Editor’s Note: Massive wildfires in Washington State may cause us to see some plants as “gasoline on a stick” (bitterbrush or Purshia tridentata) or as “grassoline” (cheatgrass or Bromus tectorum). But fire is a natural part of many ecosystems, and some plants have developed characteristics that allow them to survive, even thrive, after a fire. In this Botanical Ramble, Ellen Kuhlmann guides us through some of these adaptations. –S.G.
Four Fire-Adaptive Traits in Plants
Bark thickness. This is one trait that affects resistance to fire damage. Species with thin bark are highly susceptible to dying from fire. Thick bark does not catch fire or burn easily. It also reduces the amount of heat into the growing tissue, or vascular cambium, located beneath the bark. Vascular cambium forms both phloem and xylem, the living tissues that transport water and nutrients.
Ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudostuga menziesii) are fire-adapted species with thick bark that does not burn easily.
Resprouting. Many species have the ability to grow new shoots after fire. These plants resprout from buds in parts of the plant that are belowground such as the root crown or underground stems (rhizomes). Since these buds are beneath the soil surface they are protected to some degree from fire damage. Whether they survive depends on how hot the fire is and how long it burns.
Bigleaf maple (Acer macrophyllum), quaking aspen (Populus tremuloides), and pinegrass (Calamagrostis rubescens) are examples of species that often regenerate after a burn by resprouting.
Closed or serotinous cones. This type of cone often stays on a tree after the seed matures, with the cone scales sealed by resin. The seeds remain viable due to the cone’s connection to the nutrients flowing from the tree’s vascular system. Serotinous cones can remain on a tree for decades until conditions trigger cone opening. When a fire sweeps through a stand of trees with closed cones, the fire’s heat melts the resin. If the tree dies in the fire, the nutrient flow to the cone ceases, and the cone opens, letting out the seed.
Lodgepole pine (Pinus contorta var. latifolia) has both closed and open cones, allowing for regular as well as fire-stimulated seed dispersal.
Seed germination. The seeds of some plants can remain dormant (viable but not germinating) for long periods of time well after dispersal. They germinate only after the seed coat is damaged in some way (scarified) or when dormancy is broken by some other mechanism. These seeds generally possess an tough seed coat that helps retain chemicals that maintain dormancy. A fire’s heat can crack the seed coat, allowing the chemical inhibitors to exit and germination to occur. Species with this type of seed are common in a plant community after a burn. Dormancy can also be affected by smoke and other aspects of fire such as time of year and duration.
Longsepal globemallow (Iliamna longisepala) and snowbrush (Ceanothus velutinous) are two species with seeds that germinate after a fire.
What Causes These Traits?
Plants experience environmental variability that can include changes in temperature, moisture, and nutrients as well as disturbances such as landslides, floods, and fires. Over time, plants that have traits that help them tolerate or adapt to these changes tend to increase in number.
Sometimes a trait that evolved for one reason is useful for responding to other situations. Scientists can only hypothesize the exact reason why a particular trait developed. Whether the traits discussed above developed in response to fire is uncertain; however it is known that they are give plants an advantage for surviving or recovering from fire.
Plants Can Have Indirect Responses to Fire Too
The aforementioned traits are some of the ways plants respond to fire directly. Fire also affects plants indirectly. For example, fires can make light more available when the canopy of plants overhead is reduced. Fire can create gaps of bare soil and reduce competition from other species. Fire can also change soil properties, for example nitrogen is often volatilized during high severity fire.
Pinegrass (Calamagrostis rubescens) responds to increased light availability by flowering. Fireweed (Chamerion angustifolium) readily colonizes bare soil with its lightweight, wind-disseminated seeds. Lupines (Lupinus spp.) and other species that can fix nitrogen often increase after high severity fires because they can grow in nitrogen-deficient soils.
The interactions between fire and plants are complex, with both the direct and indirect effects of fire operating at multiple scales in time and space. By examining how plants respond to fire, we can learn how to better manage fire-prone landscapes, and foster increased plant community resiliency and health.
Ellen Kuhlmann serves on the editorial board of the Washington Native Plant Society and is active in the Koma Kulshan Chapter. This article is adapted from a special issue of Douglasia, the journal of the Washington Native Plant Society. The special issue, for which Ellen served as guest editor, focused on fire ecology. A subscription to Douglasia is a benefit of membership in the Washington Native Plant Society.