Longer-Term Effects of Selective Thinning on Microarthropod Communities in a Late-Successional Coniferous Forest

COMMUN~TYAND ECOSYSEM ~UX:Y Longer-Term Effects of Selective Thinning on Microarthropod Communities in a Late-Successional Coniferous Forest

ROBERT W. PECK'. 27 AND CHRISTINE G. NIWA'

Environ. Entomol. 34 (3): 646-655 (2005) ABSTRACT Microarthropod densities within late-successional coniferous forests thinned 16-41 yr before sampling were compared with adjacent unthinned stands to identify longer term effects of thinning on this community. Soil and forest floor layers were sampled separately on eight paired sites. Within the forest floor oribatid, mesostigmatid, and to a marginal extent, prostigmatid mites, were reduced in thinned stands compared with unthinned stands. No differences were found for Collem- bola in the forest floor or for any mite suborder within the soil. Family level examination of mesostigmatid and prostigmatid mites revealed significant differences between stand types for both horizons. At the species level, thinning influenced numerous oribatid mites and Collembola. For oribatid mites, significant or marginally significant differences were found for seven of 15 common species in the forest floor and five of 16 common species in soil. Collembola were affected less, with differences found for one of 11 common species in the forest floor and three of 13 common species in soil. Multivariate analysis of variance and ordination indicated that forest thinning had little influence on the composition of oribatid mite and collembolan communities within either the forest floor or soil. Differences in microclimate or in the accumulation of organic matter on the forest floor were likely most responsible for the observed patterns of abundance. Considering the role that microarthropods play in nutrient cycling, determining the hnctional response of a wide range of taxa to thinning may be important to effective ecosystem management.

