The world cultivated land is 80% dedicated to rainfed agriculture, with the remaining 20% allocated to irrigation (Rockstrom, 2003). Nevertheless, irrigated agriculture is a major consumer of water resources and 40% of the food and agriculture commodities are produced in irrigated areas. With the predicted growth in human population and climate change scenarios of increasing water scarcity, especially in the interior of continents and semi-arid regions, achieving a better efficiency of use of water in agriculture has become a major issue for farmers and researchers. Furthermore, land degradation reduces the soil water holding capacity and many irrigation systems waste large amounts of water. For example, more than 50% of the water allocated to irrigation in the southern and eastern Mediterranean may be wasted (Araus, 2004). One of the main aims of the 2000 World Water Council in the Hague was to increase water productivity for food production from rainfed and irrigated agriculture by 30% until 2015 (FAO, 2002). Additionally, increasing plant water use in agriculture is limited because sufficient run-off has to be guaranteed to sustain river ecology and other water uses, especially in drought-prone environments. It was suggested that globally only approximately 17% of the fresh water can be used for agricultural production (Rockstrom, 2003).
Many practices developed over the history of agriculture aimed at increasing the availability of water (such as irrigation, rainwater harvesting, mulching and contour ploughing) and enhancing the share of crop use in ecosystem water balance (such as ploughing, weeding, adjusting spacing to water availability). Plant selection and breeding for water-limited environments has resulted frequently in greater crop competitiveness with weeds and more thorough use of water resources (Blum, 1984). However, as mentioned above, especially in drought-prone environments, increasing plant water use in agriculture may be limited by other social and ecological needs.
Concerns for a more efficient use of water resources led to the development of new management strategies that bring to the field agronomical and plant physiology concepts that may improve crop WUE while maintaining or even improving crop production and quality. New approaches may exploit plant sensing and physiological signalling of mild water deficits that coordinate plant adaptive responses to water shortage, as it is provided by controlled irrigation (Loveys et al., 2004). Attempts to manage crop source/sink balance by fine-tuning agricultural practices are also important (Goodwin & Boland, 2002) as harvest indices are often sensitive to water deficits. Plant breeding to develop genotypes with improved water uptake or better WUE without penalising yield is also taking place (see Chapter 5). Plant plasticity under water deficits is large, with some genotypes showing a high potential to deal with periods of water shortage (Centritto et al., 2004; Chaves & Oliveira, 2004).
Among the options to improve productivity in rainfed agriculture, is increasing the ratio between plant transpiration and non-productive evaporation losses through (1) avoiding the early season soil evaporation (or consumption by weeds or fallow) before full emergence of the crop and (2) maintaining high canopy cover throughout the growing season (Rockstrom, 2003). Increasing WUEt by increasing yields through improved agricultural management and plant breeding is possible and desirable (Wallace, 2000; Gregory, 2004). However, as discussed above, increasing WUEt has limitations. Moreover, constitutive high WUEt is sometimes associated with a low productivity syndrome that may limit the scope for breeding crops for higher transpiration efficiency.
Table 6.2 Improving water economy in rainfed crops*
Optimising canopy development to increase the ratio of crop transpiration/soil evaporation Reducing water losses by drainage and increasing water capture Improving WUE at the leaf level
Improving harvest index per unit of water used
* Adapted from Passioura (2004).
Agronomic and breeding practices
Early crop cover, deep root systems (genetic or nutrition)
Need to overcome pests and diseases and nutrient limitations, breeding
Adjusting crop phenology to the environment, specially flowering time
Climate change may have contrasting impacts on rainfed agriculture depending on geography and technology. While droughts may reduce crop production, warming and elevated atmospheric CO2 may act positively on production potential. But even in water-limited environments, precipitation may not be the major determinant of crop productivity. In Australia, wheat seldom reaches the yield potential of 20 kg ha-1 mm-1 of water supply due to a combination of several limiting factors, such as low soil fertility or pests and diseases (Passioura, 2004). To come closer to the yield potential in rainfed crops in a changing climate, adaptation techniques should be adopted in the short-term and in the long-term (Pinto & Brandao, 2002). The former include the adequate choice of cultivars, timely planting, correct densities and harvest dates, as well as proper soil and nutrient management. Based on the Australian experience with dry land wheat, Passioura (2004) lists some practices that can ensure efficient water use (Table 6.2). On the other hand, land degradation may intensify the effects of drought to disaster levels.
