MICROARTHROPOD RESPONSE FOLLOWING CABLE LOGGING AND CLEAR-CUTTING IN THE SOUTHERN APPALACHIANS 1

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1 Ecology, 62(1), 1981, pp by the Ecological Society of America MICROARTHROPOD RESPONSE FOLLOWING CABLE LOGGING AND CLEAR-CUTTING IN THE SOUTHERN APPALACHIANS 1 T. R. SEASTEDT 2 AND D. A. CROSSLEY, JR. Institute of Ecology and Department of Entomology, University of Georgia, Athens, Georgia USA Abstract. Litter and soil microarthropod populations were monitored following cable logging and clear-cutting of a forested watershed (WS 7) at the Coweeta Hydrologic Laboratory in the southern Appalachians of North Carolina. Annual mean densities of microarthropods in litter bags were reduced over 50% on the clear-cut watershed when compared with an adjacent forested watershed (WS 2), and averaged 8.4 individuals/g of litter on WS 7 vs. 20.4/g on WS 2 (P <.01). Density estimates obtained from 5 cm deep sections of litter and soil indicated a 25% reduction in densities on WS 7, with a 17-mo average of microarthropods/m 2 on WS 7 vs microarthropods/m 2 on WS 2 (P <.001). In contrast, densities of microarthropods increased over 100% in deeper soil horizons (5-55 cm), averaging microarthropods/m 2 on WS 7 vs microarthropods/m 2 on WS 2 (P <.001). Clear-cutting by methods that do not destroy the integrity of the litter-soil environment engenders two kinds of perturbations: (1) removal of the canopy, and (2) addition of organic matter inputs, primarily woody litter debris, to the forest floor. Without a protective canopy, temperatures at the litter-soil interface on WS 7 were increased; daily maximum summer temperatures averaged over 40 C. Thus, lethal or nearly lethal temperatures reduced microarthropod densities in the top 5 cm of litter and soil on WS 7; declines were attributed primarily to oribatid mite mortality. Increased densities below 5 cm were attributed primarily to increased prostigmatid mite abundance. The relationships between microarthropod densities, standing crops of organic matter and litter and microbial production were examined. The amount and vertical distribution of organic matter appears to be the best predictor of microarthropod densities and vertical distributions in most forest ecosystems. Key words: abiotic factors; Acarina; clear-cutting; Collembola; Cryptostigmata; litter; microarthropods; oribatid miles; Prostigmata; soil mites; vertical distribution. INTRODUCTION Litter and soil microarthropods are considered regulators of forest litter decomposition and nutrient cycling processes (Crossley 1977). Microarthropods, particularly oribatid mites and collembolans, process a portion of the annual litter input, converting detritus and microbial biomass into smaller fragments and feces. Microarthropods therefore create additional surface area and modify substrates for continued microbial colonization and use (Webb 1977). The net result is the disappearance of litter, an increase in microbial activity and perhaps a more rapid return of certain nutrients to roots or mycorrhizal fungi via mineralization (Witkamp and Ausmus 1976). Microarthropod preferences for certain food resources and for specific moisture and temperature regimes have been identified for a number of species (e.g. Lebrun 1965, Mitchell 1978). However, factors that influence total microarthropod densities and vertical distributions in forests are poorly understood, and the literature contains conflicting reports. McBrayer et al. (1977) suggested that litter production determines microarthro- 1 Manuscript received 8 August 1979; revised 24 March 1980; accepted 15 April Present address: Biological Station, University of Montana, Bigfork, Montana USA. pod densities while vertical distributions are influenced by moisture gradients. However, Harding and Stuttard (1974) noted that microarthropod densities increase with latitude; hence, densities are correlated with standing crops of organic matter and not with litter production. Vannier (1970) presented evidence that normal variations in forest soil moisture do not affect microarthropod vertical distributions. Huhta et al. (1967) monitored microarthropod response to a number of silvicultural practices in Finland. They reported increased oribatid mite and collembolan densities following fertilization of a spruce forest. However, Kelly and Henderson (1978) reported declines in microarthropod densities in litter following fertilization of a hardwood forest in Tennessee, and Dindal et al. (1975) also reported a negative numerical response of litter and soil microarthropods to applications of sewage effluent in Pennsylvania forests. Huhta et al. (1967, 1969) and Huhta (1976) also studied the impact of clear-cutting on microarthropod densities. They reported temporary increases in densities of most microarthropod taxa followed by long-term declines. Huhta (1976) attributed this response to increased amounts of food resources represented by organic matter in soil after clear-cutting followed by subsequent declines in organic matter in clear-cut areas. No similar studies have been conducted in North America.

2 February 1981 MICROARTHROPOD RESPONSE TO CLEAR-CUTTING 127 I The present study reports changes in densities, vertical distribution and species composition of the microarthropod community in response to cable logging and clear-cutting in the southern Appalachians. In comparison to the common practice of tractor skidding of logs, cable logging partially lifts logs off the ground and resulted in less disturbance to the forest floor. Following the removal of merchantable timber, the remaining trees were cut and left on the watershed, along with branches and leaves of those boles removed. Density estimates reported here are compared with those of other studies to address questions concerning factors that determine densities of microarthropods in forest ecosystems. STUDY AREA The Coweeta Hydrologic Laboratory occupies an area of 2270 ha in southwestern North Carolina in the southern Appalachians. Since 1924 the United States Forest Service has controlled access to the area and allowed no activities except experimental treatments on various watersheds. The chestnut blight during the 1930's did, however, significantly alter the composition of canopy vegetation. Since 1968 the area has been the location of an intensive study of nutrient dynamics of southern forest ecosystems (Monk et al. 1977). The area experiences cool summers and relatively mild winters with a mean annual temperature of 12.8 C (Swank and Henderson 1976). Considerable rainfall occurs in all seasons. Average annual precipitation ranges from 170 cm at lower elevations to 250 cm on upper slopes (Johnson and Swank 1973). The watersheds used in this study have previously been subjected to little human disturbance, at least during this century. Watershed 7 (WS 7) is 59.5 ha and ranges in elevation from about 716 m at its base to about 1060 m at the top of its primarily south-facing slopes. During the 1940's the watershed was used in a woodland grazing experiment, and the six cattle which were kept on the area did little to alter the watershed (Oils 1957). An adjacent 14.3-ha watershed (WS 2) served as a control and was similar with respect to slope and history to WS 7. Elevations range from 705 m at its base to 975 m at its top. The vegetation of both watersheds was similar to that of a nearby forest (WS 18) described in detail by Day and Monk (1974). A cover hardwood association occupied lower elevations. Oakhickory forest dominated intermediate elevations and a pine-hardwood association occupied upper elevations and ridges. Precut and postcut vegetation on WS 7 has been described by Boring (1979). In general, rapid regrowth from sprout regeneration along with the appearance of opportunistic plant species such as black locust (Robinia pseudoacacia) and wild grape (Vitis spp.) resulted in the establishment of a shrub-like vegetation on WS 7 after the first summer (1977) following clearcutting. The canopy was 2-3 m high on WS 7 during the summer of The region of organic enrichment of Coweeta soils (the A! horizon) is very shallow, extending to only about 5 cm in depth (Yount 1975). Chandler loams, a coarse-loamy micaceous typic Dystrochrept, dominate the upper portions of both watersheds while Tusquitee stony loam, a humic Hapluoult, is found near streams and lower reaches of the watershed (D. Neary, personal communication). The regolith in the Coweeta area averages about 7 m in depth and consists primarily of the above-mentioned loams interspersed with larger rocks from the granitic-metamorphic bedrock (Johnson and Swank 1973). Small rock outcrops are common on the steeper slopes of both watersheds. METHODS The litter and soil environment Data on litter and soil moisture, temperature, and organic inputs and content of the clear-cut watershed were obtained from United States Forest Service personnel and from investigators who were concurrently studying other aspects of the perturbation. Litter and soil temperatures were obtained from a single recording station located in former pine-hardwood habitat on WS 7, and from biweekly, 24-h measurements taken at the litter-soil interface from several sites on the two watersheds by J. B. Waide and R. L. Todd (personal communication). Litter and soil moisture data were obtained biweekly from 0.25-m 2 samples of litter and soil cores. Woody litter inputs to the forest floor were measured by sampling permanent plots, and leaf-litter inputs were measured with 0.40-m 2 litterfall collectors (J. B. Waide and W. T. Swank, personal communication). Samples were taken at or adjacent to all of the microarthropod sampling sites described below. Microarthropod densities in litter and soil Sampling of microarthropod densities in the top 5 cm of litter and soil was initiated in June 1977, while clear-cutting was still in progress, and continued through December Clear-cutting was not completed on WS 7 until October Three sampling sites on WS 7 were located in clear-cut areas during summer, 1977, while the other two sites on this watershed were from areas where only merchantable timber was removed. After mid-august, all sites on WS 7 were on clear-cut areas. Samples were taken biweekly through December Beginning in January 1978 the number of sampling sites was expanded to seven sites on WS 7 and five sites on WS 2, and samples were taken monthly. A single litter and soil core, 4.8 cm in diameter by 5 cm in depth, was taken at random from each site per date within a plot of approximately 300 m 2. The purpose of the large plots was to insure that most local microhabitats were sampled over time. Samples that

3 128 T. R. SEASTEDT AND D. A. CROSSLEY, JR. Ecology, Vol. 62, No. 1 did not contain 5 cm of substrate were discarded; thus, rocky areas and tree stump areas were not sampled. No attempt was made at obtaining equal or known amounts of litter, humus and soil in each core, and these probably varied between sites and within sites over time. Samples were obtained from the two watersheds at approximately the same time of day to minimize possible watershed variations in diurnal vertical distributions of the microarthropods (e.g., McBrayer et al. 1977). Cores were wrapped in foil and placed on ice to immobilize microarthropods as well as to equilibrate litter and soil temperatures (Mitchell 1974). After about an hour, samples were placed in a high-gradient extractor. Details of extractor design and extraction techniques have been reported by Merchant and Crossley (1970) and Seastedt and Crossley (1978). Microarthropods reported in this study were those capable of passing through a 1.5 x 1.5 mm screen mesh. Almost all Acarina, Collembola, Protura, Symphyla and Pauropoda found on the study area were capable of passing through this mesh. Ticks and some adult trombidiids were not obtained. Only smaller species of immatures of spiders, pseudoscorpions, millipedes, centipedes and insects were collected with this procedure. Microarthropods were identified to suborder for mites (Acarina) and to order for other arthropods. Less abundant groups were combined for analysis. Spiders, pseudoscorpions and centipedes are referred to as the spider group. Protura, Symphyla, Pauropoda, Diplopoda and a number of orders of insects were grouped as miscellaneous arthropods. This last group was dominated by fungivores and detritivores, particularly proturans, symphylans and pauropods, and did include a few predaceous insect taxa. Densities below 5 cm Densities of microarthropods below 5 cm of litter and soil were measured at one site similar in slope and aspect on each watershed. Soil cores were obtained at approximately 3-mo intervals beginning February Samples were obtained by first removing the over-burden with a shovel, checking the depth with a metre stick, and then taking two, 5 cm deep samples. The process was repeated until five paired samples were obtained representing 5-cm increments, from 5 cm to 30 cm in depth. After inspection of February results, the sample design was changed to obtaining paired samples at 5-10, 25-30, and cm depth. As before, cores were placed in a high-gradient extractor to collect microarthropods. Litter bags Leaves from dogwood (Cornus floridd) and chestnut oak (Quercus prinus) were collected from trees on WS 7 and WS 2 during the peak of litter fall (late October and early November) These species were selected because of their abundance on the watersheds. Chestnut oak is a dominant canopy species in hardwood forests at Coweeta and composes about 20% of the total basal area, while dogwood composes about 3% (Day and Monk 1974). Dogwood became more dominant on WS 7 following clear-cutting and composed about 11% of the total basal area (Boring 1979). Senescent leaves of these species were allowed to air dry and were placed in preweighed, nylon mesh bags (=3-mm mesh). About 3 g of leaves were placed in each bag. Approximately 50 bags were placed on each watershed on sites with similar slope, elevation and aspect. Four bags per watershed were harvested at approximately 3-wk intervals beginning in early February, 53 d after placing the litter in the field. These samples were placed in refrigerated Tullgren funnels for 7 d to extract microarthropods. Litter was reweighed and microarthropods were counted, identified and grouped into classifications used for taxa obtained from soil cores. Statistical procedures Microarthropod numbers obtained in small samples of litter and soil are highly variable, and large sample sizes are generally required to obtain any reasonable amount of precision (Macfadyen 1962). Data obtained in this study were therefore pooled into seasonal estimates for summer (July through September), autumn (October through December), winter (January through March) and spring (April through June). These intervals, though arbitrary, approximate the four seasons at Coweeta. Populations of microarthropod species and groups usually exhibit contagious distributions (Crossley and Bohnsack 1960, Hartenstein 1961, Berthet and Gerard 1965). A natural log transformation was therefore applied to normalize the data (Gerard and Berthet 1966). Percent data were transformed with an arcsine function prior to using analysis of variance (ANOVA) techniques. RESULTS Densities and composition prior to clear-cutting No data on microarthropod populations were available prior to June 1977 to assess whether populations on WS 7 and WS 2 were similar prior to clear-cutting on WS 7. However, samples taken on WS 7 in partially cut areas were compared with samples from WS 2 to obtain some measurement of initial similarity (Table 1). No significant differences (P >.05) were observed between watersheds on non-clear-cut areas; hence, the assumption is made that the populations on the two watersheds were similar prior to clear-cutting. No statistical differences in percentage composition of major taxa of arthropods were found on the two watersheds. Cryptostigmata (Oribatida) composed over 60% of total microarthropods; Prostigmata (Actinedi-

4 February 1981 MICROARTHROPOD RESPONSE TO CLEAR-CUTTING 129 TABLE 1. Microarthropod densities in 0-5 cm of litter and soil on portions of WS 7 partially cut and on WS 2 during June through August Arthropod group Acarina Cryptostigmata Prostigmata Mesostigmata Collembola Total* 10 3 individuals per square metre (95% confidence limits) WS7 n = ( ) 63.9 ( ) 7.5 ( ) 6.4 ( ) 17.1 ( ) ( ) WS 2 n = ( ) 54.1 ( ) 7.3 ( ) 5.2 ( ) 12.8 ( ) 92.8 ( ) * Includes other arthropod taxa not listed separately. ANOVA F-value da) and Collembola composed about 8 and 15%, respectively. Acarina and Collembola together accounted for about 95% of all microarthropods on the two watersheds. Changes in the litter and soil environment following clear-cutting The removal of the canopy and subsequent reduction in plant transpiration markedly altered the temperature and moisture regime of the clear-cut watershed. Litter moisture was reduced on WS 7 during the summers of 1977 and 1978, while moisture content of the soil below 10 cm was concurrently enhanced (J. B. Waide and R. L. Todd, personal communication). The lowest soil moisture values recorded for any season on the two watersheds were 22.8 and 29.0% of dry mass for WS 2 and WS 7, respectively. Daily maximum summer temperatures in 1978 at the litter-soil interface averaged 41.7 C on WS 7 vs. only 26.1 on WS 2 (J. B. Waide and R. L. Todd, personal communication). A maximum temperature of 49 was recorded on several dates at the litter-soil interface on WS 7 by the Forest Service temperature recorder. The standing crops of litter and organic matter in the soil increased on WS 7 following clear-cutting (Table 2). Leaf litter inputs in 1977 and 1978 declined as a result of cutting (Boring 1979); however, the organic matter in litter and soil greatly exceeded either precut amounts or those found on WS 2. Roots were not sieved from soil prior to measuring organic content; thus, organic enrichment was attributed primarily to small fragments of bark and wood deposited by the clear-cutting and site preparation activities. These increases in organic matter stimulated microbial activity as measured by nitrogen fixation measurements, but no changes were noted in ATP amounts or CO 2 evolution (R. L. Todd and J. B. Waide, personal communication). Response of microarthropods in litter and humus to clear-cutting Densities of microarthropods on the clear-cut watershed (means of three sites during summer, 1977, and all sites on the watershed after August) were generally lower than those found in the top 5 cm of litter and soil on WS 2 (Fig. 1, Table 3). Differences between watersheds appeared to diminish with time; however, an analysis of variance of only 1978 data indicated that total microarthropod densities on WS 7 remained lower than those on WS 2 (F 1>130 = 5.34, P <.05). Cryptostigmata dominated the fauna and the reduction of this taxon on WS 7 was largely responsible for the overall reduction of microarthropod densities (Fig. 1). Two fairly large groups of microarthropods showed little or no long-term response to the perturbation. Prostigmata (Actinedida) showed no significant differences in densities after 1977 (Fig. 1). Collembola densities showed no response to clear-cutting; any real differences that may have occurred were obscured by the high variability in sample densities (Fig. 1). The decline in densities of specific microarthropod taxa resulted in significant changes in the percent com- TABLE 2. Estimates of litter and organic materials on WS 7 before and after clear-cutting and on WS 2. Material Woody litter 01 litter 02 litter Soil organic matter (0-10 cm) Soil organic matter (10-30 cm) Pre-cut 4600 ± ± ± ± ± 130 WS7 Litter and organic matter (g/m 2 ± SE)* Post-cut ± ± ± ± ± 260 WS 2 no data 470 ± ± ± ± 560 J. B. Waide and R. L. Todd, personal communication; n > 30 for all samples except woody litter where n = 18.

5 T. R. SEASTEDT AND D. A. CROSSLEY, JR. Ecology, Vol. 62, No. 1 TOTAL MICROARTHROPODS CRYPTOSTIGMATA I II» TT Ml fe 40 u > PROSTIGMATA tf COLLEMBOLA FIG. 1. Mean densities (with 95% confidence limits) of total microarthropods, Cryptostigmata (oribatids), Prostigmata and Collembola found in the top 5 cm of litter and soil on clear-cut watershed, WS 7, (solid circles) and control, WS 2, (open circles) during the last half of 1977 and Sample size = 14, 24, 26, 21, 21, and 21 for the six intervals on WS 7 and 11, 17,21, 14, 15 and 15 on WS 2. position of the fauna. Oribatids composed 57.2% of showed only a short-term decline in densities on WS total microarthropods on WS 7 but averaged 64.6% on 7, and Collembola, which showed no response to WS 2 (ANOVA of arcsine transformations of arith- clear-cutting, increased in dominance on WS 7 (16.9 metic percentages, P <.001). The Prostigmata, which and 16.2%, respectively, on WS 7, vs and 1 1.6% TABLE 3. Comparisons of mean microarthropod densities on 0-5 cm of litter and soil, July 1977-December Total microarthropods Acarina (total) Cryptostigmata Prostigmata Mesostigmata Collembola Spider group Miscellaneous arthropods P <.05; ** P <.01; *** P < individuals/m 2 (95% confidence limits) WS7 (n = 127) 98.9 ( ) 78.3 ( ) 53.8 ( ) 12.9 ( ) 5.1 ( ) 13.7 ( ) 0.1 ( ) 0.3 ( ) WS2 (n = 93) ( ) ( ) 82.6 ( ) 15.1 ( ) 7.7 ( ) 15.0 ( ) 0.1 ( ) 0.8 ( ) ANOVA Watershed (W) Idf 26.35*** 35.20*** 37.25*** 10.26** 29.12*** *** F-value (Type Time (T) 5df 8.35*** 9.50*** 7.26*** 23.95** 2.27* 3.41** IV SS) Interaction (W xt) 5df 3.23** 4.05** 2.75* 3.66** 3.54**

