Pseudotsuga, being the most shade tolerant, has the greatest successional amplitude and acts as the climax tree. Although various factors often preclude the entire replacement sequence, the relative successional amplitudes have been established for classification purposes. Figure 2 suggests that possible species diversity of the tree layer is greatest in the early seral stages. Here all three species could be well represented, although usually this is not the case. In the climax stage, however, only Pseudotsuga will be well represented, with all other tree species poorly represented or absent. A diminishing species diversity during secondary succession becomes more apparent in the shrub and herb layer classifications where more species occur.

Figure 3 shows the possible seral conditions in the tree layer that may converge to a common climax layer of Pseudotsuga. Populus tremuloides forms the base of the triangle because it has the least successional amplitude. Other species are arranged in ascending order as a reflection of their progressively greater successional amplitudes. Each taxonomic unit in figure 3 consists of a seral indicator species followed by the dominant species. In order to maintain a systematic structure, each taxonomic unit in figure 3 is called a layer type, and each group of layer types having the same seral indicator is called a layer group. Layer groups denote the various seral stages that are possible within a given habitat type or phase. Layer types within one layer group such as PIPO-PIPO and PIPO-PSME in the PIPO layer group denote the structural conditions that are possible in that particular seral stage. Similar classifications were developed for the shrub and herb layers. If desired, taxonomy of the tree, shrub, and herb layers can be combined to characterize the entire plant community.

In the PSME phase, only Pseudotsuga is well represented, so only a PSME-PSME tree layer type can result. This phase has an environment less favorable for other tree species than the PIPO phase so fewer species are well represented and consequently fewer tree layer types exist. The phase also occupies less area than the PIPO phase and experiences considerably less management activity. Consequently, there are insufficient data to develop specific management implications for this phase.

Delineating the vertical axis (successional time) into layer groups (fig. 3) provides an ecological basis for segmenting the succession. As succession progresses, a stand's classification status will progress from one layer group to a successionally older layer group. For instance, Pinus ponderosa (well represented) may dominate the tree layer (PIPO-PIPO) or may be dominated by Pseudotsuga (PIPO-PSME). But the presence of Pinus ponderosa can always be interpreted as a specific segment of the succession because the potential to be outcompeted by Pseudotsuga always exists. Pinus ponderosa is unable to replace Pseudotsuga without the aid of disturbance but can always outcompete Populus tremuloides.

Figure 3 serves as a classification diagram (not a succession model) for seral tree layers in the PSME/ PHMA h.t. These and the other diagrams herein do not outline actual successions for a given site, but rather illustrate the possibilities within the habitat type. Actual

successions skip many layer types and even layer groups within their respective diagrams. A succession can be described in terms of the layer types shown (fig. 3), but is determined by species composition of the stand and available seed sources.

Figure 3 also serves as a basis for constructing a simple key to tree layer types for field use. This is done by starting with the earliest layer group in figure 3 and progressing along the time gradient to climax (table 3). Keys to the shrub and herb layer types are constructed the same way. These keys are intended to be used in the same manner as the habitat type keys (Pfister and others 1977; Steele and others 1981).

SIZE CLASS NOTATIONS

The basic classification approach used in the tree, shrub, and herb layers was presented in figures 2 and 3 and table 3, but the tree layer progresses through recognizable size classes of development such as sapling (0.1-4 inches [0.25-10.2 cm] d.b.h.), pole (4-12 inches [10.2-30.5 cm]), mature (12-18 inches [30.5-45.7 cm]), and old-growth (>18 inches [45.7 cm]). These notations should be added so as to delineate stand structure even though they do not necessarily denote stand age. These notations are best added to each tree species after the tree layer type (1.t.) is identified, such as: mature PIPO-sapling PSME 1.t. For consistency, the smallest size class that is well represented should be noted for the seral indicator because it usually reflects the most recent regeneration of that particular species. For the dominant species, the dominant size class should be used. When the seral indicator or the dominant species is well represented in the stand but not in any one size class, the size class with the most coverage should be noted. For convenience, size class notations can be abbreviated as follows: s. - sapling; p. - pole; m. - mature; and o.g. - old-growth.

It may be difficult, at first, to visualize some tree layer types in their appropriate successional position. For instance, a s. PSMEs. PSME 1.t. may not seem to be successionally older than an m. POTR - p. PSME 1.t., because we normally think of time-related situations on a yearly (or age) scale. On a successional scale, however, a pure stand of sapling Pseudotsuga is closer to climax than a mixed older stand of Populus and Pseudotsuga because it will not go through the earlier seral stages of the POTR and PIPO layer groups. In fact, an s. PSME - s. PSME l.t. may even reach climax in fewer years because no species replacement (succession) is needed whereas an m. POTR – p. PSME 1.t. must first lose the Populus and if Pinus ponderosa is well represented must also pass through a PIPO-PSME 1.t. before reaching climax.

The three possible tree layer groups in PSME/PHMA (fig. 3) are described below and delineate tree layer succession into relatively broad segments. Because layer group delineations are usually based on a single indicator species, their origin can be related to a somewhat consistent set of site conditions. But progression from one layer group to another (and one layer type to another) depends on composition of the individual stand and is therefore predictable only from field observation. The following layer group descriptions are presented in the order they

appear in the key (table 3). Constancy and average cover of species within sampled layer types appear in appendix A.

POPULUS TREMULOIDES LAYER GROUP (POTR L.G.)

