Evolution of succulent life forms in the Portulacineae
Trained botanists and amateurs alike have regarded the cacti with awe for centuries. The copious production of spines, lack of leaves, bizarre architecture and impressive ability to persist in the harshest environments on Earth are all traits that have entitled this lineage to be named a true wonder of the plant world. Pereskia, a (now likely paraphyletic) lineage of relatively non-succulent, leafy trees and shrubs, has long been recognized as the evolutionary link between leafless cacti and other, "normal" plants. However, it is now evident that Cactaceae is only one of many highly specialized plant lineages emerging from what was once the Portulacaceae. Within what is currently referred to as the Portulacineae (sensu Nyffeler 2007) there has been the evolution of the diminuitive, leaf succulent Anacampseroteae in South Africa, as well as the diversification of the pachycaulescent Didiereaceae, a lineage spanning Africa and Madagascar, and that like Cactaceae, evolved stem succulence, but maintained their leaves as the primary sites of photosynthesis. This new phylogenetic picture raises some interesting questions. What did the ancestor of these bizarre groups of plants look like, where did it live, and how did it function? What do the cacti functionally have in common with Didiereaceae and Anacampserotae, and how do they differ? Are there particular ecological or anatomical traits that these specialized lineages share with the more "typical" herbaceous Portulaca-type plant? Can we infer the ecological conditions that may have triggered such dramatic morphological innovation in these lineages, and were they similar in each case? Our lab is involved in ongoing work on the phylogenetics and ecological/functional characterization of this enigmatic group of plants, in close collaboration with Reto Nyffeler.
ejeleafy shoot of Pereskia guamacho, northwestern Venezuela ejeCaitlin Dunn leaning on Pereskia marcanoi, Dominican Republic ejeflowering stand of Pereskia portulacifolia, Dominican Republic
Biogeography and evolution of C4 grasses
"C4 photosynthesis" refers to a suite of biochemical and anatomical traits that increase photosynthetic efficiency in high light and high temperature environments. The most prominent C4 plants are C4 grasses, which account for up to ~25% of global terrestrial primary production and include important crop and weed plants and potential biofuels such as maize, sugarcane, sorghum, and switchgrass. In the wild, C4 grasses dominate tropical and subtropical grasslands and the herbaceous understory of savannas, but are conspicuously absent from the world's cooler regions. Mechanisms directly attributable to the C4 pathway have been invoked to explain global C4 grass distribution, such as higher quantum yields at higher leaf temperatures compared to species with the more common "C3" pathway. However, it is also true that C4 photosynthesis evolved exclusively in grass lineages of tropical origin, suggesting that C4 grasses may instead be adapted to warm conditions due to other traits they inherited from their non-C4 ancestors. This conundrum has been acknowledged for decades, but never explicitly investigated. We are testing the role of photosynthetic pathway variation in the ecological sorting of grasses, using georeferenced specimens of the Hawaiian grass flora and phylogenetic comparative methods. We are also interested in dating the origins of C4 photosynthesis in grasses, and in using phylogeny and niche modeling to build better distribution models of particular grass clades around the globe. In close collaboration with Chris Still, at UCSB.
ejea lowland plant community dominated by C4 grasses, Volcanoes National Park, Hawaii eje Chris Still measuring photosynthesis in Eragrostis grandis, a C4 grass endemic to the Hawaiian Islands eje Lucy and Erika carefully id grasses, Volcanoes National Park, Hawaii
Form and function of leaves: evolution of leaf shape in Viburnum
The role of the environment in shaping leaf evolution is undoubtedly important, and decades of comparative ecological work have delineated several general predictions about correlations between leaf form and habitat. In hot, arid, and high light environments, for example, it is common to find plants with smaller, thicker leaves, often with trichomes or other cuticular appendages, and low stomatal pore areas. We infer ecological trends such as this to be adaptive; in other words, we infer that a hot, sunny environment provides a strong selective force to evolve a small, thick leaf. On the other hand, a great diversity of leaf forms can be found living alongside the "quintessential leaf" in any given environment, suggesting that many species successfully survive and reproduce there without evolving the traits assumed to be optimal. In addition, evolutionary response to external selection pressures is complicated by the fact that leaves are developmentally and functionally integrated parts of the whole plant. Recent advances in phylogenetic comparative tools provide an alternative, improved approach to address the evolution of leaf form and function. We are reconstructing the history of leaf evolution in Viburnum (Adoxaceae), a clade of ~140 species of trees and shrubs with a wide geographical distribution. Viburnum is perfectly suited to this problem as we have a solid understanding of evolutionary relationships within the group, and Viburnum species exhibit a remarkable diversity of many "key" leaf traits. In collaboration with Michael Donoghue (Yale), we are addressing the following questions:

1. Are major habitat/climate shifts driving evolutionary divergences in leaf form?

2. Does the hydraulic capacity of stems and leaves evolve as a unit, and is this hydraulic pathway linked to leaf carbon assimilation rate?

3. How do changes in leaf form affect these key functional traits?

4. Are traits pertaining to hydraulic capacity and traits pertaining to drought tolerance evolving independently? Do some leaf traits exhibit greater evolutionary lability than others?

5. How are evolutionary changes in leaf form integrated with other, often overlooked organismal-level traits, such as shifts in branching architecture and/or leafing and flowering phenology?

eje Viburnum foetidum eje Viburnum furcatum eje Viburnum kansuense eje Viburnum propinquim eje Viburnum punctatum (Photographs of cleared leaves were taken by R. Winkworth)