Journal of Applied Ecology - University of Georgia

Journal of Applied Ecology (1988), 25, 683-698

LITTER DECOMPOSITION, NITROGEN DYNAMICS

AND LITTER MICROARTHROPODS IN A SOUTHERN

APPALACHIAN HARDWOOD FOREST 8 YEARS

FOLLOWING CLEARCUTTING

BY JOHN M. BLAIR* AND D. A. CROSSLEY, JR.'

Department of Entomology, Institute of Ecology, University of Georgia, Athens,Georgia 30602, U.S.A.

SUMMARY

(1) Litter decomposition rates, nitrogen dynamics and litter microarthropods on thexeric slopes of a watershed 8 years after clearcutting (WS 7) and on an adjacent referencewatershed (WS 2) at the Coweeta Hydrologic Laboratory were measured using litterbags.Litter of Comusflorida L., Acer rubrum L. and Quercusprinus L. was used as experimentalsubstrates in three plots on adjacent areas of both watersheds. Results from this study werecompared with those obtained before and after cutting to assess the longer-term changesinduced by canopy removal.

(2) Reduced litter decomposition rates were associated with clearcutting. Litter nitrogendynamics were also affected. Net immobilization of nitrogen in litter substrates was loweron WS 7. Lower net nitrogen immobilization was related to slower increases in nitrogenconcentration per unit mass lost in litter on WS 7.

(3) Mean annual densities of total litter microarthropods remained 28% lower on WS 7than on WS 2, 8 years after cutting. Previous studies indicated clearcutting initiallyreduced mean annual densities of litter microarthropods by>50%. Mesostigmata andOribatei densities averaged 50 and 54% lower, respectively, than on WS 2. Prostigmataand Collembola densities averaged 20 and 24% lower, respectively, than on WS 2. Thisdifferential response changed the relative abundances of major groups. Changes in litterdecomposition rates and nitrogen dynamics were consistent with effects associated withlower microarthropod densities and suggest that reduced microarthropod densities maybe important.

(4) Results of this study differ from those at northern hardwood forest sites whereclearcutting caused increased decomposition rates. This suggests that generalizations fromnorthern hardwood forests may not apply to other regions. Instead the effects of canopy Iremoval depend on both the nature of pre-disturbance processes and on the 'site-specific' !effects of disturbance on the processes studied. j

INTRODUCTION \r

Decomposition of litter and mineralization of nutrients contained in it strongly influence \soil nutrient availability and ultimately ecosystem primary productivity. Disturbanceswhich alter decomposition and other soil processes can allow nutrient losses and a decline |in site productivity (Bormann et al. 1974; Swank & Waide 1980). One commondisturbance of eastern deciduous forests in the U.S. is clearcutting following timberharvesting. Several studies have examined the effects of forest vegetation removal on |hydrologic flows, nutrient fluxes, primary production, litter decomposition and soil 'organic matter turnover. Some have documented increased losses of nutrients in ;

* Present address: Department of Entomology, Ohio State University, 1735 Neil Avenue, Columbus, Ohio43210, U.S.A.

683

684 Decomposition following clear cutting

streamflow following clearcutting of northern hardwood forests (i.e. Pierce et al. 1972;Bormann & Likens 1981). Increased nutrient output after clearcutting was partlyattributable to a large input of litter along with increased decomposition andmineralization rates due to a more favourable microclimate for decomposers (Stone 1973;Bormann et al. 1974). Longer-term (i.e. > 15 years) decreases in surface organic matterfollowing clearcutting of northern hardwood forests may also result from increaseddecomposition rates (Covington 1981; Federer 1984; Hix & Barnes 1984). Based on theseobservations, increased decomposition rates and decreases in forest floor mass have beenincorporated in models of forest floor dynamics following disturbance (Aber, Botkin &Melillo 1978).

Many studies of the effects of forest cutting are based on a chronosequence approach,using measurements of forest floor mass at sites in different stages of recovery followingclearcutting (Covington 1981; Federer 1984). Fewer studies have followed changes in soilsurface organic matter through time at a single site (but see Johnson, Smith & Burger1985), and fewer still have measured changes in litter decomposition rates using litterbagsor a comparable method. The experimental clearcutting of a forested watershed(catchment) at the Coweeta Hydrologic Laboratory in the southern Appalachians gavethe opportunity to measure changes in forest floor processes through time at a single site.

