BEGINNING A NEW CHAPTER - Threats to Forests

Fire,

Two kinds of fire

Most people agree there are two kinds of fires, the good kinds that cook our food and warm our homes and the bad kind that burns down our homes.   Of course, today, electricity does the cooking and warming, but most of that electricity is generated by a fire at a power plant at the other end of the power line, we just don't see it.

Hurting, but still some green.
Resilience !

 

  

Forests and individual trees view fire a lot like people do, there are good fires and bad fires.  From the tree's point of view, good fires are low intensity, that is they are not too hot.  The flames stay close to the ground, mostly less than eight feet high, they creep around burning the grass, bushes, limbs and trees that have fallen.  These “cool” fires will kill small trees, but larger trees with thick insulating bark, like Ponderosa pine, will survive low intensity fires – no problem.  A year after a “cool” fire, the grass grows back from its roots, so do most shrubs and the forest looks clean and fresh.  The small trees that were burned were killed, no coming back from the roots for them.  That's generally good because it makes fewer trees in the area so each live tree has more space to use available sunlight, soil and water.  That makes for fewer and bigger trees rather than a lot of small trees crowded together where none get enough sun, water and soil nutrients to grow big — we like forests with big trees.

Fires in forests aren't normally a solid wall of flame moving across the landscape.  Within the perimeter of a fire there are many areas that are not touched by flames.   There are lots of reasons for unburned areas inside a fire's perimeter,  the humidity may come up causing the flames to quiet down, the wind may change, or fuels become sparse, topography affects fire behavior, or night comes on and the fire lays down. All these and more factors are at work determining what burns and what doesn't burn.  That's why fire is so unpredictable.  This is the way it is for both hot fires with flame lengths twice as high as the trees and for cool fires with flame lengths that don't reach the low limbs.  Of course, small trees and everything else survive in unburned areas, smoky air doesn’t kill trees like it does people.

Low intensity fires and forests get along very well, in fact, they need each other.  Hot fires are a different story, forests fear them and so do people - for good reasons, they’ll kill you.  Why are some fires so hot that flame lengths may be 200 feet or more and trees are destroyed?  There are three big factors in that that question, fuels, topography and weather.  Fuels are the factor we can do something about.  Natural fuels, of course, come from the forest itself, the fallen limbs, needles, dead trees, small live trees with limbs near the ground - they’re all fuel.  Fires start on the ground, either by lightning or by people.  If there is an accumulation of dry fuels at the point of ignition and if the weather is hot, dry, and windy, the fire spreads.  The flames may stay on the ground if there are no ladder fuels.  Ladder fuels are limbs on small or medium size trees that grow clear to the ground like a good Christmas tree.  When the needles or leaves are dry enough to carry fire, flames explode up the small trees, reach the crowns of the big trees and rush up forested mountain sides, tree top to tree top - that’s a crown fire - flame lengths may be twice the tree heights moving with frightening speed, intensity, and heat.  

Crown fires, kill everything in their path.  Intense crown fires may bake the soil so even the roots of grasses and shrubs are killed, only the tree skeletons remain standing, but they’re dead, the cones and seeds burned.  When a hot crown fire has past, the forest has experienced what foresters call “a stand replacement fire.”  The forest that was there before the fire is no more, it will be replaced with something else.  It may be different kinds of trees, it may be brush, it may be grass, it may be weeds, and in severe burns only bare soil and rock may be left.  It’s no wonder forests fear these hot fires, so do people.