Fire Ecology

Overview

In this lesson, we explore how fire shapes the structure and function of forest ecosystems in the southwestern United States. We also consider how fire influences the air, water, soil, and living things we rely on.  Students will perform an experiment to determine how fire can impact soils, as well as study tree rings and analyze graphs to understand how fire has changed over time.

Learning Outcomes

Students will know the following:

• Fire is a normal disturbance in southwestern forest ecosystems

Students will understand the following:

• The positive and negative ecological effects of fire

Students will be able to do the following:

• Age a tree by its rings
• Interpret trends in line graphs

Essential Questions

• How does fire shape the structure and function of forest ecosystems in the southwestern United States?
• How does fire interact with the earth’s four spheres (i.e., atmosphere, biosphere, hydrosphere, lithosphere)?

NextGen Science Standards

• MS-LS2-1 Ecosystems: Interactions, Energy, and Dynamics
  • Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
• MS-LS2-2 Ecosystems: Interactions, Energy, and Dynamics
  • Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
• MS-LS2-3 Ecosystems: Interactions, Energy, and Dynamics
  • Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
• MS-LS2-4 Ecosystems: Interactions, Energy, and Dynamics
  • Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
• MS-LS4-6 Biological Evolution: Unity and Diversity
  • Use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time.
• MS-ESS3-5 Earth and Human Activity
  • Ask questions to clarify evidence of the factors that have caused the rise in global temperatures over the past century.

Materials

• 3 large clear plastic cups
• Knife/scissors
• 3 soda bottles
• Trowel
• Handful of leaves/detritus
• 5” x 3” rectangle of grass and soil
• 3L of soil
• 2L of water
• Graduated cylinder

Glossary

• Biodiversity: The variety of living organisms and ecosystems
• Biomass: All the material that is or once was alive
• Canopy: The upper layer of a forest, consisting largely of tree crowns
• Climate change: Long-term changes in average temperature and precipitation
• Dendrochronology: The science of dating events and changes in the environment using tree rings
• Disturbance: An event that make a drastic change in an ecosystem
• Disturbance regime: The typical size, frequency, and severity of disturbances in an ecosystem
• Ecosystem: A set of relationships between the living organisms and non-living environment in a given place
• Ecosystem structure: The physical arrangement of living and nonliving components in an ecosystem
• Ecosystem functions: The processes that occur in an ecosystem that support living organisms
• Fuel ladder: Biomass that allows a fire to climb from the forest floor into the canopy
• Intermediate disturbance hypothesis: The interpretation that biodiversity is maximized when ecological disturbance is not too rare or too frequent.
• Understory: The layer of forest under the canopy but above the ground
• Palmer drought severity index: a measure of drought based on temperature and rainfall; lower values indicate hotter, drier conditions
• Positive feedback loop: An amplifying process that continually magnifies a process
• Productivity: In ecology, productivity refers to how quickly an ecosystem produces biomass
• Soil erosion: The process by which soil is moved by wind or water
• Snowpack: The mass of snow on the ground
• Tree crown: The branches and leaves/needles of a tree

Activating Strategy

Time: 5 minutes

Have students think of something that was a change in their life that they initially worried was going to be bad but turned out to be okay.

Learning Approaches

Time: 45 minutes

An ecological disturbance is an event that changes ecosystem structures and functions.  All ecosystems experience some sort of disturbance. In New Mexico and much of the southwest, one of the primary disturbances that occur in forest ecosystems is fire.

Although we often think of disturbance as something negative, ecological disturbances are not necessarily bad.  Disturbances can be important for renewing ecosystems and ensuring they can bounce back from future changes.  For example, depending on their behavior, fires have several positive ecological effects that make forests healthier:

1. They clear out the forest understory, limiting the ability of future fires to climb into tree crowns and kill trees.
2. They release nutrients, invigorating future plant growth.
3. They produce char, which can help the soil hold water.
4. Certain plant species require fire to grow. For example, aspen trees are unable to grow in shady forests, so this tree often grows quickly in the clearings created by fire.

However, disturbances can also be damaging to ecosystems, limiting biomass growth and reducing the biodiversity of plants and animals.  In forests that have too much biomass at multiple levels, fire can climb fuel ladders and burn in the canopy. This kind of fire disrupts forest ecosystems in many ways:

1. They clear soil cover, which accelerates soil erosion, promotes nutrient loss, and increases water runoff and flooding.
2. Eroded sediment enters streams, rivers, and lakes, reducing water quality and harming aquatic organisms, as well as terrestrial biota that depend on these systems.
3. They kill many plants, and they destroy habitat that makes it harder for wildlife to survive.
4. They release carbon dioxide into the atmosphere, which contributes to climate change.