Other studies of clearcut forests have indicated that generalizations from northernhardwood forests may not apply to forests in other regions (Whitford et al. 1981; Will,Hodgekiss & Madgwick 1983; Binkley 1984; Mattson 1986). Disturbances of coniferousforests in the north-western U.S. produced only modest hydrologic exports of nutrients(Fredriksen 1975, cited in Swank & Waide 1980) while clearcutting at Coweeta producedthe lowest nutrient losses via streamflow recorded in the U.S. (Swank & Waide 1980;Swank 1986). Factors likely to reduce nutrient losses in the southern Appalachiansinclude rapid recovery of net primary productivity (Boring, Monk & Swank 1981) andreduced decomposition rates (Seastedt 1979). Cable-logging and clearcutting of awatershed (WS 7) at Coweeta produced immediate decreases in rates of leaf litterdecomposition (Seastedt 1979) and rates of woody litter decomposition on xeric sites(Abbott & Crossley 1982), while surface organic matter has subsequently increased(Mattson 1986). Densities of litter microarthropods also decreased after clearcutting(Abbott, Seastedt & Crossley 1980; Seastedt & Crossley 1981). Microarthropodsinfluence decomposition rates and forest floor nutrient fluxes (Anderson & Ineson 1984;Seastedt 1984), and reductions in their densities could affect decomposition processesfollowing clearcutting.

The objectives of this study were (i) to measure decomposition rates, nitrogen dynamicsand microarthropod densities in three litter substrates on the xeric slopes of a watershed inits eighth year of regrowth following clearcutting and on an adjacent uncut referencewatershed; and (ii) to compare these results with those collected before and immediatelyafter clearcutting to assess its longer-term effects on litter decomposition and littermicroarthropods.

MATERIALS AND METHODS

Site description

The U.S. Forest Service Coweeta Hydrologic Laboratory, located in the southernAppalachian Mountains of south-western North Carolina (35°00' latitude, 83°30'longitude), is a 2185-ha forested basin consisting of numerous small watersheds that serveas experimental units. The native hardwood forest of the area is a mixture of deciduous

J. M. BLAIR AND D. A. CROSSLEY JR 685

TABLE 1. Initial N concentrations, C:N ratios, lignin concentrations, lignin :Nratios, and water- and ethanol-soluble concentrations of Cornus florida, Acer

rubrum and Quercus prinus litter used in this study*

% Water % EthanolSpecies % N C:N % Lignin Lignin:N soluble soluble

C. florida 0-75 62-9 4-6 6-1 35-1 6-7A.rubrum 0-56 85-7 9-6 17-3 47-8 11-1Q. prinus 0-87 55-7 23-6 27-2 25-2 1-7

* For details on methods of analysis see Blair (1988).

trees dominated by Quercus, Gary a and Acer spp. Mean annual rainfall in the basin variesfrom 1700 mm at lower elevations to 2500 mm on some ridges. Rainfall is evenlydistributed throughout the year. Mean annual temperature is 13 °C.

The two watersheds studied are in the north-eastern portion of the Coweeta basin.Watershed 7 (WS 7) is a 59-ha, south-facing catchment with elevations of 700-1000 m andslopes of 20-80%. Prior to 1977, WS 7 was covered by uneven-aged Quercus-Caryaforest. Watershed 7 was divided into three areas based on previous vegetational studiesand site preparation (Boring & Swank 1986). Pre-cut vegetation consisted of a mesichardwood cove association dominated by Liriodendron tulipifera, Tsuga canadensis andAcer rubrum (approximately 10% of the area), mesic slopes dominated by Carya andQuercus spp. (approximately 36% of the area) and more xeric slopes and upper ridgesdominated by Q. coccinea, Pinus rigida and Kalmia latifolia (approximately 54% of thearea) (Boring, Monk & Swank 1981). In the summer of 1977, merchantable timber was ]cut and removed by cable-logging to minimize forest floor disturbance and compaction. iFollowing removal of merchantable boles, all remaining trees were felled and the slash left Iin place. There was rapid regrowth from stump and root sprouts along with earlysuccessional herbaceous species (Boring, Monk & Swank 1981; Boring & Swank 1986).

