The Kawesas Watershed Assessment

III: Vegetation: Distribution, Characteristics, and Dynamics

Peter K. Schoonmaker & James Powell

The vegetation of the Kawesas is primarily dependent on climate and soils, but is also a product of disturbance — especially wind and in the floodplain, river dynamics. This section describes the distribution of different vegetation types and unusual species. It also aims to understand forest landscape and stand dynamics, including disturbance effects, at low elevations in the coastal western hemlock zone. We focus on this vegetation type because it is highly valuable for wildlife, recreation, research, and cultural values, and because parts of this vegetation zone are being considered for harvest — as low elevation coastal forests are throughout the province. This chapter sets the stage for the following chapter on salmon, in which we examine the flux of large logs that fall into the Kawesas River and provide much of its structure and habitat.

The Kawesas watershed comprises three broad biogeoclimatic zones: coastal western hemlock (CWH), mountain hemlock (MH), and alpine tundra (AT) (Clement 1988). The CWH subzones specific to the Kawesas include the leeward submontane and montane variants (CWHvm3 and CWHvm4) of the Very Moist Maritime CWH. These variants occur at elevations of 0–400 m and 300–700 m respectively in the watershed. The mean annual temperature in British Columbia for this subzone is 8.2 degrees C with coldest and warmest month mean temperatures of 0.3 and 16.0 degrees (Pojar et al. 1991). The mean annual precipitation in this subzone is about 2,800 mm with 73% of the precipitation falling from October through March.

The CWH zone covers 47% of the Kawesas. Characteristic CWH coniferous species in the Kawesas include western hemlock (Tsuga heterophylla), amabilis fir (Abies amabilis), and western redcedar (Thuja plicata) with Sitka spruce (Picea sitchensis) limited to low wet sites, and scattered Douglas-fir (Pseudotsuga menziesil) on dry sites. The dominant riparian floodplain, terrace, and lower slope (up to 400 m) species are amabilis fir and western hemlock. Occasionally, western redcedar is a dominant species. There are mixed stands with Douglas-fir on a southeast slope above the Haisla cabin at the river mouth — (UTM location 583099 and north to 582105 and east to 589105). There are scattered Douglas-firs on the same slope further upstream. Douglas fir is also located on a west facing slope near the mouth of Coho Creek (UTM 593086). Red alder (Alnus rubra) and black cottonwood (Populus trichocarpa) are found in riparian habitats, and are dominant on frequently disturbed substrates on the mainstem floodplain. Sitka spruce is mostly restricted to river margins or the lower valley floodplain in the middle and upper Kawesas. However, it is sometimes the dominant tree near/in tidal influenced floodplains in the lower Kawesas. Alaska yellow-cedar (Chamaecyparis nootkatensis) and mountain hemlock (Tsuga mertensiana) are common in sphagnum bogs.

The MH zone occurs at elevations of 500–900 m in the Kawesas and occupies 23% of the watershed. The subalpine climate of this zone is significantly cooler and wetter than the CWH zone, with mean annual temperatures of 0 to 5 degrees C, and sub-freezing temperatures for one to five months. Precipitation can range as high as 5,000 mm, of which 20% to 70% falls as snow mainly from October through March (Pojar et al. 1991). The growing season is short, as the snowpack is slow to melt. Mountain hemlock and amabilis fir are the most common tree species in the MH zone of the Kawesas. Occasionally, Alaska yellow-cedar is a dominant tree in this zone. Western hemlock is often present to near tree-line. As one approaches tree-line, mountain hemlock is the dominant tree. Because the MH zone is generally marginal timberland, and is unlikely to be directly impacted by development, we did not establish vegetation plots in this zone, but rather made preliminary plant collections. These collections are complemented by more extensive collections made in the Kitlope in 1993 and earlier (Schoonmaker and Kellogg 1994).

The lower limits of the alpine tundra zone begin as low as 700 m in the Kawesas, but generally occur above 900 m. Alpine tundra (excluding glaciers) occupies 12% of the Kawesas. This treeless zone is characterized by a harsh cold climate, with mean annual temperatures ranging from –4 to 0 degrees C, and subfreezing temperatures for 7–11 months. Mean annual precipitation probably approaches 3,000 mm in coastal mountains and maintains extensive glaciers. About 16% of the Kawesas is glaciated. A few reconnaissance-level inspections of the Alpine Tundra zone were made to identify plants, which included krummholz trees, shrubs, herbs, bryophytes and lichens (only the first three of these were identified to species).

