Whole-plant energy capture depends not only on the photosynthetic response of individual leaves, but also on their integration into an effective canopy, and on the costs of producing and maintaining their photosynthetic capacity. This paper explores adaptation to irradiance level in this context, focusing on traits whose significance would be elusive if considered in terms of their impact at the leaf level alone. I review traditional approaches used to demonstrate or suggest adaptation to irradiance level, and outline three energetic tradeoffs likely to shape such adaptation, involving the economics of gas exchange, support, and biotic interactions. Recent models using these tradeoffs to account for trends in leaf nitrogen content, stornatal conductance, phyllotaxis, and defensive allocations in sun v. shade are evaluated.A re-evaluation of the classic study of acclimation of the photosynthetic light response in Atriplex, crucial to interpreting adaptation to irradiance in many traits, shows that it does not completely support the central dogma of adaptation to sun v. shade unless the results are analysed in terms of whole-plant energy capture. Calculations for Liriodendron show that the traditional light compensation point has little meaning for net carbon gain, and that the effective compensation point is profoundly influenced by the costs of night leaf respiration, leaf construction, and the construction of associated support and root tissue. The costs of support tissue are especially important, raising the effective compensation point by 140 pmol m -s -' in trees 1 m tall, and by nearly 1350 pmol m -s -' in trees 30 m tall. Effective compensation points give maximum tree heights as a function of irradiance, and shade tolerance as a function of tree height; calculations of maximum permissible height in Liriodendron correspond roughly with the height of the tallest known individual. Finally, new models for the evolution of canopy width/height ratio in response to irradiance and coverage within a tree stratum, and for the evolution of mottled leaves as a defensive measure in understory herbs, are outlined. IntroductionA central objective of plant ecology is to understand the causes of patterns in the distribution and abundance of species. Physiological ecologists advance this goal by studying how various morphological and physiological properties permit a plant to survive and compete successfully in certain environments but not in others. Physiological ecology thus provides an important window on the proximal mechanisms that underlie species differences in distribution and habitat-specific competitive ability.Photosynthetic energy capture provides green plants with almost all of their chemical energy, and is central to their ability to compete and reproduce. Photosynthesis, in turn, is directly and dramatically influenced by the amount of light striking a plant's leaves. Many investigators have therefore studied how different levels of irradiance by photosynthetically active radiation affect photosynthe...

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Nearly two-thirds of extant bromeliads belong to two large radiations: the core tillandsioids, originating in the Andes ca. 14.2 Ma, and the Brazilian Shield bromelioids, originating in the Serro do Mar and adjacent regions ca. 9.1 Ma.

Orchids are the most diverse family of angiosperms, with over 25 000 species, more than mammals, birds and reptiles combined. Tests of hypotheses to account for such diversity have been stymied by the lack of a fully resolved broad-scale phylogeny. Here, we provide such a phylogeny, based on 75 chloroplast genes for 39 species representing all orchid subfamilies and 16 of 17 tribes, time-calibrated against 17 angiosperm fossils. A supermatrix analysis places an additional 144 species based on three plastid genes. Orchids appear to have arisen roughly 112 million years ago (Mya); the subfamilies Orchidoideae and Epidendroideae diverged from each other at the end of the Cretaceous; and the eight tribes and three previously unplaced subtribes of the upper epidendroids diverged rapidly from each other between 37.9 and 30.8 Mya. Orchids appear to have undergone one significant acceleration of net species diversification in the orchidoids, and two accelerations and one deceleration in the upper epidendroids. Consistent with theory, such accelerations were correlated with the evolution of pollinia, the epiphytic habit, CAM photosynthesis, tropical distribution (especially in extensive cordilleras), and pollination via Lepidoptera or euglossine bees. Deceit pollination appears to have elevated the number of orchid species by one-half but not via acceleration of the rate of net diversification. The highest rate of net species diversification within the orchids (0.382 sp sp 21 My 21) is 6.8 times that at the Asparagales crown.

Patterns in the dominance of evergreen vs. deciduous plants have long interested ecologists, biogeographers, and global modellers. But previous models to account for these patterns have signifi cant weaknesses. Bottom-up, mechanistic models -based on physiology, competition, and natural selection -have often been non-quantitative or restricted to a small range of habitats, and almost all have ignored belowground costs and whole-plant integration. Top-down, ecosystem-based models have succeeded in quantitatively reproducing several patterns, but rely partly on empirically derived constants and thresholds that lack a mechanistic explanation. It is generally recognized that seasonal drought can favor deciduous leaves, and that infertile soils can favor long-lived evergreen leaves. But no model has yet explained three great paradoxes, involving dominance by 1) evergreens in highly seasonal, boreal forests, 2) deciduous larch in many nutrient-poor peatlands, and 3) evergreen leaf-exchangers in nutrient-poor subtropical forests, even though they shed their leaves just as frequently as deciduous species. This paper outlines a generalized optimality model to account for these and other patterns in leaf longevity and phenology, based on maximizing whole-plant carbon gain or height growth, and building on recent advances in our understanding of the quantitative relationships of leaf photosynthesis, nitrogen content, and mass per unit area to leaf life-span. Only a whole-plant approach can explain evergreen dominance under realistic ecological conditions, or account for the boreal paradox, the larch paradox, the leaf-exchanger paradox, and expected shifts in shade tolerance associated with leaf phenology. Poor soils favor evergreens not merely by increasing the costs of nutrient acquisition, but also by depressing the maximum rate of photosynthesis and thus the seasonal contrast in photosynthetic return between leaves adapted to favorable vs. unfavorable conditions. The dominance of evergreens in western North America beyond the coastal zone of mild winters and winter rainfall appears related to the unusually long photosynthetic season for evergreen vs. deciduous plants there. Future models for optimal leaf phenology must incorporate differences between evergreen and deciduous plants in allocation to photosynthetic vs. non-photosynthetic tissue, rooting depth, stem allometry, xylem anatomy, and exposure to herbivores and leaching, and analyze how these differences interact with the photosynthetic rate, transpiration, and nutrient demands of leaves with different life-spans to affect rates of height growth in specifi c microsites.

