Regional Patterns of Small Mammal Abundance and Community Composition in Oregon and Washington Authors Ordination analysis of small mammal communities, both KEITH B. AUBRY is a research wildlife biologist and 1 among and within provinces, revealed few ecologically MARK J. CRITES is a wildlife biologist, USDA Forest interpretable patterns or differences among forest age- Service, Pacific Northwest Research Station, Olympia, classes. Most variation was attributable to differences in Washington 98502; STEPHEN D. WEST is an associate species composition resulting from zoogeographic barriers. professor, College of Forest Resources, University of The shrew-mole and red tree vole were closely associated Washington, Seattle, Washington 98195. with old-growth forests in all provinces where they occurred. These species should be given high priority in the develop- Abstract ment of management strategies for small mammals in Pacific Pitfall traps were used by a team of investigators to study Northwest forests. forest-floor small mammal communities in 144 different- aged Douglas-fir forest stands in three physiographic prov- Introduction inces in the Pacific Northwest: the Oregon Coast Range, The fate of old-growth Douglas-fir forests in the Pacific Oregon Cascade Range, and Southern Washington Cascade Northwest has become one of the most controversial re- Range. All three small mammal assemblages were numer- source management issues of this century (see Norse 1990, ically dominated by a few species. The Trowbridge’s shrew Norse and others 1986, Wilcove 1988). Research on the com- was the most abundant species in all three provinces and position, structure, and function of these forests, however, accounted for 47 percent of 8661 small mammals captured. has only recently been undertaken (see Franklin and others Most remaining captures were of only one or two other 1981, Gutierrez and Carey 1985, Meehan and others 1984, species: in the Coast Range, the western red-backed vole Raphael 1984, Raphael and Barrett 1984, Spies and others and Pacific shrew; in the Oregon Cascades, the western 1988). red-backed vole; and in the southern Washington Cascades, the southern red-backed vole and montane shrew. Other species each accounted for less than 6 percent of total 1 Present address: wildlife biologist, USDA Forest Service, Pacific Northwest captures in all three provinces. Region, Portland, Oregon 97208. 285 Before these studies, lists of small mammal species believed Methods to be associated with old-growth forests were based primarily on natural history observations and speculation (for example, Study Areas Franklin and others 1981, Lumen and Neitro 1980, Meslow Forest-floor small mammals were studied in 144 stands in and others 1981). The comprehensive program of research Oregon and Washington in 1984 and 1985 by a team of described in this volume was undertaken to provide quantita- investigators. Study sites were located in three physiographic tive information on wildlife habitat relationships in Douglas- provinces (Franklin and Dymess 1973): 45 in the Oregon fir forests, and to provide a scientific basis for the manage- Coast Range, 54 in the Oregon Cascade Range, and 45 in the ment of old-growth forests (Carey and Spies, this volume; southern Washington Cascade Range. Data were collected in Ruggiero and Carey 1984). all stands in both years, with the exception of the Oregon Cascades province in which only 15 stands in the central The primary objectives of the small mammal studies con- Cascade Range were sampled in 1985. ducted in the Oregon Coast Range (Corn and Bury, this volume a), Oregon Cascade Range (Gilbert and Allwine, this Homogeneous forest stands of at least 20 ha with Douglas-fir volume a), and southern Washington Cascade Range (West, as the dominant tree species were selected for study. None of this volume) were to identify small mammal species that are the sites had been logged; all had resulted from natural regen- associated with old-growth Douglas-fir forests within each eration after catastrophic wildfires. All stands are within the province, and to investigate potential correlations between Western Hemlock Zone and lower elevations of the Pacific small mammal abundances and vegetative or structural Silver Fir Zone (Franklin and Dyrness 1973). features of unmanaged Douglas-fir forests. The purpose of this paper is to combine these three data sets and compare Stands were classified into three broad age-classes: young and contrast observed habitat association patterns among (35-79 yrs), mature (80-195 yrs), and old-growth (200-730 physiographic provinces to evaluate the extent to which yrs). Ages were based on growth-ring counts, either by observed patterns vary on a regional scale. Data from studies increment coring