By Kristen Nelson and R. Douglas Stone
I wait for fire, yet I fear the blaze,
My seeds lie hidden from the heat’s bright gaze.
At 932 degrees, they shall arise,
From ash and flame, a new life flies.
Cypress trees burn, their lives ended,
But through their death, my seeds are extended.
What am I, who thrives in fire’s wake,
When all around, the world does quake?
If anything could inspire a riddle, it’s the fascinating California native plant McNab cypress (Hesperocyparis macnabiana), which has ingeniously evolved to adapt to its environment. In collaboration with the U.S. Forest Service, the CNPS Rare Plant Program is kicking off research and conservation work for McNab cypress, a conifer that is endemic to California.
Because of the real possibility that populations of McNab cypress may not be able to replace themselves in the future, Forest Service ecologist Kyle Merriam completed an International Union for Conservation of Nature (IUCN) Assessment for the species. She estimates that there are 28 stands of McNab cypress in California, with the majority occupying less than 50 acres.
Getting to know McNab cypress
Those who live in southern California may not be familiar with this plant, which can take the form of a large shrub or small, multi-trunked tree. That’s because McNab cypress is primarily found in the foothills surrounding the Sacramento Valley, from Amador up to Shasta County on the east side, and back down through Sonoma County on the west. It can be found mainly on nutrient-poor soils (such as serpentine) and is named for the Scottish botanist and horticulturalist James McNab (1810–1878) who was the head gardener at the Royal Botanic Garden Edinburgh for nearly 30 years. When it was first named in 1855, McNab cypress was placed in the genus (meaning a group of related plants) Cupressus, then moved to the genus Callitropisis in 2006, and then moved to Hesperocyparis in 2009. To make matters a bit more complicated, some botanists modernly place it in the genus Neocupressus. Whew! Perhaps McNab cypress has been a riddle for scientists, too.
Fire brings life, but also great risk
Living in mid-elevation foothills around the Sacramento Valley means that the McNab cypress is no stranger to fire. As the riddle above hinted, the plant is specifically adapted to fire. McNab cypress bears serotinous cones, so called because the cones only open and release seeds after a blast of heat. In the case of this cypress, the air temperature has to hit 500 degrees Celsius (932 °F) for at least two minutes. Let’s pause and think about that level of heat: It’s hotter than the blistering surface of Venus. This kind of intensity is required for McNab cypress to reproduce. However, the frequency of fire is also important, because these trees are thin-barked and easily killed by fire. After a fire moves through a grove of McNab cypress, most of the mature trees die, leaving space and plenty of sunlight for the new crop of seeds to thrive. The seeds will germinate nearly all at once in the first year, resulting in a dense, even-aged stand of young seedlings.
This natural cycle is successful as long as the time between intense fires is at least 30 years, which is the length of time required for the stand to accumulate enough cones to reproduce the entire stand. The trees can start making cones at around 18 years of age, but it takes time to accumulate large numbers of cones. If a fire were to hit a young stand five to 10 years post germination, it would kill all the plants, and there would be no seeds to replace the grove. This can cause an entire grove to “wink out,” a situation called “immaturity risk.” Today, more frequent fires (shorter fire intervals) are occurring due to an increasingly hot and dry climate, making immaturity risk a growing concern for the long-term survival of this species. Four McNab cypress groves have burned multiple times in the past 10 years. Yet, if fire is entirely suppressed, groves of old McNab cypress trees may start to senesce (meaning to deteriorate with age) and die out without ever reproducing. Scientists estimate that 42% of McNab cypress groves have not burned at all in recorded history and now face “senescence risk.”
Safeguarding McNab cypress
Because of the real possibility that populations of McNab cypress may not be able to replace themselves in the future, Forest Service ecologist Kyle Merriam completed an International Union for Conservation of Nature (IUCN) Assessment for the species. She estimates that there are 28 stands of McNab cypress in California, with the majority occupying less than 50 acres. In addition to threats posed by immaturity risk and senescence risk, all the groves are threatened by the way vegetation has historically been managed in California, especially in the wildland-urban interface, where 55% of the groves occur. This includes a long history of complete fire suppression, a policy which substantially altered natural fire regimes. More recently, the pendulum has swung to forest vegetation management practices that include substantial vegetation removal to create buffers around rural communities. Some groves are also threatened by urban and commercial development, OHV use, and other impacts.
Following her assessment, the IUCN may formally update the ranking of McNab cypress to Endangered in accordance with their criteria. To help conserve, protect, and manage McNab cypress, CNPS Associate Rare Plant Botanist R. Douglas Stone and other members of the Rare Plant team will work together with Kyle Merriam and other collaborators to gather data on the plant’s habitat and populations. The team will collect robust habitat data (such as associated plant species and geology and landscape information) and tissue samples from selected groves for a population genetics study. This will evaluate genetic diversity within and between populations. The team will also collect seed cones for use in both long-term conservation seed banking and short-term restoration and recovery efforts. At the end of the study, CNPS Rare Plant staff will conduct a status review for this species to assign Global and State ranks used in the California Natural Diversity Database, as well as a California Rare Plant Rank. The team will also write a Species of Conservation Concern document for use by the Forest Service in its long-term management and conservation efforts.
Over the coming year, the CNPS Rare Plant team will post further updates from the field, which will be shared on social media. Stay tuned for updates. If you’d like to support the research and conservation work CNPS is doing for McNab cypress, please consider making a donation. Perhaps you’ll feel inspired to write your own riddle or poem about the fascinating McNab cypress!
Kristen Nelson is the CNPS Rare Plant Program Manager
R. Douglas Stone is the CNPS Associate Rare Plant Botanist