Understory

The most common shrubs in the CWH zone are Alaska blue and oval-leafed huckleberry (Vaccinium alaskense and V. ovalifolium), salmonberry (Rubus spectabilis), and devil's club (Oplopanax horridum). Other common shrubs from floodplain to subalpine are red-osier dogwood (Cornus stolonifera), western crabapple (Malus fusca), red elderberry (Sambucus racemosa), stink currant (Ribes bracteosum), copper bush (Cladothamnus pyroliflorus) and slide alder (Alnus crispa). Common sphagnum bog shrubs are Vaccinium species (huckleberries, blueberries and cranberries), bog-laurel (Kalmia microphylla) and Douglas spiraea (Spirea douglasii).

The most common herbs in the tidal estuary are cow-parsnip (Heracleum lanatum), Pacific silverweed (Potentilla anserina) and asters (Aster spp.) The most common valley or slope herbs are lady fern (Athyrium felix-femina), bunchberry (Cornus canadensis), foamflower (Tiarella trifoliata) and twisted-stalk (Streptopus sp.). In wet pockets skunk cabbage (Lysichiton americanum) and water-parsley (Oenanthe sarmentosa) are quite common. Common sphagnum bog herbs are bunchberries (Cornus canadensis), sundews (Drosera sp.) and bog orchids (Platanthera diliata).

Rare or unusual plants encountered include: Two-edged water starwort (Callitriche anceps), four-parted gentian (Gentia propinqua), leafy dwarf knotweed (Polygonum minimum), an alpine plant found on a river bar; Cooley buttercup (Ranunculus cooleyae), an alpine plant found on a granite river bluff; and prickly rose (Rosa acicularis), an uncommon boreal forest rose. Only two introduced species were found: spring water starwort (Callitriche verna) and curly dock (Rumex crispus). A complete plant list is available in the Kawesas Watershed Assessment Technical Report.

Forested landscape patterns

We focused on documenting composition, structure, and dynamics of the coastal western hemlock (CWH) zone, as this area is most likely to be impacted by development. We used circular plots, transects, and species collections to document vegetation characteristics in the field and to ground-truth a 1976 forest cover map of the Kawesas produced for the B.C. Ministry of Forests. This 1:20,000 map provides stand to landscape-scale information on tree species distribution, volume, age-class, height, and site class. A simplified version of this map was used to select sample plots for detailed investigation.

We found that the forest cover map was remarkably accurate, with only a few stands out of several dozen checked that differed significantly from their mapped characterization. This forest cover map indicates that 68% of the forested landscape of the lower Kawesas (below river kilometre 26) is old-growth forest over 250 years in age, that about 22% is in 100 to 250-year age classes, and that the remaining 10% is less than 100 years old. Less than 1 % had been disturbed in the last 20 years (Table 2). The pattern for low-elevation and floodplain forests is quite different, with substantially less forest area in late seral age classes (Figure 2). "Low elevation" forests (including floodplain forests) correspond in area (approximately 1,500 hectares) and extent with "operable productive" forest as defined in TFL 41 Management Plan No. 5 (1993), and as depicted on the forest cover map for the Kawesas watershed.

Very few of the forest stands were over 45 metres in height (8% by area) and most of the forested area is less than 30 m tall (60% by area). Taller height classes indicate good site productivity and are generally found on lower slopes. Good, medium, poor, and low site classes occupied 9%, 37%, 28% and 26% of the productive forested area. Most forest harvesting would likely take place on good and medium sites, stocked with high volumes of amabilis fir and some spruce (Map 6).

Over 55% of the forested area is dominated by hemlock, fir, or a mixture of the two. Western redcedar stands (usually with other species) account for 10% of the forested area, as do mixed yellow-cedar stands. Spruce-dominated stands occur on 7% of the landscape. Mountain hemlock/yellow-cedar stands account for 12% of the forested area, and western hemlock/western redcedar predominates on about 6% Kawesas forests.

Past logging activities

In addition to traditional Haisla use, limited harvesting has taken place in the Kawesas during the twentieth century; single tree selection and small patch cut systems appear to have been used. Douglas-fir was the preferred tree for harvesting many decades ago. Therefore, standing Douglas-firs next to the river are absent. Douglas-fir stumps are present near the northwest bank along the tidal influenced Kawesas (UTM 583099 near the Haisla Nation cabin), as well as up the slope to 200 metres elevation. Cedar stumps are also on the same slope above the cabin. Occasional stumps are scattered throughout the lower Kawesas valley. Most are spruce. One area of high stump concentration is southeast of the Kawesas estuary near Coho Creek. Bucked Douglas-fir logs were found in some debris jams along the Kawesas not far below the mouth of Cole Creek. There are also a few bucked Douglas-fir logs along Cole Creek (500 m above mouth). All these trees were harvested decades ago. Recent harvesting activity in the upper Kawesas valley appears to have been used to estimate stand volume. A dozen or more dominant western hemlock and amabilis fir averaging 1 metre diametre, were felled and bucked near the river right slope (UTM 902597). On the basis of their condition, these trees were felled 10 or more years ago.

