Science & Society Box 1. Atmospheric flows The atmosphere draws moisture from the oceans as water vapour. Of the rain, snow, and hail (‘precipitation’) that condenses from this vapour and falls on land, approximately two-thirds returns to the atmosphere as water vapour, most of which falls on land once more [1,2,4]. Some terrestrial regions are major sources of water vapour (e.g., Amazon Basin, East Africa, Western North America, and central Eurasia), whereas others are sinks (e.g., south of the Amazon Basin, West Africa, Northeast North America, and much of Mongolia and neighbouring China and Siberia). Other regions, hemmed in by mountains and prevailing winds, cycle water within a more limited area (e.g., eastern slopes of the Andes and the Tibetan Plateau) [4]. Decreasing evaporation in one region may reduce precipitation in downwind regions whereas measures that conserve atmospheric moisture flows will help maintain downwind rain.
occurs at lower moisture levels than in air without them. These differences are sufficient for such particles to influence cloud formation and rainfall. Most atmospheric particles detected over the Amazon forest are biological (e.g., bacteria, pollen, and fungal spores). Poehlker and colleagues [8] recently detected that many such particles contain potassium salts, these being an indicator of their biological origins given the general scarcity and tight nutrient cycling of potassium in these ecosystems. Particle dynamics are poorly characterised, but we know that small atmospheric particles tend to grow with deposition of partially photo-oxidised volatile organic compounds (VOCs) [9]. As the particles grow larger, they become more effective in gathering liquid water or ice, thus seeding clouds or rain and returning to Earth. An estimated 90% of VOCs also have a biological origin. The distribution of these compounds is known to be highly variable, and each has its own behaviours, relations, and impacts [10]. Isoprene is the most abundant and best characterised: being produced mainly by certain plants when under heat stress. Many tree species emit isoprene, but C4 grasses do not; therefore, atmospheric concentrations of this compound are generally higher above tropical forests than above neighbouring grasslands. The ability of isoprene to increase cloud cover via increased condensation nuclei during periods of heat stress, thus lowering temperatures and perhaps stimulating rain, may help to regulate regional
Box 2. Trees are special All higher plants actively control transpiration to ensure efficient water use (controlling stomata in response to light and other environmental conditions) [15], but trees provide the main conduit of transpired moisture for the atmosphere. Leafy tree canopies produce flows of water vapour that, per unit land area, are typically more than ten times greater than from herbaceous vegetation, and also significantly surpass those from wet ground or open water. Most tropical forests evaporate the equivalent of more than 1 m of water each year and some achieve more than 2 m [3]. Deep roots can access deep moisture when surface soils are desiccated. Roots and litter ensure improved infiltration (reduced runoff) and some roots facilitate vertical movements of moisture within the soil profile that can benefit surrounding shallow-rooted vegetation. When rain is scarce, moisture intercepted by mountain or coastal forest canopies from cloud or fog may sometimes be the principle source of water. Trees can transpire moisture at higher rates than they can draw it from the ground for periods of several hours by using water stored in their stems and replenished overnight [3]. 210
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temperatures and climates more generally. Overall relations may be much more complex. Park and colleagues [10] recently used a mass spectrometer to detect and track 555 distinct VOCs over a Californian citrus orchard. Based on their chemical properties, the authors judged that most of these compounds should have an important role in the formation, growth, and dynamics of aerosols. Much remains to be clarified, but a biologically significant influence on local climate and rainfall appears likely. Winds and regional patterns The relations among land cover, wind, and rainfall have been subjected to recent scrutiny [2]. Spracklen and colleagues [11] combined satellite observations with other data to track air movements and rainfall across the tropics. For each location, they identified the trajectory of the local wind over the previous 10 days and estimated the mean leaf-area index (LAI) of the vegetation traversed. Winds that traversed forest (high mean LAI) typically produced more than twice as much rain as those that had traversed open land (low mean LAI). The authors used their results to estimate crudely how forest loss might reduce rainfall assuming (among other things) that wind patterns would remain unchanged. They predicted a 12% and 21% decline in wet and dry season precipitation, respectively, by 2050. However, if regional winds are also vulnerable to forest loss, the reality may be worse. Makarieva and colleagues developed a theory describing how evaporation and condensation generate atmospheric pressure gradients [12]. (For an overview aimed at biologists, see [3].) One implication is that regions, such as forests, that maintain high evaporation rates relative to surrounding regions become low-pressure zones. These zones draw in moist air that converges and rises, generating annual rainfall that surpasses (typically at least double) local evaporation. The implied ability of forests to draw in moist winds explains why rainfall remains high thousands of kilometres from the ocean in the continental interior of the Amazon, Congo, and Siberia, whereas elsewhere, over level nonforested land, rainfall declines with distance inland (halving every few hundred kilometres). According to Makarieva and colleagues, this inland rainfall is due to forest transpiration and any significant loss of forest, especially on the coast, risks switching the continent from wet to dry. Using monthly rain data, Makarieva and colleagues showed that, during the boreal summer in Siberia, when trees are actively transpiring, rainfall declines little with distance inland, whereas a decline appears in winter when transpiration has ceased (Figure 1) [12]. In the same study, the authors also looked at the seasonal variation in rainfall over extensive tropical forest regions and found less variation deep within continents far from the oceans than at more coastal locations, which is exactly what is expected if forests actively stabilise local climates [12]. Looking forward Considering how vegetation might influence atmospheric moisture, rainfall, and climate more generally, highlights many potential implications. Declining forest cover will lead to reduced flows of moisture to the atmosphere and