KEY WORDS microarthropods, Acari, Oribatida, Collembola, forest thinning

MICROARTHROPODSDOMINATE FQ~FLOOR faunas in forest floor decomposition and mineralization (Santos both diversity and abundance (Wallwork 1976). They et al. 1981, Kajak 1997, Lawrence and Wise 2000). are functionally diverse, with many contributing sig- Despite the importance of microarthropods in for- nificantly to both short- and long-term productivity of est ecosystems, relatively little attention has been forest ecosystems by facilitating processes involved in given to the impact of forest management practices on nutrient cycling (Seastedt 1984, Moore et al. 1988, their abundance and community structure. Most work Shaw et al. 1991). Primary contributions to nutrient has investigated shorter-term effects of clearcutting cycling include reducing the particle size of organic (Huhta et al. 1967, Vlug and Borden 1973, Huhta 1976, material through comminution (Berthet 1967), regu- Abbott et al. 1980, Seastedt and Crossley 1981, Marra lating microbe numbers through grazing (Hanlon and and Edmonds 1998). Although results from these stud- Anderson 1980), releasing nutrients immobilized by ies vary, they generally show that populations of many bacteria and fungi (Ineson et al. 1982, Seastedt and taxa characteristic of the mature forest decrease after Crossley 1983),and disseminating microbes via move- timber harvest. Such reduced abundances are largely ment throughout forest floor and soil (Behan and Hill attributed to microclimatic stresses that occur on the 1978, Kitchell et al. 1979). Field studies that manipu- forest floor after canopy removal, such as higher tem- lated microarthropod abundances suggest that peratures and lower moisture levels. To some degree, changes in population sizes can influence rates of microclimatic impacts are influenced by regional climatic differences. For example, within forests of the southeastern United States, declines in oribatid mite The use of trade or firm names in this publication is for reader abundance was attributed to prolonged exposure to information and does not imply endorsement by the U.S. Department temperatures exceeding tolerance thresholds (Seast- of Agriculture of any product or service. USDA-Forest Service, Pacific Northwest Research Station, For- edt and Crossley 1981). Conversely, within the more estry Sciences Laboratory, 3200 W Jefferson Way, Corvallis, OR moderate climate experienced on the Olympic pen- 97331. insula, Washington, temperature and moisture stress Wurrent address: USGS Biological Resources Division, Kilauea did not limit microarthropod densities in clearcut Field Station, P.O. Box 44, Hawaii National Park, HI 96718. 3Corresponding author: USGS Biological Resources Division, stands (Marra and Edmonds 1998). Changes in the Kilauea Field Station, P.O. Box 44, Hawaii National Park, HI 96718 standing crop of organic material also may contribute (e-mail: [email protected]) , to postharvest differences in microarthropod abun- June 2005 PECKAND NIWA:THINNING EFFECLS ON MICROARTHROPODS 647 dances (Mitchell 1978, Seastedt and Crossley 1981). with 70% of that amount falling as snow in November- However, Huhta (1976) suggested short-term in- March. creases in soil invertebrate abundances were associ- Eight pairs of late-successional stands (USDA et d. ated with an input of logging debris, whereas longer 1993) were chosen for study. Each pair consisted of a term decreases in abundance were driven by the ac- thinned stand and an adjacent, or nearby, unthinned celerated decomposition of organic matter. McIver et stand. Paired stands were contiguous (separated by a al. (1992) illustrated the pattern of forest floor spider road) in all stands but two; both of these exceptions community succession with increasing time after were separated by 4.5 km. All paired stands were clearcutting. It is possible that microarthropod com- similar in elevation, aspect, and slope. Thinned stands munities show similar successiond patterns. were generally selectively harvested for overstory re- In the Pacific Northwest, a shift in emphasis from duction, mortality salvage, or commercial thinning. timber production to ecosystem management has re- Thinning occurred between 16 and 41 yr before this sulted in selective thinning largely replacing clearcut- study; two stands were thinned on two occasions. ting as the principal method of timber extraction (Mc- Although some large-diameter trees were harvested Comb et al. 1993). Although thinning can be an from most stands, dl stands contained large trees as effective tool for maintaining wildlife populations, it is well as numerous snags and much downed coarse not known how forest floor and soil microarthropod woody material. Overstory canopy cover, forest floor communities are affected. Short-termeffects are likely (litter and duff) depth, and tree basal area were sig- to be less extreme than those caused by clearcuttin& nificantly higher in unthinned stands (81.7 + 2.1% because microclimatic changes on the forest floor are [mean + SEMI, 4.4 + 0.9 cm, and 7.4 + 0.3 m2/ha, theoretically less severe. However, longer term effects respectively) compared with thinned stands (53.7 + are poorly understood. Studying the response of soil 4.0%, 3.2 + 0.6 cm, and 4.6 + 0.2 m2/ha, respectively) arthropods 14 yr after selective harvest of California (Table 1).Thinned stands were still classified as late- coastal redwoods, Hoekstra et al. (1995) found pred- successional forest under current ecosystem manage- ator abundance to be lower in thinned stands com- ment guidelines (USDI 2000). pared with old-growth and 75+ year-old second Microarthropod Sampling. Within each stand, 16 growth stands and phytophagous taxa were no less forest floor and soil samples were taken at 12.5-m abundant in thinned stands. It is unknown what role intervals along transects. Transects originated >50 m climate played within the relatively moderate condi- from a stand boundary and generally ran along terrain tions found in the coastal redwood zone of central contours. No sample point was closer than 0.5 m to a California tree or log with a diameter >10 cm. Forest floor sam- The objective of this study was to determine the ples were collected using a putty knife guided by a 10 longer term effects of selective thinning of late-suc- by 10-cm plastic panel. Forest floor included freshly cessional forests on forest floor and soil microarthro- fallen needles and leaves as well as the packed, dark- pod communities in the Cascade Mountains of south- colored fermentation and humus layers that extended ern Oregon. We compared arthropod abundance and down to the mineral soil. After forest floor collection, community composition of stands thinned 16-41 yr the top 5 cm of soil was collected using a stainless steel ago to adjacent unthinned stands. corer (7.8 cm in diameter) lined with a 5-cm-long thin-walled polyvinyl chloride plastic sheath (=240 cm3). Forest floor and soil samples were sealed in zip-lock plastic bags, immediately placed into chilled Materials and Methods coolers, and transported to the laboratory where they Study Sites. The study area was located within the were maintained at S°C until microarthropod extrac- Medford Resource Area, Bureau of Land Manage- tion took place. Extraction of forest floor samples be- ment, of the Cascade Mountains of southern Oregon. gan immediately upon return to the laboratory, and Study sites were located within a 10-krn radius of 42" soil samples were processed 2-3 d later. From each 07', 122" 26'. Elevation ranged from 1,092 to 1,556 m. transect, sets of four consecutive samples from each Sites were dominated by white fir, Abies concoh depth were composited in the laboratory for extrac- (Gord. & Glend.) Lindl. ex Hildebr., and Douglas-fir, tion (thus, four collections of extracted microarthro- Psewlotsuga menzWi (Mirb.) Franco, but some also pods were obtained from each site). Microarthropods included sugar pine, Pinus Zumbertiana Dougl.; pon- were extracted from both forest floor and soil samples derosa pine, Pinus punderosa Dougl. ex. Laws.; and by using 40-W incandescent light bulbs atop 20-cm- incense-cedar; Libomdm decurrens Torr. Geological diameter Berlese-type funnels (Moldenke 1994). features consisted of tertiary volcanic rocks of the Lights were alternated on and off at 2-h intervals Western Cascade Range, tertiary and quaternary vol- during the kt24 h and then were left on continuously canic rocks of the High Cascade Range, and quater- until the samples became desiccated (dry to touch). nary surficial deposits and intrusive rocks (USDI Forest floor and soil samples were taken on two oc- 2000). Soils were loam in nature, ranging from very casions: 21-23 June and 4-6 October 1999. October cobbly to gravelly (Johnson 1993). Climate is charac- samples were taken adjacent to those collected in terized by warm, dry summers and cold, moist winters. June. Mean monthly temperatures range from -2.O"C (Jan- Due to the large number of microarthropodswithin uary) to 16.4OC (July). Annual precipitation is -87 cm, many samples, it was often necessary to reduce sample 648 ENVIRONMENTALENTOMO~ Vol. 34, no. 3

Table 1. Year of thinning and measurements of forest ~troetureand microarthropod densitiee(no.1400 em2 of forest floor) for thinned and nnthinued sites

yr Canopy Forest floor Basal Microarthropod group Treatment thinned cover depth area (%) (em) (me/ha) Oribatida Mesostigmata Prostigmata Astigrnata Collembola Thinned 1983 54.7 + 9.2 4.1 + 1.0 4.0 + 0.3 322.9 + 107.9 73.6 + 35.9 485.3 + 180.3 Unthinned 89.1 + 2.0 6.2 + 0.7 9.0 + 0.4 1,370.0 + 338.6 119.3 + 42.9 2,928.3 + 1,148.6 Thinned 1983 53.8 + 9.3 2.7 2 0.5 3.7 + 0.6 327.5 + 100.6 51.5 -C 24.3 980.5 + 504.9 Unthinned 88.5 + 6.9 4.3 + 0.4 9.5 + 0.8 745.5 + 181.3 113.5 + 53.2 1,380.0 + 235.2 Thinned Late 43.4 + 9.5 3.3 + 0.5 4.7 + 0.8 270.9 + 106.6 26.3 + 19.1 260.9 + 104.7 1960s Unthinned 80.0 + 5.3 4.7 + 0.6 8.5 + 0.6 996.5 + 135.7 132.5 + 28.1 1,595.0 + 208.8 Thinned 1978 and 53.2 + 9.3 2.3 + 0.7 4.8 + 0.8 357.5 + 116.4 56.3 + 32.1 706.5 + 226.8 1991 Unthinned 71.7 + 3.0 4.1 + 0.7 5.4 + 0.5 571 + 116.4 162.0 + 41.1 1,902.0 + 410.4 Thinned 1963 46.2 + 6.6 3.3 + 0.6 3.7 + 0.5 140.0 + 51.3 19.5 + 15.0 624.3 + 234.7 Unthinned 76.1 + 4.9 3.6 + 0.4 7.4 + 0.7 727.0 + 260.8 109.0 + 42.4 1,243.0 2 133.7 Thinned 1964 and 77.7 + 6.0 3.7 + 0.4 6.4 + 0.6 659.0 2 146.4 74.0 + 31.5 1,147.0 + 213.6 1990 Unthinned 84.5 + 5.3 3.3 + 0.4 7.0 + 0.7 850.0 + 118.9 107.0 + 25.9 1,139.0 + 232.4 Thinned 1958 62.257.0 3.250.9 4.120.5 313.8276.1 395220.1 662.35227.0 Unthinned 84.4 + 5.1 4.5 + 0.8 4.8 + 0.3 211.5 + 20.3 10.5 + 7.6 838.0 2 210.7 Thinned 1979 66.8 + 7.7 2.6 + 0.5 5.7 + 0.6 569.0 + 105.3 65.0 + 28.4 1,438.0 + 198.9 Unthinned 79.4 + 4.4 4.2 + 0.9 8.6 + 0.7 563.0 + 139.7 78.0 + 41.8 1,111.0 + 216.7

Measurement values presented are means + SEM. Methodology for measurement of forest structure is described in Peck and Niwa (2005). Microarthropod densities are average values between June and October sample dates (n = 4 samples per site per date). a BLM site names: BC, South Fork Beaver Creek; EC,East Chinquipin;HO, Hobart; SC, Soda Camp; JC, Jenny Creek; YS, Yew Springs; RO, Rosebud; MC, Mill Creek. Differences between thinned and unthinned stands were statistically significant (Wilcoxon's signed ranks test for paired comparisons; P < 0.05). sizes by systematically dividing samples into smaller that failed to meet the assumptions of the test were fractions for identification.Generally, one-half to one- In-transformed (Y + 0.5) and residuals reexamined. eighth of each sample was sorted and enumerated. Taxa with too few samples containing counts were not This procedure likely led to the underrepresentation analyzed, regardless of their overall abundance (39 of rare species. All microarthropodswere identified to oribatid mites and 46 Collembola were not analyzed). at least the suborder level by using a dissecting mi- For each depth, a split-plot analysis of variance croscope. Whenever possible, mesostigrnatid and (ANOVA) was used to analyze microarthropod abun- prostigrnatid mites were identified to the family level, dances. Within blocks (paired stands), stand type and adult oribatid mites and Collembola were iden- (thinned or unthinned) was the whole-plot factor, and tified to species or morphospecies level. Oribatid date of collection (June and October) was the subplot mites were cleared with lactic acid, temporarily factor. Interactions between date and treatment were mounted on cavity slides and identified using a com- examined to determine if responses to thinning dif- pound microscope. Their classification follows Mar- fered with season. Only statistically significant inter- shall et al. (1987). Collembola were cleared in 10% actions are reported. The relationship between time KOH for 5-10 min, mounted in polyvinyl alcohol on since thinning and microarthropod abundance was glass slides, and identified using a phase contrast com- investigated for each mite suborder and Collembola pound microscope following Christiansen and Bell- by using regression models by comparing the age of inger (1998). Voucher material was deposited in the the thin with the difference in abundance of each USDA-Forest Service, Western Forest Insect Collec- taxon between paired thinned and unthinned stands. tion (Oregon State University, Corvdis, OR). This analysis was only performed on forest floor sam- Data Analysis. Before analysis, average values for ples because no significant differences were found microarthropod counts from the four collections between thinned and unthinned sites in soil for these within each site, depth and date were calculated, groups. General linear models within SAS statistical yielding a value representing the density of microar- software (PROC GLM;SAS Institute 1991) were used thropods within the forest floor (400-cm2 surface for ANOVA and regression analyses. Strong evidence area) and soil (960 cm3). Counts from soil samples of statistical differences was accepted when P < 0.05, were later standardized to represent 400-cm2 surface and evidence was considered marginal, but suggestive, area for presentation. The density values presented when 0.05 < P < 0.10. can be converted to number of individuals per square Multivariate techniques were used to further eval- meter by multiplying by 25. For each taxon analyzed, uate the influence of thinning on oribatid mite and residuals (estimates of model errors) were examined Collembola communities within the forest floor and to determine whether transformation of the counts soil layers. Principle Coordinate Analysis (ORD) , was necessary to meet the assumptions of the para- followed by Nonmetric Multidimensional Scaling metric test (Sabin and Stafford 1990). Response data (NMDS), were used to ordinate each stand into spe- June 2005 PECK AND NIWA:THINNING EFFECLS ON MICRO~RTHROPODS 649

