September 2024, by guest author Logan Ridenour

In recent decades, fires have increased in size and frequency throughout California. The most recent of these record-breaking wildfires, named Park Fire, is the fourth-largest recorded wildfire in the state’s history. Park Fire started in Bidwell Park in Chico, California, as the result of arson. So far, it has burned through over 429,000 acres, destroyed more than 600 structures and burned through protected ecosystems, like Bidwell Park, the Big Chico Creek Ecological Reserve, and over 113,000 acres of Lassen National Forest. Many factors contribute to California’s ever larger and more frequent fires, but two big ones are climate change – with its resultant dry spells and record-breaking temperatures – and shifts in fire management strategies. In the last century, fire suppression practices, as opposed to controlled burns, have become the norm. Smaller, more frequent controlled burns decrease excess vegetation, reducing a fire’s ability to spread, while strict fire suppression can lead to a build-up of vegetation, increasing the frequency of large-scale wildfires. Here we’ll see how fires can drive evolution, helping plants survive and thrive after fires – and how more frequent fires can make some of these fire-adapted traits a liability.

Where's the evolution?

Adaptations are attributes shaped by natural selection that improve an organism’s ability to survive and/or reproduce in a particular environment. Plants have evolved an array of traits that help them survive in fire-prone environments. Fossil evidence of fire dates back to plants’ early colonization of land approximately 400 million years ago, so fire has long been a challenge to plants’ survival and reproduction. Some of plants’ fire-adaptive traits can be difficult to attribute to fire specifically because they may have originated in regions that aren’t fire-prone or exist in plants that undergo different disturbances.  Whether such traits originated because of fire specifically or a broader array of challenges, in the wake of a fire, these adaptations can be the deciding factor in how an ecosystem bounces back, or doesn’t!

In different regions, fires tend to burn in different ways – part of the area’s ‘fire regime’ – and this can drive natural selection, leading plants to develop fire-adaptive traits that help them survive or regenerate. Specific fire regimes began emerging in the late Paleozoic Era. For example, crown fires impact most above-ground material. We often see these high-intensity fires affect chaparral ecosystems in California. Surface or understory fires burn through dead foliage on the surface and don’t affect taller trees. These fires burn with a lower intensity than crown fires. Ground fires burn underground and, therefore, kill plants at the roots. Fire regimes help determine the adaptive traits plants pass to future generations.

In California, chaparral ecosystems, with evergreens, shrubs, and small trees, are often threatened by crown fires. Chaparral vegetation can be found throughout the state, in both high and low elevations. The majority of shrub genera found in chaparral communities are obligate resprouters, meaning their seeds are destroyed by fire and they replenish themselves after a fire by resprouting from surviving vegetation. The fossil record shows us that several shrub genera have had this adaptation since the Eocene Epoch, which began 56 million years ago. However, over time, some species have lost their ability to resprout, and have evolved into obligate seeders.

Obligate seeding plants do not resprout after fire, and instead rely on seeds surviving the blaze and growing into new plants. These seeds can come from a few sources: (1) dormant seed banks in soil, (2) serotinous cones, or fruits that don’t release seeds until they’re exposed to fire, and (3) seeds that surviving plants and populations can disperse. Obligate seeding may offer an evolutionary benefit by leading to an increase in genetic variation and a more rapid rate of evolution for those species that depend on it. However, with increased fire frequency, obligate seeders also face a risk. They may not have enough time to mature or build up a sufficient seed bank before the next fire rolls through. And if there aren’t any or enough viable seeds around after a fire, that species of obligate seeder may not be able to return. Obligate seeding started showing up in shrub genera during the mid-Miocene, and most species in the Arctostaphylos and Ceanothus genera, found in chaparral ecosystems, are obligate seeders, putting these community members at high risk as California wildfires become ever more frequent. 

Traits like obligate resprouting and seeding have developed over millions of years when fire frequency allowed for a natural recuperation period of 30-130 years for plants in chaparral ecosystems. This allowed plants to mature, reach a larger population size, and build up seed banks before another fire occurred. However, with fire frequency increasing to 10-15 year intervals due to factors like human activity and climate change, plant species in chaparral ecosystems are at great risk for biodiversity loss. Despite eras worth of evolution, they may be unable to keep up with the pace humans have set.