6 February 1981 MICROARTHROPOD RESPONSE TO CLEAR-CUTTING TOTAL MICROARTHROPODS CRYPTOSTIGMATA 28 : i 12- < cc IS D Z o PROSTIGMATA COLLEMBOLA o summer 1978 autumn winter spring summer autumn 1978 FIG. 2. Mean densities (with 95% confidence limits) of total microarthropods, Cryptostigmata (oribatids), Prostigmata and Collembola obtained from litter bags on clear-cut watershed, WS 7, (solid circles) and control, WS 2, (open circles). Sample size = 12 litter bags (6 dogwood, 6 chestnut oak) per watershed per interval. on WS 2; P <.001 for both). Within the major groups, trends were noted for increased dominance of smaller soil forms. For example, Oppia minus replaced Oppiella nova as the dominant oribatid mite on WS 7. Small tydeids replaced eupodids as dominant prostigmatids, and Rhodacarus sp. replaced species of Ologamasidae and Parasitidae as dominant mesostigmatid (gamasid) mites (Seastedt 1979, Abbott et al. in press). Microarthropod densities in litter bags Microarthropod densities in litter bags were reduced on the clear-cut watershed during much of 1978 (Fig j; M CONTROL (WS2) / thousands/m 2 l_ [2.4 r l 1 pf^ 4,1 CLEARCUT (WS7) // ~ i FIG. 3. Vertical distributions of microarthropod densities recorded for the clear-cut watershed, WS 7, and control, WS 2. Dashed lines indicate no data for soil increment. 2). All taxa showed a similar response; winter, spring and summer densities were lower on WS 7 than WS 2, and no watershed differences were observed in autumn. Mean annual oribatid mite densities in litter were not significantly different on the two watersheds (.1 > P >.05; Table 4), while annual means for Prostigmata and Collembola were significantly reduced in litter (P <.001). Thus, in spite of increased amounts of litter (Table 2), densities of microarthropods per gram of leaf litter material were, on an annual basis, reduced >50% on WS 7. Litter and soil core samples exhibited only a 25% reduction on the clear-cut watershed. Hence, surface microarthropods appeared most affected by clear-cutting. Microarthropods in soil below 5 cm depth A preliminary analysis of variance of samples below 5 cm in depth showed no effects of time on total densities (F 3i34 = 1.1, P >.3). Samples from the four dates (February, June, September and December 1978) were therefore pooled to increase sample size for subsequent analyses. Analysis of the pooled data revealed significant differences in densities and composition of the fauna on the two watersheds (Table 5). Soil microarthropod densities were greater on the clear-cut watershed, and this increase was attributed to prostigmatid mites. Oribatid mites were most abundant; however, no significant differences were ob-

7 132 T. R. SEASTEDT AND D. A. CROSSLEY, JR. TABLE 4. Mean annual microarthropod densities in litterbags, Ecology, Vol. 62, No. 1 Arthropod taxa * WS7 («= 46) Acarina (total) 5.3 ( ) Cryptostigmata 3.9 ( ) Prostigmata 1.8 ( ) Mesostigmata 0.7 ( ) Collembola 1.1 ( ) Miscellaneous arthropods 0.3 ( ) Spider group 0.0 ( ) Total arthropods 8.4 ( ) : P <.05; ** P <.01;*** P <.001. Individuals/g litter WS2 (n = 46) 14.2 ( ) 6.6 ( ) 5.0 ( ) 2.3 ( ) 3.2 ( ) 0.3 ( ) 0.1 ( ) 20.4 ( ) Watershed (W) Idf 15.87*** *** 15.38*** 11.67*** * 11.44** ANOVA F- value (Type IV SS) Time (T) 3df 13.19*** 12.90*** 3.98* 13.44*** 3.46** 4.37** *** Interaction (W x T) 3df 2.93* 2.82* * served between watersheds (.10 > P >.05). More microarthropods were found at deeper levels on the clear-cut watershed (Fig. 3). Only about 60% of the total microarthropod population was found in the top 10 cm of the clear-cut watershed, while about 90% of the total population on WS 2 was found at this level. Members of the family Tydeidae were the most abundant prostigmatid mites in deeper soil. These mites included species of Coccotydaeolus, Microtydeus or immatures of these and other genera. Unlike oribatids, these mites are liquid feeders, and may be feeding on bacteria or fungal hyphae contents, or be predaceous on nematodes and nematode eggs (Santos and Whitford 1981). DISCUSSION A major conceptual error in interpreting microarthropod response to a given perturbation is to assume that density changes are equivalent to changes in microarthropod-resource relationships. Microarthropod metabolic activity and behaviors are doubtless modified by changes in physical parameters such as soil temperature and moisture, regardless of density changes (e.g., Mitchell and Parkinson 1976). Clearcutting resulted in a shift towards smaller forms of microarthropods, and a slight shift