INTERNAL REPORT 102 FOREST PLANTCOMMUNITIESOF THELOWER CEDARRIVER WATERSHED D.R. M. Scott and J. Long University of Washington INTRODUCTION The Cedar River Watershed is one of two principal sites for the Western Coniferous Biome of the International Biological Program (IBP). Various physical add biological components of the watershed are being intensively studied as part of the Analysis of Ecosystems objectives of IBP. The purpose of this report is to present the results of an initial recon- naissance-level study of the forest vegetation of the watershed. The . tentative delineations and descriptions of this vegetation may be useful in the stratification of the watershed for intensive ecological research. A proposal for an inventory of terrestrial ecosystems of the Cedar River drainage by Scott and Del Moral (1971) was not funded for 1972. However, a half-time graduate student salary for six months and some transporta- tion were made available. This limited funding made possible the initiation of a survey of the forest communities of the watershed. The emphasis during this first field season was placed on the vegetation of the lower watershed. This decision was based on several factors, including the uncertainty of, transportation to the upper watershed and the fact that some study of the upper watershed, specifically the Findley Lake Basin, had already been completed (Del Moral, unpublished). DESCRIPTION OF THE LOWER WATERSHED The lower watershed can be defined, for the purposes of this report, as that part of the Cedar River Watershed below 600 m in elevation. On this basis it represents approximately 14,000 ha or 40% of the total watershed. The surficial geology of the lower watershed is typical of much of the Puget Sound Basin. The glacial history of this area has resulted in the build-up within the lower valley of glacial outwash and till--this material overlaying and forming terraces and drumlins. The topography is, with a few local exceptions, gentle. Cole and Gessel (1968) state that there has been little -1- topographical modification of he area since the Fraser Glaciation (10,000- 18,000 years before present). Most of the soils and weather data available come from the Thompson Research Site which is locatedat the extremewest end. of thewatershed. It is presumed that these data provide a reasonable approximation of conditions throughout most of the lower watershed. There aretwo primary soil types on theThompson Site (Cole and Gessel 1968). Everett soilsdominate areasof outwashmaterial and are characterized by excessivedrainage. In contrast, the poorlydrained Alderwood soil is found on the compacted basal till ofdrumlin areas.Cole andGessel(1968) attribute the greater soil moistureand cationexchangecapacity of the Alderwoodsoils to its higher siltand clay fraction. The climateof the lowerwatershed is mild with January and July average tempera- tures at the ThompsonSite of 3°Cand 17°C, respectively.The average annual precipitationat theThompson Site is approximately 130 cm, of which most falls as rain and over70%of which falls between October and March.The lower water- shed occurs entirely withintheTsuga heterophylla Zone as defined by Franklin and Dyrness (1969). The original forest had been removed from nearly all of the lower watershed by 1924 (Winkenwerder and Thompson 1924).At least one sera]stand,the product of a prelogging erafire,now contains Pseudotsuga menziesii approximately 150 years old. However,the majority of the forest cover represents planted or natural regeneration 35-40 years of age. METHODS The reconnaissance method of forest site classification described by Franklin, ,Dyrness and Moir (1970) was adopted for this study.This reconnaissance approach allows the description of vegetation units over large areas in a relatively short time. It is a particularly useful method for the investigator who must work without field assistance. The technique entailed the location of study plots within what were considered to be homogeneous areas. Information measured or estimated on each 15-20 m circular plot included: (1)elevation,(2) slope andaspect,(3) topography and landform,(4)density of forestcanopy,(5) basal area (estimated with a wedgepr,ism),abundance and canopy coverage of mature trees by species (mature trees were defined as those thought to have reached sexualmaturity,(6) height, DBH and age of the largest tree of eachspecies,(7) abundance and coverage of tree reproduction byspecies,(8),projection of climaxspecies,(9) abundance and coverage of each species of shrub, herb andmoss. Abundance and coverage were estimated without the aid ofa plot frame. The estimates of cover presented here probably represent over-approximations. This is particularly true for the less abundant speciesas is made