Abundances of Small Mammals in Fir Forests in Northeastern California
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ABUNDANCES OF SMALL MAMMALS IN FIR FORESTS IN NORTHEASTERN CALIFORNIA JEFFREY R. WATERS AND CYNTHIA J. ZABEL United States Forest Service, Pacific Southwest Research Station, 1700 Bayview Drive, Arcata, CA 95521 We compared abundances of seven species of forest rodents among three types of fir (Abies concolor and A. magnifica) forest: unlogged old-growth, unlogged mature, and shelter- wood-logged old-growth. Small mammals were livetrapped during summers 1991 and 1992 in four grids within each type of forest; grids were located in the Lassen National Forest in northeastern California. Shelterwood-logged forests had been logged 6-7 years previ- ously. Differences in capture rates between unlogged, old-growth and shelterwood-logged, old-growth forests suggest that logging led to significant increases in populations of golden- mantled ground squirrels (Spermophilus lateralis), yellow pine chipmunks (Tamias amoe- nus), and lodgepole chipmunks (Tamias speciosus) but may have led to reduced populations of western red-backed voles (Clethrionomys californicus). Capture rates of Douglas’ squir- rels (Tamiasciurus douglasii), Allen’s chipmunks (Tamias senex), and deer mice (Pero- myscus manicukatus) did not differ significantly between unlogged and shelterwood-logged forests. Capture rate of T. douglasii was significantly greater in mature forests than in old- growth forests in 1992, but we did not detect significant differences between old-growth and mature forests for the other six species. These results illustrate how opening of the canopy and disturbance of the forest floor can lead to significant changes in patterns of abundance of forest rodents. Key words: Clethrionomys californicus, Peromyscus maniculatus, Spermophilus lateralis, Tamias amoenus, Tamias senex, Tamias speciosus, Tamiasciurus douglasii, logging, small mammals Four silvicultural systems traditionally changes in abundances of small mammals have been recognized for harvesting and re- following clearcutting. Traditional clearcut- generating forests (Smith, 1986): clearcut, ting, in which all or nearly all live trees are seed-tree, shelterwood, and selection. Nu- cut, is being used less frequently on public merous studies have evaluated effects of forests in the western United States. Silvi- clearcutting on populations of small mam- cultural practices in which live trees and mals (Corn and Bury, 1991; Gashwiler, other forest components such as large snags 1959, 1970; Gunther et al., 1983; Hooven and logs are retained at the time of harvest and Black, 1976; Kirkland, 1977; Raphael, are being used more frequently (Franklin et 1988; Sims and Buckner, 1973; Tevis, al., 1997). 1956a; Van Home, 198 1), but published In a previous paper, we compared den- studies on effects of seed-tree, shelterwood, sities of northern flying squirrels (Glauco- and selection harvests are lacking. Losses mys sabrinus) among three types of fir of food and cover associated with harvested (Abies) forest in northeastern California trees and disturbance of the forest floor, as (Waters and Zabel, 1995): unlogged old- well as increases in food and cover asso- growth, unlogged mature, and shelterwood- ciated with increased production of herba- logged old-growth. In the shelterwood sil- ceous vegetation and shrubs following dis- vicultural system, the mature stand is re- turbance, typically lead to significant moved in at least two cuts, so that regen- 1244 eration of the new stand occurs under a par- was referred to as young by Waters and Zabel, tial forest canopy (Smith, 1986). In this pa- 1995). These forests were dense and had closed per, we compare abundances among the canopies, and virtually no herbaceous plants or three types of forest of seven additional shrubs were present in the understory. Old, dead species of small mammals captured during stems on the forest floor indicated that shrubs were abundant for some period of time after the study of flying squirrels: golden-man- wildfires occurred. The organic soil included tled ground squirrel (Spermophilus lateral- layers of litter and humus. Based on counts of is), lodgepole chipmunk (Tamias specio- growth rings from cored trees, we estimated that sus), yellow pine chipmunk (T. amoenus), most codominant and dominant trees in this for- Allen’s chipmunk (T. senex), Douglas’ est type were 80-100 years old. squirrel (Tamiasciurus douglasii), deer The shelterwood-logged, old-growth forest mouse (Peromyscus maniculatus), and type (hereafter, shelterwood-logged forests) was western red-backed vole (Clethrionomys located in Swain Mountain Experimental Forest. californicus). Our objectives were to eval- The experimental forest was dominated by old- uate effects of these shelterwood harvests growth forests until 1984-1985 when large areas on abundances of forest rodents and deter- were logged to study natural regeneration rates mine differences in abundances of forest ro- using the shelterwood silvicultural system. dents between old-growth and mature fir These timber harvests left a park-like stand structure of widely spaced, large-diameter trees. forests. The ground was intentionally disturbed to ex- pose mineral soil for natural regeneration. Trac- MATERIALS AND METHODS tors with brush blades were used to remove logs Study area.