in dominance of the microarthropod taxa. Changes in metabolic activity, behavior, biomass and trophic status of the microarthropod population likely occurred as a result of clear-cutting, and these changes may be of more importance from a functional standpoint than the density and community composition data presented here. The study remains useful, however, in addressing questions concerning factors that affect numerical abundance of microarthropods. Clear-cutting by methods that do not directly destroy the integrity of the litter-soil environment engenders two kinds of perturbations: (1) removal of the canopy, and (2) addition of organic matter inputs, primarily woody litter, to the forest floor. In southeastern United States forests, canopy removal results in both moisture and temperature modifications of the litter and soil environment. Data from Vannier (1970) suggest that the soil moisture values observed in this study would not adversely affect the microarthropod TABLE 5. Microarthropod densities in soil at 5-55 cm depth. Arthropod taxa Acarina (total) Cryptostigmata Prostigmata Collembola Total arthropodsf 10 3 individuals per square metre (95% confidence limits) WS7 (n = 28) 72.5 ( ) 30.5 ( ) 27.7 ( ) 4.6 ( ) 89.8 ( ) WS 2 (n = 23) 32.0 ( ) 18.1 ( ) 11.4 ( ) 2.8 ( ) 43.7 ( ) ANOVA F- value 21.19*** 3.22 (P =.08) 13.57** *** t Includes other arthropod taxa not listed separately. ** P <.01; *** P <.001.

8 February 1981 MICROARTHROPOD RESPONSE TO CLEAR-CUTTING 133 fauna. Temperature changes appear more important. Average summer maximum temperatures of >40 C measured at the litter-soil interface in 1978 on WS 7 approached or exceeded lethal temperatures reported for oribatid mites (Madge 1965). Temperatures may also have affected the reproductive success of this taxon. Exposure to temperature no higher than 35 for several hours is sufficient to kill gametes of oribatid mites (Woodring and Cook 1962). Soil temperatures on the clear-cut watershed below 5 cm of litter and soil did not have an adverse affect on microarthropod densities. The reduction of microarthropod densities was limited to the surface litter bag samples, and to a lesser extent, to samples obtained from the upper 5 cm of litter and soil. Densities of microarthropods increased below the 5 cm depth, and this numerical response was similar to that observed for microarthropod densities in the litter and humus of northern European and Scandinavian forests following clear-cutting (e.g. Huhta 1976). The amount of organic matter and certain indices of microbial activity were enhanced in all litter and soil horizons following clear-cutting on WS 7 (R. L. Todd and J. B. Waide, personal communication), and were likely the causal factors for increased microarthropod densities in deeper soil horizons. A similar density response might also have occurred in upper soil and litter horizons had temperatures not adversely affected the fauna. We predict that clear-cutting by cable logging methods in more northern forests of North America, like those of the northern European and Scandinavian studies, will not cause a short-term negative numerical response of microarthropods as observed in this study. Microarthropod densities show a positive response to increased organic matter and/or increased microbial activity provided that the physiological tolerance limits of the microarthropod fauna are not exceeded. The numerical response observed in this study was correlated with increased amounts of organic matter rather than increased productivity of the microflora. Lussenhop (1976) demonstrated that higher microarthropod densities occurred in prairie areas that were more productive but had less organic matter than adjacent areas. Assuming equal biomass and turnover rates for the microarthropod populations in Lussenhop's studies, then litter inputs and microbial production did determine microarthropod production and, in that particular case, microarthropod densities. That study may have been the exception, however. Comparing productivity (amount per area per time interval) with densities (numbers or biomass per area) requires clarification when turnover time of the microarthropod population varies