Thursday, August 9, 2007

PS 61-82: Morphological variation of Yucca brevifolia (Agavaceae) among seven populations in the Mojave Desert

Tracy R. Valentovich and Darren R. Sandquist. California State University, Fullerton

It has been observed that the current distribution of Yucca brevifolia Engelm. (Joshua tree) seems to form a disjunct ring at higher elevations around the Mojave Desert. This distribution warrants investigation because it may represent the result of post-glacial migration that now restricts gene flow between eastern and western populations of this species. We examined the tree and leaf morphology of seven populations of Yucca brevifolia spanning the Mojave Desert. Genetic relationships cannot be directly tested using morphology; however it may be valuable in evaluating the established taxonomic treatments of this species. Two taxonomic varieties, Y. brevifolia var. jaegeriana and Y. brevifolia var. brevifolia, are commonly distinguished based on general morphological characteristics including tree and leaf sizes. Morphological variance among populations was expected to be minimal within eastern or western regions, while variance between regions was expected to be greater. The key taxonomic characteristics of trunk and leaf dimensions were compared among all populations using an ANOVA, while a broader evaluation using 11 tree and leaf traits were examined with a principal components analysis. Results showed that tree sizes and leaf characters of three populations always conformed to the classification of Y. brevifolia var. jaegeriana; one population showed characteristics of var. jaegeriana for leaf characters but not for trunk size, and three western populations exhibited characters similar to the more widespread Y. brevifolia var. brevifolia.  Principal component analysis using morphology revealed scatter among all populations but consistent grouping within most populations. Additionally, there was a clear separation between the western populations and those of the eastern Mojave Desert. Climate variables did not significantly correlate with morphological traits of the populations.  In general, these results support the varietal classifications of Y. brevifolia and suggest that populations that exhibit different morphology may have become discontinuous during the most recent post-glacial period.

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