-Thethree types of forest in and disturb soils, and slash was piled and burned which we sampled (unlogged old-growth, un- or broadcast burned. At the time of our study, logged mature,and shelterwood-logged old- grasses, forbs, and low shrubs (primarily Cea- growth) were located within and around Swain nothus cordulatus and Ribes roezlii) had become Mountain Experimental Forest, Plumas Co., in established, and the little organic soil present on the Lassen National Forest in northeastern Cal- the forest floor primarily was characterized by ifornia. This area lies at the southern extent of undecomposed litter. the Cascade Range, and soils are of volcanic or- Sampling-We selected four areas within igin. The unlogged, old-growth forest type each of the three types of forest that were similar (hereafter, old-growth forests) was characterized in elevation and composition of trees, 2 150 m by forests with multi-layered canopies and nu- apart, relatively homogeneous, and sufficiently merous large, decayed logs, stumps, and snags. large. These restrictions precluded random se- Dense patches of small firs occurred in the un- lection from a larger number of potential areas, derstory, but herbaceous plants (e.g., Pyrola pic- so we do not attempt to make inferences beyond ta, Viola purpurea, and Corallorhiza maculata) the areas studied. Elevations of the 12 grids and shrubs (primarily Chrysolepis sempervirens) ranged from ca. 1,800 to 2,000 m. Each area was were uncommon. The organic soil included lay- dominated by white fir (Abies concolor) or a ers of litter and humus and large pieces of bur- mixture of white and red fir (A. magnifica). Jef- ied, decayed wood. Based on counts of growth frey pine (Pinus jeffreyi), ponderosa pine (P. rings on cut stumps in shelterwood-logged areas ponderosa), sugar pine (P. lambertiana), and within Swain Mountain Experimental Forest, we lodgepole pine (P. contorta) occurred at low estimated that the majority of codominant and densities in some stands. The four areas in shel- dominant trees in the old-growth and shelter- terwood-logged forests were located within wood-logged forest types were 200-400 years Swain Mountain Experimental Forest. Three of old. the four areas in the old-growth forest type were The unlogged, mature forest type (hereafter, located in residual stands of old-growth within mature forests) was characterized by even-aged the experimental forest, and the fourth area was forests that grew back after stand-replacement located 6 km southwest of the experimental for- wildfires. Few residual logs, snags, or trees from est. One of the areas in the mature forest type the previous forest were present (this forest type was located in Swain Mountain Experimental 1246 JOURNAL OF MAMMALOGY Vd. 79, No. 4 Forest, and three were located 8 km southwest cm) on the ground at each grid point in 1991. of the experimental forest. Sherman live traps were not used in 1992 be- Within each of the 12 areas we established an cause of damage to traps in 1991 by bears. Bait 11.5-13.4-ha, rectangular-to-square grid (7 by was a mixture of rolled oats and peanut butter 13, 8 by 13, 8 by 12, 9 by 11, or 10 by 10) with (a small amount of molasses was used in 1991 40-m spacing between grid points (size and but not in 1992). After marking and weighing shape of grids varied depending on size and individuals, we determined sex and reproductive shape of stands). Each grid was surrounded by status. S. lateralis and T. douglasii were ear- a strip of similar forest 240 m wide. During tagged, and all other species were toeclipped. surnrner 1991, we sampled vegetation at every Data analyses.-Numbers of captures and re- third point within each grid. Within circular captures varied widely among species and grids. plots, we measured diameter at breast height For several species, there were few or no recap- (dbh) of trees >5-cm dbh and length and mid- tures in at least some of the grids, so we cal- point diameter of all logs with a diameter 2 12 culated capture rate as a measure of relative cm at the midpoint. Trees <5-cm dbh were tal- abundance for each species. Capture rate (CR) lied, not measured. Size of vegetation plot dif- was expressed as number of individuals/l,000 fered among the three types of forest because of trap nights and was determined using the for- large differences in the densities of trees. In old- mula of Nelson and Clark (1973): growth forests, small trees (< 18-cm dbh) were CR = I X l,OOO/(T - S/2), measured within a 6-m radius and larger trees and logs within a 16-m radius. In mature forests, where I = number of individuals captured, T = all trees and logs were measured within a 6-m number of traps multiplied by number of nights radius.