Plant-Environment Interactions : FIRE

 Photo 1: 1988 / Historic Yellowstone Fire of 1988

 Photo 2: 1989 / One year later

Photo 3: 1999 / Ten years later (All photos taken from nps.gov)

Everyone has seen news footage some time in their life of massive wildfires burning across thousands of acres.  Whether the fire is a high intensity wildfire or low intensity prescribed fire, the results on the land following fire are similar. The soil is an ashy black, the entire green thick understory has been burnt away, and the tall tress remain standing another day but with distinct black scars around their bases. Following these fires the news trucks pack up and move on to another "story," but what is to become of the forest following the fire? How does this natural disturbance effect the environment after it burned through?

Many effects of fire are similar to those of other natural disturbances. What makes a disturbance a disturbance is no matter the type of ecosystem or how old the forest, it is an event that sets back succession.  After any natural disturbance, there is a significant rate of mortality of current plants.  After a fire, this is mainly restricted to the understory of herbaceous plants and small woody shrubs. Tree species range in fire adaptability, so certain tree species will die, while others are unaffected or benefited. Old or diseased trees also have an increased chance of mortality during or after a fire. With all of the plant mortality comes light. Suddenly light is hitting the ground in places it may have not hit in a hundred years! With more light there is more growth. In one study "Direct and Indirect effects of Fire on Shrub Density and Aboveground Productivity in a Mesic Grassland," the species monitored was Cornus drummondii. Following fires, C. drummondii's stem density was increased 600% (Heisler). With regular fires on the landscape, eventually grasses and herbaceous plants will dominant the landscape, not because the woody shrubs aren't there, but because they are regularly suppressed (Heisler).

           

With the increase in growth, one would assume there would also be an increase in competition. This is actually not the case immediately following fires. With limiting factors, such as light, more available following fires there is no statistically significant data supporting higher competition after fires (Clarke).  Another reason reducing post-fire competition could be microsite disturbance following fires (Vila). Microsite disturbance is critical for many seeds.

Fire actually has many positive effects for seeds, other than just increased light and microsite disturbance.  Fire helps disperse seeds in a variety of ways.  The strong winds during fires help release seeds from the canopies (Lamont). Other seeds from fire-adapted species actually need a certain high temperature only reachable from fire to release the seeds. Seeds already in the seed bank are also benefited due to the soil disturbance and increased light. Although fire may positively impact seeds, seedlings do not have the same positive responses. If the seedling manages to survive the fire, the reduction of nutrients, stress, and disturbance will greatly reduce its growth rate (sometimes up to 35%), and eventually lead to it being outcompeted by the newly released seeds that will germinate in optimal conditions and have higher growth rates (Lamont). However, the one major disadvantage to being a post-fire seed is the same thing that benefits, more exposure. The exposure that brings light and space also brings a higher risk of predation (Clarke).

Besides light, fire also has a large effect on water in the environment, and often not a positive one.  With removal of many plants, and disturbance to the soil, there is often an increase in surface runoff, erosion, sediment transport, and deposition following fires (Ice). Another issue with water following fires is soils becoming "water-repellent." Normal infiltration of water into the soil is rapid, with bare soil being moderate to slow. Post-fire water repellent soil makes infiltration very slow to none (Neary). With incredibly low rates of infiltration of water into soil, this makes it incredibly difficult for new seedlings or surviving plants to access water. The reason post-fire soil becomes water repellent is due to many factors influencing the soil. Fire removes binding organic matter from the soil and reduces porosity (Neary).

Figure 1: Infiltration Rates Under Varying Surface Conditions (Adapted from Hewlett 1982, copyright University of Georgia Press, Athens, GA, Figure taken from U.S. Forest Service)

		Infiltration
		Rate	Description
Surface Conditions	in/hr	mm/hr
1. Intact forest floor	>6.3	>160	Very Rapid
2. Vegatation	0.2-2.0	5-50	Slow to Moderate
3. Bare soil	0.0-1.0	0-25	Very to moderately slow
4. Water repellent soil	0.0-0.04	0-10	Very slow to none

           

Evapotranspiration is normally a large loss of water from the hydrologic cycle. With fire removing many woody plants from the landscape and replacing them with grasses and herbaceous plants, loss of water from evapotranspiration is greatly decreased (Neary).  Both lower evapotranspiration and lower rates of infiltration lead to increase stream flow as well as soil water storage. So while less water can enter the soil, less water is also leaving it. Unlike light, water has both numerous positive and negative responses from fire, and is greatly dependent on factors such as fire intensity and topography.

Nutrients available for plants also go through a variety of changes post-fire.  Many nutrients vital to plant growth, especially nitrogen, are lost in the upper surface layers during a fire. Also, soil pH levels increase and mineral elements move to the ash layer (Boerner). The increase in erosion following fires actually helps transport nutrients and microorganisms back to the soil.

With all the negative and positive influencing factors, as well as variability within nature, it can be difficult to draw a conclusion as simple as fire being "good" or "bad" for plants, communities, or ecosystems. Regardless of fire being a positive or negative there are many plant species and even ecosystems that have evolved over time to be adapted to better survive fires, and even be benefited from them. Many species need heat or smoke to help release seeds, re-sprout, and/or germinate. Other species simply have adapted more flame resistant barks, and higher limbs (Keeley).  Overall, fire is a major natural disturbance that affects light, water, and nutrient availability for plants in a variety of complex reactions that often leads to earlier successional communities.

           

Works Cited

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