Function of living landscapes

  Regional cooling strategy to secure agricultural productivity
                                                             Haruka Yoshimura, Ph.D.

Fig. 1. Landscape of high primary productivity for the cooling strategy in agricultural areas. To construct high LAI vegetation structure, spatial design with diverse plant species of canopy forming tall trees, mid-story small trees/shrubs and under-story/ground covers (e.g., small shrubs, herbs, grasses, climbers) should be sought out.

Urgent need for regional cooling strategy to secure agricultural productivity 

Intensifying Climate Change impacts: altered precipitation patterns and increase in temperature especially during the summer with the higher solar radiation energy are already realities. Negative impacts of the climate trends on crop production are already evident in several regions of the world (IPCC: Porter et al. 2014).

Recent studies have demonstrated a great negative sensitivity of cereal productivity to high temperatures—high temperature stress. Rice (Oryza sativa) is one of the most important food crops and is a staple food for much of the world’s population. Although the strains of rice are native to tropical and subtropical regions, they are strikingly vulnerable to high temperatures. The negative impacts of high temperatures on rice production are already being seen in China, India, Indonesia, the Philippines, Thailand, Vietnam (e.g., Welch et al. 2010), and Japan (e.g., Sakata et al. 2012).

In the spikelets during the flowering period, exposure to high temperatures exceeding around 34°C induces sterility (Jagadish et al. 2007) owing to poor anther dehiscene and low pollen production (Matsui et al. 2000, 2001; Prasad et al. 2006). Increased nighttime temperatures during the critical stages of development reduce rice yield (Peng et al. 2004; Welch et al. 2010) and quality (Ambardekar et al. 2011), through spikelets sterility, increased respiration rates (Mohammed and Tarpley 2009), and decreased assimilate accumulation (Morita et al. 2005). In major Asian rice-growing countries, the night temperatures have increased faster than the day temperatures (Peng et al. 2004; Padma Kumari et al. 2007), and the trend is projected to continue into the future (IPPC: Christensen et al. 2007).

Using regional-scale statistical analyses, Schlenker and Roberts (2009) have shown that yields increase with temperatures up to 29°C for corn (Zea mays), 30°C for soybeans (Glycine max) and 32°C for cotton (Gossypium spp.) but that temperatures above these thresholds are very harmful.

Wheat (Triticum aestivum) is very sensitive to high temperatures. High temperatures during anthesis or the “flowering period” (e.g., Ferris et al. 1998) and grain-filling phase can reduce yield and quality (e.g., Farooq et al. 2011). The optimum temperature for wheat anthesis and grain filling ranges from 12 to 22°C (Farooq et al. 2011). In wheat, high nighttime temperatures are detrimental, decreasing gain yields through spikelet sterility and decreased photosynthesis (Prasad et al. 2008a).

Sorghum (Sorghum bicolor) is an important grain crop in semiarid regions in the world, because of its tolerance to drought and water-saving qualities, the cereal has been called the “camel of crops.” However, high temperature stress can cause significant decreases in sorghum grain yields. The optimum temperatures for maximum dry matter and seed yields of grain sorghum are around 27/22°C (daytime/nighttime temperature). Although season-long (from emergence to maturity) growth temperatures in the range of 36/26 to 44/34°C do not influence leaf photosynthetic rates, high temperature (≥36/26°C) significantly decreases seed set, seed number, seed size, and seed yields (Prasad et al. 2006). Even short periods of high temperature exposure during the pre-flowering and flowering stage and early stages of seed development reduce sorghum grain yields (Prasad et al 2008b).

High temperature is a crucial constraint to yield and the quality of crops. Even short-term extreme temperature events can be critical, especially during the key stages of development. Only a few days of extreme temperature (above 32°C) at the flowering stages of many crops can drastically reduce yield (Wheeler et al. 2000).

Most cereal grains are currently grown in regions where current temperatures are already close to optimum for production. Therefore, any further increases in the temperature that exceeds thresholds during the sensitive stages or extreme climate episodes could threaten food security (e.g., Gornall et al. 2010).

Construction of the cooling landscapes: integrated with mitigation and adaptation strategy for climate change

Urbanization is an important component of global land modification (Vitousek et al. 1997) and urban heat island phenomenon is an extreme case of regional climate change owing to land use modification (Foley et al. 2005). As a fundamental solution for urban-heat-islands mitigation, increasing tree canopy cover is becoming regarded as a major promising strategy (Akbari et al. 2001; Akbari 2002; Mcpherson et al. 2005, 2011).

Recent studies have shown that the density of vegetation or leaf area index (LAI) is an important indicator in the cooling effect (Hardin and Jensen 2007; Jenerette et al. 2007; Peters and McFadden 2010). LAI is generally defined as one-sided green leaf area per unit ground area in broadleaf canopies, and variously defined (projected or total) in needle canopies. The LAI is zero on bare soil, farm land prior to crop emergence and civil engineering structures made from concrete and asphalt without vegetation cover and is low in grasslands, as the leaf stratified structure is thinner and simpler. A recent field measurement shows tree shade can provide effective cooling, whereas grass has little effect (Armson et al. 2012).

All life on earth, including human beings, stems primarily from the conversion of solar radiation energy by photosynthesis into biochemical energy. Primary production is the estimated amount of biomass fixed atmospheric carbon dioxide using solar radiation energy through photosynthesis and subsequent biochemical activities. Primary production and LAI roughly correlate, as primary production is closely related to light interception, which is mainly determined by LAI. Terrestrial ecosystems have spatial variation in structure, depending on LAI. Biomes with high LAI have high primary productivity, as the highly stratified structure of canopy foliage effectively intercept solar radiation for photosynthesis. A high LAI of over 8 is the level found in mature forests such as temperate broad-leaved evergreen forests and tropical rain forests (Odum 1971; Whittaker and Likens 1973; Chapin 2003). The mature forests with high LAI have evolved in such a way that the vertically stratified structure of canopy foliage effectively alters the solar radiation balance (e.g., Yoshimura et al. 2010) and cools the air by transpiration.

Micro climate amelioration in agricultural areas is essential, as high temperatures already threaten food security. It is well known that ‘urban heat island’ effect takes place at night (e.g., Grimm et al. 2008), when buildings and streets radiate the solar heat absorbed during the day (Kalnay & Cai 2003).

Even in agricultural areas, land has been modified by expanding human infrastructure such as highways, roads, and irrigation canals lined with concrete. Consequently, nighttime temperatures are elevated as concrete, asphalt and water bodies without riparian forests absorb the solar radiation during the day and radiate it back out as heat at night. Tree foliage shields solar radiation and keeps soil (Jones et al. 2003) and water bodies (Dan Moore et al. 2005) cool underneath the canopy, which in turn keeps the nighttime temperature cool. The cooling strategies incorporating landscapes with high LAI vegetation structure is applicable in agricultural areas (Fig.1.).

Restoring wetlands and streams in agricultural areas for high LAI vegetation

The cooling strategy of high LAI tree coverage through the interception of solar radiation and transpiration requires adequate water. Endreny (2008) advocates naturalization of urban watersheds hydrology to ensure the cooling function of vegetation. The cooling strategy of irrigated naturalized watersheds hydrology is applicable in agricultural areas. Infiltration of human-made wetlands such as irrigation channels, ponds, canals, and ditches provides a low-cost, self-sufficient method of natural irrigation for high LAI vegetation structure. While the Ramsar Convention (www.ramsar.org) is an inter-governmental conservation treaty primarily for the importance of wetlands as a habitat for migratory birds, human-made wetlands such as irrigation channels, ponds, canals, and ditches utilized for ecosystems’ cooling function result in a wide range of values and functions including groundwater recharge and discharge, storm water control, maintenance/restoration of clear water, and a habitat for valuable fishery species (Fig. 2.). The cooling strategy should be integrated within broader catchment management strategies (Bernhardt and Palmer 2007).

Biodiversity and primary production (LAI) are linked (e.g., Loreau et al. 2001). To support biodiversity, land planning of diverse habitats is essential (Fig.1.).

Strategic planning and regional design solutions appropriate for agricultural areas are urgently needed, as current trends of high temperatures already threaten food security.  

                   
Fig. 2. Cool water creek to cool down irrigation water and to ameliorate microclimate. Human-made wetlands with riparian forests provide integrated ecosystem services. See text for explanation

Acknowledgements This research was partially supported by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in-aid for Scientific Research (C)(2); Project number: 11650553.

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