Fall Rates of Prescribed 

 Fire-Killed Ponderosa Pine 



Michael G. Harrington 



The immediate, visible impacts of forest fires can 

 vary from partial consumption of the surface litter and 

 minimal plant charring to high consumption of all fuel 

 classes and complete mortality of large stands of 

 trees. When fire passes through a stand, trees can be 

 killed outright if all or a large portion of the foliage, 

 buds, and branches are killed, or they can be mortally 

 wounded if a smaller portion of the living tissue is 

 killed. Trees in this latter group will die over several 

 years subsequent to the fire with the mortality rate 

 generally decreasing over time. Another category in- 

 cludes trees whose vigor has been reduced by fire 

 injury, thereby permitting successful insect and dis- 

 ease attacks, which commonly result in tree death. 



Standing dead trees have distinctly different influ- 

 ences on ecosystems than living trees do. In the first 

 half of this century, snags were primarily thought of 

 as fire hazards (Dahms 1949; Keen 1955). Manage- 

 ment activities at that time included felling dead trees 

 in timber sales and along roads. More recently, man- 

 agers have realized the economic and ecological value 

 of snags. Products with direct economic value include 

 house logs, firewood, and even lumber (Lyon 1977). As 

 decay spreads within the tree, these values diminish 

 and are generally lost when the tree falls. 



Apart from economics is the value of standing dead 

 trees for wildlife habitat. Birds use such trees to perch, 

 feed, nest, overwinter, and hide (Bull 1978). Mammals 

 also use snags to nest or den, feed, and overwinter. 

 The value of dead trees for habitat dramatically changes 

 when standing trees become fallen logs because bird 

 nest and perch use is greatly reduced. These logs, how- 

 ever, are important for hiding and thermal cover, 

 feeding, and nesting for ground birds (grouse) and 

 small mammals (rabbits and squirrels) (Thomas and 

 others 1979). As they decay, logs also ameliorate soil 

 moisture and nutrient condition for plant growth 

 (Harvey and others 1988). 



Dead trees are also an important forest fuel. They 

 ignite more easily than live trees and, therefore, repre- 

 sent a higher potential source of burning embers that 

 can aid wildfire spread. After falling, logs represent a 

 potentially high concentrated heat source causing 

 problems for fire suppression and sometimes result- 

 ing in microsite soil degradation. 



Knowledge of the transition rate from standing dead 

 trees to fallen logs would be important for those who 



manage timber and soil resources, wildlife habitat, 

 and wildland fuels. Others have reported on the fall 

 rate of beetle-killed ponderosa pine (Pinus ponderosa) 

 (Keen 1955; Schmid and others 1985), beetle- and fire- 

 killed Jeffrey pine (Pinus Jeffrey i) and white fir (Abies 

 concolor) (Raphael and Morrison 1987), and fire-killed 

 lodgepole pine (Pinus contorta) (Lyon 1984). 



This study describes 10 year fall rates of second- 

 growth ponderosa pine following mortality from differ- 

 ent levels of crown scorch sustained in different sea- 

 sons of prescribed burns. 



Study Site 



This research was conducted on the San Juan Na- 

 tional Forest in southwestern Colorado on a southern 

 aspect at 7,600 ft elevation. The nearly pure ponderosa 

 pine stand averaged 124 square feet of basal area and 

 300 trees per acre with about 75 percent occurring in 

 the 4- to 11-inch diameter at breast height (d.b.h.) 

 range. Gambel oak (Quercus gambelii), which domi- 

 nated the understory, averaged about 2 to 4 ft tall. The 

 nearest weather station, 5 miles to the southwest at a 

 similar elevation, receives an annual average of 17 

 inches of precipitation. The least precipitation falls in 

 the late spring and the greatest in mid-summer. 



Methods 



This research was established initially to compare 

 the impacts of prescribed burning in three distinct 

 seasons on understory Gambel oak, which is a highly 

 competitive, aggressive sprouter in this region. Fire 

 treatments were applied in the autumn, late spring, 

 and mid-summer. A description of pine phenology cor- 

 responding with these three seasons follows: entering 

 dormancy in late October (autumn), during bud break 

 with emerging foliage in early June (late spring), and 

 during the moist, growing season in mid-August (mid- 

 summer). Fires were applied at 2- and 4-year intervals 

 over 8 years in an attempt to suppress oak sprouts. 

 Burn conditions and oak response have been detailed 

 in Harrington (1985). In general, air temperatures 

 during burning were 10 to 15 °F cooler in the autumn 

 burns, rates of fire spread were slightly higher in the 

 spring burns, but flame lengths and fuel consumption 

 were similar in all three seasonal treatments. 



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