Table 2. Comparison of mean f SEM deneitieaj (no.14.00 em2 of surfaee area) of forest floor and soil microarthropods between thinned and tmthinued stands

June Oct. P valuea Taxon Layer Thinned Unthinned Thinned Unthinned Date Treatment Oribatid mitesb Forest floor Soil Mesostigmatid mites Forest floor Soil Ologamasidae Forest floor Soil Zerconidae Forest floor Soil Other families (3)" Forest floor Soil Prostigmatid mites Forest floor Soil Alicorhagiidae Forest floor Soil Eupodidae Forest floor Soil Nanorchestidae Forest floor Soil Rhagidiidae Forest floor Soil Tydeidae Forest floor Soil Other families (9)" Forest floor Soil Astigmatid mites Forest floor Soil Collembolab Forest floor Soil

" NS, not signscant; *, 0.05 < P < 0.10; **, < 0.05. See Tables 3 and 4 for densities of the most common species of oribatid mites and Collembola, respectively. " Digamasellidae, Polyaspididae, and Uropodidae. Too few individuals were collected to test for statistical differences. " Bdellidae, Bimichaeliidae, Cunaxidae, Parat~deidae,Pediculidae, Penthalodidae, Pygrnephoridae, Scutacaridae, and Trombidiidae. cies-space and to graph for visual comparison. Multi- nearly three times more extracted from soil than the variate analysis of variance (MANOVA) was then forest floor. used to determine whether these communities dif- Based largely on identifications of adults, 15families fered significantly between thinned and unthinned of prostigmatid mites and five families of mesostigma- stands. Only those species whose relative abundance tid mites were identified (Table 2). Prostigmatid mites was 10.1%of the total were included in the analyses. were dominated by Eupodidae and Nanorchestidae, Thus, the number included were 39 and 33 oribatid each comprising -24% of the total, followed by Ty- mites in litter and soil, respectively, and 30 and 28 deidae (13.5%),Alicorhagiidae (12.3%),and Rhagidi- Collembola in litter and soil, respectively. PC-ORD idae (8.5%).Within mesostigmatid mites, Ologamasi- was used for ORD and NMDS analyses (McCune and dae comprised 55.2% of the individuals identified to Mefford 1997).BioStat (Sigmasoft 1993) was used for family followed by Zerconidae, comprising 39.7%. MANOVA. Seasonal differences in abundance were found for most taxa (Table 2). For mesostigmatid mites, no seasonal difference was found within soil, but signifi- Results cantly more individuals were collected from the forest Microarthropod densities within both the forest floor in June than in October. The pattern within the floor and soil were dominated by prostigmatid mites forest floor was reversed for the dominant families, (1,152.6 + 160.0 and 1,681.1 + 162.51400 cm2 surface because Zerconidae was more abundant in June and area, respectively), followed by oribatid mites Ologamasidae was marginally more numerous in Oc- (562.2 2 81.6 and 631.1 + 38.7), Collembola (101.8 + tober. Juvenile mesostigmatid mites, not identified to 19.0 and 300.7 + 35.0), mesostigmatid mites (77.3 + the family level, were more numerous in June, rep- 10.6 and 115.9 + 8.2), and astigmatid mites (38.0 5 9.3 resenting 54.6% of the total, compared with 35.7% in and 57.6 + 12.1) (Table 2). Overall, microarthropod October. This difference in juvenile abundance led to density was slightly greater in soil than in the forest the greater numbers of mesostigmatid mites in the floor, with 2,786.4 + 185.5 individuals1400 cm2 in soil forest floor in June. In contrast, prostigmatid mites and 1,929.7 5 258.2 in the forest floor. The difference were more abundant in both horizons in October. between horizons was greatest for Collembola, with Between-family differences also existed for adult 650 ENWRONMENTAL ENTOMOLOGY Vol. 34, no. 3

Table 3. Comparison of mean * SEM dedtiea (no.lP00 em2 enrfaee area) of the most abundant oribatid mite epeciee in fomt floor and mil between thinued and nnthinued etands

