Plant Reactions to wildfire

Wildfires are devastating to landscapes. However devastating we think wildfires are, some plants have adapted to wildfires and rely on them. In the past, the plains and foothills areas experienced low to medium intensity fires every 10 to 30 years. These fires served to clean up the duff and dead wood, and killed young tree seedlings that had not developed fire resistance. Due to these low intensity fires, ponderosa tree density in the past was only 20 – 30 trees per acres as opposed to the hundreds per acre that we have now. Higher tree density increases the occurrence of higher intensity fires that require longer recovery times. Fires were caused naturally by lightning strikes or started by man. As you go higher in elevation, fire frequency decreased with highest elevations experiencing fires every 100 to 300 years.

Forestry experts have defined plant responses to wildfire as Resistors, Sprouters, Seeders, Invaders or Avoiders. Plants may fall into more than one category.

Resistors are plants with attributes to resist low to moderate intensity fires. They have thicker bark, deep roots, shed lower branches so that the fire cannot utilize lower branches to climb to the crown, and may have branches that contain more moisture or foliage that is shorter or moister. Ponderosa pines can have bark that is three inches thick and they naturally lose their lower branches. Douglas fir and aspens are also Resistors. Aspens, due to their higher moisture content and lack of resinous needles, do not readily burn. A crown fire drops to the ground and decreases in intensity when it comes to an aspen grove.

Sprouters are plants that either have a root system that survives fire or may have a seed that either endures the fire or utilizes the fire to expose the seeds for germination. Aspen are Sprouters and may produce 50,000 to 100,000 sprouts per acre. Post fire, most sprouts do not survive to become mature trees. Lodgepole pines produce non-serotinous and serotinous cones. Serotinous cones may stay on the tree for 15 to 30 years and do not open to release the seed until they are exposed to higher temperatures due to a wildfire. Grasses and shrubs also sprout post fire as long as the buds at the soil surface were not damaged.

Seeders produce a lot of seeds after fire. Lodgepole pines are seeders, as is fireweed. The lodgepole pine serotinous cones need heat to open up and release the seeds. Fireweed (Epilobium sp.) produces many seeds that have a fluffy chaff attached to them, so they readily move through an area post fire. The seeds take advantage of the nutrients post fire to germinate and populate burned areas.

Invaders take advantage of the lack of competitive vegetation to get a foothold in the disturbed area. While fireweed is a native, it is considered an invader due to the fact that it may not have been in the area prior to the fire, but seed has come in and sprouted. Non-native invaders are noxious weeds such as Canada thistle, musk thistle and cheatgrass. Non-native weeds are brought in on fire-fighting equipment, homeowner vehicles or in mulch, straw or seed.

Avoiders tend to grow in areas around water or higher elevations where fires do not normally burn. They have thin bark, shallow roots, lower branches that are not shed and lots of resin which can help fire spread. Avoiders are those plants that grow along the streams and riparian areas. They are plants such as blue spruce, cottonwoods and willows.

While the effects of wildfires are devastating, plants have the capability to recover and to thrive post fire.