THE OLDEST FOSSIL OF a ponderosa pine tree (Pinus ponderosa) is over 600,000 years old. Since then, the species has adapted to thrive in lightning-ignited fires that are common in the intermountain west. Today, ponderosa pines have numerous physical characteristics that allow them to survive frequent, low-intensity fires.
Prior to the arrival of European settlers, fires occurred in Arizona ponderosa pine forests every five to ten years. When fire happens that often, it is of a low intensity that creeps along the ground burning grasses and small young shrubs and rarely affects any tree that is more than 4-5 feet tall. It will be spotty, miss some of the small trees and other ground foliage which aids in forest rejuvenation.
The ponderosa pine is a common species in Arizona’s montane conifer forests. It is a rosy-barked tree that has long, 4-5” needles bunched in groups of two or three. The bark has a delightfully sweet scent that resembles vanilla or butterscotch for those willing to give it a sniff. Not far north of Prescott is the largest ponderosa pine stand in the world, which can be found in the Coconino National Forest surrounding Flagstaff.
When a ponderosa is young, it spends several years channeling energy to develop its root system. This deep system, growing several feet into the soil, offers heat protection from ground fires. At this time, the sapling has a thin, corky bark only 0.3 to 0.6 inches thick. As the young tree grows, its bark will thicken and the tree will drop its lower branches, giving the tree its characteristic exposed red and flaky trunk topped with heavily needled limbs.
Once the tree reaches maturity, its branchless lower trunk keeps the highly flammable, resin-filled needles high above the heat of the flames. The lower bark, which could be up to six inches thick, will flake off during a fire to protect the live cells of the inner cambium layer. Because ponderosa pines naturally grow apart from each other, their crowns do not touch. This prevents the flames from jumping from one crown to another during a lightning fire in the upper branches of a single tree.
Fires have been suppressed in montane conifer forests for the past 150 years. This has led to fires of greater intensity that burn hot and fast over large areas, a pattern that is beyond the trees’ adaptations. But looking at the blackened fire scars on old ponderosa pines, we can still observe this trees resilience. Maybe by looking close at the Pinus ponderosa, we can better understand these natural processes and learn to accept them.
Contributed by Frank Croft
Edited by Jo Wurst