I U"1"‘---/318Reprinted from PROCEEDINGS, SOCIETY OF AMERICAN FORESTERS, Detroit, Michigan, 1965

DIVISION OF FOREST-WILDLIFE MANAGEMENT

Tree Seed Abundance vs. Deer Mouse Populations inDouglas-Fir ClearcutsJay S. Gashwiler

THE DOUGLAS-FIR (Pseudotsuga men-ziesii) region of the Pacific Northwestruns from northern California throughOregon and Washington to centralBritish Columbia. It includes most ofthe west slope of the Cascade Moun-tains and extends westward nearly tothe Pacific Ocean. That part occurringin this country is of great importancesince it contains an estimated 40 per-cent of our standing sawtimber. InOregon, about 52 percent of the yearlyincome is derived from the lumberingand wood products industry; a highpercentage of this comes from theDouglas-fir belt.

As the old-growth timber in theDouglas-fir type is harvested, regenera-tion must be started on the clearcutsat an early date if a maximum sus-tained yield is to be maintained. Theearly re-establishment of desirable spe-cies, at a reasonable cost, is hamperedby many things. One of the more acuteproblems is the adverse effect of wild-life on the establishment and produc-tion of new crops. These forest-wild-life problems in the Douglas-fir belthave been well documented by Moore(15); Isaac (7) ; Kangur (10); Law-rence (13); Hooven (6); Kverno(12); Hagar (4) ; Lawrence, Kverno,and Hartwell (14); Radwan (17) andothers. A brief outline of the typesof wildlife damage in the Douglas-firbelt and the more common vertebrateanimal depredators follows :

Seed destruction by—Deer mice (Peromyscus maniculatus)Shrews (Sorex spp.)Tree squirrels (Tamiasciurus spp.)Juncos (Junco oreganus)

Foliage and twig clipping, debudding,and browsing by—

Deer miceVoles (Microtus spp.)Snowshoe hares (Lepus americanus)Brush rabbits (Sylvilagus bachmani)Mountain beaver (Aplodontia rufa)Deer (Odocoileus spp.)Elk (Cerullo canadensis)Sooty grouse (Dendragapus obscurus)Pocket gophers (Thomomys spp.)

THE AUTHOR is wildlife research biolo-gist, Bureau of Sport Fisheries andWildlife, U. S. Dept. of the Interior,Oregon State Univ., Corvallis. He wishesto thank members of the Pacific, North-west Forest and Range Expt. Sta., andthe Willamette National Forest for theircooperation on this work. Their protec-tion of study areas, loan of seed traps,permission to use file data, and helpfulsuggestions on technical matters are allgreatly appreciated.

(Gophers will probably cause dam-age at higher population levels)

Root, stem, and branch barking by—VolesMountain beaverSnowshoe haresWoodrats (Neotoma spp.)Tree squirrelsBlack bears (Ursus americanus)Pocket gophersPorcupine (Erethizon dorsatum)

(This animal is extending its rangeinto western Oregon and may eventu-ally be a serious forest problem)

During the last several years I havebeen engaged in an investigation ofthe ecology and biology of birds andsmall mammals as related to forest-wildlife problems on the H. J. An-drews Experimental Forest. Thesestudies are only concerned with a smallsegment of a large complex problem.The Andrews Forest is located in Linnand Lane counties in central Oregonon the west slope of the CascadeMountains. It includes nearly the en-tire watershed of Lookout Creek withelevations ranging from about 1,400 to5,250 feet. Timber on the area islargely old-growth Douglas-fir, someof which is nearly 400 years old. TheAndrews Forest is under the jurisdic-tion of the Pacific Northwest Forestand Range Experiment Station, U. S.Forest Service, and within the boun-daries of the Willamette National For-est.

In northern California, Jameson (8)found deer mice eating seeds (includ-ing conifer and broad-leaved tree,shrub, forb, and grass seed), fruits,arthropods, leaves, fungi, and miscel-laneous items. These animals are veryadaptable and when a preferred foodbecomes unavailable they turn to onewhich can be secured even though notso well liked. This ability to adjustto a variety of situations helps tomake these mice one of our most suc-cessful species. Forest managers re-sponsible for regeneration work in theDouglas-fir belt frequently ask if highdeer mouse populations can be pre-dicted. Since deer mice increase rapid-ly in clearcut areas, are one of themost abundant small mammals, andfeed heavily on tree seed, they are aspecies causing much concern. If thehigh populations can be predicted, itwould permit advance planning for apotentially greater reinvasion problemand thus aid in getting maximum re-turns from artificial and natural seed-ing.

