Current Research

I am currently involved in three research projects:

Population genetics and evolutionary history of Arctostaphylos uva-ursi (Ericaceae)

Arctostaphylos uva-ursi (bearberry) is a circumboreal evergreen tundra shrub and the only species of Arctostaphylos to grow outside of North America, and one of only about a half a dozen species (of about fifty total) to grow outside of California, most of them shrubs that characterize Californian chaparral communities. The fossil record of Arctostaphylos is primarily centered in California and Western Nevada and despite its sparse nature is fairly continuous from the middle Miocene onward, concurrent with the development of the California Floristic Province. Yet, there are some enigmatic Eocene fossils from Wyoming that have been putatively identified as A. uva-ursi, although this identification is uncertain. Did Arctostaphylos originate as a tundra plant, of which A. uva-ursi is representative of the ancestor of the genus, that opportunistically radiated into the Mediterranean climate in California as it developed? Or is the genus a product of the radiation within the California Floristic Province, one lineage of which stumbled upon an ecological niche that allowed it to spread circumboreally? This project will involve a study of the population genetics of the approximately twenty populations of A. uva-ursi, as well as potentially the creation of a temporal diversity curve using museum collections of Arctostaphylos fossils. My collaborator on this project is my advisor, Kristina Shierenbeck.

The Latitudinal seed size gradient in Arctostaphylos (Ericaceae)

A well-known problem in biogeography is the origin of the latitudinal seed size gradient: plants living in the tropics tend to have larger propagules than plants living at higher latitudes, but the reason that this relationship exists is unclear. One possibility is that animal-mediated dispersal is more widespread in the tropics than at temperate latitudes, thus releasing tropical plants from selection for propagules small enough to be dispersed effectively by wind. All species of Arctostaphylos (manzanita) are animal-dispersed: the fruit of Arctostaphylos is a drupe (stonefruit) that is spread by birds, bears, rodents, and numerous other animals. As a result, if animal-mediated dispersal is the primary driving factor behind the latitudinal propagule size gradient, then there should be no pattern in propagule size of Arctostaphylos as a function of latitude. Currently, this research is in a meta-anaysis state comparing published fruit sizes as well as latitudinal and altitudinal ranges of Arctostaphylos species in California to determine if (1) the latitudinal gradient exists in the genus, and if (2) the gradient also exists altitudinally, wherein if the ultimate driving factor is temperature, then seed size should decrease with altitude as with latitude, but if the driving factor is moisture, then seed size should increase with altitude. Preliminary results suggest that the gradient exists and that fruit size increases with altitude. If results bear out, then a possible field study is anticipated this coming spring and summer to determine of the elevational gradient exists within a single wide-ranging species (likely A. patula), or if it is only visible between species. My collaborators on this project are Kristina Schierenbeck and Colleen Hatfield. If the field study is feasible, I will also be working with Chris Ivey.

Biogeography, dispersal mode, and evolutionary succession in gymnosperms

Sepkoski’s evolutionary faunas are complemented in the plant world by Niklas, Tiffney, and Knoll’s evolutionary floras, the two most recent of which to appear are the gymnosperms that dominated the Mesozoic and the angiosperms that dominate the late Cretaceous and Cenozoic. Yet, espite being surrounded by angiosperms, many of the major gymnosperm genera were still widespread and fairly successful until the great aridification of the world’s climate during the middle Miocene about 15 million years ago. However, three groups of living gymnosperms, the Pinaceae (pine, spruce, larch, etc.), the genus Juniperus (juniper), and the Podocarpaceae (a southern conifer family), diversified primarily during the Cretaceous and Cenozoic, together with the angiosperms, and also like the angiosperms, these three clades rely heavily on biotic dispersal. Is the lack of an effective long-distance dispersal mechanism the cause of the gymnosperms’ decline, relative to the angiosperms? Do those gymnosperms that do presently exhibit biotic dispersal actually belong in the Cenozoic flora, with the angiosperms, and not in the Mesozoic flora with the other gymnosperms? To do this, I am using a literature meta-analysis of published geographic range maps of existing gymnosperm species, and recording both geographic range and maximum disjuncture, that is, the distance between the most widely separated populations. Results indicate that conifers possessing animal-mediated (biotic) dispersal were slightly more widespread than conifers that disperse via wind or other abiotic means. However, the distance between disjunct populations is far greater in biotically-dispersed conifers than in abiotically dispersed conifers. Cycad species are less widespread than conifer species and much less likely to have disjunct populations than even abiotically-dispersed conifers, but when cycad populations are disjunct, they are only marginally less widely-separated than disjunct populations of abiotically dispersed conifers. In addition, families of conifers that live primarily in the tropics are less widespread than families that live in the extratropics, even among conifer groups that have the same dispersal mechanism (biotic or abiotic). I conclude from these results that biotic dispersal does indeed play a role in the success and probably the survivorship of certain conifer groups such as pines and junipers, and that this is a trait that they share with many of the angiosperms that they successfully compete with. Moreover, most cycads seem bereft of any effective means of dispersal, although at the meeting it came to my attention that this is not true for all cycads, and a reassessment of means of dispersal of cycad seeds is in progress. I am currently collaborating with Bruce Tiffney on this project.

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