Watershed 2 (WS 2) is an adjacent 14-3-ha catchment, with similar slope, elevation, l

aspect and climate, which was used as an unmanipulated reference. The vegetation wasdominated by uneven-aged Quercus, Carya and Acer spp. (Berish & Ragsdale 1985).

Decomposition dynamics and litter microarthropods i

Litter decomposition rates, nitrogen dynamics and litter microarthropod densities 8 Iyears after clearcutting (1985) were measured using litterbags. Litterbags with an inside Iarea of 10 x 10 cm were made of fibreglass window-screen material with a 1-6 x 1-8 mm !mesh. Senesced leaves of Cornus florida L. (flowering dogwood), Acer rubrum L. (red jmaple) and Quercus prinus L. (chestnut oak) were collected in October 1984 and air-dried !at 22 °C. These species were abundant on both watersheds, comprising about 45 % of leaf Ilitter inputs in. 1985-86 (Risley 1987), and were chosen to give a range of resource quality i(Table 1). Newly senesced litter was collected from several sites at similar elevations in theCoweeta basin, pooled by species and mixed prior to filling litterbags to ensure thathomogenous litter substrates were used on both watersheds. Approximately 2-5 g of air-dried leaves were placed in pre-weighed bags. Five bags of each species were oven-dried at50 °C to develop equations relating air-dry to oven-dry mass. On 4 January 1985 fifty-twolitterbags of each species were placed in each of three plots (1-5 x 1-5 m) arranged in atransect and spaced 100-150 m apart on adjacent areas of both watersheds. All six plotswere located on similar elevations, slopes and aspects on both watersheds. Plots on WS 7

686 Decomposition following clearcutting\

were located on the xeric sites formerly occupied by the Quercus-Pinus-Kalmia Icommunity because it was the predominant type and because we expected the effects of |clearcutting to be longer lasting there. Five bags of each species were immediately !retrieved to determine mass loss due to handling. These values were used to adjust initialmasses of litterbags before determining subsequent mass loss.

Litterbags were retrieved randomly from within each plot at 2-week intervals fromJanuary to November and in mid-December 1985, and every 2 months in 1986 for a totalof twenty-nine collection dates over 2 years. The last collection date was 6 January 1987.Litterbags were weighed before processing to determine percentage moisture. Maximumand minimum temperatures at the soil-litter interface between collection dates wereobtained using max-min thermometers with remote probes at one plot on eachwatershed. Microarthropods were extracted from litterbags using Tullgren funnels andseparated into five broad taxonomic groups: Prostigmata, Mesostigmata, Oribatei,Collembola and 'others'. Densities were expressed as numbers of individuals g~' of litterremaining. Seasonal densities were calculated by grouping collection dates during the firstyear as follows: winter (January-March), spring (April-June), summer (July-26September) and autumn (27 September-December). Litter moisture and temperaturedata were also pooled to calculate seasonal averages.

Following microarthropod extraction, intact litterbags were oven-dried at 50 °C, thelitter reweighed and ground, and subsamples ignited at 500 °C for 4 h to determinepercentage ash-free dry mass (% AFDM). Any obvious extraneous material was removedbefore weighing. However, after 6 months in the field, soil contamination was evident insome litterbags on each collection date. Therefore, litter masses were corrected for soilinfiltration before determining % mass loss using the following soil correction equation(Walther 1983):

FLi = (SaAFDM-SlAFDM)/(LiAFDM-SlAFDM)

where FLi is the fraction of litterbag content that is actually litter; SaAFDM is the %AFDM of the entire litterbag sample; S1AFDM is the average % AFDM of the soil at thesite; Li AFDM is the initial % AFDM of the litter substrate. The underlying assumption isthat organic matter content (% AFDM) of the litter and the soil remain constantthroughout the study. The equation calculates the fraction of litterbag content that isactually litter based on any reduction in % AFDM of the sample caused by soilcontamination. The equation is preferable to simply calculating litter mass loss as % .AFDM remaining when soil moving into the litterbags has a high organic matter content,since organic matter from the soil would contribute to the apparent. AFDM of the litter.First-year decay rate constants were calculated from percentage mass remaining data, H

using a single negative exponential decay model X/X0=e~kt, where X/Xo=percentagemass remaining at time t, *=time elapsed in years, and fc=the annual decay constant(Olson 1963). The single negative exponential model was fitted to the data by least squaresregression of the natural logarithm of mean percentage mass remaining over time.