A comparison of the annual seedcrop with deer mouse populations onthe Andrews Forest over an 11-yearperiod strongly suggests a relationshipbetween seed abundance and the fol-lowing fall's deer mouse populations.This paper will present the data andinterpretations.

MethodsThe seed traps used in this study

are the common U. S. Forest Servicetype. They are 2 X 3 feet in size andhave an effective area of 5.7 squarefeet. The western redcedar (Thujaplicata) framework is covered with or-dinary wire screen with 14 X 16 meshesper square inch for the floor and withtops of %-inch hardware cloth. Thetop mesh is small enough to excludebirds and small mammals yet largeenough to permit Douglas-fir, westernhemlock (Tsuga heterophylla), andredcedar seeds to enter easily. Theseed traps were located to sample theentire seed fall in the 4 cuttings : 2A(24 acres), 3B (21 acres), 3G (41acres), and 9A (25 acres). Fifteentraps were used on 2 cuttings in 1954,16 on 1 cutting in 1955, and 24 onsingle cuttings the rest of the time.Forest Service personnel (11, 16)checked the traps in 1954 and in Octo-ber 1955; the writer has tended themthe remaining time. The cutting test wasused to determine soundness ; only fill , -seeds were included in the corntions. Since redcedar seedsreadily taken by deer mice (15), theabundance of these seeds probablydoes not affect the populations. Con-sequently, only Douglas-fir and hem-lock seeds which are preferred by deermice (15) were used in the computa-tions. The estimated seed crop peracre was converted to pounds using40,000 Douglas-fir and 300,000 hem-lock seeds per pound (7).

Deer mouse population estimateswere secured on grids with 10 X 10trap sites spaced 50 feet apart. LargeSherman type live traps were usedand were baited with whole oats sup-plemented in cold weather with wheatand cracked yellow corn. Dry woolwas kept in the traps at all time fornesting material. Trapping was for a6-night period in April or May andagain in September each year. Cap-tured animals were ear tagged withmonel fingerling tags and released atthe trap site. The "Lincoln Index"

Table 1.—Year, Estimated Pounds of Filled Douglas-Fir and Hemlock Seed,and Estimated Deer Mouse Population in Spring and Fall on Clearcuts

Mice per acreYear Cutting Pounds of seed per acre Spring Fall1954 2A & 3B 0.811955 3G 0.04 101956 30 2.38 41957 3G 0.02 121958 3G 0.10 71959 3G 4.82. 01960 3G 111960 9A 0.03 41961. 9A 0.231962 9A 0.93 a1963 9A 0.03 4 91964 91 0.28 0

'Seed traps were moved from 3G to 9A in 1960: however, live-trapping was con-tinned on 3G.

220 PROCEEDINGS -

•

method was used to compute dailypopulations; the last 3 daily figureswere averaged for the final estimate.The grids were expanded on all sidesby 1;4, the "Adjusted Range Length"(18). This delineated the effective areacovered by the grid traps. All popula-tion estimates are rounded to the near-est whole figure.

Many factors influence the numberof deer mice in a population. Sinceit is very difficult to separate eachfactor and measure its direct influenceon wild populations, indirect methodsmust often be used. If a certain con-dition (in this case, a large seed crop)is associated with a specific response(a high deer mouse population) andthis is repeated several times, a cause-effect relationship is assumed. This isthe type of interpretation used to ana-lyze the data in this paper.

ResultsFrom Table 1 it can be noted that

data were secured annually from 1954until 1964 inclusive. In 1954, the seedproduction estimate was obtained byKoenig (11) from the 2A and 3Bclearcuts which are about 700 feet low-er than 3G. Cone crop estimates (1)for a cutting adjacent to 3G wereabundant for both Douglas-fir andhemlock. It seems very likely the 3Gcutting also had a good seed crop al-though the pounds estimated on 2Aand 3B may not be representative.From 1 9 5 5 until 1 9 5 9 seeds weretrapped on the 3G cutting; in 1960the traps were moved to the 9A cut-ting and are presently there.

