Research

"The red-rock forest may seem hellish to us, but it is a refuge to its flora....it is the obdurate physical (and chemical) adversity of things such as peridotite (serpentine) bedrock which often drives life to its most surprising transformations."

David Rains Wallace (The Klamath Knot, 1983)

Plants that are restricted to extreme edaphic (soil) conditions such as those found on heavy metal rich serpentine outcrops and mine tailings, nitrogen rich guano deposits, high salt containing saline and alkaline flats, and calcium rich limestone provide unique opportunities for the study of plant evolution. Adaptation to such extreme edaphic conditions is not a rare phenomenon and closely related plants can often be distinguished by their distinct edaphic tolerances. Closely related plants that grow under contrasting edaphic conditions provide an ideal setting to study the forces and mechanisms that may be involved in the process of evolution.  Studies of edaphic specialists suggest that adaptations to extreme edaphic conditions can often have effects on reproductive isolation (i.e. on reducing gene flow) between the specialists and their closest relatives, setting the stage for further diversification. Thus, the study of edaphic differentiation is fertile ground for understanding the role of natural selection in speciation (i.e., adaptive evolution). Much of my past research has focused on using edaphically divergent and closely related plants to study the process of speciation (i.e. adaptation, reproductive isolation, and genetic divergence). At College of the Atlantic I conducted descriptive and experimental studies of lichens, bryophytes, and vascular plants found on extreme substrates with a team of highly enthusiastic undergraduate students.

Research in Maine (2004-2008)

Serpentine Soils and Plants of Maine

Research Team

Research on California's Flora (1996-2004)

The study of plants growing on unusual geologies has provided much insight into evolutionary biology. Examination of models of speciation shows that unusual soil conditions can serve as environmental triggers for most modes of speciation. Speciation clearly occurs as a natural process. The point at which new species can be defined, however, is a matter of opinion. Nevertheless, factors that may play a part in the process can be studied. Adaptation to extreme soil conditions is a common phenomenon, and closely related taxa can often be distinguished by their distinct edaphic tolerances. Studies of edaphic specialists suggest that adaptations to extreme edaphic conditions (e.g., heavy metals, drought) can often have effects on reproductive isolation between the specialists and their progenitors. Thus, the study of edaphic differentiation is fertile ground for understanding the role of adaptation in speciation.

My research focused primarily on Lasthenia californica (Asteraceae: Heliantheae), the most widely distributed taxon of a predominantly Californian genus (one species, L. kunthii, occurs in Chile). This obligatory outcrossing, spring annual ranges from south-central Oregon throughout California, from the foothills of the Sierra Nevada to the coast, east into Arizona, and in northern Baja California. The distribution of the species may be limited by its preference for a Mediterranean-type climate, characterized by mild, wet winters and long, hot, dry, summers. The species has wide ecological tolerance: it can be found on coastal bluffs in open grasslands, oak woodlands, alkali flats, chaparral, pastures along roadsides, in desert habitats, and on serpentine outcrops. The most extensive serpentine population so far encountered exists at Jasper Ridge Biological Preserve at Stanford University. Lasthenia californica shows a high degree of morphological, cytological and biochemical diversity and is clearly the most variable taxon in the genus consisting of approximately 20 species. Recent phylogenetic work suggests that the species formerly recognized as L. californica consists of a species complex representing two-geographically based, non-sister clades. The two clades are now recognized as two cryptic species, L. californica sensu stricto representing the northern clade and L. gracilis representing the southern clade.

My Ph.D. research with Dr. Jeannette Whitton (Department of Botany, The University of British Columbia, Canada) involved the study of parallel evolution of two edaphic races in the two closely related species belonging to the Lasthenia californica complex (both races are found in both species). The two races (A, C), characterized by their flavonoid pigments, are physiologically differentiated to deal with key variables that are associated with their distinct edaphic habitats.  Race A is better adapted to deal with ionic stresses, specifically with sodium and magnesium, ions that characterize their edaphic habitat. In contrast, race C is better adapted to drought, a feature that characterizes their edaphic habitat. Both races achieve higher fitness under conditions that best match their natural environment, strongly suggesting adaptive differentiation. The edaphic races are also reproductively isolated via flowering time differences and pollen incompatibility reactions pointing to an ideal model system for the study of the relationships between adaptation and speciation. Phylogenetic data reveal that neither race is monophyletic, and that race C populations have originated multiple times from race A (Rajakaruna et al., 2003, Rajakaruna and Whitton, 2004). Thus, the L. californica complex provides a unique opportunity to address the role of edaphic factors in parallel speciation.

As part of my ongoing research on Lasthenia, I have now included other species from the genus also appearing to have undergone intriguing ecological diversifications. The closely-related species pair of L. minor and the guano endemic L. maritima is of primary interest. This system provides an excellent opportunity to characterize the physiological/genetic basis for guano tolerance, a feature that has not been examined in any detail in the current literature. The L. minor-L. maritima are also reproductively isolated since L. maritima is predominantly self-compatible. Whether the switch to self-compatibility is associated with traits conferring guano tolerance is unknown. This species pair provides another opportunity to further my studies on adaptive evolution.

Tolerance to this unusual substrate may involve multiple functional traits relating to morphology and ecophysiology, making the study of serpentine chaparral an ideal model system to enhance our understanding of the relative importance of adaptive evolution and ecological sorting processes in the evolution of plant communities.

During my post-doctoral tenure with Dr. David Ackerly, Stanford University (now at University of California, Berkeley), I conducted a comparative study of functional traits of serpentine tolerant species and their serpentine intolerant relatives from several common chaparral lineages to address the following questions:

1. What are the key traits relating to functional morphology and ecophysiology that differ between the serpentine species and their closest relatives found on non-serpentine soil.

2. Do the findings support the hypothesis of convergent evolution (i.e., traits giving adaptation to serpentine have evolved independently in different lineages) or exaptation (i.e., traits were already present in the ancestors and ecological sorting processes have allowed the formation of serpentine chaparral) or both.

The preliminary phase of my research involved field measurements of functional traits of eight species pairs belonging to six plant families. Measures were taken twice a year, in the spring and late summer of 2003. I am currently in the process of analyzing this data set. The second phase will include common garden studies involving hydroponic and greenhouse experiments to characterize potentially adaptive traits relating to the tolerance to heavy metals (Ni, Cr), nutrients (N, Ca/Mg) and water stress, features that appear to be distinct between the various species pairs.