September 28 2017

5:00 pm 1100 TLSB

EcoEvoPub Series

Graduate Student Presentations


Scott O'Donnell
Department of Ecology and Evolutionary Biology, UCLA

"Is hybridization a boon or a burden\? The evolutionary consequences of hybridization and introgression between three sympatric oak (Quercus spp.) taxa in southern California."

Hybridization has long been proposed to be an important evolutionary process in plant lineages. Recent work has attempted to quantify the ramifications of hybridization, including the potential for gene flow across species boundaries through backcrosses with either of the parental taxa, otherwise known as introgression. Increasingly, attention has been directed to determining the role that introgression may play in local adaptation through the exchange of potentially adaptive alleles. Oak (Quercus spp.) species have long been known to possess relatively weak reproductive isolation mechanisms between closely related taxa and have been used as a model system to study the structure and maintenance of species boundaries in light of gene flow between genetically and ecologically distinct species. Selection has been shown to play a key role in the formation and maintenance of hybrid tension zones between oak species. This offers a unique opportunity to test the role and function that adaptive introgression may play in the evolutionary histories of sympatric oak taxa. While other recent studies have provided evidence for potential adaptive introgression between various oak taxa, few studies have explored this topic through the use of whole genome sequence data. Through mapping whole genome sequence data to an available reference genome, it will be possible to explore the extent and adaptive significance of introgression between three sympatric oak species in southern California.

Grace John
Department of Ecology and Evolutionary Biology, UCLA

“Leaf hydraulic vulnerability influences sapflow responses to vapor pressure deficit in Los Angeles urban trees”

Authors: Grace P. John, Christine Scoffoni, Elizaveta Litvak, Diane Pataki, Tom Buckley, and Lawren Sack

Urban trees contribute strongly to the water balance of cities rendering essential a quantitative understanding of their water use and its regulation. Tree evapotranspiration and sapflow and their sensitivity to atmospheric drought (vapor pressure deficit; VPD) have been described by several empirical, mathematical functions representing the plant water transport system. Leaves represent a major bottleneck in the control of plant water transport yet their contribution to the regulation of sapflow and its response to VPD has remained unclear. For seven common urban trees of Los Angeles, we tested hypotheses for the role of leaf hydraulic vulnerability and functional traits in determining the response of sapflow to VPD. We measured the response of leaf hydraulic conductance (Kleaf) to declining water status (leaf bulk water potential; Ψ) and quantified the sensitivity to hydraulic decline as the leaf water potential at 50% loss of Kleaf (P50). We hypothesized that (1) Kmax would strongly predict whole tree transpiration rates and sensitivity to VPD and that (2) P50 would drive whole tree sensitivity to VPD. To expand this framework, we also compared the strength of relationships between these leaf and whole plant water use performance traits and associated hypothesized functional traits measured in the field.
Across the 7 species of urban trees, we found substantial variation (>27-fold) in leaf hydraulic conductance for well-hydrated leaves (Kmax), and in P50 (from –0.3 to –2.3 MPa), as well as in the sensitivity of sapflow to VPD as estimated using the transpiration sensitivity model described by Litvak et al. in 2012 (>2-fold). We found novel relationships between the control of sapflow and leaf physiological and functional traits. The sensitivity of sapflow to VPD was positively related to Kmax and P50 across species, indicating that species with leaves that transport water more efficiently when hydrated, yet reduce their conductance during dehydration, also reduce their transpiration and sapflow most strongly at high VPD. Whole tree water use and its sensitivity to VPD showed weak coordination or independence from other functional traits, such as leaf mass per area, leaf density, or the water potential at which leaf turgor is lost (Ψtlp). This study provides novel insights into the hydraulic basis for tree water use and an improved ability to predict whole tree behaviour from leaf physiology across urban tree species.











































































































































































































































































































































































































































































































































































































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