Pounds of seed per acre varied froma low of 0.02 to a high of 4.82. Thecrop was considered to be abundantin 1956 and 1959. In 1954 and 1962it was classed as being moderate andwas rated light in 1955, 1957, 1958,1960, 1961, 1963, and 1964. Thus, inan 11-year period there were only 4moderate to abundant seed crops. Theyear after each moderate or abundantcrop the production of seeds dropped

to a low level. Isaac (7) reported thatDouglas-fir trees often failed to flowerthe year following a heavy seed crop.This would account for the greatly re-duced production.

Spring deer mouse populations variedfrom 0 to 4 animals per acre with amode of 2. All of the spring trappingwas done in April except in 1956 whenheavy snow cover delayed trappinguntil May. Although the yearly dataare variable, other studies indicateMay is probably near the low point inthe yearly population cycle of the ani-mals. May was also reported as thelow point in the yearly deer mousepopulation in northwest Oregon byHooven

The spring deer mouse populationsgenerally remained at low levels. Thereis one modest exception in 1963 whenthe estimate was 4 mice per acre. Sam-pling variability could have been re-sponsible for this exception. However,there was a heavy snowfall in 1962-1963 and it may have favored over-winter survival and thus a greaterpopulation.

Fall populations ranged from 2 to12 deer mice per acre. There were 4falls with relatively high populations,4 with moderate, and 2 with low dur-ing the 10-year period. These Septem-ber populations are probably not themaximum. Unpublished data indicatethe peak population is reached in No-vember or even later. Hooven (5) alsoreported that deer mouse populationestimates in northwest Oregon gen-erally reached peak abundance invember. Fall populations were eon-sistently greater than the correspond-ing spring estimates; they also fluctu-ated greatly and were more variable.

DiscussionOrdinarily, seed dissemination on

the Andrews Forest starts in Septem-ber, builds quickly to a peak in Octo-ber or November, and gradually de-clines until the following summer. Theexact pattern, however, is determined

largely by variable weather conditions(7)-

Since the fall trapping was alsodone in September there was littlechance for the current seed crop tohave a measurable impact on the deermouse population at that time. Ben-dell (2) demonstrated that a food sup-ply could have a determining influenceon the ability of wood mice (Peromys-ens lencopus) to survive and thrive ona timbered island. Thus, it could heexpected this preferred food wouldhave some influence on the populationthe following spring. My data, how-ever, reveal no such response duringthe good seed years of 1956 and 1959.Stickel and Warbach (19) give in-stances of wood mice in a wooded areaapparently responding quickly to goodcrops of mast. They also report arapidly declining population during agood pine seed crop. The spring popu-lation of 1963 which followed a moder-ate seed fall was the highest recorded.Even so. it was only twice as great asthe mode and was only 1/3 of the larg-est fall population.-

It could also he reasoned that asmall seed fall would have an adverseeffect on the size of the followingspring's deer mouse population. Thisresponse, however, is not evident fromdata in the table. Generally speaking,the tree seed crop, regardless of itssize, had very little if any effect onthe size of the following spring deermouse population in this study. Ben-dell (2) reported, "The most impor-tant effect of food supply is on thesurvival of young from birth to ap-proximately 1 month of age," but deermouse breeding in western Oregon ismuch restricted if not suspended al-together during late fall and winter.Consequently, tree seed would not havemuch opportunity to affect breedingactivities or survival of young at thattime.

Bendell also found a lower lifelongmortality rate with ample food com-pared to inadequate food. Improvedfood resources should increase the car-rying capacity and thus maintain alarger overwinter population. It isnot known why this pattern did notprevail. Some additional field data,not yet compiled, may help explainthis condition.

