Thomas J. Givnish

Ecological Determinants of Species Loss in Remnant Prairies

Recensuses of 54 Wisconsin prairie remnants showed that 8 to 60 percent of the original plant species were lost from individual remnants over a 32- to 52-year period. The pattern of species loss was consistent with the proposed effects of fire suppression caused by landscape fragmentation. Short, small-seeded, or nitrogen-fixing plants showed the heaviest losses, as did species growing in the wettest, most productive environments. The interruption of landscape-scale processes (such as wildfire) by fragmentation is an often overlooked mechanism that may be eroding biodiversity in many habitats around the world.

On the Adaptive Significance of Leaf Form

although it is clear that leaves are central to a plant’s adaptation for growth and competitive survival, the actual nature of the contribution of leaf form to plant adaptation is complex. For example, the size and shape of leaves can affect the rates at which leaves exchange heat, take up carbon dioxide, and lose water vapor. Leaf size and shape can also affect the efficiency with which the total photosynthetic surface can be arranged, supported, and supplied. Adaptations in leaf form thus touch on several aspects of plant form and function, with implications for thermoregulation (Gates et al., 1968), efficiency of water use (Parkhurst and Loucks, 1972), photosynthetic potential (Cunningham and Strain, 1969), branching and rooting strategies (Givnish and Vermeij, 1976), productivity (Tsunoda, 1972), and, presumably, competitive ability.

CARNIVORY IN THE BROMELIAD BROCCHINIA REDUCTA, WITH A COST/BENEFIT MODEL FOR THE GENERAL RESTRICTION OF CARNIVOROUS PLANTS TO SUNNY, MOIST, NUTRIENT-POOR HABITATS

Brocchinia reducta is the first documented case of carnivory in the Bromeliaceae. Its erect leaves form a yellowish cylinder with a cuticular lubricant on its inner surface, impound fluid that emits a nectarlike fragrance, and bear trichomes capable of absorbing amino acids from this fluid in which numerous insects, mainly ants, drown. Trichome absorptivity and aspects of trichome structure appear unique in the primitive subfamily Pitcairnioideae. We present the first rigorous definition of carnivory in plants, and discuss its implications for the identification of cases of carnivory and protocarnivory in bromeliads. A cost/benefit model for the evolution of carnivory is developed to analyze why carnivorous plants are restricted mainly to sunny, moist, nutrient-poor sites and seasons, and why carnivory is rare in epiphytes and other bromeliads. The relative advantages of carnivory and ant-fed myrmecophily are discussed in terms of this model, and predictions made regarding the nature of the ant-plant mutualism in understory myrmecophytes.

Leaf and Canopy Adaptations in Tropical Forests

Ecological patterns in several aspects of leaf form and canopy structure in tropical forest plants are reviewed and analyzed in terms of their potential significance for competitive ability. Cost/ benefit models for traits that directly influence gas exchange — such as the size, inclination, and reflectivity of leaves and the profile and aerodynamic roughness of canopies — suggest a basis for the paradoxical duality of morphological adaptations to drought and nutrient poverty. Models based on the balance between photosynthesis and mechanical efficiency predict various patterns in leaf shape, and analyze the functional significance of orthotropy and plagiotropy, asymmetric leaf bases, anisophylly, alternate vs. opposite leaves, and simple vs. compound leaves. Brief comments are made on the potential importance of biotic interactions for trends in plant form.

SIZES AND SHAPES OF LIANE LEAVES

We have developed evolutionary models to account for ecological patterns in the size and shape of vine leaves. Leaf size is predicted via an economical model in which photosynthetic gains due to enhanced gas exchange are balanced with the metabolic losses incurred in replacing the concomitant water loss. Mechanical efficiency and appropriate orientation in a light field form the basis of our model for leaf shape. Data from montane rain forest in Venezuela and from varied habitats in Costa Rica illustrate the predicted trends along vertical and horizontal microclimatic gradients. Large, cordate-based leaves with long petioles are favored in sunny situations, while small, narrow-based leaves with short petioles are favored in more shady environments. Compound or deeply lobed leaves of the smallest effective size are expected in the most exposed positions.

Phylogeny, adaptive radiation, and historical biogeography in Bromeliaceae: Insights from an eight-locus plastid phylogeny

PREMISE Bromeliaceae form a large, ecologically diverse family of angiosperms native to the New World. We use a bromeliad phylogeny based on eight plastid regions to analyze relationships within the family, test a new, eight-subfamily classification, infer the chronology of bromeliad evolution and invasion of different regions, and provide the basis for future analyses of trait evolution and rates of diversification. METHODS We employed maximum-parsimony, maximum-likelihood, and Bayesian approaches to analyze 9341 aligned bases for four outgroups and 90 bromeliad species representing 46 of 58 described genera. We calibrate the resulting phylogeny against time using penalized likelihood applied to a monocot-wide tree based on plastid ndhF sequences and use it to analyze patterns of geographic spread using parsimony, Bayesian inference, and the program S-DIVA. RESULTS Bromeliad subfamilies are related to each other as follows: (Brocchinioideae, (Lindmanioideae, (Tillandsioideae, (Hechtioideae, (Navioideae, (Pitcairnioideae, (Puyoideae, Bromelioideae))))))). Bromeliads arose in the Guayana Shield ca. 100 million years ago (Ma), spread centrifugally in the New World beginning ca. 16-13 Ma, and dispersed to West Africa ca. 9.3 Ma. Modern lineages began to diverge from each other roughly 19 Ma. CONCLUSIONS 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.

Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)

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.

ANCIENT VICARIANCE OR RECENT LONG-DISTANCE DISPERSAL? INFERENCES ABOUT PHYLOGENY AND SOUTH AMERICAN-AFRICAN DISJUNCTIONS IN RAPATEACEAE AND BROMELIACEAE BASED ON ndhF SEQUENCE DATA

Rapateaceae and Bromeliaceae each have a center of diversity in South America and a single species native to a sandstone area in west Africa that abutted the Guayana Shield in northern South America before the Atlantic rifted. They thus provide ideal material for examining the potential role of vicariance versus long‐distance dispersal in creating amphiatlantic disjunctions. Analyses based on ndhF sequence variation indicate that Rapateaceae and Bromeliaceae are each monophyletic and underwent crown radiation around 41 and 23 Ma, respectively. Both exhibit clocklike sequence evolution, with bromeliads evolving roughly one‐third more slowly than rapateads. Among rapateads, the divergence of west African Maschalocephalus dinklagei from its closest South American relatives implies that Maschalocephalus resulted via long‐distance dispersal 7 Ma, not ancient continental drift; only its sandstone habitat is vicariant. Rapateads arose first at low elevations in the Guayana Shield; the earliest divergent genera are widespread along riverine corridors there and, to a lesser extent, in Amazonia and the Brazilian Shield. Speciation at small spatial scales accelerated 15 Ma with the invasion of high‐elevation, insular habitats atop tepuis. Among bromeliads, Pitcairnia feliciana diverges little from its congeners and appears to be the product of long‐distance dispersal ca. 12 Ma. Brocchinia/Ayensua and then Lindmania are sister to all other bromeliads, indicating that the Guayana Shield was also the cradle of the bromeliads. Three lineages form an unresolved trichotomy representing all other bromeliads: (1) Till andsioideae, (2) Hechtia, and (3) a large clade including remaining genera of Pitcairnioideae and all Bromelioideae. The last includes a clade of pitcairnioid genera endemic to the Guayana and Brazilian Shields; a xeric group (Abromeitiella/Deuterocohnia/Dyckia/Encholirium/Fosterella) from southern South America and the southern Andes, sister to Pitcairnia; and Andean Puya, sister to Bromelioideae, with many of the latter native to the Brazilian Shield. Both Rapateaceae and Bromeliaceae appear to have arisen at low elevations in the Guayana Shield, experienced accelerated speciation after invading dissected mountainous terrain, and undergone long‐distance dispersal to west Africa recently. Bromeliad acquisition of key adaptations to drought (e.g., CAM photosynthesis, tank habit, tillandsioid leaf trichomes) 17 Ma appears to have coincided with and help cause the centripetal invasion of drier, more seasonal regions beyond the Guayana Shield, resulting in a wider familial range and dominance of the epiphytic adaptive zone. Geology, past and present climate, and proximity to South America help account for both families occurring in nearly the same area of Africa. We present a new classification for Rapateaceae, including a new tribe Stegolepideae, a new subfamily Monotremoideae, and revisions to tribe Saxofridericieae and subfamily Rapateoideae.