Nitrogen concentrations in the residual litter were measured monthly in 1985 andbimonthly in 1986. Nitrogen concentrations were determined with a Technicon Model IIAutoanalyzer following micro-Kjeldahl digestion (Technicon 1970). Nitrogen concentra-tions for litterbags contaminated with soil (as indicated by reductions in % AFDM of thesample) were corrected using the following equation:

LiN = [SaN - (FS1 x SIN)] ~ FLi


TABLE 2. A summary of first-year decomposition rates on pre-cut WS 7 and otherunmanipulated watersheds in the Coweeta basin

k (% remaining, t= 1 year)*Watershed Comusflorida A. rubnan Quercus prinus

WS 181969-70 -1-18(30-8) -0-83(43-6) -0-55(57-8)WS 181970-71 -1-35(26-0) -0-72(48-6) -0-66(51-4)WS2 1975-76 -0-71 (47-8) -0-48 (57-9) -0-29 (72-2)WS7 1974-75 -1-31(27-8) -0-53(49-0) -0-34(69-3)

* Rates are expressed as annual decay constants (k) derivedfrom a single negative exponential model (Olson 1963). Percentmass remaining at the end of one year is indicated in paren-theses. WS 18 data are from Cromack (1973). WS 2 and WS 7data are from J. B. Waide, R. L. Todd, J. R. Webster, and D. A.Crossley, Jr, unpublished.

where LiN is the nutrient concentration in the residual litter; SaN is the nutrientconcentration of the entire sample; SIN is the average nutrient concentration in the soil;FS1 is the fraction of the sample that is soil (1 — FLi from the above soil correctionequation). Net nitrogen immobilization or release was calculated as the product ofpercentage mass remaining and nitrogen concentration in the residual material divided bythe initial nitrogen concentration of that litter type.

Pre-cut and first-year post-cut decomposition rates and litter microarthropod densitieswere based on similar litterbag studies. For further details on methods see Seastedt (1979)and Seastedt & Crossley (1981).

Statistical comparisons of data from clearcut and reference plots are presented, whereappropriate, to indicate significant differences. However, because only one watershed wasclearcut, reliance on inferential statistics to imply treatment effects at the watershed levelwould be inappropriate (Hurlbert 1984). Instead, comparisons of pre-cut and post-cutdifferences between the clearcut and reference watersheds and long-term changes inrelevant parameters are used to assess the effects of clearcutting.

RESULTS

Decomposition rates

Litter decomposition rates on WS 2 and WS 7 were believed to be similar before theclearcutting of WS 7 in 1977 (J. B. Waide, R. L. Todd, J. R. Webster & D. A. Crossley Jr,unpublished data). Decomposition rates of litter of selected species on pre-cut WS 7 weresimilar to rates obtained on other unmanipulated watersheds (WS 2 and WS 18) in theCoweeta basin (Table 2). Decomposition rates on WS 18, a north-facing, more mesicwatershed were expected to be somewhat higher than rates on watersheds 2 and 7.Measurements of decay rates on WS 2 and WS 7 were not taken in the same year beforeclearcutting. The available data do indicate, however, that litter decayed at least as fast onWS 7 as on WS 2 before the clearcut. Decay rates measured in successive years withsimilar climates were somewhat higher on WS 7 (Table 2). Similar litter inputs andstanding stocks of surface organic matter on comparable areas of WS 2 and WS 7 beforeclearcutting indicate similar decomposition rates. Given similar litter inputs anddecomposition rates, standing mass of surface organic matter should be similar (Olson1963). Pre-cut measurements of surface organic matter (L and F layers) were 816 g m~2 on


TABLE 3. A comparison of annual decay rates (k) of Cornusflorida, Acer rubrum andQuercus prinus litter on clearcut WS 7 and uncut WS 2 (1985 data)*

