A fine addition to the ecophysiology bookshelf

The deserts of the world have particular significance for plant physiological ecologists. In deserts, the influence of abiotic factors over structure and function is enhanced relative to biotic factors, making it relatively easy to identify the environmental significance of many features of desert plants. Deserts thus have been a proving ground for developing theories and techniques in physiological ecology, and many if not most of the leading figures in the field have at some point spent time in “the desert.” In recognition of the recent advances in desert ecology and physiology, the publisher Springer has produced 20 short books in a series entitled Adaptations of desert organisms. Seven of the titles address major advances in desert plant biology, with three focusing on the physiological ecology of desert plants. Of these, the Physiological ecology of North American desert plants by Stan Smith, Russ Monson, and Jay Anderson warrants special attention, as it attempts an integrated synthesis of leading theories and research advances in physiological ecology. This review will focus on Physiological ecology of North American desert plants, after a brief discussion of the other six plant biology books in the Adaptations of desert organisms series. These six titles are fairly specialized, addressing germination biology (Seed germination in desert plants, Y. Guttermann, 1993, 253 pages), mineral nutrition (Mineral Nutrition of Desert Plants, A. D. Day and K.L. Ludecke, 1993, 117 pp.), psammophytes (Plants of Desert Dunes, A. Danin, 1996, 177 pages), structure (Structure-function relations of warm desert plants, A.C. Gibson, 1996, 215 pages), economic plants (Ecophysiology of economic plants in arid and semi-arid lands, G. E. Wickens, 1998, 343 pages), and dispersal (Dispersal Biology of Desert Plants, K van Rheede van Oudtshoorn and M. W. van Rooyen, 1999, 242 pages). Gibson's Structure-Function Relations of Warm Desert Plants initially appears to be a synthesis of structure and physiology, however, it is primarily an overview of the anatomy and morphology of desert species, with a relatively limited treatment of the associated physiology. With the exception of Plant Nutrients in Desert Environments (which is a very general summary of the mineral nutrition of economic plants and says little about the nutrition of wild plants of arid regions), all of these specialized books are valuable references. Although their price of $99 to $192 may preclude most folks from purchasing them, they provide valuable examples and case studies of the unique manner in which plants survive harsh, arid environments. Most of these titles are richly illustrated to show how specific features control the timing of germination and dispersal, or promote survival in shifting sands and during severe drought. Many wonderful examples are to be found for teaching or term papers, or simply enriching a field trip into an arid region. One notable contribution of these books is that they include desert species from around the world and thus allow for comparisons of many of the exotic forms to be found in diverse extremes of the planet. Wicken's Ecophysiology of Economic Plants of Arid and Semi-Arid Lands is particularly useful in this regard. For those familiar with only North American deserts, his comparisons of plants from the Asian, Australian, and African deserts will be a valuable, mind-expanding experience. The ethnobotanical value of Wicken's text is also substantial. The last third of the book describes the biology and use of some 170 species, most of which will be unfamiliar to readers in the developed world. Because of the specialized focus of these six titles, however, the synthesis and integration of physiology and ecology are not emphasized, and in this light Smith et al.'s Physiological Ecology of North American Desert Plants stands out as a unique contribution. Physiological Ecology of North American Desert Plants emphasizes the carbon, water, energy, and nutrient relations of desert plants of varying growth form, with comparatively little treatment of structural adaptations, herbivore defense, and reproductive strategies. The emphasis on carbon, water, energy, and nutrient relations—the bread and butter topics of plant physiological ecology—is a legacy of developments from the 1960s and 1970s that focused the discipline on biochemical controls and the dynamics of material and energy flux (Mooney, Pearcy and Ehleringer, 1987). Desert studies were instrumental in the development of this new focus, as they were among the first to incorporate biophysical theory such as the water potential concept into experimental protocols. With the development of sophisticated gas exchange equipment, desert biologists were quick to exploit the new technology to assess trade-offs between water use and carbon acquisition. Later, when the theory linking carbon isotope discrimination to water use efficiency was developed, desert ecophysiologists were again pioneers in using this understanding to integrate seasonal and lifetime water use profiles of various functional types in heterogeneous environments (for example, see Ehleringer and Cooper, 1988). This ability to integrate over multiple levels of complexity is key to current efforts to scale to the global level and explains in part the major contribution that physiological ecology has made to global change research. In some ways, it is odd that desert research has had a major influence over the development of physiological ecology. From an anthropocentric viewpoint, deserts have been historically considered barren wastelands, with little intrinsic value. North American deserts were no different, and up until the 1950s, they were underpopulated backwaters that engendered little interest from society at large. So how is it that the deserts have had such an important role in plant biology, particularly physiological ecology? At one level, the focus on desert plants results from the extreme biology of so many resident species. An understanding of why these plants are unusual has led to a better understanding of adaptive mechanisms to environmental stress, and thus a more thorough view of evolution and physiological responsiveness. However, this has also contributed to a mistaken notion in the popular mind that deserts are freak shows populated by botanical oddballs. While an interest in extreme adaptations explains some of the work in desert systems, it does not explain how it was that so much research