The long-term measures will be the search for new genotypes with a better adaptation to heat and drought and increased water- and nutrient-use efficiencies. Biotechnology may play a fundamental role in this context, although it must be acknowledged that a significant gestation time is still required before its impact is realised, as far as genetic modified crops are concerned (InterAcademy Council, 2004). There are, however, major breakthroughs utilising conventional breeding - good examples are the drought tolerant maize and wheat lines developed by CIMMYT through marker-selected breeding. Another example is the New Rice for Africa (NERICA), interspecific hybrid rice obtained by crossing Oryza sativa (Asian rice) with Oryza glaberrima (African rice), that gives 35% higher grain yields than the upland African rice varieties, when cultivated with traditional rainfed systems without fertilizer (InterAcademy Council, 2004). In addition to higher yields, the NERICA varieties are richer in protein and they are claimed to be more disease and drought resistant than local varieties of the West African savanna region.
In irrigated agriculture there is a strong need to increase efficiency, avoiding unnecessary water spending while improving product quality (Araus, 2004). These are the objectives of fine-tuning irrigation practices such as deficit irrigation, whereby water is supplied below the full plant demand, allowing a mild stress to develop with only small negative effects on yield (FAO, 2002). This strategy may lead to greater economic gains than that by maximising yields. In general, deficit irrigation has been more successfully applied to crops less sensitive to water deficits (such as cotton, maize, groundnut, grapevine, peach or pears) than to sensitive crops like potato (Kirda et al., 1999).
Regulated deficit irrigation (RDI) is a type of deficit irrigation where the amount of water applied is not constant throughout crop development, taking into consideration the needs at each stage. This method is used in high-density orchards to reduce excessive growth and to optimise fruit size and quality (Chalmers, 1986). RDI may also improve the extent of soil water uptake as mild deficits during vegetative growth may have a favourable effect on root growth, improving water acquisition from deeper soil layers, as observed in studies with groundnuts in India (FAO, 2002).
In the partial rootzone drying approach, each side of the root system is irrigated during alternate periods. The plant water status is maintained by the wet part of the root system and stomatal closure is promoted by the dehydrating roots of the other half of the root system (Davies et al., 2000), using less water per plant. This type of deficit irrigation will be efficient in canopies where stomatal control over shoot water status through transpiration is important (Kang & Zhang, 2004). This is the case in crops with isohydric behaviour, where stomata do respond to root signalling, most likely through ABA synthesised in the roots and modulated via xylem pH, such as grapevines (Santos et al., 2003b; Loveys et al., 2004; Souza et al., 2005, see Chapter 5).
An efficient monitoring of plant performance is an essential component of the water-saving strategy. Several techniques are available, although most of them are time-consuming and demanding as far as equipment is concerned, such as monitoring soil water or plant water relations (sap flow meters or leaf water potential). Thermal imaging is emerging as a potential tool to monitor canopy water status. The use of indices such as crop water stress index, calculated from canopy temperatures in relation to references, can give us estimates of stomatal aperture and therefore be used for irrigation scheduling (for a review see Jones, 2004a).