Forest floor Soil Species" P value P value Thinned Unthinned Date Treatment Thinned Unthinned Date Treatment Brochy&us sp. June** June** EpidamaarP a&icoZus (Hammer) June* NS Eremaeus oocidentalis Behan-Pelletier NS NS Eueremaeus stiktos (Higgins) Oct.** -b GUskEoM sp. June** NS JmhueUa sEriata Wallwork NS NS LMcmus bidentutus Ewing NS Oct.** MePrioppM .mgntmh Woolley & Hig. June** NS Mycobates rncuroatus Hammer NS NS Neorizeh ficgosuln (Ewing) m.** NS Odm~ueriornntus (Higgins) NS NS OppkUa nooa (Oudemans) June** June** OuadroppM qumkminata (Michael) - June** Scheiin-fbates palzidulws (Koch) NS NS ScmbbeIba sp. - Oct.** Tedocephacs velatus (Michael) NS NS Trhypochdhonius kctorum (Berlese) NS June*

June and October sample dates have been combined to show treatment means but were separated for ANOVA analyses. NS, not significant; *, 0.05 < P < 0.10; **, < 0.05. " Species not analyzed statistically:Achiptera sp., Amerh-oproctuPsp., Aphekdnw (Berlese), Atupdbniws mtiodadylus Grandjean, Auto- sp., BeIba sp., CanisM horrida (Hermann), Cam, spinifm (Koch), Car- sp., CeratoppM bipilis (Hermann), Cer- pocijkw Behan-Pelletier, C3mdwniw lanatus (Michael), Epidnl~e~dip (Hammer), Ep. hammerue Behan-Pelletier & Norton, Ep. koyukon Behan-Pelletier& Norton, Er- cal~~~khan-Pelletier, Er. gradlis Behan-Pelletier, Er. orebbs Behan-Pelletier,Er. sp., Hematankh mhtaEwing, KodMkella lutea Hammer, K. sp., Lepidozetm tifoliws (Fujikawa), Wocwus latus Ewing, Wmsp., Liebstudiu similis Michael, Maerkebtritia alaskensis Hammer, Moritzoppiu clauigera (Hammer). Mycobata amitus Hammer, Nabkelaga& Berlese, Not-hella gildersh (Hammer), Nothsp., Palaeocarus sp., Phthir- anonymow Grandjean, PropeZups catumhsis (Hammer), Pi-. gydandicus (Sellnick), ScheIoribates rotundatus Hammer, Z&x&inibates sp., Zygoti?natulusp. Too few individuals were collected to test for statistical differences. prostigmatid mites, because Eupodidae and Rhagidi- mites, or Collembola within the forest floor on either idae were more numerous in June, whereas sample date. Nanorchestidae and Tydeidae were more numerous in Oribatid Mites. Fifty-five species of oribatid mites October. An abundance of juveniles in October Sam- from 33 families were collected in thinned and un- ples resulted in greater overall numbers on this date. thinned sites. Most species were found in both stand Oribatid mites and Collembola were most numerous types, with 53 and 48 species collected in thinned and in June in both horizons. For astigmatid mites, no unthinned sites, respectively. Species overlap be- seasonal difference was found in the forest floor, but tween horizons was also high, with 42 species (76.4%) greater densities were found in October in the soil. found in both the forest floor and soil. Oppiellu ma Thinning had a greater impact on microarthropod dominated the samples, comprising 30.4% of all adult densities in the forest floor than in the soil (Table 2). oribatid mites collected (25.3% in forest floor and Within the forest floor, oribatid mites, mesostigmatid 33.9% in soil), followed by Scheloribates pallidulus mites, prostigmatid mites, and astigmatid mites were (7.5%), Techxpheus oelutus (7.3%), Brachychthonius significantly more abundant in unthinned stands than sp. (7.0%), and Gustavk sp. (6.3%).Most species were in thinned stands, although the difference was mar- relatively rare, because 39 species each comprised ginal for prostigmatid mites. In contrast, no significant 4%of the total. thinning effect within soil was found at the suborder Seventeen species of oribatid mites were abundant level for mites. For mesostigmatid mites in the forest enough to analyze statisticab. Significant seasonal floor, both Ologamasidae and Zerconidae were more abundance patterns were found for some taxa in both abundant in unthinned stands than in thinned stands, the forest floor and soil (Table 3). In the forest floor, whereas for prostigmatid mites, Alicorhagiidae were Brachychthonius sp., Epidamueus arctioolus, Gustauk more numerous in unthinned stands. Within soil, Eu- sp., Met+ oregd,and 0. mawere all sig- podidae and Nanorchestidae were more common in nificantly more abundant in June than in October. In thinned stands. Although this difference was marginal contrast, Euer- stiktos and Neori;zetes rugosuh for Eupodidae, >6 times more individuals were col- were more abundant in October than in June. Within lected in thinned sites than in unthinned sites. No the soil, Brachgchthonius sp., 0. ma, Quadroppia difference between thinned and unthinned sites was ipudricarinatu, and Trhypochthonius teetorurn were found within either horizon for Collembola more abundant in June, whereas Lhm&tatus No significant relationship was found between time and Suctobelba sp. were more abundant in October. since thinning and the abundance of oribatid mites, Densities of several oribatid mite species differed mesostigmatid mites, prostigmatid mites, astigmatid between thinned and unthinned sites (Table 3). June 2005 PECK AND NIWA:THJNNING Ehmm ON MICROARTHROPODS 651

Table 4. Comparison of mean f SEM densities (no.l.200 em2 surface area) of the moet abnndaut eollembolan species in forat floor and mil between thinned and uutbiuued stande