There appears to be a direct rela-tionship between moderate to good seedcrops and high deer mouse populationsthe following fall. However, tree seedsapparently are not a critical mousefood since the table shows the popula-tions reached moderate abundance evenduring periods of near seed failure.The moderate to heavy seed crops of1954, 1956, 1959, and 1962 were allfollowed by high deer mouse popula-tions the succeeding fall. The mouse

DIVISION OF FOREST-WILDLIFE MANAGEMENT 221

population on another clearcut about1.5 airline miles distant and at a slight-ly higher elevation, for which no seedcrop data are available, followed essen-tially the same pattern. Since it isgenerally accepted that good seed cropsin the Douglas-fir region are oftenuniform over relatively large areas,these additional data corroborate theexperimental results.

It is of interest to note the micewere able, under field conditions, toincrease nearly 6 times from April toSeptember. Most of this incrementprobably resulted from increasedbreeding success and higher spring andsummer survival rates. However, it ispossible some movement may havecontributed to the increase. The exactway seed abundance influences thepopulations has not been determined.One can only conjecture, but it seemsprobable the seeds supplied just enoughadditional nutrients in winter or springto trigger a spring and summer breed-ing and/or survival response in theanimals. The table shows that 2 of the3 available spring estimates which pre-ceded the large fall populations were2 animals per acre each. In northernCalifornia, Jameson (9) also foundbrushfield Peromyscus maniculatusreaching peak abundance about 1 yearafter a heavy mast crop. He consid-ered the changes in numbers of deermice to be due to varying rates ofreproduction.

The 7 poor seed years which rangedfrom 0.02 to 0.28 pounds of seed peracre caused no apparent response inthe fall deer mouse populations. Themoderate to abundant years whichranged from 0.81 to 4.82 pounds ofseed per acre all caused approximatelythe same degree of increase the follow-ing fall. No data are available onpopulation response to seed abundancebetween 0.28 and 0.81 pounds per acre.It would seem that the lowest pound-age which might cause a measurableresponse would fall somewhere betweenthese values.

The number of deer mice in thespring had little influence on the sizeof fall populations. Blair (3) report-ed similar findings for deer mice inblue-grass habitat in southern Michi-gan. In 1957 and 1960 the high fallpopulation was preceded by only anormal spring population of 2 animalsper acre. The high 1963 spring popu-lation of 4 animals per acre did notcause a corresponding increase thefollowing fall. Although the fall popu-lation was high, it was less than in1957 and 1960 when the spring figurewas 2 animals per acre. Althoughbreeding stock is necessary to producea population, the two are not alwaysdirectly proportional. Many interact-

ing factors operate to determine thepopulation levels.

It can also be noted the high fallpopulations of 1955, 1957, and 1963were followed by spring populationsof 3, 3, and 0 deer mice per acre re-spectively. The high 3G population of1960 was followed by a spring one(not shown in the table) of only 1mouse per acre. These spring popula-tions are comparable to those pre-ceded by smaller fall populations. Theonly possible exception would be thefall population of 5 deer mice peracre in 1962 which was followed by aspring population of 4. It seems evi-dent that size of the fall populationshas little influence on that of the fol-lowing spring. These data do not agreewith the 5-year study of Blair (3) whofound the carryover of deer mice tothe following spring was greater inyears of comparative abundance thanfrom those of scarcity. It is not knownwhy the two sets of data disagree.

Summary and ConclusionsTree seed abundance had only a

small influence on spring deer mousepopulations in this study. The springpopulation was roughly the same re-gardless of the amount of seed. Theeasons for the lack of response to

seed abundance have not been deter-mined.

Fall deer mouse populations dis-played no response to current tree seedabundance. However, there was amarked response by the following fallto moderate or good seed crops. Sincethis was replicated 4 times, and du-plicated on another cutover, it is as-sumed to be a valid relationship.

Deer mouse response to moderateand good seed abundance was consid-ered to be nearly the same. This studyindicated that approximately 0.8pounds of seed per acre will cause ahigh mouse population the followingfall. Since there was no available in-formation between 0.28 and 0.81 poundsof seed per acre the lowest activatingquantity may be between these figures.

These data suggest that size of thespring deer mouse population has littleor only a modest influence on the fallpopulation and vice versa. High breed-ing potential of deer mice is capableof increasing a small population tohigh levels in a short time under favor-able conditions.

