Sunday, September 1, 2013

9/1/13 9:00 pm: Each species of tree has its own responses to wildfire

Each species of tree has its own response to fire.  Fire also burns with different frequencies in different landscapes.  The severity and frequency of wildfires vary depending not just on the weather and on the physical landscape, but on what is burning.

Yellowstone does not have as many species of trees as do forests in moister lowland regions nearer the East Coast or West Coast.  Most of the trees in the park are either lodgepole or whitebark pine, Englemann spruce, or white fir or subalpine fir.

Most of the Alum fire is burning in mature lodgepole pine forest, which is the tree type that burned across the park in 1988.  Lodgepole grows at about 7000 to 8500 feet elevation at this latitude.  About half of lodgepole pine trees have a unique relationship with fire:  they depend on wildfire in order to reproduce. Their serotinous pine cones, covered in gummy resin, cannot open without being heated, either by a hot summer day, or by fire, or by about 15 seconds in a microwave oven.  After exposure to heat, the sealed bracts or scales of the mature cones peel apart so the tiny winged seeds can emerge.  The cones stay viable (with live seeds) for six to eight years on the trees, just waiting for a fire to open them.  In other words, lodgepoles usually produce seedlings when the parents burn.

Thus because the lifetime of a lodgepole pine tree is about 200 or 250 years, fire usually burns through the crowns (treetops) of older, weaker lodgepole pine forests only about every 150 to 200 years.  Adult lodgepoles lose their lower branches, so in mild weather, lodgepole trees do not burn.  Only their fallen needles burn along with underbrush.  The physical design of lodgepole crowns is an open ladder that can carry fire from lower to topmost branches within seconds.  In dry enough, windy enough weather, lodgepole fires race across treetops, releasing fresh seeds to repopulate the landscape as the older trees turn to ash.

Whitebark pine trees, in contrast, live in extreme high mountain areas up to 12,000 feet.  Their large pine nuts are a favored food of grizzly bears.  Clark's nutcrackers have a symbiotic relationship with the wihitebark pines and plant new stands of trees by caching the seeds.  On their cool, moist mountainsides, whitebark pines have a short fire season and are used to small, irregular fires that burn only a few drier trees.

Spruce trees, like whitebarks, are vulnerable to fire due to their thin bark, but usually avoid fire by growing in high moist mountain environments.  They like to grow in shade.  Shade-loving Douglas fir trees have thick platy bark, so they survive many low-intensity fires. High on the mountainside overlooking the Lamar valley, the Druid Fire is burning in heavy spruce-fir forest.  Spruce-fir fires usually do not kill every tree, but burn instead in a raggedy patchwork or mosaic of burned areas.  Fir trees throw lots of sparks as they burn, so they often send small fires burning in spots ahead of the rest of a fire.  After a wildfire in these forests, first herbaceous plants like fireweed and ceanothus brush must grow to provide shade for seedling trees.  Decades can pass until existing seeds in the soil regenerate enough to provide a new tree canopy.

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