Ecological dynamics

The Mesic Xeric Loamy hills, Ponderosa Pine Warm Dry Grass ecological site is comprised of the modal Ponderosa pine (Pinus ponderosa)/Idaho fescue (Festuca idahoensis) plant association, as well as the Ponderosa pine/bluebunch wheatgrass (Pseudoroegneria spicata) and the less extensive Ponderosa pine/pinegrass (Calamagrostis rubescens) associations. This ecologic site was identified and mapped in Asotin, Lincoln, and Spokane Counties, and in the Coleville Indian Reservation soil surveys.

The acres associated with each plant association in this ecological site are as follows:
Ponderosa pine/Idaho fescue (PIPO/FEID) 17,006*
Ponderosa pine/bluebunch wheatgrass (PIPO/PSSP) 15,890
Ponderosa pine/pinegrass (PIPO/CARU) 1,034
TOTAL: 33,930 Acres
* Another assessment of the data shows this ecologic site at 43,274 acres. The final ESD (not provisional) will need to rectify the difference.

The reference site is on warmer and drier slopes within the MLRA. Ponderosa pine stands are nearly pure and self-replacing. Ponderosa pine makes up all seral stages; in early seral through the mid seral and into the climax plant community. An occasional Douglas-fir (Pseudotsuga menziesii) may be encountered, but only on favorable microsites. Tree stocking is low, resulting in an open canopy structure in the mid and late stages of forest development. Age classes are variable, and small amounts of sapling/pole size cohorts typically exist in the understory or in patch openings of various sizes.

Frequent surface fires dominated these forests in the pre-European time period, occurring with an average fire return interval (FRI) of 2 to 10 years. Surface fires returning in such a short time period seriously hinder young Ponderosa pine seedling recruitment and development, while at the same time promoting a robust expression of grasses.

The understory is comprised of low growing shrubs and grasses, including bluebunch wheatgrass, Idaho fescue, and lesser amounts of needle and thread (Stipa comata) and rough fescue (Festuca campestris). Shrubs include very limited amounts of dwarf rose (Rosa gymnocarpa), spirea (Spiraea betulifolia), rose (Rosa woodsia) and some serviceberry (Amelanchier alnifolia). Forbs species include silvery lupine (Lupinus argenteus), basalmroot (Balsamorhiza serrata), pussytoes (Antennaria spp.), hawkweed (Hieracium albertinum), and common yarrow (Achillea millefolium lanulosa).

The modal pine/Idaho fescue association is encountered on sites with the most effective moisture in the ecological site, although the advantage is slight. This fescue types occur in favorable positions such as on lower slopes and more level positions across the landscape. The pine/bluebunch wheatgrass association is restricted to the slightly dryer and warmer locations; however, both the Idaho fescue and bluebunch wheatgrass types are often in close association.

The pine/pinegrass association is included in this ecological site because it shares common historical fire attributes and the nearly pure Ponderosa pine stand expression of the dominant fescue and bluebunch wheatgrass types. It is limited in extent. The pinegrass type is slightly more favorable in climate expression than the dominant two, although it is still warm and dry in the overall context within the ecologic site. This pinegrass association will occupy slightly higher elevational positions, is more productive and is more apt to have small expressions of Douglas-fir and perhaps western larch (Larix occidentalis).

Upland forests in this ecological site are warm and dry and exist in an ecotone position that is transitional between the hotter and drier Ponderosa pine/antelope bitterbrush (Prushia tridentata) ecosystems, and the cooler and more moist mixed conifer forests that are gradually able to support increasing levels of Douglas-fir and western larch along with early seral Ponderosa pine. The forests in this particular warm-dry grass ecological site are classified as savanna types, representing a transition between grasslands and a more typical upland mixed conifer forest.

In the early successional stages, the ESD is comprised of seedlings, saplings, and poles, which progresses in time to older age and size classes of dominant/co-dominant and mature/over-mature stands of trees. Disturbance events are likely to occur at any successional stage, but overall wildfire was the main disturbance element in the pre-settlement period. Mature canopy coverage was open to medium, with a typical range of 40 to 60 percent coverage.

These forests are typically un-even aged. On a larger scale, this ecological site is characterized overall by a heterogeneous spatial pattern across the landscape, including some small “patch” openings occupied by grass and limited shrub species, with or without recruitment of Ponderosa pine seedlings.

Naturally occurring fires in the pre-European settlement period (i.e. the historic reference) were caused by late summer lighting strikes. In addition, many fires were deliberately ignited by indigenous peoples; the early adoption of prescribed burning. Human-caused fires, commonly expressed as surface fires, were typically ignited in late spring or early fall to alter the understory plant community for a variety of purposes which were important to the local native population.

Surface fires would create areas of exposed mineral soils for a short period of time. This seedbed condition is a prerequisite for seedling germination and establishment of Ponderosa pine. The low canopy coverage of the site allowed ample sunlight to reach the forest floor, also aiding in recruitment. Those young seedlings that escaped another quick round of surface fire would develop to the sapling and pole stages where they would rapidly acquire thick fire-resistant bark. Surface fires after this point was often beneficial in terms of naturally thinning thick patches of advanced regeneration.

The long-term perpetuation of this relatively dry conifer forest type in MLRA 9 depends on the successful regeneration of Ponderosa pine as seedlings, some of which eventually grow to provide replacement for the mature component of overstory pine across the landscape. Pine recruitment is limited to a short dispersal distance away from the seed tree source because the seed is relatively heavy. Viable seed crops occur intermittently, and a succession of favorable growing season conditions is required for a new cohort of pine to develop. Ponderosa pine recruitment can be a tenuous process but is offset to a large degree by the longevity attribute of the species.

In addition to the direct impacts of wildfire, disturbance factors that resulted in the demise of mature pine included injury from lightning strikes, wind events, weather extremes, and the collective influence of various damage agents such as bark beetles, pine engraver, mistletoe, and other adapted insects and diseases. Tree age and stand stocking levels often predispose stands and individual trees to mortality. Overall, bark beetles of the genus Dendroctonus (western and mountain pine beetle) are the most destructive biological agent in terms of biotic mortality.