Stand characteristics and dynamics

Twelve 0.1 ha circular plots were sampled in three topographic positions on the lower slopes of the Kawesas River valley: floodplain, low terrace, and low- to mid-slope, mostly in hemlock, hemlock-fir, and fir-hemlock stands. Tree species, diametre at breast height (dbh), condition, and canopy position were noted for all trees greater than 10 cm dbh. Regeneration was noted for all stems taller than 0.25 metres and less than 10 cm dbh. Understory plants were tallied by percent cover into five dominance classes. Snags were measured at breast height, estimated for total height, and were classified into 5 decay classes. Coarse woody debris was tallied by species, classified into five decay classes, and measured for volume determination.

Nearly all stands were dominated by western hemlock and amabilis fir, with spruce occurring in and occasionally dominating the floodplain forests. Across all plots, hemlock accounted for more than a third of the basal area (33 square metres per hectare) with amabilis fir occupying 29 square metres per hectare, and spruce averaging 19 square metres per hectare. Spruce populations tended to consist of fewer, larger individuals, with only modestly more mid- to small-size stems. Hemlock and amabilis fir populations were skewed toward smaller individuals with size distributions centered on 30 and 40 cm dbh, versus 65 cm for spruce. Hemlock and amabilis fir stem density was far greater (190 and 115 stems per hectare) than spruce (29 stems per hectare).

Snags accounted for 33 square metres per hectare of basal area and 118 stems/hectare, compared to 85 square metres/hectare and 334 stems/hectare for live stems. Thus dead wood made up 39 percent of standing basal area, and 35 percent of all stems. Volumes were not calculated for standing trees, but given the ratio of dead:live basal area and density, dead and downed trees likely account for over one third of above ground wood volume and biomass in the lower elevation forests of the Kawesas. Since most of these dead and downed stems have died within the last two hundred years, it is evident that disturbance is an important process in the Kawesas, as it is elsewhere on the northwest coast.

It is difficult to determine disturbance agents without detailed chronologies based on tree-ring analysis, but size class distributions and field observations indicate that small patch disturbances are prevalent, along with occasional larger scale wind events. Several 1-3 km blow-down tracks were detected running parallel to the valley bottom; their configuration across the landscape was discontinuous and patchy. Wind and landslides appear to occur mainly in small patches, and less frequently in larger patches (1–100 ha). Fire may also be a significant, if infrequent, disturbance agent in the Kawesas, as indicated by several soil humus cores taken during the summer of 1995 (K. Lertzman pers. comm.).

Both CWHvm and MH zones are usually characterized by uneven-age or multi-storied even-aged stands generated by rare stand initiating events. Disturbance events are generally small in extent and result in small gaps which are filled by shade-tolerant saplings. The Kawesas appears to fall into this natural disturbance type (NDT1), as shown by the high proportion of older trees, few young stands, multiple age classes and uneven canopy. Larger disturbance patches and occasional stand initiating fires may occur in this watershed; in addition, the Kawesas River itself is a significant disturbance agent, as indicated in the preceding and following chapters. A more detailed disturbance history is needed before any development occurs in the Kawesas.

Although not tallied in this study, we observed that many of the hemlock and amabilis fir stems grew on old wind-throw mounds and logs on floodplain and terrace slopes, whereas on mid slopes stems were more often rooted on organic soil. This observation indicates that many low elevation hemlock/amabilis fir stands in the Kawesas, as elsewhere in British Columbia tend to self-perpetuate following wind storms and small blowdown events (Lertzman et al. 1996). In contrast, upper slope stands with more cedar tend to reach older ages and are more wind-firm. Thus LWD on floodplain and terrace forests is important not only as a source of large wood for riparian systems, but also for maintaining the regenerative capacity of low elevation forests themselves. One clear implication of this observation is that tree removal from floodplain/terrace surfaces, and consequent increased decay and fluvial removal of LWD on the forest floor would decrease the regenerative capacity of these forest stands over the long term. Given these circumstances, a prudent management regime would eliminate harvest activities from annually flooded forest stands and limit harvesting on terrace slopes to single tree selection and small (<1 ha) patch cuts.

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