between sites. Microarthropods in forest soils tend to increase in density with latitude (Harding and Stuttard 1974). Given the latitudinal gradients for litter production and decomposition (Reiners 1973, Meentemeyer 1978), microarthropod densities appear inversely correlated with production and decomposition rates and directly correlated with amounts of undecomposed organic materials found on the forest floor. In tropical forests, where decomposition is very rapid, the standing crop of dead organic matter and microflora is small and supports relatively small microarthropod densities of x 10 3 individuals/m 2 (Madge 1969, Block 1970). In temperate systems the standing crop of organic matter is greater and supports higher densities of x 10 3 individuals/m 2 (Harding 1969, Price 1973, McBrayer et al. 1977). Boreal systems are characterized by high standing crops of dead organic matter and high microarthropod densities of x 10 3 individuals/m 2 (Huhta and Koskenniemi 1975, Mitchell 1977). Turnover of the organic matter in boreal systems is slow, and microarthropod feeding rates are limited in part by temperature constraints (Mitchell and Parkinson 1976). Within-site variations in microarthropod densities are also often correlated with standing crops of organic matter (e.g., Mitchell 1978, Santos et al. 1978). Thus, while litter and microflora productivity may determine microarthropod productivity, densities appear largely determined by the standing crop of organic matter. Densities observed at various depths in the soil profile also appear correlated with the amount of organic matter. McGinty (1976) reported an exponential decline in root biomass with depth for a nearby forest at Coweeta (WS 18), and this exponential decline is illustrated in the distribution of microarthropod densities observed on the control watershed (Fig. 3). Temperature and moisture regimes of southeastern hardwood forests are generally moderate. Data from Madge (1965), Lebrun (1965) and Vannier (1970) indicate that these parameters, as estimated in the present study, would not adversely affect oribatid mite distributions in soil, and this conclusion is probably valid for the microarthropod fauna as a whole. This does not preclude the existence of diurnal or seasonal vertical distributions of microarthropods in litter and soil; however, these are likely determined by resource availability (Mitchell 1978) or other factors affecting survivorship and reproduction. The amount and vertical distribution of organic matter in soils may therefore be the best predictor of microarthropod densities in Southeastern forests as well as in other ecosystems. Clearly, this is not true for perturbed or more harsh systems such as WS 7 where environmental factors in upper soil and litter approach or exceed the tolerance limits of microarthropods. ACKNOWLEDGMENTS We are grateful for the comments and suggestions of Drs. Wayne T. Swank, Preston E. Hunter, Robert L. Todd, David T. Abbott, and two anonymous reviewers. Drs. Robert L. Todd, J. B. Waide, Walter G. Whitford and P. F. Santos provided access to unpublished data, and climatic data were provided by the United States Forest Service, Coweeta Hydrologic Laboratory, Krista Gridley, Lee Reynolds and

9 134 T. R. SEASTEDT AND D. A. CROSSLEY, JR. Ecology, Vol. 62, No. 1 Cathy M. Tate provided laboratory and field assistance. Funding was provided by National Science Foundation Grant #DEB A01 to the University of Georgia. LITERATURE CITED Abbott, D. T., T. R. Seastedt, and D. A. Crossley, Jr., in press. The abundance, distribution and effects of clearcutting on oribatid mites (Acari: Cryptostigmata) in the southern Appalachians. Environmental Entomology. Berthet, P., and G. Gerard A statistical study of microdistribution of Oribatei (Acari). I. The distribution pattern. Oikos 16: Block, W Microarthropods in some Uganda soils. Pages in J. Phillipson, editor. Methods of study in soil ecology. UNESCO, Paris, France. Boring, L. R The role of successional vegetation in nutrient conservation on a clearcut Appalachian watershed. Thesis, University of Georgia, Athens, Georgia, USA. 64 p. Crossley, D. 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