Myself and several colleagues at the Academy have been working with scientists at the United States Forest Service, exploring how resilience may be maintained in forests of the Sierra Nevada in the coming century. The major concern is fire and its management. The natural regime of the Sierra Nevada is one of frequent fire, promoted by highly seasonal precipitation (intensely wet winters, very dry the rest of the year), thus with many fires during a year. The high frequency of fires would maintain a sparser density of trees, and fuel accumulation would be regulated by regular burning. The first human residents of the Sierra Nevada, Native American peoples, utilized natural resources in the Sierra Nevada, and developed a system of management using frequent fires, thus maintaining to a great extent the pre-anthropogenic regime. In later historic times, however, beginning in the late 19th century, fire management was implemented in various forms and levels in the interest of the timber industry, as well as the mining and expanding high elevation communities. Significant deforestation was replaced in many areas by increasing tree densities driven by fire suppression, conservation and re-planting efforts. Unfortunately, the result has been unnaturally high tree densities in many areas of the Sierra Nevada, and the dangerous accumulation of fuels. This is a common problem throughout forested areas of the United States, and one of the dangerous outcomes are fires of sizes and intensities well beyond what would be natural for the forest systems. California is at high risk because of the extent of its montane forested regions, its seasonal precipitation, drying and drought due to climate change (global warming), and extensive human use of the forests. The USFS and other agencies, as well as residents, communities and industries, are therefore engaged in developing strategies to maintain resilience of the forests, and sustainable use of forest resources. Academy scientists have become involved both as advisors about communication and education efforts, as well as working on the ecology of the system.
One of the first steps that we took was to begin work on a simple conceptual model of tree growth, fuel and fire dynamics based on ordinary differential equations. Most recently I’ve been translating the model into a tractable system of difference equations for simulation on a landscape. The model is still too preliminary to get into much detail here (and not yet peer reviewed), but here are some cool visualizations. The plot at the top of the post shows a landscape of four forest patches. Each patch consists of reproducing trees. Patches may export a small fraction of seedlings to neighbouring patches, including out into nothingness (tree bare patches), and will eventually asymptote density-dependently to a stable equilibrium (based on a standardized carrying capacity of 1). Trees also produce fuel, which accumulates unless a fire is ignited, at which point a large fraction of the fuel is lost. At each time step, trees reproduce and produce fuel, and there is a probability of fire. The top plot shows the forest (and individual patches), where the probability of ignition is fairly high, mimicking pre-historic conditions. The “jaggedness” of the population trajectories indicate fires and recovery. This second plot is the same forest, but this time with a much lower incidence of fire, corresponding to a modern regime of fire suppression. Notice the lower frequency of burns, but when they do occur they are larger and losses of trees are greater. These would correspond to our unfortunate current megafires.
Finally, the latest bit of work has been to scale the model up, and here are the results of two 10×10 forest grids run for 500 years (with the first 100 years discarded as transient burn in) (see the YouTube links below). “Greener” indicates higher tree density, and yellow indicate lower densities. The main thing to get from these is that the landscape is overall greener when the incidence of fire is lower, that when fires occur in that regime they are indicated by the immediate appearance of bright yellow patches of tree loss, and that there is a more moderate density of trees in the high fire landscape, and correspondingly greater frequency of smaller fires. That’s pretty much it for now, but working on this has been a lot of fun so far, and who doesn’t like nice visualizations?! (If you don’t like them, feel free to comment…)