
David Douglas, the great English botanist, for whom the Douglas Fir, and many other plants is named, collected in the Pacific Northwest, the Eastern seaboard, and Hawaii, in the years 1823-1834. He also spent some time in California. (čəbidac is the Lushootseed name for D. Fir. Another Coast Salish name for the tree, used in the Halkomelem language, is lá:yelhp https://shoreline.libguides.com/treecampus/douglas_fir)
He traveled in there during the years 1830-32, collecting many species then unknown to botanists. He visited the colonial settlements from Santa Barbara to Sonoma, measuring the latitude and longitude at many of them. While in Sonoma he probably spent some time with the Mission fathers, as he had further south. He appreciated them as educated men who spoke fluent Latin.
Back in London in 1827, Douglas had trained in surveying with geographer and astronomer Edward Sabine. On his 1829 return voyage around the Horn to North America, Douglas practiced with the instruments and studied the math. He became proficient at the difficult task of measuring longitude, given the unreliability of chronometers at that time.

A Harrison chronometer, late 18th century. By Bjoertvedt – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8483950

The longitude as measured today: 38° 17′ 41.2368” N.
According to Willis Jepson, the dean of California botany, writing in 1933, “He was the first botanical collector in California in residence for any extended period and during this time he traveled through the Coast Ranges from Monterey north to the Mission San Francisco De Solano (Sonoma) and south to the Mission of Santa Barbara. He was not only the first traveler to collect the extensively rich and varied spring flora of the Coast Ranges, nearly all the species of which were new to botanical science, but also the first to leave some written description of it. Hundreds of new species, our most familiar plants, were based on the Douglas collection…” And “It has been suggested that Douglas visited Mt. Diablo, but the form of Calochortus pulchellus which he obtained might have been collected in the Sonoma region.”
Willis Linn Jepson was a co-founder of the Sierra Club and founder of the California Botanical Society. The oldest known California Bay Laurel (Umbellularia californica) is named after him. https://www.smcgov.org/parks/what-see-crystal-springs-trail

Calochortus pulchellus is a rare species of flowering plant in the lily family known by the common name Mt. Diablo fairy-lantern or Mount Diablo globelily.
Of the first plant he encountered in California, Douglas stated (one-upping his mentor Menzies?), “Early as was my arrival on this Coast Spring had commenced. The first plant I took in my hand in full flower was Ribes Staminum, (Smith) remark able for the length and crimson splendour of its stamens, a plant not surpassed in beauty by the finest Fuchsia, for the discovery of which we are indebted to the good Sir Arch. Menzies in 1779.” The epithet Ribes staminum is way out of date. It has been suggested to me by the esteemed botanist Steve Acker, Phd. That the species described by Douglas would be Ribes divaricatum var. pubiflorum. However, given Douglas’ description it could be Fuchsiaflower Gooseberry, Ribes speciosum

David Douglas died while trekking over Mauna Kea on Hawai’i Island in 1834. He was 35 years old. An interesting article about the circumstances of his death was published in The Plantsman in 2014: https://www.rhs.org.uk/about-the-rhs/publications/the-plant-review/the-plant-review-back-issues/2014-issues/december/the-suspicious-death-of-david-douglas.pdf
My thanks to Peter Meyerhoff and Jack Nisbit for assistance. Jack is the author of two books about David Douglas, “The Collector: David Douglas and the Natural History of the Northwest” and “David Douglas: A Naturalist at Work” available from Sasquatch Books: https://sasquatchbooks.com/