came about in biomes that just a few decades ago were outside the view of what most funding sources considered worthy of attention. Why the change? In the North American case, the research emphasis on desert systems has resulted in considerable part from a combination of political and social factors associated with major historical events on the world scene. For one, the arid west of North America was the home of the cowboy, who became the romantic icon of the ranching industry. Ranching in the west was valuable as a means of initially populating the interior, both because it provided jobs and was the main source of protein for the many mining and railroad towns established throughout the region (Young and Sparks, 1985). Because of the peculiarities of the American political system that gives disproportionately high political influence to the western states in general, and ranching in particular, substantial federal resources were allocated to the establishment of one or more land grant universities in each state of the arid region. Many of these land-grant universities were also associated with federal agriculture research stations, whose focus was the conservation of arid rangelands. Together, the land-grant universities and federal research stations brought to the region not just basic scientists, but also applied scientists, many of who were quick to adopt the new techniques of ecophysiology. Dwight Billings, for example, who is considered by many to be the father of modern physiological ecology, began his academic career at the University of Nevada in the early 1940s. Later, a number of his students conducted significant work at universities throughout the arid west. Both World War II and the Cold War played an important role. World War II led to substantial economic, population, and political growth in the western states, which later paid dividends in that the land grant institutions were better funded and staffed (Nash, 1985). After the war, much of the arid west became a locale for military research, most notably at the nuclear test facility in the Nevada desert. Associated with this activity was the funding of research on desert ecosystems. For example, the atomic tests of the 1950s through early 1970s required assessments of radiation effects on local biota, but the funding for this work also supported background work on the physiology and ecology of desert vegetation (Wallace and Romney, 1972). Importantly, the establishment of the Carnegie Institute of Washington Desert laboratory in Tucson, Arizona, and later the desert work of the Carnegie laboratory team at Stanford, California provided leadership and advanced training in desert ecophysiology (Billings, 1985). One of the important contributions of the Carnegie group is that they networked extensively with leading international researchers, thereby bringing the latest advances from around the world into the local arena. While Smith et al. do not address this background history, their book is a nice testament to the work of the many hundreds whose opportunities to study in the “arid wastes” resulted from much larger historical contingencies. As Smith et al. demonstrate, the consequence of the desert research in North America is a much-improved biological understanding that leaves us well poised to deal with some of the serious global challenges of the coming century. Physiological Ecology of North American Desert Plants is loosely organized into three sections. The first section covered in Chapter 1 provides a background description of the geography, climate, soils and vegetation types within the deserts of North America. Herein, Smith et al. set forth many important distinctions. First, the deserts of North America are generally not true “deserts” as rigorously defined, but instead reflect a common perception of the general public. They are clearly arid systems, more properly called steppes or scrubland, with levels of precipitation and primary production that are above that of true deserts such as the Sahara. In recent decades, however, the North American deserts have been substantially desertified by overgrazing, biological invaders, and possibly climate change, so that the current systems increasingly resemble true deserts. The pattern and mechanism for the desertification of the arid lands of North American are common themes throughout the book and are treated here better than almost anywhere else. Second, the North American deserts are young by world standards. Aridification of the western region has occurred over the past 15–20 million years, driven in large part by long-term drying of the earth's climate and uplift of the Sierra/Cascade mountains, which created a rain shadow blocking moist air flow off the Pacific. The role of climate factors in distinguishing the deserts is well delineated and sets up the discussion of plant functional roles. The Great Basin, or “cold” desert, is distinguished from the others by harsh winters and dry summers, while the “warm” deserts (the Chihuahuan, Mojave, and Sonoran) are distinguished from each other by the timing of precipitation. The Mojave desert receives most of its precipitation in winter and thus has a rich, dominated flora with relatively few perennial grasses. In contrast, the Chihuahuan desert has significant summer precipitation arising from monsoon air flow off the Gulf of Mexico. This supports a diverse flora of C4 grasses, many of them rhizomatous perennials that once formed extensive grasslands. The Sonoran desert has a bimodal pattern of winter and summer precipitation and is the warmest of the four. Its mild winters, coupled with high evapotranpiration, enable a subtropical flora of succulents and cacti to extend into the southwestern United States, thus providing the image of the lonesome saguaro and other “spiny, stunted and tenacious” xerophytes that dominate the popular literature of desert ecosystems (for example, see Abby, 1968). As Smith et al. point out, the plants actually found in the North American deserts represent a far greater collection of life forms than cacti and succulents, and Chapters 2 through 10 detail mechanistic explanations for the success of these various functional types. Chapter 2 represents the second section of the book in that it provides a general background of the major processes affecting performance of desert plants, particularly in relation to stress. Thus, C3, C4, and CAM (crassulacean acid metabolism) photosynthesis, drought adaptations, the role of osmotic adjustment, root/shoot