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 1:The 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.