Succession of a Riparian Forest
Andrea M. Shea
Supervisor: Kermit the Frog
Department of Biological Sciences
Andrea
California State University, Sacramento
Dr. Frog
ABSTRACT
This model displays possible succession changes that might
happen in a riparian forest (bay, oak, douglas fir, maple, and
pine trees) in Santa Barbara County, CA. Current forest
composition and transition probabilities were calculated from
collected data. Modeled disturbances were fire and sudden
oak death. According to the model the forest is not currently at
its stable state, and a loss of oak trees leads to a bay tree
monoculture.
INTRODUCTION
Succession is a change in species composition over
time. The stable state is interrupted by disturbances such
as fire, disease, extreme weather, etc. A study was done to
predict the succession of a riparian forest at Figueroa
Mountain in Santa Barbara County, CA. A succession model
was built under the assumptions that all the trees have the
same generation time, each seedling has an equal
probability of making it to the canopy, and transition times
do not change with forest composition. The idea is that if an
adult tree dies, the tree species that replaces it is directly
proportional to the ratio of tree saplings currently present
under the living adult tree. Fire and sudden oak death were
used to model disturbance to the stable state.
Oak
Douglas Fir
RESULTS
After projecting the initial abundances for 10 years, the
expected species abundances were 85.2% oak, 6.3% bay,
3.7% maple, 3.1% douglas fir, and 1.7% pine. After
simulating one and then multiple fires, the species
abundances would return to the above abundances after
just two generations. After simulating sudden oak death, the
bay trees would dominate the forest with a 2% abundance
of maple trees.
Bay
Grey Pine
0.9
0.8
Maple
Oak
0.6
Bay
0.5
Maple
Figure 1:The tree species present in the forest sampled.
Doug Fir
0.4
Pine
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
10
Generation
Figure 3: Fire simulation at generation 7. Forest returns to stable state after
3 generations.
Figure 2: Stable state projected from original population composition and
transition matrix.
Oak Death
0.9
0.8
1.2
0.7
Oak
0.6
Bay
0.5
Maple
0.4
Doug Fir
0.3
Pine
0.2
0.1
Proportion Occupancy
Proportion Occupancy
An area of the forest containing oak, douglas
fir, bay, maple, and pine trees was sampled. The
current percent cover of each tree species was
calculated. The germinating saplings under each
species of trees were counted. The data
included the species of adult trees and the
proportion of each tree species that was
germinating under them.
The succession model was constructed using
matrix math to project the species composition
over time. The initial abundance vector was
calculated as 57.8% oak, 15.6% bay, 10.9%
maple, 10.9% douglas fir, and 4.7% pine. The
5x5 transition matrix contained the probabilities
of any one tree species transitioning to any
other tree species after its death. The after fire
abundances were 10% oak, 3% bay, 3% maple,
42% douglas fir, and 42% pine. Sudden oak
death was modeled by completely removing oak
trees at generation 15.
Proportion Occupancy
METHODS
0.7
1
Oak
0.8
Bay
0.6
Maple
Doug Fir
0.4
Pine
0.2
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21
Generation
Generation
Figure 4: Multiple fires at generations 1, 4, 7, and 8. Each fire changes
community composition differently.
Figure 5: Sudden oak death model. Oak trees eliminated at generation 15. Bay
trees reach a near monoculture.
DISCUSSION
This model is consistent with the inhibition model of
succession. The oak trees simply increase in abundance
regardless of the abundances of any other species. It
would be interesting to core the trees in search of
evidence in their wood for past fires. This would allow a
real life event to compare to the succession patterns
predicted by this model.
Multiple fires, or at least one fire per tree generation,
will lead to species abundances that never match the
predicted end of succession abundances. This is likely
what happens because the forest is not currently at the
predicted abundances.
In the sudden oak simulation, all of the oak trees
were killed. The bay trees achieve almost absolute
cover, but the maple trees are able to stay at about 2%
abundance. This change in the abilities of the other tree
species to coexist without the presence of the oak tree is
due to the changes in their ability to get saplings to the
canopy. The oak trees deaths were essential for the
douglas fir and pine saplings to mature.