The structure of the open, park-like old-growth ponderosa pine forest
differs from the Pacific Coast old-growth forest that grows west of the Cascade
crest. Prior to 1900, old-growth ponderosa pine forests typically had widely
spaced clumps of trees, few shrubs, often grassy, with relatively few dead
branches and large logs. Forest structure was the result of both the dry climate
and the fire regime.
The open structure of these old-growth forests made walking within them
easy. Since 1900, this openness has been replaced by a thick understory and
higher tree density and tree diversity. Fire suppression has allowed shrubs
to become established and allowed more shade tolerant species to encroach.
Timber harvests removed the large trees thus creating gaps for shrubs and competing
trees to grow. Interestingly, the arrival of timber harvests has actually increased
the density of growth (although trees are rarely reaching the size and age
they did in earlier times). The introduction of livestock also impacts the
forest structure by the grazing of native grasses which further aid in the
establishment of shrubs.
o Moisture and fire and not the only major structural forces of the ponderosa
pine forests, but pathogens as well. Some common afflictions are
• The inner bark fungus, comandra blister rust, which causes growth
reduction, stem deformity, and mortality of hard pines.
Pandora moth larvae which eat the needles of ponderosa and lodgepole pine
and western pine beetle, which is capable of killing apparently healthy Ponderosa