Ecological dynamics

The Ponderosa pine dry shrub and grass ecological site (ES) is comprised of the modal Ponderosa Pine (Pinus ponderosa)/Antelope bitterbrush (Purshia tridentate)/ Idaho fescue (Festuca idahoensis) plant association (PIPO/PUTR/FEID), along with the Ponderosa Pine/Antelope bitterbrush (Purshia tridentate)/bluebunch wheatgrass (Pseudoroegneria spicata) plant association (PIPO/PUTR/PSSP ).

Collectively these two plant associations are referred to as Ponderosa pine/Antelope bitterbrush types in this MLRA. This ES is best described as an open pine/woodland site with low overall occupancy of Ponderosa pine in most plant community phases.

This ecological site is the driest conifer forest in eastern Washington, occurring in the transition between the steppe grassland and those ponderosa pine habitat types which are more fully occupied under increasing favorable moisture and temperature regimes (for example, the “Warm Dry Ponderosa pine” type).

Forests in this ecologic site consisted of nearly pure, self-replacing stands of Ponderosa pine. Historic stands are described as open, with overstory canopy coverage typically 40 percent or lower. The fire regime of the ecologic site was dominated by surface fires, which occurred historically on a mean fire return interval (MFRI) of approximately 8 years. Surface fires were the most common type of wildfire event in this ES, accounting for approximately 75 percent of all fires. Fire ignition sources came from naturally occurring lightning strikes, as well as from intentional fires started by indigenous tribal persons.

Fires were, and still are, often accompanied by windy conditions and can also be influenced by the neighboring steppe fuel characteristics. At times, relatively isolated individuals or small clumps of mature ponderosa pine would experience crown touching.
Ponderosa pine is the only conifer in this ecologic site (e.g. it is a “climax dominant” species), although Douglas-fir (Pseudotsuga menziesii) may be found in limited extent on very favorable microsites.

Ponderosa pine is well adapted to frequent fire having a thick, platy bark which begins to develop in the sapling size class, and by developing natural fire-pruned lower branches as a result of beneficial low intensity non-lethal surface fires when they occur in the smaller pole size class. Other fire adaptations include a mature open crown configuration coupled with high foliar moisture retention and protective bud formations, deep rooting structure, and the overall longevity of individual Ponderosa pine trees.

Following stand replacement events, Ponderosa pine recruitment is dependent on viable cone/seed production, coupled with a period of favorable germination and establishment conditions which will enable the seedlings to become established. Reproduction from seed occurs occasionally, and at irregular intervals (“episodic” reproduction by definition). Ponderosa pine regenerates best on mineral soils, under abundant sunlight.
Over the larger landscape level scale, Ponderosa pine forests exhibited a mosaic of densely populated stands of various ages and heights.

Ponderosa pine is subject to a variety of biotic and abiotic sources of injury and mortality. Animals such as rabbits, gophers, squirrels, along with porcupines, damage seedlings, saplings and pole sized trees. Deer will often browse the terminal and lateral branches of young plants. Western and mountain pine beetle will infest stands of various ages and densities of Ponderosa pine, and may become epidemic in scope when forest health is adversely impacted by fire stress, age, or from the stress of over-stocking within crowded patches of pine. Ipps beetles will attack Ponderosa pine, especially when green slash is carried overwinter and into spring and early summer by events such as windstorms.

Dwarf mistletoe is the most common disease in the Ponderosa pine dry shrub and grass ecological site. It is especially prevalent in this grass-dominated site and will reduce growth and vigor of pine from a relatively young age forward.

Abiotic damage comes from the impact of wildfire, and from wind as well as from the structural damage caused by heavy, wet snowfall especially in sapling and pole size classes.
Antelope bitterbrush is a long-lived shrub of medium height. It is highly susceptible to fire kill but plants may sprout after a wildfire, depending on the ecotype and severity of the fire. Young or older plants will not typically respond well in terms of sprouting. Antelope bitterbrush will be more abundant when fire return intervals stretch out to decades, but when fire is excluded from the site for long periods of time the plants will become decadent and begin to fade from the plant community. As the bitterbrush recedes, a greater expression of ponderosa pine will occur in the overall plant community mix. Shorter fire return intervals will correspond with an increase in the abundance of understory grass species with lesser amounts of newly established pine.

Antelope bitterbrush re-seeds well after mixed fire episodes from seed dispersion by birds, rodents and ants to adjacent disturbed patches. The seed is encapsulated in a hard shell which requires cool-moist stratification to break down the protective capsule which then releases the dry fruit to the soil.

Antelope bitterbrush is an important browse species for wildlife of all kinds, and especially during wintertime for big game species. It is important as well for various livestock animals during the grazing season. It is very palatable.

Under historic disturbance regimes, Idaho fescue was the dominant understory grass, accompanied by variety of adapted shrub species.

Present day Ponderosa pine/antelope bitterbrush forests are drastically altered from the historical conditions due to a multitude of factors, including fire exclusion, conversion to agriculture, grazing, and to a lesser extent orchards and urbanization conversions.