Forest floor Soil Speciesa P value P value Thinned Unthinned Date Treatment Thinned Unthinned Date Treatment Antmphorus leeptentrionalfsPalissa Oct.** Entomobrya conha Christiansen Oct.** Fohia cundidu (Willem) June** Fohia oze~urYosii Oct.* Hypgastrwa virga Christ. & Bellinger June* Hypgastrwa vulgaris Yosii June** htoma sp. nr. mucleani Fjellberg - Isotoma sensibilis Tullberg June** Onychiurus cocklei (Folsom) Oct.** Ongchiurus eisi Rusek Oct.** 0nychiwu.s eous Christ. & Bellinger - Onychiurusjiimmms Kinoshita June** Onychiurus fblsomi (Shaer) - Onychiurus mil& Chamberlain June** Onychiurus ranwsus Folsom - Tomocerus vulgaris (Tullberg) June** Tullbergia nulla Christ. & Bellinger June**

June and October sample dates have been combined to show treatment means but were separated for ANOVA analyses. NS, not significant; *, 0.05 < P < 0.10; **, < 0.05. " Species not analyzed statistically:Anbpalites diversu~Mills, Bowldie& luridQSnider, Cryptopygus deoemoculalus (Folsom), Enhnobtya ah-ocinctu (Schott), E. grkeoolivata (Packard),E. sp., E. tiangularis Schott, Fohia macroseta Ford, F. nfvalfs (Packard), E sp. A., E sp. B., E stehChristiansen and Tucker, Hypogastrura brevis Christiansen and Bellinger, H. isabellae Fjellberg, H. pratonmz (Packard),H. pPeudarmata (Folsom), H. horridu Yosii, H. krafti (Scott), H. sp. A, H. sp. B, Isotoma albella Packard, I, a.rborva (L),I. brucreaUa Wray, I. sp. nr. mmuna MacGillvray, I. japonica Yosii, I. monochaeta Kos, I. negb (SchafTer), I. pwdxinerea (Fjellberg), I. sp., Mdkrotomcr grandbps (Reuter), Morulina sp., Neanwajiigida Yosii, Neelus minimus Willem, Odontella coronifer Mills, 0nytAiut-w sp., Orchm& ahaGuthrie, Prokotomn minuta (Tullberg), Pseudachorutes simplex Maynard, Sminthurinus hhad (Folsom), S. moculosus Snider, S. sp., Tafallia mbusta (Scott), To- grahumi Christiansen, To. sp., To. nr. wilkeyi Christiansen, XenyUa sp. Too few individuals were collected to test for statistical differences. "A significant (P < 0.05) date by treatment interaction was found. F, ozwma was significantly (P < 0.05) more abundant in unthinned stands in June, but more abundant (but not significantly) in thinned stands in October.

Within the forest floor, E. ardicolus, Er- occi- were uncommon or rare, as 44 species each comprised dentalis, and Eremaeus stiktos were found in signifi- 4%of the total collected. cantly greater densities in unthinned stands compared Seasonal differencesin density were found in one or with thinned stands. Only T. kctorum was more abun- both horizons for 16 of the 17 Collembola species dant in thinned stands. Within the soil, Nycobates abundant enough to be analyzed statistically (Table incurvatus, T. velatus, and T. tectorum were more abun- 4). Within the forest floor, eight of 11 species showed dant in thinned stands, whereas Neorizetes rugosala significant seasonal differences in density; all eight was marginally more abundant in unthinned stands. species were collected in greater numbers in June Ordination revealed no obvious separation between than in October. The seasonal pattern was even stron- thinned and unthinned stands for oribatid mite species ger in the soil, with differences in density found for all in either the forest floor or soil. Variation among sites 13 species analyzed. In contrast to the forest floor, was relatively high because the cumulative amount of however, five species were more abundant in October variation explained by Principal Coordinate axes one than in June. Only one species, Anurophorus seph- and two was 53.1% in the forest floor and 51.7% in the soil. Overall, MANOVA failed to find differences be- Erionalis, revealed seasonal patterns that differed between centroid values representing oribatid mite com- tween horizons, with a significantly greater density munities in thinned and unthinned stands for either found in June in forest floor and in October in soil. the forest floor (F= 2.93; df = 10,5; P = 0.12) or the Thinning impacted only a few species of Collem- soil (F = 2.18; df = 10, 5; P = 0.20). bola (Table 4). Within the forest floor, only Hypogas- Collembola Overall, 61 species of Collembola from trura virga was affected, being marginally less abun- seven families were identified. Slightly more species dant in thinned sites than in unthinned sites. Similarly were found in unthinned sites than in thinned sites, within soil, FoIsomiu candida, 0. mil&, and Tullbergia with 54 and 46 species identified, respectively. Species nulla were found in fewer numbers in thinned sites overlap between horizons was lower than that for than in unthinned sites. A significant date by treat- oribatid mites, with 25 species (41.0%) found in both ment interaction was found for Folsomiu o;zeana; sig- the forest floor and soil. OnychiuTus mil& was most nificantly more individuals were collected in un- abundant species, comprising 23.1% of the fauna, fol- thinned sites in June but on average (but not lowed by Zsotoma sensibilis (21.1%),Entomolnya con- statistically different), more individuals were col- firsa (7.9%),and On~chiuruseisi (6.7%).Most species lected in thinned sites in October. 652 ~O~ALENTOMOLOGY Vol. 34, no. 3