In addition to adding to our under-standing of forest ecology some find-ings from this study may be of prac-tical use. The most obvious would ap-pear to be in direct seeding programs.When the filled Douglas-fir and hem-lock seed crop amounts to about 0.8pounds of seed per acre it would seemreasonable to expect a high deer mouse

population the following fall. If seed-ing is attempted then, additional pre-cautions should be taken to periodi-cally census the mouse populations be-cause of the possibility of heavy re-invasion and thus seed destruction. Itmight even be prudent to hold the seedanother year in anticipation of a de-cline in the mouse population. It isassumed that reinvasion would be lessduring periods of low population.

It is generally believed that controloperations during low periods in ananimal's life cycle are most effectiveand economical. The data indicatethat early spring is a low point in thedeer mouse yearly cycle. Where fea-sible from an operational viewpoint,early spring would seem to be a goodtime to reduce mouse populations.

Literature CitedANoNvmous. 1954. H. J. Andrewsexperimental forest seed crop record.From the files of the H. J. AndrewsExpt. Forest.BENDELL, J. F. 1959. Food as acontrol of a population of white-footed mice (Peromyscus leucopusnoveboracensis Fisher). Can. Jour.Zool. 37:173-208.BLAIR, W. F. 1948. Population den-sity, life span, and mortality ratesof small mammals in the blue-grassmeadow and blue-grass field associa-tions in southern Michigan. Am.Midi Nat. 40:395-419.HAGAR, DONALD C. 1960. The inter-relationships of logging, birds, andtimber regeneration in the Douglas-fir region of northwestern California.Ecol. 41:116-125.HOOVEN, EDWARD F. 1958. Deermouse and reforestation in the Tilla-mook burn. Ore. Forest Lands Res.Center. Res. Note 37:1-31. . 1959. Dusky-footed wood-rat in young Douglas-fir. Ore. ForestRes. Center. Res. Note 41:1-24.ISAAC, LEO A. 1943. Reproductivehabits of Douglas-fir. Charles Lath-rop Pack Forestry Foundation,Washington, D. C. pp. 1-107.JAMESON, E. W., JR. 1952. Food ofdeer mice, Peromyscus maniculatusand P. boylei, in the northern SierraNevada, California. Jour. Mamm.,33:50-60. . 1953. Reproduction ofdeer mice (Peromyscus maniculatusand P. boylei) in the Sierra Nevada,California. Jour. Mamm., 34:44-58.KANGUR, RUDOLPH. 1954. Shrews astree seed eaters in the Douglas-firregion. Ore. State Bd. of Forestry.Res. Note 17:1-23.KOENIG, R. C. 1955. Regeneration,seed studies, H. J. Andrews Forest,1954 crop. Typewritten Report, Filesof H. J. Andrews Expt. Forest.KVERNO, NELSON B. 1964. Forestanimal damage control. Vert. PestControl Conf. Proc. 2:81-89.LAWRENCE, WILLIAM H. 1958. Wild-life-damage control problems on Pa-cific Northwest tree farms. NorthAmer. Wildl. Conf. Trans. 23:146-152. , NELSON B. KVERNO, andHARRY D. HARTWELL. 1961. Guideto wildlife feeding injuries on coni-

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fers in the Pacific Northwest. West.Forestry and Conserv. Assoc. 44 pp.MOORE, A. W. 1940. Wild animaldamage to seed and seedlings oncut-over Douglas-fir lands of Oregonand Washington. U. S. Dept. Agr.Tech. Bul. 706:1-28.MUERLE, GERHARDT F. 1955. Regen-

eration, seed studies, seed fall. Type-written Memo, Files of H. J. An-drews Expt. Forest.

17. RADWAN, M. A. 1963. Protectingforest trees and their seed fromwild mammals U. S. Forest Serv.Pac. NW. Forest and Range Expt.Sta. Res. Paper PNW-6:1-29.

STICKEL, Li rciLLE F. 1954. A corn-narison of certain methods of meas-uring ranges of small mammals.Jour. Mamm., 35:1-15. , and OSCAR WARBACH.1960. Small-mammal populations ofa Maryland woodlot, 1949-1954.Ecol. 41: 269-286.