evident by the summationof theindividual speciesestimates. Total coveragegreater than100%within a stratum is not unreasonable due to overlapof individuals of different heights.Totals as great as those reportedhere,however, are probably too high.This astumption is supportedby theconcurrentin %) iation of analytic plots in the same area as part of another study. Nevertheless, this source of error should not have a major effect on the conclusions derived from these data. The errors are assumed to be fairly uniform for the entire study and the differences between plots ofa particular forest type will be small, relative to the differences between plots of dissimilar forest types. Species diversity was calculated with Brillouin's formula: 1 N. H (l N1092 (nl !n2!....ns) ) where N equals the total coverage of a particular sample, n. equals the coverage of the ith species and st-equals the total number o'P species. Coverage values were normalized as cm2/100 m2 (Reiners et al 1970). Evenness was calculated as J = H/log2 S. RESULTS ANDDISCUSSION Three vegetation units, each of which is seral, were delineated from the vegetation of the lower watershed.They are, in the assumed order of increasing soil moisture and decreasing total area, Pseudotsuga menziesii/ Gaultheria shallon, Pseudotsuga menziesii/Polystichum munitum and Alnus rubra/ Polystichum munitum. The approximate location of each plot is indicated in figure 1.Tables 1-4 summarize the environmental and floristic data for each plot of the three different forest communities.Data presented for each plot in Table 1include total basal area for the trees, aspect, slope, stand age and elevation.Tables 2-4 represent coverage for each species on each plot; average cover values are given for each type. The Pseudotsuga/GauZtheria community represents the most extensive vegetation type on the lower watershed.The data from the eleven reconnaissance plots established within this type are summarized in Tables 1and 2.The ages of the stands sampled ranged from 34to 73 years of age.This type, like the others, represents a pioneer stage ofa Tsuga heterophylla sere.It is characteristically found, but not restricted to moderate slopes and the Everett soil series. Diversity, richness andevenness of this type are lower than either of the other two communities (Table 5). The overstory tree layer of the Pseudotsuga/GauZtheria Community is dominated by Pseudotsuga menziesii,with over 75% cover. WhileTsuga heterophyllaoccurs as a mature tree in over 50% of the stands sampled, it occurs in the under- story in over 80% of these stands. The climax status of'this species within thePseudotsuga/GauZtheriacommunity is apparent. Thuja plicataon the other hand seems to play a relatively minor role, bothas an overstory species (less than 20% constancy) andas reproduction (less than 10% constancy). Basal area for this community averages only 9 m2/ha--lowerthan thePseudotsuga Polystichumcommunity. It should be noted that this basal area results from a higher density of stems than is found in stands of the other types. -3- The tall shrub layer is poorly developed in each of the types.In the Pseudotsuga/ GauZtheria community there are species which occur in over 40% of thestands but no species averages more than 4% cover.The most common species, Vaccinium parviflorwn, has91%constancy and only 3% average cover.Acer circinatwn and Ho odiseus discolor are the other two species which commonly occur in these stands(54% and 45%constancy, respectively). Diversity and richness of this stratum are low relative to the same stratum in the other twocommunities,a fact which may be symptomatic of the generally poor site quality of which the Pseudotsuga/Ga,,ltherii community is found. The low shrub layer is strongly developed in the Pseudotsuga/GauZtheria community. The understory of these stands is dominated by GauZtheria shaZlon.This species is ubiquitous within the type and averages over50% cover. Rubus ursinus with 91% constance and Berberis nervosa with75%constancy are common associates of GauZtheria shallon. However,both associates average only 3% cover.The total cover for this stratum is over60%,compared to less than 20% for the same stratum in the other two forest types.When Gaultheria shallon is discounted, however,the total cover of this stratum-is lowest for the Pseudotsuga/GauZtheria community. This discrepancy is probably due in part to the rather arbitrary delineation ofstrata,but also probably reflects the high concentration of dominance in Gaultheria shallon in the Pseudotsuga/GauZtheria community. While fourteen species were found within the herb layer of thistype,over 50% of the coverage is contributed by only twospecies,Polystichum munitum and Pteridium aquilinum.Linnaea borealis and Viola sempervirens