Clearcut UncutWS 7 WS 2

Species Slope r2 Slope r2

C.florida -0-705 0-951 -0-850 0-973 P<0-0025A.rubrum -0-548 0-952 -0-638 0-954 P<0-025Q.prinus -0-185 0-938 -0-274 0-843 P<0-01

* Slopes (k) were calculated by fitting the data to asingle negative exponential model (Olson 1963). Coeffi-cients of'determination (r2) indicating goodness-of-fit arepresented. Probability values are based on a one-tailed t-test of the hypothesis that rates obtained on WS 7 plotsare slower than those obtained on WS 2 plots (Zar 1984).

TABLE 4. Comparisons of litter decomposition rates on WS 2 and WS 7 prior toclearcutting and in the first and eighth years following cutting of WS 7*

SpeciesPre-cut

WS 7 WS 2Post-cut (1 year)

WS 7 WS 2Post-cut (8 years)

WS 7 WS 2Cornusflorida -1-31 (27-8) -0-711 (47-8) -0-634 (55-6) -1-17 (34-3) -0-705 (51-3) -0-850 (45-8)Acer rubrum -0-529 (49-0) -0-477 (57-9) -0-374 (70-5) -0-618 (53-7) -0-548 (62-4) -0-638 (52-9)Quercus prinus -0-336 (69-3) -0-285 (72-2) -0-290 (76-7) -0-431 (65-2) -0-185 (78-1) -0-274 (73-1)

* Decompositions rates are expressed as first year annual decay rates (k) derived from a negative exponentialmodel (Olson 1963). Percentage mass remaining at the end of 1 year is indicated in parentheses. Pre-cut and firstyear post-cut rates are composite values based on collection of litterbags from several sites on both watersheds(J. B. Waide, R. L. Todd & D. A. Crossley Jr, unpublished data).

WS 7 and 880 g mr2 on WS 2 (Seastedt & Crossley 1981), indicating similardecomposition rates on the two watersheds.

Decreased litter decomposition rates were associated with clearcutting. The three littersubstrates examined decayed in the following order on both watersheds: C.florida>A.rubrum > Q. prinus. However, annual decay rates were lower on the WS 7 plots relative torates obtained for the same substrates on WS 2 plots (Table 3). Measurements of decayrates in the first year following clearcutting also indicated lower decomposition rates onWS 7 relative to WS 2 and to pre-cut rates on WS 7 (Table 4). This reduction was initiallymost pronounced for the more rapidly decaying C.florida and A. rubrum litter. In the firstyear after clearcutting, annual decay rates (k) of C.florida, A. rubrum and Q. prinus litteron WS 7 were 46,44 and 33% lower, respectively, than on WS 2. Decomposition rates onWS 7 remained lower on the xeric WS 7 sites 8 years after clearcutting, although themagnitude of the difference had decreased substantially for C.florida and A. rubrum litter(Table 4). Annual decay rates of C.florida, A. rubrum and Q. prinus in the eighth year ofregrowth on WS 7 were 17, 14 and 32% lower, respectively, than on WS%2.

Nitrogen dynamics

Litter nitrogen dynamics were also altered on the xeric WS 7 plots. Absolute increasesin nitrogen content of litterbags (net nitrogen immobilization) were reduced on WS 7


140

120

100

80

60

40

20

—„ mass

III 225 345 528 732Days in the field

FIG. 1. Mean percentage of initial mass and nitrogen remaining over time in (a) Cornus florida,(b) Acer rubrum and (c) Quercusprinus litter on uncut WS 2 ( ) and clearcut WS 7 ( ) from

January 1985 to January 1987.

relative to WS 2. Mass loss curves and net nitrogen fluxes in litter on WS 7 and WS 2 arepresented in Fig. 1. The pattern of net nitrogen immobilization/release varied among litterspecies in a similar way on both watersheds. Initial net nitrogen losses in A. rubrum(Fig. Ib) and Q. prinus (Fig. Ic) litter were followed by increased net immobilization.Cornus florida litter (Fig. la), did not exhibit an initial loss phase but did immobilizenitrogen. All species appeared to begin a net release phase by the end of the study.

