Drought Management and Resilience Building: From Ecosystem to Plant Functional Traits
by David Ramirez
Challenges posed by climate change and land use
As part of its strategic objective of enhancing food security, CIP is proposing a diversification and intensification of cereal-based cropping systems in Asia using early maturing, or “agile” potatoes. The potatoes that will be introduced into cereal cropping systems need to be agile, or precocious genotypes with growing periods of 70-80 days and the ability to endure various biotic and abiotic stresses. At the same time, intensification will require responsible agricultural practices to avoid water waste, nutrient loss, carbon emissions among others.
Central Asia is dominated by grassland and rangeland, followed by sparse vegetation deserts. Agricultural land covers more than 21% of Central Asia, and roughly 30% of this cultivated land is under irrigation. Uzbekistan has the largest irrigated area (>80% of the cultivated land), whereas Kazakhstan and Kyrgyzstan have the lowest (<13%).
The Intergovernmental Panel on Climate Change (IPCC) scenarios for the region predict that atmospheric temperature will increase, which will result in greater evapotranspiration, canceling out the moderate increase of precipitation predicted for some areas. Increases in annual temperature have already been reported for the region, which has experienced a noticeable reduction in ice and permanent snow cover as glaciers melt and snowfall decreases. Another problem that the region faces is increasing soil salinization (both in rate and extension), which is the result of water withdrawal (e.g. Aral Sea reduction), saline groundwater levels and suboptimal irrigation and drainage management. The abandonment of irrigated areas has consequently increased.
Resilience, Drought and Scales
Resilience is the capacity of an ecosystem to absorb shocks without losing its structure and function. Ecosystems move into different states through succession, and some of those states are more resilient than others. However, external factors such as resource overuse, inappropriate management and natural disasters can push an ecosystem into an undesirable state that is difficult to move out of. According to the Millennium Ecosystem Assessment classification, ecosystems provide “provisioning services,” which include food, fiber, fuel, genetic resources and clean water, and “supporting services,” which include carbon sequestration, nutrient or water cycling and soil formation. Inappropriate management practices or resource overexploitation erode the capacity of an ecosystem to provide these services and can push it into a highly stable, degraded state. One of the mechanisms ecosystems have for avoiding this is response diversity: if a perturbation causes the loss of a function, response diversity allows for it to be compensated by a homologue function.
Response diversity reduces sensibility to disturbance and fosters the capacity to adapt to varied scenarios, which helps to enhance operating resilience across different scales.
CIP’s Durbek Khalikov showing an experiment with the Partial Root-Zone Drying (PRD) technique
In the context of resilience, there are two types of droughts: meteorological drought and agricultural drought. Meteorological drought is when total precipitation is below the minimum required to maintain fundamental ecosystem services. Agricultural drought is when there is not enough plant-available soil moisture in the root zone. Agricultural drought is driven by inappropriate land management and is very common. For example, in rain-fed savannah cropping systems, only 30% of rainfall reaches the root zone because 70% is lost to evaporation, interception, drainage and surface runoff. Agricultural drought can be reduced by good soil management. For example, some soils in the Sahel are prone to the formation of a crust that reduces water infiltration. De Goede and Brussaard (2002) tested the incorporation of mulch (wood shavings) and termites into the soil and found that after three years, agricultural drought was reduced and plant cover and diversity increased significantly. The termites accelerated the decomposition of the wood shavings, adding organic matter to the soil, while the canals they constructed to link their colonies increased soil porosity and water infiltration. However, if water scarcity crosses a threshold, nothing can be done to produce more food or other ecosystem services, and the only solution is to rely on “resilience parachutes” such as food relief, cereal banks, social networks and conservation.