Populus tremuloides can establish by seed on newly exposed mineral soil that remains moist during the critical germination period. Viability of freshly fallen seed usually exceeds 90 percent but lasts only about 3 weeks (Brinkman and Roe 1975). Occasional Populus seedlings have established in well-scarified areas, some drier than PSME/ PHMA, but usually the young trees occur as root sprouts following fire or logging. If large Populus trees are cut or burned, their roots can produce numerous sprouts if sunlight is adequate. The sprouts provide excellent browse for deer and elk.

In the PSME/PHMA h.t., the POTR 1.g. consists of three possible layer types (fig. 3). These layer types usually result from resprouting of scattered, often decadent, Populus following overstory removal by wildfire or logging. When Populus is present in the stand and no conifers establish soon after disturbance, a POTR-POTR layer type can result (fig. 4). In this layer type, subsequent invasion by conifers may be slow even when seed sources are nearby. Reasons for this are unclear, but Younger, Koch, and Kapustka (1980) have shown that leaf litter of Populus tremuloides can chemically inhibit seedling growth of several grasses. Possibly, conifer seedlings are also affected. Because the POTR-POTR layer type creates only light shade it can allow lush development of the shrub and herbaceous layers which also hinder conifer establishment. Simultaneous establishment of P. ponderosa, or Pseudotsuga with the resprouting of scattered Populus, can produce a POTR-PIPO or POTR-PSME layer type. Both of these can progress to a pine or Douglas-fir layer group more quickly than the POTR-POTR layer type.

Although relatively uncommon in PSME/PHMA, the POTR 1.g. can be found on old landslides, the leeward sides of windswept ridges, and possibly elsewhere. It could develop on any site supporting Populus tremuloides if competing conifers are killed. Only four stands in this layer group have been found; all were in the POTR-POTR layer type. Stand ages ranged from 18 to 75 years; none showed recent invasion by conifers even though the oldest stand was bordered by an adequate seed source of Pseudotsuga.

PINUS PONDEROSA LAYER GROUP (PIPO L.G.)

By definition, Pinus ponderosa occurs throughout the PIPO phase of PSME/PHMA and, unless planted, is absent in the PSME and PAMY phases. Pinus ponderosa is the only major seral tree species found throughout the PIPO phase and is often prevalent in old-growth stands, yet it seldom colonizes recent clearcuts. Poor dispersion of the heavy seed and unsuitable seedbeds limit the pine regeneration. Distance to seed source and infrequent cone crops are often responsible for a scarcity of seed. Logging and high-intensity burning stimulate several shrub and herb layer species, which can quickly dominate potential pine seedbeds. As a result, natural establishment of P. ponderosa in large clearcuts is often slow and sporadic.

The PIPO layer group consists of two layer types in the PSME/PHMA h.t (fig. 3). Both of these layer types are quite common and were extensively sampled. The PIPOPIPO layer type has resulted mainly from plantations, especially where the dominant size class is now saplings or small poles (appendix A). PIPO-PIPO in the larger size classes resulted either from shelterwood and seed-tree cuts or wildfire within the past 50 years (fig. 5). PIPO-PSME layer types were dominated mainly by pole-size or larger trees. This layer type resulted mostly from wildfire 50 to 100 years ago.

Prior to settlement by Euroamericans, wildfire burned PSME/PHMA sites at frequent intervals. In west-central Idaho, fire frequency ranged from about every 13 years in the dry extremes of PSME/PHMA to about every 22 years in the moist extremes (Steele and others 1986). Wildfires at these frequencies were most likely low-intensity surface fires, that killed most of the young Pseudotsuga in the stand but not the more fire-resistant pine (Arno 1976). Even young pines are fairly resistant to low-intensity surface fires, and the eventual result was an accumulation of pine dominating the site. Today, in the absence of frequent surface fires, these PIPO-PIPO layer types are difficult to achieve naturally but are being replaced by many pine plantations. Unlogged sites that now burn infrequently often develop high coverages of Pseudotsuga, resulting in the PIPO-PSME layer type. Selective cutting of the pine in PIPO-PSME has quickly advanced succession to the PSME layer group.

PSEUDOTSUGA MENZIESII LAYER GROUP (PSME L.G.)

Pseudotsuga is the only tree species that occurs throughout the range of PSME/PHMA and, being the most tolerant, acts as the climax species. In general, climax species are more difficult to establish than seral species, and Pseudotsuga is no exception. Most plantings of Pseudotsuga have failed in the PSME/PHMA h.t., and natural regeneration is usually slow to establish and appears to need protection from sun and wind. It is likely that most existing stands of Pseudotsuga developed gradually beneath the canopy of either seral trees or shrubs.

As climax, the PSME layer group consists of only one layer type, PSME-PSME (fig. 3). This layer type can occur in nearly pure stands of sapling, pole, and mature trees as well as old-growth. Regardless of tree size, the PSME-PSME layer type is considered closest to climax on a successional scale because no successional replacement of tree species will occur and the climax species is already dominant (fig. 6). Compared to other tree layer types in PSME/PHMA, PSME-PSME generally has the greatest hazard potential for catastrophic fire, insects (spruce budworm), and disease (dwarf mistletoe). Silvicultural options also become more limited due to the prevalence of these hazards and the scarcity of seral tree species in the stand.

MANAGEMENT IMPLICATIONS

The following management implications were derived from data and repeated field observations taken during this study and the habitat type study (Steele and others 1981). Because of the often small sample size of the data set and the minimal amount of field testing and user response, the reader should exercise caution in implementing trace findings. Nevertheless, trends reflected by these data are logical and support the management implications.

Pocket Gophers-It has long been known that pocket gophers (Thomomys talpoides) can damage pine plantations (Dingle 1956; Moore 1943). Reasons for this damage have been studied at length. In summarizing gopherrelated studies, Teipner and others (1983) suggest that gopher damage to young pines may be related to amount