The coastal sage scrub/soft chaparral that is the dominant plant community in Daley Ranch is influenced by several abiotic factors. First, there's the low levels of precipitation that arrives mainly during the winter "rainy" season. Escondido is right in the middle of a semi-arid environment, and we average just about 15" of rain annually (and more than half of that falls in January-March). Second -- and because it's so dry through the summer and fall -- many of the perennial plants and shrubs go dormant in the warm, dry months. Look at the hills of Daley Ranch in August and September, and the only green you might see will be from the deep-rooted Laurel sumac (Malosma laurina); everything else, all of the annual grasses, flowers and non-native weeds and all of the herbaceous perennials, will just be shades of brown. Third -- and something that is directly linked to the first two -- our plants have evolved with and to an extent have come to depend on periodic wildfires.

Chaparral communities in Southern California -- and reaching down into Baja Mexico, too -- have a wildfire "disturbance regime" with a frequency of about 35-50 years. In other words, in a natural cycle (and without the man-made fires from arsons, catalytic converters lighting grasses on the side of the road, arcs from power lines, etc.), the hillsides will be swept clean of brush after about 40 years or so. These natural fires are typically started by lightning strikes. In the summer of 2008, "dry" lightning started nearly 2,000 fires in California, leading to over 1.5 million acres being burned.
One thing that plants do very well is grow. This growth is powered by energy in sunlight and the "fixing" or taking up of carbon, assisted by a little moisture from the soil. This is the process of photosynthesis, where plants convert the kinetic energy of sunlight into stored, potential energy in glucose -- a simple sugar. Plants use glucose as a building block for many other chemical structures, and the amount of carbon they're able to sequester and immobilize in their tissues is a measure of evolutionary fitness. The mass of carbon that an entire plant community (like the shrubs in the soft chaparral of Daley Ranch) collectively takes out of the atmosphere is a particular measurement known as "Net Primary Productivity" -- and it's usually expressed in terms of hundreds of metric tons per square kilometer! That plant material accumulates on the hillsides, either in the form of still-living plants, annuals that die off after their cycle is done, and plant "litter" (spent leaves, flowers, and stems). Let's turn that around for a moment, and look at that from a different perspective: Every ton of carbon that is added to the plant material on a hillside is another ton (disregarding cellular respiration in the decomposers for a moment!) that is waiting to burn.

Our native shrubs are patient, slow-growing plants. In the spring, after the winter rains have come, they'll put out their leaves, present flowers to the pollinators, produce some seeds, and then just sort of shut down and go into the slumber of dormancy until the rains return. On the other hand, the herbaceous perennials and the non-native annual grasses (in particular) are fast-growing opportunists; even before the rains stop, the seeds their predecessors left in the soil last year germinate and spring to life. The grasses sink down roots, send up shoots, start harvesting solar energy and CO2 from the atmosphere, put out flowers and make seeds -- and then die. This process repeats year after year, leading to ever more biomass (also known as a "fuel load") on the hillsides. We get occasional multi-year droughts around here, too, meaning that even formerly-healthy shrubs can die off, adding their biomass to the fuel load, too.
In Daley Ranch, the last big wildfire that swept through was the Paradise Fire, in October 2003. This fire was likely human-caused (perhaps by power lines) in the Valley Center area, and ended up consuming nearly 57,000 acres -- and also lead to the deaths of two civilians. The Paradise fire swept west out of Valley Center, crossed Valley Center Road and down the hills into the northern reaches of Escondido, and cleared the hillsides around Caballo and Sage and westward as far as East Ridge and the eastern side of Jack Creep Meadow Loop. Some 700 acres or so of Daley Ranch coastal sage scrub were destroyed.
The process of regrowth occurs quickly, however, in spite of the loss of sequestered carbon going up into the air as smoke (particulates) and gasses (the CO2, and also the oxides of nitrogen when the proteins and DNA in the plant's cells are burned). The fall "fire season" is typically followed closely by the winter rains; if the rains aren't too heavy -- causing mudslides that carry the remaining topsoil and organic material away -- the hillsides start greening up again.

Recovery from wildfire is a form of secondary succession. There is usually a lot of organic material still left in the soil, as long as the fires weren't too intense. In addition to loose, free organic matter, there's also a lot of life, including vertebrates hiding in tunnels, and invertebrates, fungi and other microbes in the soil. Many of our shrubs and trees, being tolerant of and evolved with fire, have stored a lot of energy (remember that glucose I mentioned above?) in their roots. Many shrubs, such as the ceanothus, sages and laurel sumacs, have large root burls, organ structures that usually survive the fire burning above the surface of the soil. The plants use the energy (glucose) stored in this still-living tissue to produce new shoots that sprout above the surface and renew the process of photosynthesis.

Other plant species rely on germination of seeds remaining in the seed bank in the soil. Many of these plants are fire-dependent, too, even though the parent plant may be completely consumed by the fire, or perhaps was an annual that had already died long ago. The seeds left behind had been lying dormant, waiting for the soil chemistry to change -- either through the application of the heat of a wildfire, or by a residue of ash or the compounds in smoke reacting with moisture in the soil. Some seeds actually need to have their tough seed coats cracked open ("scarified") by the heat, allowing moisture to enter to the embryo and initiate germination.

Shortly after a fire, one of the most important functions that occurs in the restoration of the plant community is the capture of nitrogen and carbon from the atmosphere, translocating those essential nutrients into the soil. Fire-following plants include deerweed (Lotus scoparius) and Ceanothus species, whose genomes include the genes that can issue chemical "signals" from their roots into the surrounding soil, beckoning nitrogen-fixing bacteria (such as Rhizobium spp.). Once the bacteria arrive, the plants form nodules from their roots, enclosing the bacteria. The plant's roots are "leaky"; bacteria get the excess glucose from photosynthesis, and the plant gets excess nitrogen (in the form of ammonium) from the bacteria. All living things need nitrogen to survive. The plants can't use the N2 that's so abundant in the atmosphere (nor can any animal) -- it's the nitrogen-fixing bacteria in the soil that help to make terrestrial life possible.
Something that's happened over the past century or so, at least here in the U.S., is that the frequency of the renewing wildfires in chaparral communities has changed. Changes in land use -- such as urban sprawl, and lots more people moving out into rural or wildland areas -- has lead to management decisions to completely and immediately suppress fires. Consequently, some areas go for many decades without burning (adding to their fuel loads year after year). Global climate change is also affecting weather patterns; some areas are hotter and drier than recorded trends. Studies have been done comparing the incidences of wildfires in "managed" regions in the southwestern U.S. to "natural" areas in Baja. Some of those studies have shown that the fires that start here in California -- and that can't be quickly suppressed -- turn into huge, catastrophic and much more destructive blazes. This is in contrast to smaller, lower-intensity fires that are observed in Mexico, were fires are often allowed to simply burn themselves out.

Thinning of brush on hillsides in southern California, notably by prescribed burns, help to restore natural processes. Unfortunately, these burns are small-scale operations, and leave many thousands of acres of untouched "fuel load", waiting for that next spark...
Okay, that's more than enough text for now! I promise to go back and add some pretty pictures later on. In the meantime, have a look at Mountain Bike Bill's web page on Daley Ranch (scroll down to his text on the 2003 Paradise Fire Damage). Also, Prof. Wayne Armstrong has some very interesting material at his Ashes to Wildflowers page -- a very good reference!

If anyone has any material on earlier fires on Daley Ranch, please let me know!