Conversion of natural systems for agriculture causes depletion of soil organic carbon (SOC) pools (by approximately 60% and 70% in temperate and tropical soils respectively). Grasslands and sparse vegetation are the main land cover in Central Asia, and data for the last 80 years show that SOC has increased in these ecosystems, which indicates that rangelands are carbon sinks. However, there was a net loss of soil carbon in cultivated areas, both rainfed and irrigated. Central and South Asia are considered global soil degradation hotspots, making these regions high priorities for soil restoration and the resumption of carbon sequestration. The concept of response diversity can inform the implementation of alternative mechanisms to compensate for the loss of carbon sequestration. Some authors propose the conservation, restoration, and appropriate management (avoiding overgrazing) of the rangeland ecosystems in order to compensate for carbon emissions from the intensification of agriculture. At the same time, it will be necessary to improve the soil carbon sequestration capacity of cropping systems through practices such as no-till farming, efficient irrigation (drip irrigation, partial root-zone drying) water conservation and harvesting, nutrient management (mulching) and other conservation agriculture strategies.
Two studies provide insight into to how to measure drought resilience in breeding programs. Chapuis et al. (2012) analyzed 14 environmental contexts (France, Chile and Hungary) and 19 maize hybrids, and estimated a drought index as the mean of soil water potential (a physical variable related to soil water availability) during a period spanning from 10 days before to 10 days after male flowering (a key developmental stage). The resilience for each hybrid was estimated as the slope of the response curve between seed numbers per plant and drought index in each environment. Low slope (high resilience) means that the variation of drought index (in different environments) has low effect in yield. This result was coherent with the resilience estimated in a phenotyping platform (phenodym) of potted plants in greenhouse conditions. In another study, Kahiluoto et al. (2014) defined 12 phenology-based agro-climatic parameters that are critical for barley performance in Finland. Two kinds of diversities were calculated using the Shannon Diversity index: a) Type Diversity was based on the numbers of cultivars used in an area, while b) Response Diversity to weather was based on the yield response of genotypes to the agro-climatic parameters. Despite a steady increase in barley cultivar diversity, the diversity of response to weather declined during the last decade in the regions with highest barley productivity in Finland.
Experiment with mulch
CIP’s strategic objective number two (SO2) highlights the requirement of “resilient and competitive” potato varieties. However, what kind of traits must be prioritized to get these resilient genotypes? Some authors highlight two main reasons for choosing a trait: It must be scalable at different levels (from the leaf to the canopy) and it must be integrative across the growing period. François Tardieu from the French National Institute for Agronomic Research (INRA) stresses that the selection of a trait should depend on the drought scenario. Thus, in mild water stress conditions (which is the most common scenario in agriculture) it is much better to prioritize individuals with a risky or opportunistic behavior, such as higher carbon assimilation, stomatal conductance and transpiration, that enables to take advantage of water pulses. However, under terminal or severe drought, it is more important to select plants that reduce stomatal conductance to save water, which increases the intrinsic water use efficiency, defined as assimilated carbon per transpired water. It doesn’t make sense to search for genotypes with high water use efficiency in scenarios of mild water stress. A resilient genotype is one that has a high diversity of traits to respond to different drought scenarios.
- Response diversity is crucial to keeping an ecosystem in desirable states. Some proxies used to measured response diversity can help the assessment of resilience at different scales.
- SOC is a component that reflects environmental health, and its measurement and monitoring are crucial for assessing land management. CIP and EMBRAPA have developed some techniques for measuring SOC stocks in the field using Laser-Induced breakdown and Fluorescence Spectroscopy technologies.
- The definition of drought scenarios is important for prioritizing the choice of traits, drought targeting and phenotyping. The identification of key environmental factors during some critical phenological stages (i.e. tuber initiation onset in potato, conversion of adventitious to reserve roots in sweetpotato), type of agriculture (rain-fed or irrigation) and the simulation of water demand via modeling will be crucial for the drought targeting of root and tuber crops based on experiences in cereals.
- Research on the interaction of drought with biotic (microorganism infections, pests) and abiotic (heat, salinity) stresses will be necessary for achieving SO2 goals.
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Our research is conducted under the CGIAR Research Programs (CRP) on Climate Change, Agriculture and Food Security (CCAFS), Water, Land and Ecosystems (WLE), and Root, Tuber and Bananas (RTB).