants that are strict drought avoiders with their life cycle tuned to the duration of the period with enough soil moisture (Walter, 1973). Although soil water may be exhausted up to the grass/shrub rooting depth during the dry season, enough water is usually available for woody plant transpiration, except in extremely dry sites or after severe droughts.
Deep rooting (> 1 m) is more likely to occur in sandy soils, as opposed to clayey or loamy soils (Schenk & Jackson, 2002a) and depends on plant type, increasing from annuals to trees (Schenk & Jackson, 2002b). In extreme arid environments, rooting depth is limited by the small infiltration depth that results from low-rainfall events on very dry soils (Schenk & Jackson, 2002b).
6.3.3 In situ water redistribution - hydraulic redistribution
Root architecture and distribution in the soil is of utmost importance as it determines plant access to water (Ryel et al., 2004). However, roots have also the role of water redistribution. The passive movement of water through roots from wetter, deeper soil layers into drier, shallower layers along a gradient of water potential (Caldwell et al., 1998; Horton & Hart, 1998) is known as hydraulic lift. A similar concept was developed to include the downward (Schulze et al., 1998) or even lateral transport of water by roots. Together they are called hydraulic redistribution (Burgess et al., 1998). These processes typically occur when stomatal aperture is minimal (e.g. at night), otherwise the atmospheric draw on water for transpiration is stronger than that provided by the water potential gradients in the soil. Hydraulic redistribution seems to be more effective in plants with dimorphic root distributions (e.g. shallow lateral and deep tap roots) and where soil water infiltration is limited as in more fine-textured soils (Ryel et al., 2004).
Hydraulic redistribution has been proposed as a mechanism that can buffer plants against water deficits during seasonal drought (Richards & Caldwell, 1987; Ryel, 2004). The downward water transport increases infiltration, reducing run-off losses and may help plants to use water in a more conservative way and may facilitate root growth through dry soil layers (Schulze et al., 1998), as well as allowing nutrient uptake from deep soil horizons (McCulley et al., 2004). In hydraulic lift, water absorbed by deep roots is redistributed back to shallow roots, enabling them to survive and absorb water and nutrients even when the soil is dry and to take advantage of precipitation pulses (Seyfried et al., 2005). The quantity of water redistributed by the upward movement of water may amount to 14-33% of plant daily transpiration (Richards & Caldwell, 1987). The movement of water to the shallower soil layers, where most of the soil nutrients and microbes are, can improve plant water and nutrient status (Caldwell et al., 1998), as well as provide benefits to mycorrhizal mutualists (Querejeta et al., 2003) and neighbouring plants (Dawson, 1993) (but see Ludwig et al. (2004); see also Section 6.4).
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