Assembling the Tree of the Monocotyledons: Plastome Sequence Phylogeny and Evolution of Poales1

Abstract The order Poales comprises a substantial portion of plant life (7% of all angiosperms and 33% of monocots) and includes taxa of enormous economic and ecological significance. Molecular and morphological studies over the past two decades, however, leave uncertain many relationships within Poales and among allied commelinid orders. Here we present the results of an initial project by the Monocot AToL (Angiosperm Tree of Life) team on phylogeny and evolution in Poales, using sequence data for 81 plastid genes (exceeding 101 aligned kb) from 83 species of angiosperms. We recovered highly concordant relationships using maximum likelihood (ML) and maximum parsimony (MP), with 98.2% mean ML bootstrap support across monocots. For the first time, ML resolves ties among Poales and other commelinid orders with moderate to strong support. Analyses provide strong support for Bromeliaceae being sister to the rest of Poales; Typhaceae, Rapateaceae, and cyperids (sedges, rushes, and their allies) emerge next along the phylogenetic spine. Graminids (grasses and their allies) and restiids (Restionaceae and its allies) are well supported as sister taxa. MP identifies a xyrid clade (Eriocaulaceae, Mayacaceae, Xyridaceae) sister to cyperids, but ML (with much stronger support) places them as a grade with respect to restiids + graminids. The conflict in resolution between these analyses likely reflects long-branch attraction and highly elevated substitution rates in some Poales. All other familial relationships within the order are strongly supported by both MP and ML analyses. Character-state mapping implies that ancestral Poales lived in sunny, fire-prone, at least seasonally damp/wet, and possibly nutrient-poor sites, and were animal pollinated. Five subsequent shifts to wind pollination—in Typhaceae, cyperids, restiids, Ecdeiocoleaceae, and the vast PACCMAD-BEP clade of grasses—are significantly correlated with shifts to open habitats and small, inconspicuous, unisexual, and nectar-free flowers. Prime ecological movers driving the repeated evolution of wind pollination in Poales appear to include open habitats combined with the high local dominance of conspecific taxa, with the latter resulting from large-scale disturbances, combined with tall plant stature, vigorous vegetative spread, and positive ecological feedback. Reproductive assurance in the absence of reliable animal visitation probably favored wind pollination in annuals and short-statured perennials of Centrolepidaceae in ephemerally wet depressions and windswept alpine sites.

PHYLOGENY, CONCERTED CONVERGENCE, AND PHYLOGENETIC NICHE CONSERVATISM IN THE CORE LILIALES: INSIGHTS FROM rbcL AND ndhF SEQUENCE DATA

Abstract Calochortus and the family Liliaceae s.s. have often been considered each others closest relatives, based partly on their shared possession of bulbs, visually showy flowers, winged wind‐dispersed seeds, and narrow parallel‐veined leaves. We present a well‐supported molecular phylogeny for these groups and their close relatives in the core Liliales, based on sequence variation in the chloroplast‐encoded rbcL and ndhF genes. This analysis identifies Liliaceae s.s. as monophyletic, including one clade (((Lilium, Fritillaris, Nomocharis), Cardiocrinum), Notholirion) that appears to have diversified in the Himalayas roughly 12 million years ago and another ((Erythronium, Tulipa), (Gagea, Lloydia)) that arose in East Asia at about the same time. Medeola and Clintonia are sister to Liliaceae s.s. and bear rhizomes, inconspicuous flowers, fleshy animal‐dispersed fruits, and broad reticulate‐veined leaves. Calochortus is sister to Tricyrtis; both Tricyrtis and the neighboring clade of Prosartes‐Streptopus‐Scoliopus share several of the traits seen in Medeola‐Clintonia. The core Liliales thus provide compelling examples of both concerted convergence and phy‐logenetic niche conservatism. Invasion of open, seasonal habitats was accompanied by the independent evolution of bulbs, showy flowers, wind‐dispersed seeds, and narrow parallel‐veined leaves in Calochortus and Liliaceae s.s. Conversely, persistence in shady habitats was accompanied by the retention of rhizomes, inconspicuous flowers, animal‐dispersed seeds, and broad reticulate‐veined leaves in their sister groups. We advance arguments for the context‐specific adaptive value of each of these traits, as well as evidence of parallel trends in other groups. Concerted convergence—convergence in several different traits, favored by the same shared set of ecological conditions, in two or more lineages—is an important evolutionary process that can mislead evolutionary analyses based solely on phenotypic variation.