Succession of a Riparian Forest

1
Succession of a Riparian Forest
Andrea M. Shea Supervisor Kermit the
Frog Department of Biological Sciences
California State University, Sacramento
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.
Andrea
Dr. Frog
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
Maple
Figure 1The tree species present in the forest
sampled.
METHODS 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.
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.
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.
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.