allocation, and its influence on growth are all reviewed. of Chapter 2 as a general ecophysiology in a and be useful background for a plant ecology or ecophysiology However, because it substantial of photosynthesis, gas exchange and water this and much of what is advanced for many who have not a in plant physiology. For those who have the much is to be or for those this is not the book to about desert systems or physiological ecology. The third section of the book is by Chapters through which provide case studies of the major plant life forms in the deserts, and species of from deserts, or species of within the desert. is in these that one an for the in life and physiology in the Chapter the of the the major of the desert, with the of the of each of the warm deserts. While cacti and succulents the common image of deserts in the popular it is these that more than the vegetation of the North American deserts. The of these species is to their ability to severe drought on the soils of and that throughout much of the while of these species into of is to from physiological The of into the warm desert is limited by while the of is limited by In this the of the physiological performance of desert species are Thus, for example, that both have high levels of potential on a but because of that these species actually have The biology of desert species is also developed in this Both are but the are short (for example, only for a in is as few in a of may reflect from only one to but because of the of the may be over 10 their survival is As Smith et al. this change if land on attempts to economic of arid attempts are a common legacy in the arid have been of vegetation to Chapter and a that is common to warm deserts, is the first species emphasized, in large part because it leaves that a with drought. 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The economic value of may explain why one of the more and is Chapter which in the desert grasses, through a case study of the wild This is its discussion of the adaptive significance of the or growth that is common in the desert, but not in the warm deserts, rhizomatous C4 its Chapter has some notable For one, the of as a desert is because it is a relatively species generally found on soils with greater water Second, while the grasses, their treatment of the perennial of the warm desert is so the of the between the rhizomatous of the desert systems is much than the between This is a to some important Why is it that desert are while warm desert are more to be is also associated with this with the perennial C4 to be while the are this have Chapters and are relatively brief of plants that into desert and plants, In the the case study on This is because is in the warm deserts, both the and they While it for an between within a species that as or plants not using the the of the such as and the Washington The discussion of does out a number of valuable the that are to such as on but to do so they to water in above the water be because is not water to during into the of ground water is water in the and control has the of Thus, the establishment of and other is a to a more of The on plants emphasizes species, and Desert are for their high growth and which are as adaptations to growth and set during the of One means of high performance is to the leaves with high levels of for this desert have the in plants. second is to protein into leaves the leaves so as to the as it the plants are to of throughout the while a for only a few of the when the is Smith et al. these to show that they in levels of carbon but the do not on the relative and of each One point they is that desert have little for species, and are to severe drought. These examples some between perennial life forms that deal with extreme stress, and others such as desert that by during a when resources are The on plants the and physiological performance of and plants that are to during the dry these have been treated as botanical limited As Smith et al. point out, this is a mistaken because by in and is a significant source of in The case study in this is the of the Chihuahuan desert. This species has a for the study of plant adaptations in the desert and their discussion on the physiology of the and its adaptive For example, the in plants appears to be a mechanism for From a the of a plant is because it shows that the reproductive is not a to life in arid as is in The case study on and is an that may be the most in western North America other than was first over a ago and has in much of the desert In to an for water, because it a of during the dry Although most of the desert is a the of with during establishment has stands to if not all of the vegetation on a In is common to of the desert a series of to stands of with only a few plants and With each the from a desert dominated by to a In the summer of 1999, of the of the state of Nevada in the United to The for the region are dominated by provides little for or and its to will to lands during the the of of The towns of the mining be by towns from as ranching and dry up from have diverse into stands that little in the of and are to Smith et al. the dispersal of these invaders, but their treatment in its discussion of the mechanisms the success of these invaders, and the associated for As they both species through to both over a of both and more and in the both relatively large systems that from They are of and are to in that Importantly, Smith et al. the for example, dry high of the as well as the will be to control developed for these of the case study has an extensive but these are to the material in the than for and valuable The of the physiological understanding are in the and this of the basic biology into the of desert and that are some of the more of the In the Physiological Ecology of North American Desert Plants is a to the of all plant physiological if they the to the deserts also of the as many of the species are not illustrated by or and thus are just that will be unfamiliar to the contrast, a number of the other in the of Desert series have that to life the features of exotic plants many readers will The book does have in but for its limited it is a of the main features of the North American deserts and the major plant functional valuable is the focus on the mechanisms by which the have found is from their treatment that the challenges in with these will be however, through the understanding that this book is to that may some be to these ecosystems to their by

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