Intentional burning awakens a fire follower: a botanical account

An aerial view of the western bewildering bushmallow, a rare San Francisco Bay area endemic.

California is one of five Mediterranean climate regions around the world, in which summers are reliably hot and dry, while winters are cold and wet. Wildfires periodically arise in the summer, and many California native plants have adapted to Mediterranean climate and to fire. Fire adaptations include resprouting, serotiny (when seed release is triggered by disturbance like fire), heat induced germination, and smoke induced germination. 

Prescribed pile burning took place in March 2024 in the chaparral and blue oak woodland at Jasper Ridge Biological Preserve, and piles are being monitored by staff to track vegetation regrowth. These burns occurred a season earlier than when native plants experience wildfires. Many plants will have flowered and deposited seeds in the soil by the time wildfires pass through, and the winter rains support their germination and growth through the spring. At Jasper Ridge, the seed bank faced smoke and heat in the spring through intentional burning, followed by a hot, dry summer – so we did not expect many, if any, seeds would germinate in the first year following the fire.

This summer, a rare fire follower was awakened from the seed bank and is flourishing in multiple piles in the chaparral. Seedlings of the western bewildering bushmallow, Malacothamnus arcuatus var. arcuatus, form rings around the edges of piles, where smoke and heat permeated the soil but was not so intense that it scorched the seeds. 

The western bewildering bushmallow, Malacothamnus arcuatus var. arcuatus, form rings around the edges of piles.

The western bewildering bushmallow was first documented at Jasper Ridge in 1962 through an herbarium specimen housed in the Oakmead Herbarium at Jasper Ridge. Its past distribution documented through specimens includes the chaparral around Searsville Lake, but it has not been observed recently or so abundantly in the preserve and has never before been documented near the current day burn piles.

Some individuals were previously identified as chaparral bushmallow, Malacothamnus fasciculatus, which is common in southern California and scattered throughout the Diablo mountain range. A recent phylogenetic analysis based on morphology and population genetics separated individuals in the Bay area as their own variety, Malacothamnus arcuatus var. arcuatus (see work by Kier Morse).

Stellate trichomes, or star-shaped hairs, adorn the leaves and stems of the western bewildering bushmallow, protecting the plant from herbivory.

This variety is found only in the San Francisco Bay Area, and nowhere else in the world. This narrow endemism (distribution) makes it a rare plant, classified as California Rare Plant Rank 1B.2 (rare, threatened, or endangered in CA and elsewhere). Where it is found, it may be abundant or sparse. Although this plant had not been found within the boundaries of the preserve for years, its seeds were apparently laying dormant in the soil.

Many species in this genus are known fire followers, meaning they germinate after fires – in this case where woody chaparral plants burned, exposing the soil to bountiful sunlight, and seeds were stimulated by smoke and heat. So it is an exciting and joyful surprise to see seedlings sprouting in the burn piles, and to see this plant at the preserve again.

The burn piles are full of new life, with many plants resprouting from trunks or rhizomes. The western bewildering bushmallow is a sign that the seed bank has also responded, and that plants who have not been seen in decades may be dormant in the seed bank. We are hopeful that more species will be awakened by the prescribed fires from March 2024 and that they will arise next spring.

Check out more details about these individual plants on Calflora.

For more about this plant and its Indigenous connections, see the accompanying post by Faculty Director, Tad Fukami.

For more information about the biology of this plant, read the description on the Jepson eflora.

 By Adriana Hernandez, Ph.D.

 Adriana is the Assoc. Director of Research at Jasper Ridge Biological Preserve. Adriana's passion for biodiversity studies, collections-based research,   and natural history has driven her to a research career in plant evolutionary biology and conservation. Read about her background here.

 Photo credit: Adriana Hernandez