Collembolan communities from the forest floor and suppressing microarthropod abundances. Bird and soil did not ordinate consistently within groups rep Chatarpaul (1986) found mites within litter to be resenting thinned and unthinned stands. Rather, ad- significantly less abundant in whole-tree harvested jacent thinned and unthinned stands often placed plots than in uncut and conventionally harvested plots similarly in space along axes one and two suggesting but found a decrease in moisture to be more strongly high community similarity between some stands. The associated with reduced mite abundance than tem- first two Principal Coordinate axes accounted for perature. In contrast, Marra and Edmonds (1998) more of the variance in the forest floor than in soil found moisture and temperature to have had little (61.7 and 47.996, respectively). Collembolan commu- influence on microarthropod densities in clearcut for- nity composition did not differ significantly between ests on the Olympic Peninsula of western Washington, thinned and unthinned stands in either the forest floor although climatic conditions in their study area are or the soil (MANOVA; F = 0.88, df = 10,s; P = 0.60 moderate compared with many other temperate en- and F = 2.08; df = 10,5; P = 0.22, respectively). vironments. Changes in microclimatic conditions within our thinned stands may have been less severe than in the clearcuts discussed above, but they still Discussion may have approached tolerance limits for some spe- Our results indicate that late-successional stands cies. Data from a nearby weather station indicated that thinned 16-41 yr prior support lower densities of July temperatures reached 35.8OC, with five consecu- mites within the forest floor than unthinned stands of tive days being at least 31.4OC. In addition, little rain similar age, but mite densities in the upper 5 cm of soil fell on the sites between mid-June and mid-August, were largely undected by thinning. Densities of on- likely causing moisture stress. Our late June and early batid, mesostigmatid, prostigmatid, and astigmatid October samples were not taken during periods of mites were all lower in thinned stands, indicating that greatest climatic stress, but lower numbers of mites longer term effects of thinning is widespread among collected in thinned stands may reflect a generalized mite taxa In contrast, the overall abundance of Col- response to less favorable conditions. lembola did not differ between stand types, suggesting The amount of organic matter on the forest floor that current conditions within thinned stands were also has been shown to influence the microarthopod not sufficiently different from unthinned stands to community. Seastedt and Crossley (1981) found a result in differences in their abundance. positive correlation between microarthropod abun- To a limited degree, our findings corroborate those dances and increased organic matter following timber of others. Within coastal redwood forests of central harvest, and proposed that when physiological toler- California, a study comparing forest floor arthropods ances are not exceeded, the amount and distribution within uncut old-growth, mature second growth (180 of organic matter within soil can be an accurate pre- yr old), and second growth stands (260 yr old) se- dictor of microarthropod abundances. Similarly, lectively harvested 14 yr before the study, found only Mitchell (1978) found significant positive correlations predatory arthropods (which included spiders, centi- between abundances of three oribatid mite species pedes, and pseudoscorpions as well as many mesostig- and the depth of organic matter within fermentation matid mites) to be significantly lower in abundance in and humus layers of Aspen woodlands soil. In our thinned stands than in unthinned stands (Hoekstra et study, amounts of fine wood (C2.5 cm in diameter) al. 1995). Micro- and pan-phytophagous arthropods and partially decomposed organic material were sig- (mostly oribatid and prostigmatid mites and Collem- nificantly greater in unthinned stands compared with bola) did not differ in abundance among the three thinned stands (Peck and Niwa 2005). These differ- stand types. In a multiregional study within coniferous ences were due to the higher abundance of trees and forests of western Oregon, Madson (1998) found no greater extent of canopy cover in unthinned stands. It significant differences in the abundance of microar- is unclear how organic matter may have influenced thropods within the forest floor and soil among old- microarthropod abundances, but plausible explana- growth, mid-aged pole stands (50-90 yr old), and pole tions include greater amounts of microbial food in- stands thinned 5-23 yr before the study. habiting the surface of the organic matter (Seastedt Physical changes on the forest floor associated with and Crossley 1981) or a buffering of temperature and thinning may influence microarthropod communities moisture stresses (Mitchell 1978). in a variety of ways. For example, thinning can result Although thinning reduced the abundance of many in more extreme temperatures and an increase in the taxa, ordination and MANOVA revealed that the over- rate at which the forest floor desiccates during dry all composition of the microarthropod community dif- periods (Abbott et al. 1980). Microclimatic conditions fered little between stand types. Most species were during these times may approach the tolerance limits found in both thinned and unthinned stands; 85% of of many species (Woodring and Cook 1962, Madge oribatid mites and 64% of Collembola were found in 1965). Seastedt and Crossley (1981) found signifi- both stand types. In some cases, greater similarity cantly lower densities of microarthropods in litterb- existed between adjacent thinned and unthinned ags, and to a lesser extent in the upper 5 cm of litter stands than within either stand type. Marra and Ed- and soil, in clearcut stands than in uncut stands. Their monds (1998) also found broad overlap in species data suggested that temperatures on the forest floor composition between clearcut and uncut stands, with might have surpassed critical