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Trends in Plant Science April 2014, Vol. 19, No. 4
Special Issue: Systems Biology
How plants water our planet: advances and imperatives Douglas Sheil1,2,3 1
Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, 1432 Ås, Norway School of Environment Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia 3 Center for International Forestry Research, Bogor 16000, Indonesia 2
Most life on land depends on water from rain, but much of the rain on land may also depend on life. Recent studies indicate that vegetation, especially tree cover, influences rain and rainfall patterns to a greater extent than is generally assumed. Here, I briefly highlight some of these findings to show that vegetation sciences will have an increasing role in understanding climate and its vulnerability to changes in land cover. Rainfall patterns in a changing climate The lives, wellbeing, and environment of most people depend on reliable rainfall. Between 1900 and 2013, over 11 million people lost their lives due to drought and over 7 million lost their lives due to floods, whereas over 5000 million required emergency assistance due to one or other (http://www.emdat.be). Vegetation influences many aspects of the climate on Earth. Large-scale loss of forest cover is typically associated with increased seasonality, reduced cloud formation, and less rain. Declining rainfall and weakening monsoons have been linked to deforestation in various regions of the tropics, although the nature and significance of these landcover influences remains uncertain [1–3]. Recent results indicate that vegetation is more important than was previously assumed. Climate researchers view the atmosphere, oceans, and land surface as components in a physical system. Despite major investments in incorporating land cover in climate simulation models, much remains uncertain, especially concerning the influence of land-cover change on cloud cover and rain [1–4]. One recent commentary on climate models noted that rainfall over land remains hard to simulate because it is largely determined by ‘unresolved processes’ [5]. This is a challenge in itself and also represents the ‘main limitation in current representations of the climate system’ and ‘a major roadblock to progress in climate science’ [5]. Nonetheless, there has been progress concerning the flows of atmospheric moisture within and among regions (Box 1) and the important role of tree cover in generating atmospheric moisture (Box 2). Here, I briefly examine some Corresponding author: Sheil, D. (D.Sheil@cgiar.org). 1360-1385/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tplants.2014.01.002
recent studies in which plant sciences cast additional light on these processes. Atmospheric moisture The share of atmospheric moisture derived from vegetation is higher than previously recognised. This transpired water (vapour emitted from plants through their stomata) can be distinguished from other evaporated water (vapour drawn from moist surfaces and water bodies) by its influence on isotopic ratios of oxygen (18O/16O) and hydrogen (2H/1H). Jasechko and colleagues evaluated these isotopic ratios in the water from 73 lakes and inland seas worldwide [6]. By using additional data to summarise the flows in and out of each catchment, they estimated that transpiration produces 80–90% of the atmospheric moisture derived from continents. This figure substantially surpasses previous estimates (20–65%; see [6]). Even in deserts, transpiration contributes approximately three-quarters of the land–atmosphere water flow [6]. In some regions, this likely reflects deep-rooted trees, such as Boscia albitrunca (Capparaceae), which reaches 68 m beneath the Kalahari Desert in southern Africa [7]. Transpiration has a profound impact on the energy budget of the atmosphere. The 62 000 8000 km3 of water released to the atmosphere each year by vegetation [6] is not only a source of cloud and rain, but also lowers local temperatures. Vaporising water consumes nearly half the solar energy reaching the surface of the Earth (approximately 33 Wm 2 of the approximately 70 Wm 2), causing local cooling [7]. Cloud cover influences both planetary albedo (a measure of the solar radiation reflected into space) and radiation of heat energy (water vapour is a powerful ‘greenhouse gas’). Until the processes underlying vegetation control of the water cycle are resolved, the potential impact of land-cover change on the regional and global temperature regimes cannot be estimated with confidence. If the share of atmospheric moisture derived from vegetation is larger than previously acknowledged, then changes in vegetation may also have greater impacts. In the next sections, I consider some of these influences in greater detail. Condensation Condensation occurs when air is saturated with water. The threshold depends on temperature and also on the presence and nature of any surfaces. All else being equal, saturation in air containing suitable surfaces (typically aerosol particles or droplets called ‘condensation nuclei’) 209
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
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Figure 1. Summer (July; red line) and winter (January; broken blue line) precipitation in the Eurasian boreal forest transect at 618 N plotted as percentage of the mean Atlantic oceanic precipitation versus distance from the Atlantic Coast (prevailing zonal winds are inland from this coast). The lower panel indicates elevation (black over grey), forest cover (green symbols), and open water (solid blue). Calculated and drawn from data presented in [12] with the assistance of the authors.
other changes due to modified condensation dynamics. More speculatively, if transpiration from continuous natural forests actively generates and stabilises winds responsible for long-range transport of moisture, then both forest loss and increased atmospheric CO2 (reduced stomatal opening leads to lower water use) will modify wind patterns and likely lead to reduced rainfall within continents. Long-term observations already suggest a general and unexplained decline in wind speeds over the tropics and mid-latitudes [13]. Forest-cover change may also explain an apparent eastward shift of the rainfall zone over South East Asia, with its potentially wide-ranging regional and global consequences [14]. As the behaviour and dependencies of the climate on Earth become increasingly well understood and captured by models, many of the remaining uncertainties are likely to involve biological systems (terrestrial and marine). Addressing these uncertainties should offer new insights into understanding many basic topics, such as large-scale hydrology, palaeoclimates, macroecological gradients, evolutionary dynamics (e.g., [15]), climate change, and alternative stable states (as discussed in [3]). However, the potential impact of tropical deforestation for people and our planet makes the search for understanding such threats a moral imperative. Plant scientists have a major role to play in this search. Acknowledgements I thank Anastassia Makarieva, Daniel Murdiyarso, Miriam van Heist, Peter Bunyard, Victor Gorshkov, and referees for help and feedback.
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