Arboretum staff work to conserve genetic diversity of rare Texas oaks

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Since 2016, the UC Davis Arboretum and Public Garden has been working on a collaborative tree gene conservation project focusing on rare and threatened oaks in West Texas with funding from the American Public Gardens Association and the US Forest Service. Through collection, propagation, and planting in public garden living collections across the US, this project helps conserve the genetic diversity of these unique and imperiled oaks.

Over the last few years, the UC Davis Arboretum and Public Garden has led a collaborative conservation project targeting five rare and threatened oak species native to West Texas. In 2016 and 2018, we received tree gene conservation partnership grants from the American Public Gardens Association and US Forest Service that supported a series of five scouting and collecting trips. The targets of the project were five oak species of conservation concern (Quercus carmenensis, Q. depressipes, Q. graciliformis, Q. robusta, and Q. tardifolia). The primary goal of the project was to increase the representation of genetic diversity of these rare oak species in living public garden collections by collecting and distributing acorns sampled from multiple individuals across their range. We also aimed to further document the current distribution of these species by identifying the locations of individuals and populations as well as any current conservation threats.

Oaks (plants in the genus Quercus) have seeds that don’t store well for more than a year or two and are not suitable for traditional seed banking. We selected our target oak species due to their rarity and their limited distribution within a concentrated area of West Texas. Because of the small population sizes and close proximity of ranges for these species, it was realistic to target multiple species for a combined scouting and collecting effort. When the project was undertaken, most of these species were not represented in the living collections within the American Public Gardens Association Plant Collections Network Quercus Multisite Collection or in institutions participating in Botanic Garden Conservation International’s PlantSearch database.

The far western region of Texas, the Trans-Pecos (referring to the area west of the Pecos River), is home to an exceptionally high diversity of oaks – 22 species occur here, representing almost a quarter of the species diversity of oaks in the United States. Oak diversity in the Trans-Pecos region is concentrated in a series of “sky island” mountain ranges – the Chisos Mountains, Davis Mountains, and Guadalupe Mountains – that are isolated from each other by lowland Chihuahuan Desert vegetation. Due to their isolation, the sky islands of Texas harbor four endemic oak species as well as relict populations of species that were previously more widespread. The West Texas mountains also have shared natural diversity with the Sierra Madre Occidental and the Del Carmen-Sierra Madre Oriental of Mexico and serve as the northernmost limit of five Mexican oak species that do not occur elsewhere in the United States.

One of the major challenges associated with assessing priorities for conserving oak genetic diversity from the sky island mountains of West Texas is taxonomic. Hybridization is common among the many species of oaks that grow in close proximity. This has resulted in confusion about species identification and definitions. Quercus graciliformis, Q. robusta, and Q. tardifolia have all been considered by some to be hybrids at some point since they were first described. Especially when a species is poorly documented, it becomes difficult to assess whether it is a rare species that should be prioritized for conservation or a chance hybrid that does not warrant high conservation attention. By increasing documentation of these species through the collection of herbarium specimens, we hope to generate more scientific resources that can be studied to shed light on these taxonomic questions.

The sky island mountains of West Texas are well protected within National Parks, Texas State Parks, and Nature Conservancy holdings. However, the oak species in these mountains are still vulnerable to climate change. As the climate becomes warmer and drier, their migratory paths are quite limited. For this reason, conservation in curated living collections will be critical for ensuring the long-term survival of these species. For those species that extend into Mexico, capturing the genetic diversity for the populations at the northern limit of their range is valuable, especially in the face of climate change.

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