thresholds for survival, 47 of 62 species of oribatid mite common to both stand June 2005 PECK AND NIWA:THINNING EFFECrS ON MICROARTHROPODS 653 types. And Madson's (1998) landscape-level study of stands. Deeper and more frequent sampling may have microarthropod response to forest thinning found detected seasonal vertical migrations if they had oc- geographic affinities to be of greater importance in curred. determining community composition than was stand Understanding the functional role of microarthro- structure. pods within forest ecosystems is important to inter- The fact that no significant relationships were found preting the results of studies involving impacts due to between time since thinning and the abundance of management practices. Although great strides have any of the mite suborders or Collembola suggests that been made determining and classifying the diets of recovery from thinning is not progressing at a steady many microarthropods (Luxton 1972, Behan-Pelletier rate over time. It is likely that temporal changes in and Hill 1983, Wallwork 1983, Walter 1987, Kaneko microarthropod abundance after thinning is influ- 1988), current levels of knowledge limit our ability to enced by a variety of complex factors, including the accurately place many forest microarthropod taxa, extent and spatial pattern of tree harvest as well as the particularly mites, into specific functional groups rate that the vegetation changes over time. It is pos- (Dindal 1990, Moldenke and Thies 1996). Primary sible that a greater sampling effort, representing ad- challenges include understanding the extent of om- ditional thinned stands of varying age, would have nivory (Walter et al. 1986, Walter 1987), understand- revealed more clear relationships between microar- ing how seasonal changes in resource availability af- thropod abundance and time since thinning. fect diet (Anderson 1975, Rockett 1980, Behan- It is clear from our results as well as those of others Pelletier and Hill 1983, Walter and Ikonen 1989) and (Luxton 1981, Bird and Chatarpaul 1986, Moldenke determining how diets change as species mature and Fichter 1988, Moldenke and Thies 1996, Marra and (Walter 1987).Largely due to these limitations,we did Edmonds 1998) that forest microarthropod abun- not analyze our results at the functional level. How- dances fluctuate considerably over the course of the ever, it is clear that thinning reduced numbers of both year. Of the 17 most common collembolan species microphytophagous (fungiand bacteria feeders; sensu collected, 16 differed significantly in abundance in Luxton 1972) and predaceous mites within the forest either the forest floor or soil between June and Oc- floor. In particular, densities of oribatid mites, gener- tober. Oribatid mite abundances were only slightly ally the dominant fungivorous microarthropod group more similar over time, with 11 of 17 species differing within many forest ecosystems, were lower in thinned between dates. Species-specific life history strategies stands than in unthinned stands. Densities of mite taxa may influence the extent to which seasonal differ- that are primarily predaceous, such as the Ologama- ences are detected. For example, oribatid mites have sidae and Zerconidae (Mesostigmata) and Alicorha- relatively low metabolic rates, long developmental giidae and Rhagidiidae (Prostigmata), were also lower times, and low fecundity. Some species take a year or in thinned stands. Considering the roles that micro- more to complete development (reviewed by Luxton arthropods play in nutrient cycling, determining the 1981), and some females lay fewer than 100 eggs in a relationship between forest management practices, lifetime (Saichuae et al. 1972). Because egg laying such as thinning, and the functional response of generally occurs over the entire season (Luxton 1981), affected microarthropod populations is critical to ef- adults and juveniles are likely to be encountered at fective ecosystem management. Further research is any time. In contrast, many Collembola and prostig- necessary to better understand these complex inter- matid and astigmatid mites are relatively shorter lived actions. and are more fecund, often completing multiple gen- erations within a single season (Crossley 1977, Dindal 1990). Because these latter taxa are better able to Acknowledgments respond more quickly to changes in resource availability, such as rapid growth of fungal mycelia after We thank D. Russell (USDI, Medford BLM) for help snow melt or fall rain, temporal differences in abun- locating study sites and P. LeBlanc (USDI, Medford BLM) dance are more likely to occur (Crossley 1977). This for providing stand data. We also thank K. Tygart, B. Kre- owski, L.Gundersen,D. Jones,D. Muir, G. Spence, and D. Hill reproductive trait also may explain why such a high for assistance in the field and laboratory. J. Battigelli (Earth- percentage of prostigmatid mites collected (-90%) works Research Group) identified mites and Collembola M. were juveniles. Due to potentially rapid population Huso (Oregon State University) assisted in developing changes of many species, we concur with Moldenke ANOVA protocol. G. Brenner (Pacific Analytics, LLC.) per- and Thies (1996) that it is important to sample on formed the multivariate analyses. We gratefully appreciate multiple occasions throughout the year to measure the comments made by A. Moldenke, J. Battigelli, and two anon- response of microarthropods to forest treatments. ymous reviewers.This work was funded by the USDI Bureau Seasonal migration within and between the forest of Land Management and the USDA-Forest Service, Pacific floor and soil is a common response to heat and des- Northwest and Pacific Southwest regions, as part of the Northwest Forest Plan, Survey and Manage Program. iccation stress (Dowdy 1944, Usher 1970, Hassall et al. 1986) and may help explain some of the differences we observed between stand types. However, we were unable to detect abundance patterns that would sug- References Cited gest that microarthropods migrated vertically to a Abbott, D. T, T. R Seastedt, and D. A. 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