How We're Helping to Save the San Diego Thornmint

Conservation Status: U.S. Fish and Wildlife Service - Threatened

Threats to survival: Habitat fragmentation; low genetic diversity; invasive species

Managing Together

Although it is considered to be a rare plant, the San Diego thornmint has a large number of occurrences in San Diego County with multiple populations. However, these populations have become isolated and each faces a plethora of threats, including a decline in genetic diversity resulting from their isolation. Under the leadership of our partners, land managers in San Diego County have come together to manage collectively and adaptively the San Diego thornmint throughout the region. Our Plant Conservation team is gathering key information on thornmint biology to help inform and ensure the success of regional management strategies.

A Common Garden

We collected seeds at eight sites where San Diego thornmint occurs, ranging from coastal Carlsbad to the mountains of Cleveland National Forest. We then conducted a common garden experiment in which seeds from different sites were grown under the same conditions in order to test whether differences in seasonality and morphology between populations are genetic or are the result of individual plants responding to their environment. We found that differences in the timing of flowering, plant height, and response to increased watering appear to be genetic, with most variation occurring along an east-west gradient. This work helps give land managers a better idea of the degree of similarity among populations and whether it would be appropriate to enhance one population with seed from another.

Seeds in the Bank

Results of the experimental and genetic studies have justified the need to better represent all San Diego thornmint populations in a seed bank. Seeds remaining from our collecting activities for the common garden experiment have been added to our Native Plant Seed Bank, where they serve as an insurance policy against extinction. Still, more populations need to be conserved in order to better capture the genetic diversity of the species. Our long-term goal is to ensure that there is seed available to enhance populations needing it most without diluting the genetic differences that promote critical adaptations to specific environments.

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