UC Riverside

Manzanitas (Arctostaphylos, Ericaceae) are the most taxonomically diverse genus of woody plants in western North America, with an epicenter of diversity in the California Floristic Province (CFP). For more than a century, botanists have recognized the high diversity of the genus, and have worked to come to a satisfactory understanding of how best to circumscribe taxa in the genus. Additionally, there has been much curiosity about how the genus became so diverse in the CFP. However, addressing these questions using the tools of investigation that have been historically available has proven difficult.

I undertook three studies, utilizing updated and modern methods, aimed at addressing these questions. In the first chapter, I present the results of a phylogeographic study using reduced representation genome sequence data to reconstruct the biogeographic history of three widespread species in western North America (A. patula, A. pungens, and A. pringlei). For each species, I tested whether they originated in the CFP, or outside of the CFP. I found a likely origin for A. patula to be within the CFP, a likely origin for A. pungens outside of the CFP, and I found an unclear geographic origin for A. pringlei with regard to the CFP - leaving the question of the origins of diversity in the genus unanswered.

In the second chapter, I present the results of a morphological study aimed at testing distinguishability and distinctiveness of manzanita taxa. I found that taxa can largely be distinguished, but also found overlap among many taxa in overall morphological variation. Together, this indicates that distinguishability is accounted for by specific, taxonomically prescribed, diagnostic traits. I also found evidence that morphological variation in the genus is not composed of naturally discrete, separated units, but rather, is relatively continuous.

In the third chapter, I present the results of a study investigating genetic differentiation among subspecies of a widespread species, Arctostaphylos glandulosa, with 10 described subspecies that are not completely separable by either morphology or geography. I found that the pattern of genetic differentiation within A. glandulosa corresponds to geography, rather than subspecies identity, with the exception of one subspecies, A. glandulosa subsp. gabrielensis, which was genetically distinct.

Taken together, this dissertation research shows the potential for improving our understanding of manzanitas with continued investigation utilizing modern methods, but also indicates that much more work remains to be done to better understand this challenging genus.