or examination of cut stumps in nearby conducted in the Northern California province were not stands (Spies and others 1988). Old-growth stands typically included because the ecological characteristics of stands contained high proportions of Douglas-fir and western hem- occurring there were considered to be too different from lock and, in wet sites, western redcedar. Mature and young those in Oregon and Washington for the data to be combined stands were dominated by Douglas-fir. In all age-classes, into a single data set (T. Spies and J. Franklin, pers. comm.). other species such as red alder, vine maple, bigleaf maple, Pacific silver fir, and western hemlock occurred in lesser By combining the data in a single analysis, results from each amounts. See Spies and Franklin (this volume) and Spies province can be compared quantitatively with those from (this volume) for more detailed descriptions of the vegetative other provinces. In addition, larger sample sizes resulting and structural characteristics of the study stands. from the pooling of data increase the power of statistical analyses, especially for relatively rare species. Knowledge of Within each province, stands of comparable moisture con- regional variation in the habitat relationships of small mam- dition were placed into an age gradient as a means of assess- mal species associated with old-growth forests will enable ing variation in small mammal abundances resulting primarily wildlife managers to set priorities for management activities from differences in stand age-class. The age gradient con- within different areas of the Pacific Northwest, and will sisted of 127 stands, including 43 from the Coast Range, 48 provide additional insights into the habitat relationships of from the Oregon Cascades, and 36 from the southern Wash- small mammals in unmanaged Douglas-fir forests. The ington Cascades. Although the range of ages sampled was specific objectives of this paper are to: similar among provinces, old-growth stands in the southern Washington Cascades contained nine stands that were 25 to 205 years older than the oldest stands sampled in either the l Describe the forest-floor small mammal communities Coast Range or the Oregon Cascades, Ranges for the age- occurring in unmanaged Douglas-fir forests in three classes in each province are: Oregon Coast Range, young, 40 physiographic provinces in Oregon and Washington; and to 75; mature, 80 to 120; and old-growth, 200 to 525 years old; Oregon Cascades, young, 35 to 79; mature, 84 to 180; l Compare patterns of community composition, species and old-growth, 200 to 500 years old; and southern Wash- abundances, and habitat associations among provinces. ington Cascades, young, 55 to 75; mature, 80 to 195; and old-growth, 210 to 730 years old. 286 Small Mammal Sampling and forest deer mouse occur only in the southern Washington Within each province, forest-floor small mammals were Cascades. One species, the montane shrew, occurs through- sampled with pitfall traps, snap-traps, and tracking stations out the southern Washington Cascades but is found only in (see Carey and Spies, this volume). Only pitfall trapping, the northernmost portion of the Oregon Cascades (Carey and I however, was used in all three provinces in both years. Other Spies, this volume; Ingles 1965). Stand sample sizes were techniques were used either in only one or two provinces, or adjusted for these species. were not equally applied in both years. To compare results among physiographic provinces, we therefore used only data Community analyses-Although yearly variations in pop- derived from pitfall trapping. ulation numbers may influence results in studies of wildlife habitat relationships, 2 years of data is not sufficient to ade- Thirty-six pitfall traps were installed in each stand in a 6 x 6 quately evaluate temporal variation in small mammal abun- square array with traps spaced 15 m apart. New pitfall grids dances. Furthermore, without corresponding demographic were established in each stand in the southern Washington information, causative factors for observed variation cannot Cascades during the second year of the study; in the Coast be determined (Van Home 1983). For these reasons, we Range and Oregon Cascades, grids were retrapped in the sec- pooled data from both years, and calculated abundance in- ond year. Comparisons of results between new grids and a dices for each stand as mean captures for both years per 100 subset of grids retrapped the second year showed no differ- trap-nights. We examined community patterns of small mam- ences in small mammal species composition or abundances mal abundance with detrended correspondence analysis (Gilbert and Allwine, this volume a). All stands were (DCA) using DECORANA (Hill 1979a). We interpreted