Skip to content

Rethinking Historic Fire Behaviors in Ponderosa Pine Forests

October 26, 2016

Contrary to popular belief, fire can actually be beneficial to forests. In fact, ponderosa forests rely on fire to create favorable germination conditions (Taylor 2009), as well as to provide structural diversity (Brown et al. 2015). The lack of fire not only gives rise to unhealthy, dense forests, but also causes highly intense fires that destroy them entirely. These fires, referred to as mega fires, are always detrimental. Ironically, the practice of fire suppression has ultimately lead to mega fires. The increasing frequency of these highly intense fires is concerning, and indicates that fire is required to maintain forest health. Scientists use models of  historic fire behaviors to determine how fire should be used to manage each forest. New studies have produced evidence that contradicts current models of ponderosa forest fire behaviors and suggests the need for an updated model.

Fire has a different niche in each forest type and affects each forest differently. The fire regime, which is the collection of fires and their characteristics and behaviors recorded over time (Odion et al. 2014), explains the variation in niches from forest to forest. A fire regime includes:

  • Frequency
  • Size
  • Seasonality
  • Severity

Frequency and severity are the two factors that are used most commonly in determining fire regimes, and each ranges from low to high. On the lowest end of the severity spectrum, fires remain close to the ground and cause no deaths among trees. The other end of the spectrum results in high mortality rates and fire that burns in the canopy as well as on the ground (Odion et al. 2014). Usually there areas within a fire that have varying fuel loads or moisture contents that cause the fire to express a gradient of severities. In this case, a fire may have areas of both high and low severity, but be classified as the most dominant severity.

This picture shows an aerial view of a fire scar. There are areas of completely black trees, indicating high severity fire, areas of red trees, indicating moderate severity fire, and areas of green trees, indicating low severity fire.

This mosaic burn pattern shows evidence of multiple levels of fire severity that is common among forest fires. Photo credit: flickr.com.

Scientists previously classified historic ponderosa pine forests as having a low or low to moderate severity fire regime. They believed the frequent, low severity fires were thought to thin out understory vegetation, such as brush and young ponderosas that weren’t entirely fire-adapted, resulting in what is referred to as “park-like conditions”(Odion et al. 2014). The picturesque scene of large, mature trees evenly spread throughout a park is a lot like how scientists imagined historic ponderosa forests looked. They also thought that they had very little structural diversity.

This picture shows a historic ponderosa forest consisting of large, mature ponderosas spread out in an even manner. There is very little understory saplings or shrubs and almost no litter.

The original model of a ponderosa forest would look like this picture from flickr.com.

However, recent studies have found characteristics of ponderosa forests that don’t fit the model of primarily low severity fire. For example, there was much more structural diversity in historic ponderosa pine forests than is suggested by the previous model. While low severity fire only slightly changes forest structure, high severity fire has the most impact on structural changes (Hessburg et al. 2016). Groupings of similar aged trees are indicative of high severity fire as it produces areas in which all of the trees die and are replaced with saplings. The successional regrowth would result in groups of young trees surrounded by groups of older trees, thus increasing structural diversity within the forest (Brown et al. 2015).

This picture shows an aerial view of a forest. Within the forest are groupings of different tree ages. There are clear areas of younger trees and older trees.

High severity fire causes similar groupings of even aged trees like shown in this picture. Photo credit: publicdomainpictures.net.

Increased understanding of fire behaviors implies a model that includes high severity fire. Scientists found that at or above wind speeds of approximately 32 kilometers per hour (20 mph), high intensity fire is almost certain to occur regardless of vegetative conditions (Odion et al. 2015). Therefore, commonly dry ponderosa forests surely experienced high intensity fire more often than is suggested by the low severity model.

With the inclusion of high severity fire, ponderosa forests can no longer be classified as having a low or low to moderate fire regime. Instead, this new evidence proposes that they be classified as having a mixed severity fire regime, which consists of much more variation. Of course, this new model isn’t denying that there were often low severity fires. Instead, it is suggesting that ponderosa forests would’ve experienced both low and high severity fires, but would’ve more commonly experienced fires that are combinations of the two (Hessburg et al. 2016).

This redefined regime is significant not only for managing the health of the forest, but also in furthering our understanding of the relationship between fire and ponderosa forests. With greater structural diversity in forests due to high severity fire, the likelihood of a mega fire, disease, or insect outbreak destroying the whole forest decreases dramatically (Brown et al. 2015). Utilizing high severity fire in managing forests will break up the continuous fuel beds of dense canopy in modern-day ponderosa forests. The structural variation within the forest will decrease the spread of wild fires and decrease the threat of the whole forest being destroyed. Scientists’ ability to understand the ponderosa forest fire regime will enable them to maintain healthy, beautiful forests.

Citations:

Brown PM, Battaglia MA, Fornwalt PJ, et. al (2015) Historical (1860) forest structure in              ponderosa pine forests of the northern Front Range, Colorado. Canadian Journal of Forest Research, 45 (11), 1462-1473.

Hessburg PF, Spies TA, Perry DA, et al. (2016) Tamm Review: Management of mixed-severity fire regime forests in Oregon, Washington, and Northern California. Forest Ecology and Management, 366:221-50.

Odion DC, Hanson CT, Arsenault A, et al. (2014) Examining Historical and Current Mixed-Severity Fire Regimes in Ponderosa Pine and Mixed-Conifer Forests of Western North America. PLoS ONE 9(2): e87852. doi:10.1371/journal.pone.0087852

Taylor AH (2009) Fire disturbance and forest structure in an old-growth Pinus ponderosa forest, southern Cascades, USA. Journal of Vegetation Science21, 561-572.

Picture Citations:

First picture:
Slide Fire Aerials 05.27.14 (102)” by Coconino National Forest is licensed under CC BY-SA 2.o.

Second picture:
321441 Ponderosa Pine near Ochoco RS, Ochoco, OR 1936” by Forest Service is licensed under CC BY 2.0.

Third picture:
Aerial Tree Tops” by Petr Kratochvil is licensed under CC0 1.0.

Advertisements
No comments yet

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: