Fire exclusion, especially in the dry forests (i.e. those dominated or potentially dominated by ponderosa pine) has most often altered tree and shrub composition and structure and, though often overlooked in many locales, the forest floor from conditions that occurred historically (pre-1900).

When fires are excluded from the dry forests, bark slough and needle fall from mature ponderosa pine are not consumed and large quantities of these materials tend to accumulate at the base of trees. Even the largest ponderosa pine with an extensive tap root acquires nutrition and water from lateral roots often extending well beyond the spread of the tree’s crown. Nutrient and water uptake in the fine lateral roots is aided and dependent on the formation of ectomycorrhizal fungi. The abnormal deep surface organic layers occurring because of fire exclusion are rich in nutrients and moisture, facilitating robust fine root development that historically occurred in the mineral soil. These organic surface layers and the roots and ectomycorrhizae they contain are vulnerable to loss from both prescribed and wildfires. In a fire, even though the inner portions of large ponderosa pine are protected by the thick bark from heat, the destruction of the organic surface layers and the roots they contain stresses trees making them susceptible to both insect and disease attacks. In addition, the mechanical removal of these organic layers through raking the material from the base of trees causes the loss of the roots and sets trees up for pest attack. Therefore, restoring forests in which fire exclusion has changed both their vegetative character and soils requires treatments applied in concert that address both of these components.