Although general patterns of net nitrogen immobilization and release were similar onboth watersheds, less nitrogen was immobilized in each substrate on WS 7 relative to WS 2(Fig. 1). Greater net nitrogen immobilization on WS 2 was due to greater increases innitrogen concentration in the residual litter (Fig. 2). A way to integrate mass loss andnitrogen concentration data is to plot percentage mass remaining as a function of nitrogenconcentration in the residual litter (Aber & Melillo 1980). Data from C. florida litter onboth watersheds are presented in this format in Fig. 3. Regressions of mean percentagemass remaining over mean nitrogen concentration in the residual litter were calculated for


2-0

1-0

£ 1-0z5?

0-51-5

1-0

0-5

(a ) Cornus f/orida

(b) Acer rubrum

. (c) Quercus prinus

0 97 139 238 345 408 464 528Days in the field

FIG. 2. Changes in the mean nitrogen concentration of decomposing (a) Cornus florida, (b) Acerrubrum, and (c) Quercus prinus litter on uncut WS 2 ( ) and clearcut WS 7 ( ). Bars

represent S.E. (n = 6 on days 13-345 and « = 3 on days 408-528).

each litter type on both watersheds. Slopes, 7-intercepts and coefficients of determinationare presented in Table 5. The slope describing the relationship between percentage massremaining and nitrogen concentration is a measure of the rate of increase in nitrogenconcentration per unit mass lost. For all three litter types this slope is 'steeper' for the WS7 data, indicating slower increases in nitrogen concentration per unit mass lost in the xericWS 7 plots.

Litter microarthropods

Microarthropod densities on WS 2 and pre-cut WS 7 are assumed to be similar. Soil/litter cores taken in clearcut (WS 7), partly-cut (WS 7), and control (WS 2) plots in thesummer of 1977 revealed no significant differences in microarthropod densities, highcoefficients of similarity between partly-cut and control plots and lower coefficients ofsimilarity between the clearcut plot and the others (Abbott, Seastedt & Crossley 1980;Seastedt & Crossley 1981).


100

D).E 80

60

40

WS2, WS 7

0 2-0% Nitrogen

FIG. 3. Slopes of the inverse relationship of mean percentage mass remaining and mean nitrogenconcentration in the residual litter for Comus florida on uncut WS 2 (O) and clear cut WS 7 (•).

The solid line is derived from the WS 2 data and the dashed line from the WS 7 data.

TABLE 5. Slopes, Y-intercepts and coefficients of determination (r2) of the inverselinear relationships of percentage mass remaining and nitrogen concentration inresidual litter on uncut WS 2 and clearcut WS 7; regressions are based on data

collected over a 464-day study period (January 1985-April 1986)

Species (WS) Slope r-intercept r2

Comusflorida (WS 2) -45-201*** 124-703 0-928(WS7) -64-275 140-692 0-937

Acerrubrum (WS 2) -56-064** 114-059 0-823(WS7) -85-715 136-831 0-864

Quercusprinus (WS 2) -48-908* 136-244 0-721(WS7) -74-940 159-620 0-784

* 0-05 <P< 0-10; **P<0-025; *** /><0-0025; valuesindicate the probability level at which slopes are statisticallydifferent based on a one-tailed r-test (Zar 1984).

In the first year following clearcutting the mean annual density of total littermicroarthropods in C. florida and Q. prinus litterbags on WS 7 was reduced > 50%relative to WS 2 (Seastedt & Crossley 1981). There was also a change in the pattern ofseasonal abundance of microarthropods. Microarthropod densities on WS 2 approxi-mated a unimodal curve, with maximum densities occurring in the summer. The patternof seasonal abundance on WS 7 was bimodal, with peak densities occurring in the springand autumn (Seastedt 1979). Summer densities were reduced, presumably due to highlyelevated temperatures and reduced litter moisture (Abbott, Seastedt & Crossley 1980).Results in the eighth year of regrowth indicated partial recovery of litter microarthropoddensities. However, mean annual densities of total microarthropods remained signifi-cantly lower on WS 7 relative to WS 2 (Table 6). Mean annual densities of totalmicroarthropods in all litterbags (C. florida, A. rubrum and Q. prinus) on WS 7 wereapproximately 28% lower than on WS 2 (Table 6). The seasonal distribution ofmicroarthropods had resumed a normal curve and the greatest differences occurred in theautumn (Fig. 4).