Let’s do an experiment to demonstrate how fires can impact soils. When water flows quickly over bare soil where a fire has cleared all vegetation, it will carry away sediment in a process called soil erosion. The eroded suspended sediment can negatively impact water quality and kill aquatic organisms. Let’s do an experiment to see how soil cover impacts erosion. Which sample do you hypothesize will be the dirtiest, and why? How might fire influence erosion?

1. Collect 2L of soil, a handful of leaves/detritus, and a 5x3in rectangle of grass with roots and soil attached
2. Use scissors to cut a 5x3in rectangle from the side of each of the three soda bottles
3. Place the soil into two of the bottles
4. Place the detritus/leaves atop one of the soils
5. Place the soil and grass sample in the final bottle
6. Balance the bottles on the edge of a desk so that their necks are over the ground
7. Holding one of the large plastic cups beneath the neck of the bottle, pour 500mL into the opening of each bottle, one by one

So what makes a disturbance like fire beneficial or damaging for ecosystems?  We don’t want disturbance to be too big or too small, too common or too infrequent.  This is known as the intermediate disturbance hypothesis.  It’s a bit like Goldilocks, but what is “just right” for any ecosystem depends on the disturbance regime it has adapted to over time.  The frequency, intensity, and extent of disturbances determine an ecosystem’s disturbance regime (Table 1).

Table 1: Factors used to define a fire regime and their application in a fire ecology context

Different ecosystems will have different disturbance regimes, which are determined in part by an area’s climate and the plants that live there. Before human settlement in pinyon-juniper woodlands, wildfires typically burned every 300 years on average because these ecosystems typically have more dispersed trees and sparse grasses and shrubs, which disrupt the spread of fire.  By contrast, in ponderosa pine forests, fires burned about every 10-20 years.  Species in this ecosystem have adaptations that allow them to survive fires.  For example, ponderosa pines have thick bark that is layered so it sloughs off when burning, the spacing between their branches limits fire movement up the tree, and they can generally survive fire so long as about 1/3 of the needles stay alive. Mixed conifer forests that contain many different types of trees can experience a diverse fire regime.  Here, fires can occur relatively frequently (every 5 to 20 years in some areas) or infrequently (every 20 to 100 years) across a range of different severities.  But how do we know how often a forest should burn to be healthy? 

Dendrochronology is the science of dating events and changes in the environment using tree rings. Each year’s new growth makes a ring in the wood of a tree’s trunk. The newest ring will be on the outside of the tree, just underneath its bark, and the oldest will be at the center. Scientists have specialized tools called corers that allow them to extract a small piece of wood without harming the tree.

How old are the trees in Figure 1 below? Be sure to count the light-colored wood between the darker rings to determine the age.

In addition to helping scientists age trees, the tree rings can also tell a story about the environmental conditions that occurred over the tree’s life. For example, a wide tree ring indicates the plant had plenty of sunlight and water, while a narrow ring suggests it was limited by poor growing conditions. Fires can be observed in tree rings as well because the outside of the tree will be charred. By determining the age of the tree, scientists can determine the dates of fires.
How long ago did the tree in Figure 2 below experience a forest fire?

Although fire is a natural component of ecosystems, fire regimes are changing. Climate change is one of the ways human activities are shifting fire regimes. The earth’s climate has always experienced changes, but human activities are accelerating these shifts. The combustion of fossil fuels and deforestation releases greenhouse gases, which trap energy from the sun in the atmosphere. This has resulted in shifts in temperature and precipitation patterns. Interpret the graphs below; how have temperature and precipitation in New Mexico changed?

In New Mexico, the average annual temperature has increased 3.3 degrees Fahrenheit and snowpack has decreased by up to half since 1955. As we continue emitting greenhouse gasses, these trends are expected to continue. By 2070, temperatures are expected to rise by 5 to 7 degrees and streamflow and groundwater recharge is expected to decline by at least 25% due to decreasing snowfall. How do you expect this to impact fire regimes? How would you interpret the graphs below?

Hotter, drier conditions have made our forests very prone to fire. While the frequency of fires has not necessarily increased, they do tend to burn larger areas and more intensely. With a 1.8°F increase in temperature, New Mexico’s burn area is expected to rise by 4 times compared to the average between 1950 and 2000. This emits carbon dioxide, which is a greenhouse gas. This is an example of a positive feedback loop: a process that amplifies itself. As temperatures increase, more fires burn, increasing greenhouse gas emissions, which causes temperatures to rise even more.  Another way humans have altered fire regimes is through our management of forests, which we will learn about in a future lesson.

Summarizing Strategy

Time: 5 minutes

Ask students to write down three things they learned from the lesson and a question they still have.

Assessing Strategy

• Age determined for trees using dendrochronological methods
• Identified trees growing in natural and modified fire regimes
• Interpreted trends in temperature and precipitation in New Mexico
• Predicted trends in fire regimes in New Mexico in response to climate change