Woodland Note No

TECHNICAL NOTE USA Department of Agriculture Natural Resources Conservation Service WOODLAND NOTE NO. 55, Arizona

FIRE ECOLOGY IN PONDOROSA PINE-GRASSLAND This technical note is abstracted from a paper published in Tall Timber and Fire Ecology Conference No. 12, dated June 8-9, 1972. The paper is authored by Harold H. Biswell, Professor of Forestry (Ecology), University of California. As Arizona has one of the largest ponderosa pine forests, it is essential that those of us who make recommendations for treatment of these lands are kept current on new developments toward a better understanding of this ecosystem. Mr. Biswell assembled a wealth of material and presented much food for thought.

Ponderosa pine is widely spread, covering some 36 million acres in the Western US. These are highly valuable multiple-use lands. My objective is to discuss fire ecology of this vegetatio type and explain how one can fit prescribed burning and management together to have a productive harmony between man and nature. One of the most interesting aspects of this vegetation type is its relationship with fire. Fires set by lightening and Indians were common before European man arrived. These were described as light and creeping surface fires; however, they planed a major role in determining the structure and composition of ponderosa pine-grassland. From accounts of early travelers and researchers, we know that the structure of that forest is one of open and park-like stands. For example, Dutton (1987) wrote in his report on the Grand Canyon District in Arizona: “The trees are noble in aspect and stand widely apart… We can look far beyond and see the tree trunks vanishing away like an infinite colonnade. The ground is unobstructed and inviting… the way here is pleasing…for it is beneath the pines standing at intervals varying from 50-00 feet, and upon a soil that is smooth, firm and free from undergrowth. All is open, and we may look far into the depths of the forest on either hand.” Tree production in aboriginal forests was small even-aged groups where fire, windfall, insect attacks or mortality from old age had created forest openings. These areas of various sizes were swept clean by fire and left….suitable for regeneration. Thus, the aboriginal forest was one of uneven-aged trees occurring in even-aged groups. In addition to this role, fire had several other important functions in the forest. Fire reduced heavy needle mats, windfalls and snags and aided in recycling nitrogen and other nutrients… Encouraged growth of grasses and nitrogen-fixing leguminous plants. Also, because the stands were kept open… more sunshine reached the forest floor and the understory was more palatable and nutritious to grazing animals… One final role of the fire in aboringinal pondorosa pine- grasslands was a natural thinning agent… Fire brought about the production of extremely large individual trees by limiting competition. Ponderosa pine-grasslands in stable condition were highly dependent on frequent fires. It is interesting to note that through the evolutionary process of natural selection, these plant communities have developed characteristics that make them highly flammable and have been referred to as fire dependent.

June 1973 Woodland Technical Note No. 55 Page 2

Fire scar and ring count studies have shown that fires were widely spread in aboriginal ponderosa pine-grasslands about every 8 years and that fires could have burned every 2-3 years with many spots burning every 1, 2, 3 or more years in succession. With the coming of European man, policies of fire exclusion and suppression were initiated. This meant the removal of one of the most important environmental factors that had functioned over the centuries to maintain balance and stability in this kind of forest. These policies have had the following effects: Fuels have accumulated making damaging wildfires difficult to control, understory has been invaded by shade-tolerant trees, the ponderosa pine overstory has been weakened, making them susceptible to insect attack and reproduction becomes stagnated in dense thickets. Prescribed burning is the use of fire for certain planned benefits and is carried out in selected places and at selected times. It should not be confused with slash burning following clear cutting. The term prescribed burning should be used in conjunction with the use of surface fires in the understory of trees. Working in harmony with nature, the land manager can use surface fires to keep reproduction out of the understory of well-stocked stands. He should seek reproduction only in the openings. Fire should be kept out of new reproduction in these openings for a few years, preferably until the trees are 12-15 feet tall. This is easy, since there is a lack of surface fuels in those areas. The common practice in forestry of having advanced reproduction in the understory of trees at the time of timber harvest is not biologically sound. It leads to buildup of heavy fuels that sooner or later will give rise to severe wildfires; it makes logging operations exceedingly difficult and produces unsightly accumulations of debris. Overstory-understory relationships are also important. With the increase of crown cover, the amount of litter on the ground increases. With this change, shading and competition for nutrients and moisture choke out most of the herbaceous vegetation. Species composition also changes with degree of shading. Some investigators found that increased degrees of shading can cause slow germination of pine seeds, slow growth and development, and even death. Side shading was found to be relatively unimportant as long as there was plenty of sunlight from directly above. The ideal situation for growing timber trees was to have taller, mature trees to establish side shading and, at the same time, an opening in the overstory to provide sunlight and ensure survival. This is very significant because this is exactly the situation that exists in the ponderosa pine-grasslands when periodic surface fires are present. There is much more detailed information in this article which will be maintained at the state office. This can be obtained for study by contacting the State Range Conservationist, Phoenix, Arizona.

Prepared by Albert P. Thatcher, State Range Conservationist, Natural Resources Conservation Service, Phoenix, Arizona

July 1973