Name :______________________________________ Date: ______________
Ecological Succession
In this lab you will apply what you learned about succession to the ecological changes that occur after a
fire. Determine the type of trees present after 5, 20, 50 and 100 years after the activity. Is this
primary succession or secondary succession? Explain your answer.
Helena National Forests As you travel around Montana, you likely notice the beauty trees provide and
also the variation among different areas. Identifying trees provides the ability to make conclusions
about the type of environment. A variety of trees grow in the Helena National Forest, each with a
specific niche or ecological role. Trees are site-specific and grow in a particular area based on climate,
soil type, disturbances, shade tolerance or intolerance, and succession.
As learned, succession refers to the replacement of one plant community by another after a disturbance
such as fire, a wind storm, or a logging or construction operation. One rule that can be applied to the
distribution of trees is the tolerance for shade. Shade-intolerant trees often occupy the site after a
significant disturbance, but as they grow and shade the site, shade-tolerant trees become established
underneath them. Without another disturbance the shade tolerant trees will eventually shade out the
intolerant trees. Trees that can reproduce under their own canopies are considered to be climax
species on the site. Trees that initially occupy the site after a disturbance are considered seral species.
For each figure 1-6, count how many of each species of tree are found in the field, using the key as a
guide. Add up all of the trees to find the total number of trees present in the field. Calculate the
percentage of each tree species at each observed time period. Use the following formula: Number of
specific tree ÷ TOTAL number of trees = ______ X 100 = ______%
Tree species
Ponderosa Pine 
Aspen
Chokecherry


Serviceberry 
Englemann Spruce 
Douglas Fir
TOTAL

Number of
Trees
% of
Total
Figure 1 (Understory and Canopy after 5 years)
Figure 2 (Understory after 20 Years)
Tree species
Number of
Trees
% of
Total
Ponderosa Pine 
Aspen

Chokecherry

Serviceberry

Englemann Spruce 
Douglas Fir

TOTAL
July 1, Noon – air temperature: 75˚F, soil surface temperature: 84˚F
Figure 3 (Understory after 50 Years)
Tree species
Number
of Trees
% of
Total
Ponderosa Pine 
Aspen

Chokecherry

Serviceberry

Englemann Spruce 
Douglas Fir

TOTAL
July 1, Noon – air temperature: 70˚F, soil surface temperature: 68˚F
Figure 4 (Canopy after 20 Years)
Tree species
Number
of
Trees
% of
Total
Ponderosa Pine 
Aspen

Chokecherry

Serviceberry

Englemann Spruce 
Douglas Fir

TOTAL
Figure 5 (Canopy after 50 Years)
Tree species
Ponderosa Pine 
Aspen

Chokecherry

Serviceberry

Englemann Spruce 
Douglas Fir
TOTAL

Number
of Trees
% of
Total
Figure 6 (Canopy after 100 Years)
Tree species
Number
of
Trees
% of
Total
Ponderosa Pine 
Aspen

Chokecherry

Serviceberry

Englemann Spruce 
Douglas Fir

TOTAL
Procedure B:
Using the data in the tables you just completed, construct two graphs. The first graph will show
the changes in the percentage of each tree species in the understory from 5 to 50 years after
the hay field was abandoned. The second graph will show the changes in the percentage of each
tree species in the canopy from 5 to 100 years after the hay field was abandoned.
Use the percentages of each tree species from figures 1, 2, and 3 to construct your graph of
the understory changes.
Use the percentages of each tree species from figures 1, 4, 5 and 6 to construct your graph of
the canopy changes.
After completing both graphs, answer the following analysis questions.
1. Describe, in terms of decreasing or increasing percentage, what happened to each of the
following tree populations from 5 to 100 years after the field was abandoned:
a. Ponderosa pine - ____________________________________________
b. Aspen - _________________________________________________
c. Chokecherry - ______________________________________________
d. Serviceberry - _____________________________________________
e. Englemann Spruce - ____________________________________________
f. Douglas fir - _________________________________________________
2. Using the understory figures 2 and 3, which tree species disappeared between 20 and 50
years after the field was abandoned? (Note: do not include trees that had already disappeared
after 20 years.)
_______________________________________________________________________
3. Use the canopy figures 4, 5 and 6 to answer the following questions:
a. Which tree species disappeared between 20 and 50 years after the field
abandoned?
was
________________________________________________________
b. What tree species appeared between 20 and 50 years after the field was abandoned?
_________________________________________________________
4. What happened to the air temperature and soil surface temperature in the understory
between 20 and 50 years after the field was abandoned?
______________________________________________________________
5. Think about the types of trees growing after 20 years in the understory. Are they tall or
short species? ________ Now think about the types of trees growing after 50 years. Are they
tall or short species? ________
6. Based on your answers to questions 4 and 5, what caused the change in the air and soil
temperature between 20 and 50 years after the field was abandoned?
_______________________________________________________________________
_____________________________________________________
7. Which trees/shrubs are shade tolerant and which trees/shrubs are not?
7. In this activity you have observed the changes that occurred in the types of plants that grew
100 years after the fire. What other types of organisms would be affected by the changes
that occurred?
_______________________________________________________________________
______________________________________________________________
8. Will the community of organisms in the forest change significantly over the next few years?
_______ Why or why not? _____________________________
_______________________________________________________________________
9. After such disturbances, would you expect to a forest to become the climax community
again? ___________ Why or why not? ________________________
Teacher Notes: In this part of Montana, the predominant type of disturbance which affects our
forests is fire. If you think about it, different types of forests are naturally exposed to different
fire return intervals. Low elevation forests are warmer and drier than high elevation forests that
are cooler and usually receive more rain and snow. So, a warm and dry forest would have a
greater number of days a year when fires could be expected to burn than would a cool and moist
forest.
If fires were frequent, fuels did not have much time to accumulate, so fires tended to be low
intensity. Trees in this environment evolved to survive frequent low intensity fires with thick
insulating bark, and branches high in the crown. Because the trees survive the fires they tended
to live a long time and become quite large.
If fires were infrequent, fuels tended to accumulate. Often, these forests can burn only on the
hottest and driest days so fires tend to be high intensity. Crown fires often develop in these
forests that kill all or most of the forest. Lodgepole pine trees in this type of forest have cones
that remain closed until the heat of a forest fire causes them to open. This seral tree may be
killed by a forest fire, but will reseed itself and grow a new forest.
Probably the most disruptive disturbances to forests involve exotic species, or the introduction of
plants, animals and diseases, which are not native to the ecosystem. These new disturbance
processes often push forest types into states of balance that are not natural to the system. This
can damage and deteriorate the quality of the ecosystem not only for plants and trees, but also
for the animals that rely on these forests as their home.
We’ll learn more about all these things as we learn how to identify the trees of the Helena
National Forest.
Seral/intolerant trees
Climax/tolerant trees
Limber Pine
Juniper
Ponderosa Pine
Aspen
Cottonwood
Lodgepole Pine
Serviceberry
Chokecherry
Whitebark Pine
Douglas Fir
Engelmann Spruce
Subalpine Fir
Shade Tolerance
Very Intolerant aspen balsam poplar black willow cottonwood jack pine
tamarack
Intolerant bitternut hickory black ash black cherry black walnut butternut
paper birch red pine white ash
Intermediate American elm black oak bur oak hackberry red oak rock elm
shagbark hickory sycamore white oak white pine white spruce yellow
birch
Tolerant basswood black spruce boxelder ironwood north, white-cedar
red maple silver maple slippery elm
Very Tolerant American beech balsam fir hemlock sugar maple From:
Silvics of North America, Vol. 1 & 2. 1990