Plant functional biology

“To explain how plants are fitted to subsist in the precise environment that they occupy demands an elaborate inquiry into the form, structure, physiology and life-history of the plants, and an equally exhaustive analysis of their animate and inanimate surroundings. But to solve the still further problem as to the original source and evolution of the plants and of the whole community, necessitates a corresponding investigation relating to the immediate allies of these plants living under other conditions.” (page xiii in Schimper, 1903). [Schimper] “…asked whether particular peculiarities of a plant were truly adaptive, that is, whether they had been evolved for the specific purpose of enabling the plant to exist in its present surroundings, or to what extent they were previously possessed by the plant, thus enabling it to enter its present home. (page xv in Schimper, 1903). It is not a coincidence that physiological ecology, the discipline that has most influenced both editors and many of the contributors to this volume, traces its own intellectual antecedents to Schimper's late 19th-century perspective on the study of adaptation. So given this promising beginning and a long run of investigations into diverse aspects of plant function, why then is there still no coherent field dedicated to the study of the evolution of plant function? Ackerly and Monson suggest that adaptation has too often been assessed by comparisons among plant species from broadly different environments rather than by formal phylogenetic analysis, which is true enough but sadly ironic given Schimper's early appreciation of the need for comparisons among allied species. Broad comparisons outside a strict phylogenetic framework invite facile interpretations of the adaptive value of traits, an error that was recognized early on by Coulter (1909) and yet 70 years later still required Gould and Lewontin's (1979) echo and elaboration of the critique. Additionally, as Ackerly and Monson point out, actually establishing the adaptive value for traits is not trivial, especially for perennial plants where fitness must be assessed over long time spans. They also point out the proclivity of physiological ecologists to credit precision of measurements over their replication; the nature of traits that have attracted physiological ecologists and the technologies for measuring them do not lend themselves to the quantitative analysis of the genetic variation on which selection works. Finally, Ackerly and Monson point out that few people are able to embrace “the contrasting traditions of physiology, population biology, quantitative genetics, and phylogeny” that studying the evolution of plant function requires. The barriers to “waking the sleeping giant” in fact go beyond those enumerated by Ackerly and Monson. They remark in passing that “the integration of plant ecophysiology and ecosystem ecology over the past two decades has been much more successful than the integration between plant ecophysiology and evolutionary biology.” At the outset the emerging discipline of plant physiological ecology had affinities with population ecology and a comparative perspective that had evolutionary elements (Billings, 1957), which were not entirely forgotten in the intervening years (Mooney, 1976), but that became subordinate to the task of estimating primary productivity in ecosystems. This latter stream of inquiry benefited from the substantial infusion of funds associated with the International Biological Program, a type of societal investment not ever likely to buoy up studies of the evolution of plant function. Another obstacle that has long challenged and confounded the study of plant adaptation is the intriguingly high degree of plasticity typical of plants. Because of plasticity, botanists have often been of two minds about the very concept of adaptation, sometimes focusing on adaptation as the outcome of selection over generations and other times on adaptation as a process of individual acclimation to environment. Frederic Clements, after publishing Plant Succession in 1916 and departing Nebraska for the Carnegie Institution, devoted the rest of his career to assessing the merits of these two views of adaptation. Given that Clausen, Keck, and Hiesey's (1948) studies of variation in Achillea millefolium figure in every elementary textbook of evolution, it is both curious and instructive that Clements, working in transplant gardens at the same institution, arrived at some decidedly non-Darwinian conclusions (Clements et al., 1950). Observing that even in his high-density experimental plantings across a wide range of species and environments, individual plants did not succumb to selection but only got smaller, Clements concluded that individual acclimation to environment sheltered plants from selection so that the Darwinian concept of adaptation did not hold! Though now more subtle, similar difficulties of disentangling acclimation and adaptation persist today in discussions of adaptive plasticity. Finally, there is the problem of identifying a tractable set of traits that effectively represent the main lines of functional diversification in plants. Only recently have serious efforts in this vein begun (Westoby et al., 2002). We are a long way from the universal morphospace that informs the work of vertebrate zoologists (Thomas et al., 2000), let alone a framework that includes Schimper's “form, structure, physiology and life-history.” In the face of these various conceptual and practical obstacles, how do Ackerly and Monson see the intellectual architecture of “plant functional evolution”? They recognize four lines of inquiry: (1) investigations of the adaptive value of traits singly or in aggregate, (2) analysis of the genetic and developmental basis of variation in functional traits within and between species, (3) investigations of the evolution and evolutionary consequences of phenotypic plasticity, and (4) phylogenetic and paleobotanical assessments of functional trait evolution. These themes organized the selection of papers in this collection, so it is fair to ask to what degree the contributions suggest “plant functional evolution” might finally coalesce as a coherent and vital discipline in its own right. Some of the papers are strong contributions toward this end. Monica Geber and Lauren Griffen provide a comprehensive assessment of recent work on selection and heritability for traits central to plant function. In an elegant and comprehensive summary of the evolution of C4 photosynthesis, Russ Monson explores the role of gene duplication and neofunctionalization that may underlie the emergence of this “complex adaptation.” In a useful complement to Monson's paper, Jon Keeley and Phil Rundel summarize the ecophysiological, environmental, phylogenetic, and paleobotanical aspects of carbon-concentrating mechanisms in CAM and C4 plants. John Sperry provides a concise but comprehensive review of the adaptive significance of key features in xylem structure. Peter Reich and colleagues summarize broad patterns of correlation among traits associated with leaf function. In terms reminiscent of Schimper's concern with the “animate surroundings,” David Ackerly explores some novel ideas about the community ecology of selection on functional traits. A fascinating paper by Taylor Field, Nan Arens, and Todd Dawson characterizes the environment and adaptations in early angiosperm evolution. These papers all reflect traditionally central issues in plant physiological ecology and draw strength and coherence from that shared intellectual legacy. While their focus on variation in form and function associated with the influence of carbon and water resources on plant productivity sets aside questions of life history strategies, there is nonetheless a nucleus here around which a discipline of plant functional evolution might coalesce. The other papers in the collection are more or less a coherent sampling, interesting and related in one way or another to the themes organizing the symposium but less immediately relevant to developing a coherent framework for inquiry into the evolutionary origins of plant function. They include a technical comment on the value of quantitative trait loci, a discussion of terpenoids as plant defenses, a review of the phylogenetic and ecological patterns of sprouting in South African shrubs, a selection of case studies of plant invasion, and an argument for plant plasticity as a form of habitat selection. One might hope here for less selective detail and more generality, but then topics such as plant defenses, invasions, or plasticity have overwhelming literatures in their own right. Somehow the essence of these topics must be distilled and carried into the discussion of the overall functional biology of plants and a tractable subset of key traits found to represent each major element of plant function. In fact the prior task may be simply to identify all the major elements of plant function, a topic never addressed in this volume. Ackerly and Monson hope that this collection of papers will be “a stimulus to the next generation of plant functional biologists who go beyond the question of How does it work? And take up the question of Why does it work that way? And How did it come to be?” The risk in posing those questions outside a comprehensive general framework for the assessment of variation in plant function is that the next generation will investigate many disparate traits in isolation one from another. That would be a mistake and an obstacle to any deep understanding of the evolution of plant function. This admonition aside, the editors and contributors to this volume deserve thanks for highlighting the interest and opportunity in studies of the evolution of plant function. At this point it is not entirely clear what the agenda for the study of plant functional evolution should be, but the papers collected here move the discussion forward. The real accolades must wait on the emergence of a definitive framework at the level of whole plants for the study of the origins and nature of plant adaptation.

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