As in agriculture, current trends in population growth and improvement of living standards leads to an increase in the global demand for forest products. The consumption of wood-based products and paper increased four times faster than the population during the twentieth century (FAO, 2000). Today these needs are partly covered by cultivated forests, but natural forests will face an increasing pressure for logging. Additionally, deforestation for agriculture and energy is likely to proceed in tropical countries. On the other hand, the forest management paradigm changed during the last decades, emphasising sustainable management and ecosystem services, rather than wood production alone. As a consequence, forests must provide raw materials, preserve biodiversity and provide other ecosystem services such as the mitigation of greenhouse gas emissions through carbon sequestration. Because natural forests are at risk and need to be preserved, there is little doubt that managed forests - both tree plantations and 'renaturalised' forests - will continue to perform these essential roles in society.
In many regions, e.g. central Europe, forest production may have increased with global change due to the effects of increased CO2 concentration in the atmosphere combined with nitrogen deposition (Bascietto etal., 2004; Kilpelainen etal., 2005) and a longer growing season due to warming (Myneni et al., 1997). However, severe droughts can offset such gains (Raffalli-Delerce et al., 2004). That is the case of France and Portugal, where assessments of the impacts of climate change in forestry at the regional level have forecasted gains in productivity in the wetter northern regions and losses in the drier southern regions (Loustau et al., 2005; Pereira et al., 2005). In addition to the generalised drought effects on NPP, the change of carbon allocation towards roots will reduce the proportion of NPP available for stem growth, resulting in a greater decline in timber productivity than in NPP.
Changes in climate, e.g. increasing drought severity, will put trees under stress and may influence the distribution of other organisms, some of them essential for ecosystem function (mycorrhizae) as well as for the preservation of biodiversity. On the other hand, many observations suggest that plants subjected to drought stress may become more susceptible to insect attacks (Mattson & Haack, 1987). For example, plant water stress had a major role in promoting survival and growth of Phorachantha semipunctata larvae, an insect pest that attacks Eucalyptus globulus outside Australia (Caldeira et al., 2002). The consequent tree mortality may lead to this crop becoming unviable in drought-prone areas.
Maintaining forest productivity with increased aridity may imply diverting to the economically interesting species the largest possible proportion of water supply. This may be achieved using deep-rooting genotypes (if possible), site preparation techniques that can improve water availability (e.g. by removing hardpans that limit rooting depth) and increasing the ratio of transpiration/actual evapotranspiration (T/AET). The main non-productive portion of AET is the evaporation loss of rainfall intercepted by the canopies, which may account for 25-75% of overall evapotranspiration (McNaughton & Jarvis, 1983). Very little has been done to increase T/AET, except manipulating tree density. Yet, as mentioned above, the option of using more water for tree production is constrained by the need to allow enough run-off and drainage to maintain ecological and socioeconomic services such as river flows and aquifer recharge.
The reduction of stand density (thinning) may decrease the interception losses and increase the amount of water available per remaining tree, enhancing their survival and growth. Thinning, however, may produce changes in the physical environment below the canopy (e.g. increasing light, higher temperature, changes in soil organic matter decomposition rates), which favour the development of under-storey vegetation. This will compete with canopy trees, thus offsetting the effects of water reallocation in the stand. Furthermore, in fire-prone environments, the development of understorey vegetation can pose an additional risk, as grasses, shrubs and juvenile trees are more quickly affected by droughts than deep-rooted mature trees, increasing the amount of highly inflammable biomass (see also Section 6.5). The association of fires with frequent severe droughts and, eventually, with pests and diseases may bring about drastic changes in the environmental settings for forest development, requiring an adaptive approach to forest management.
During the last decades forest management has emphasised sustainability of resource use and ecosystem services. While the current practices are able to cope to some degree with the effects of climate fluctuations and its associated impacts, large gaps still persist in our knowledge of forest ecosystems functioning and their responses to multiple disturbances. Furthermore, given the long timescale of forest growth, the present climate change process may be too rapid for the natural adjustment of forests to the new environments. Improved ecosystem monitoring and research are therefore key steps in management under a rapidly changing climate, and should be incorporated into the management process itself (Dale et al., 2001). The adaptive management approach, which considers learning as a part of the management process, may be essential especially because greater climatic variability and increased frequency of extreme events are expected.
Was this article helpful?