The endemic Hawaiian lobeliads are exceptionally species rich and exhibit striking diversity in habitat, growth form, pollination biology and seed dispersal, but their origins and pattern of diversification remain shrouded in mystery. Up to five independent colonizations have been proposed based on morphological differences among extant taxa. We present a molecular phylogeny showing that the Hawaiian lobeliads are the product of one immigration event; that they are the largest plant clade on any single oceanic island or archipelago; that their ancestor arrived roughly 13 Myr ago; and that this ancestor was most likely woody, wind-dispersed, bird-pollinated, and adapted to open habitats at mid-elevations. Invasion of closed tropical forests is associated with evolution of fleshy fruits. Limited dispersal of such fruits in wet-forest understoreys appears to have accelerated speciation and led to a series of parallel adaptive radiations in Cyanea, with most species restricted to single islands. Consistency of Cyanea diversity across all tall islands except Hawai`i suggests that diversification of Cyanea saturates in less than 1.5 Myr. Lobeliad diversity appears to reflect a hierarchical adaptive radiation in habitat, then elevation and flower-tube length, and provides important insights into the pattern and tempo of diversification in a species-rich clade of tropical plants.

Summary 0The number of woody species in tropical forests tends to increase with precipitation\ forest stature\ soil fertility\ rate of canopy turnover and time since catastrophic disturbance\ and decrease with seasonality\ latitude\ altitude\ and diameter at breast1 A model is presented to account for these trends[ Novel hypotheses include how increased rainfall and substrate fertility\ and decreased seasonality\ might "i# increase attacks by natural enemies\ and thus the overall level of density!dependent plant mortality^"ii# increase shade tolerance\ canopy turnover\ and stem density of the species!rich understorey^and "iii# increase reliance on relatively sedentary forest! interior birds for seed dispersal\ fostering high rates of speciation in understorey genera[ 2 High rainfall and low seasonality in the tropics favour two key groups of natural plant enemies Ð insects and fungi Ð that are directly responsible for promoting high rates of density!dependent plant mortality[ Lower rainfall\ greater seasonality\ soil infertility\ or unfavourable rooting conditions favour greater allocation to anti!her! bivore defences\ and thus lead to lower rates of such mortality and thence to lower tree diversity[ The increased number of individuals on rainier sites is a minor contributor to increased tree diversity\ accounting for only about 06) of the 7[2!fold increase with rainfall in the lowland Neotropics[ 3 Predictions of the model are consistent with many ecological patterns of variation in tropical tree diversity within regions\ and may help explain the decrease in tree diversity with elevation and the accompanying decrease in horizontal patchiness "within!habitat b diversity#[ 4 Random drift over evolutionary time in the relative e}ectiveness of density!depen! dent control of individual tree species by specialized natural enemies may better account for the observed distribution of tropical tree abundance than a random walk of species abundance through ecological time[ Keywords] anti!herbivore defences\ density!dependent mortality\ tree turnover\ tropi! cal forests Journal of Ecology "0888# 76\ 082Ð109

SUMMARYLeaves or their functional analogues provide outstanding opportunities for comparative studies. Here, I use leaves to illustrate the crucial role of ecological, biogeographic and phylogenetic comparisons in generating and testing hypotheses regarding the adaptive significance of morphological variation, the relative importance of selective pressures vs phylogenetic constraints, and the rise of adaptations within lineages. The complementary roles of comparative studies and optimality models are stressed throughout.The first section of this paper reviews 23 ecological patterns in leaf form, physiology and arrangement which have heen uncovered hy comparative studies. Three general sets of energetic trade-offs, involving the economics of gas exchange, support, and hiotic interactions, appear likely to influence the evolution of leaves and underlie these trends. The first of these trade-offs is illustrated with an analysis of the adaptive significance of leaf size, in both terrestrial and aquatic plants. The resulting predictions are compared with the actual trends observed, and the relative strengths and limitations of the approach are discussed.The second section addresses the role of selective pressures and phylogenetic constraints in determining features of leaf form and phenology in forest herbs. Ecological comparisons of 74 species from a site in the Virginia Piedmont show that members of each temporal photosynthetic guild display evolutionary convergence in several aspects of leaf form and arrangement. These convergences can each be understood in terms of models that assume that selection favours plants whose form and physiology tend to maximize whole-plant growth. Phylogenetic comparisons indicate that congeners of guild members share the same leaf phenology as the guild members themselves. This remarkable finding suggests that phenology is evolutionarily rather non-labile within genera but that, within guilds, species in several different genera and families converge strongly in other leaf traits. In this case, phylogenetic constraints appear to be important mainly in determining which lineages evolve particular phenologies and leaf adaptations, not whether they arise. The section concludes with a critical discussion of the capacity of two classes of evolutionary models, based primarily on functional considerations or phylogenetic constraints, to produce truly deductive predictions.The third section briefly reviews an analysis of adaptive radiation in leaf shape among violets of eastern North America. Each ecological group of species displays the leaf shape expected on functional grounds and separate lineages (as judged by means of phylogeny independent of leaf shape) show parallel trends. Ecological and phylogenetic comparisons, in combination with optimality models, provide insights into the sequence of habitats invaded and leaf forms evolved.I conclude with comments on the advantages and limitations of comparative studies, and speculate on avenues for future research on leaf form. An integrated app...