April 2021

by Tony Gurnoe, Director of Conservation Horticulture

Last year an alarming report was released indicating that 40% of all plant species are threatened with extinction. When considering that dire number, it is also important to know that nearly one third of all plant species exist in a botanical garden somewhere for safe keeping. Gardens like SDBG are uniquely suited to prevent plant extinction in immediate and tangible ways including the seed accessions from Encinitas baccharis and Del Mar manzanita we recently put into seed banks. As soon as those seeds were frozen, their threatened wild populations had a safeguard against catastrophe, making seed banking one of the most efficient and effective ways to conserve rare plant genetics. Not all seeds tolerate being frozen though, so how can we effectively work to conserve something like a rare species of oak?

Quercus cedrosensis or Cedros Island oak is a scrubby species, rarely exceeding 7’ tall in the United States. This rare species was previously thought only to grow in Baja California until a tiny population containing a few hundred individuals was discovered on Otay Mountain, immediately north of the border. Quercus cedrosensis is among the most highly threatened species of oak in the United States due to climate and border related stressors, yet not a single tree exists as ex-situ backup. As SDBG’s conservation horticulture capacities began to grow it became clear we had to step in to help this species before it completely disappeared from our region. However, given SDBG’s limited planting space and that the acorns will die if stored for any length of time, we also knew that we would need partners in this effort.

SDBG’s conservation horticulture team designed an ambitious plan to conserve Quercus cedrosensis which focuses on growing hundreds of new plants in a way that accurately reflects the genetic composition of this rare population and culminates in the distribution of living saplings to public gardens throughout California. Other botanical gardens were eager to participate and our project was awarded a grant from the American Public Gardens Association and U.S. Forest Service through the Tree Gene Conservation Partnership. We also received a small amount of financial support from the California Biodiversity Initiative for the work of collecting propagules. The Bureau of Land Management and California Department of Fish & Wildlife graciously approved our permits for the work of collecting rare plant material from protected habitat. Grateful for so much foundational support, SDBG’s conservation horticulture team was finally ready to get hands-on with these plants.

In early February we began collecting extensive GIS mapping data, tissue samples for DNA analysis, herbarium vouchers, and field observations throughout the Otay Mountain Wilderness to finally gain a comprehensive understanding of the range of this species. SDBG will become the primary repository for this collection of important conservation material and data, with redundant preserved specimens distributed to the herbaria at the U.S. National Arboretum and the San Diego Natural History Museum. Living plants will be shared with at least 8 other botanical gardens. We will also contribute our survey data to update the California Natural Diversity Database and will share our herbarium data for any future research purposes.

Once we complete the adventurous survey portion of this project our next step will be to collect material to propagate more of these rare plants. Although we are hoping to collect acorns later in the summer because seed is the most reliable propagation method to preserve genetic diversity, the trees have not successfully produced fruit for many years. Just in case of another unproductive seed year, we will also be working to root cuttings in SDBG’s nursery and collaborating with researchers at the San Diego Zoo to establish living conservation collections of these rare plants though tissue culture techniques. Ultimately, we expect to grow and share hundreds of these endangered oaks as part of broader efforts to make rare plants less rare, but it’s important to note that Quercus cedrosensis is just one species among thousands of varieties of threatened plants.

Given how much work is going into preserving this single species, you can surely imagine the resources necessary to address plant conservation at even a regional level, and we could not be more grateful for the support we are receiving toward these ends. Every piece of our collaborative conservation network is critical, from our volunteers and members, to those who directly fund such efforts, and all the various constituents in our community working to shine important light on the imminent need to preserve biodiversity. We will continue to provide updates on this project and other conservation projects in future articles and hope to be able to continue to count on your support for this garden as we work passionately and tirelessly to ensure that future generations find a world as full of rich diversity and wonder as we do today.