Ponderosa pines (Pinus ponderosa) are considered one of the most fire resistant conifers in the west because of their seedlings’ propensity to benefit from mineral soil seedbeds and open habitats created by wildfires. This fire resistance increases as the tree matures.
Several factors influence post-fire regeneration of ponderosa pines including the behavior of the fire, the fire intensity and severity, the degree of fire related damage to the trees, the season of burn and weather conditions.
Burn severity and intensity is described by the amount of tree mortality, degree of soil heating, organic biomass consumption and changes in the physical properties of the soil. Low severity fires results in a high proportion of surviving trees with fire scars, medium severity fires kill many small and some larger trees, with those larger trees that die taking longer to actually succumb, and high severity fires destroy most of the vegetation.
The number of filled seeds in cones produced by surviving trees is increased by burning regardless of the burn season. However, spring burns result in fewer cones and a lower seed quality (smaller seeds which produce smaller seedlings), while seeds from fall burns tend to be larger and produce larger seedlings.
About two months after the Cove Fire which burned nearly 31,000 acres in the Modoc National Forest near Adin CA (Modoc County) last July and August Leonard and I observed many ponderosa pine trees with no needles and scorched trunks bearing open cones. The cones were fire blackened on the outside, but subsequently opened to release their seeds. The ground was littered with scorched cones and undamaged seeds were scattered over the ground. In the spring these seeds should begin the regeneration process.
The future ponderosa pine seedlings should thrive. The surviving ponderosa pines will provide the intermediate shade the seedlings need while the vegetation cover will be low because of the fire. The scorched needles on the forest floor will serve as mulch helping to retain moisture and mitigate subsurface soil temperature extremes for the seedlings. Generally burned areas also show an increase in soil nutrients which should also benefit the young ponderosa pines. It will be interesting to observe the seedlings germinate and grow next spring.
Not only are the ponderosa pine seeds important for forest regeneration, the seeds also provide sustenance for a variety of small mammals including chipmunks, ground squirrels and tree squirrels. Some small animal was eating pine seeds at this midden located within the fire zone.
It was encouraging to see all the ponderosa pine seeds in the forest so recently ravaged by fire.