TABLE 6. Mean annual densities+ S.E. (H=138) (individuals g~' litter) of majormicroarthropod groups in Cornus florida, Acer rubrum and Quercus prinus

litterbags on uncut WS 2 and clearcut WS 7 (1985 data)

Total microarthropods

Prostigmata

Mesostigmata

Oribatei

Collembola

Other

(WS2)(WS7)(WS2)(WS7)(WS2)(WS7)(WS2)(WS7)(WS2)(WS7)(WS2)(WS7)

Cornus florida

(WS2) 33-89 + 2-27*(WS7) 28-34±2-46(WS2) 12-23 + 1-11(WS7) 11-06+1-06(WS 2) 2-24 + 0-37(WS7) 1-55 + 0-26(WS 2) 8-22 + 0-88***(WS7) 4-67 + 0-54(WS2) 9-70 + 1-06(WS 7) 8-56 + 0-99(WS2) 1-51+0-25*(WS7) 2-51 ±0-37

Acer rubrum

42-81+3-95***26-45+2-8415-30 + 1-44**10-94 ±1-043-40+0-59***1-26+0-21

13-64±2-06***4-99 ±0-699-14+0-947-68 + 1-321-31 ±0-181-57±0-16

Quercus prinus25-81 ±1-96**19-49 ±1-7810-12 + 0-84*8-12±0-832-01 + 0-27***0-98 + 0-176-00 + 0-68***3-24+0-376-63 + 0-665-41 + 0-671-05 + 0-20*1-74 + 0-34

* P<0-05; ** P<0-01; *** P<0-001; values indicate the probability level at whichdensities are significantly different based on a one-tailed f-test (Zar 1984).

80

60

40

20

n

Total microarthropods20

I? '5} }1 10

*J i 5_T51_i 1 1 — . i n

Oribatei-

\L

5 .

I 1- \ i

5. i i ,

" Mesostigmata

|

g 20(D

15

10

5

0

Prostigmata20

- j h •11

II

^ } ,i 1 1. J 1 n

Collembola T

' 1ll4

,i51 i i i

J

Winter Spring SummerAutumn Winter Spring SummerAutumn

Other

Winter Spring SummerAutumn

FIG. 4. Mean seasonal densities of major microarthropod groups in Cornus florida litterbags onuncut WS 2 (O) and clearcut WS 7 (•). Bars represent S.E. (n = 36). Patterns of seasonal densitiesof microarthropods in chestnut oak and red maple litterbags were similar, although densities were

higher in red maple litterbags.

J. M. BLAIR AND D. A. CROSSLEY JR

TABLE 7. Percentage composition of the total microarthropod fauna from litterbagsof Cornusflorida, Acer rubrum and Quercusprinus on uncut WS 2 and clearcut WS 7

(1985 data)

693

Group

Percentage of total faunaCornusflorida Acer rubrum Quercus prinusWS2 WS7 WS2 WS7 WS2 WS7

ProstigmataMesostigmataOribateiCollembolaOthers

38-96-5

24-226-7 :

3-8 '

47-92-3

11-629-1

9-2

36-87-7

31-121-3

3-2

42-74-6

18-228-6

6-0

39-08-0

22-925-8

4-3

42-15-0

16-327-9

8-7

TABLE 8. Mean + S.E. seasonal maximum and minimum temperatures (°C), basedon six recording intervals per season, recorded at the soil-litter interface on uncut

WS 2 and clearcut WS 7

SeasonUncut WS 2 Clearcut WS 7

Maximum Minimum Maximum MininumWinter 31-0±4-1 -8-9 + 2-9 32-3 + 5-3 -9-6 + 3-9Spring 31-8±4-4 6-5±l-9 39-2 + 3-5 3-9 + 2-4Summer 22-6+0-5 13-7+0-5 33-2±l-8 14-4±0-7Autumn 26-6+1-5 4-9 + 2-9 38-1 + 5-3 2-4 + 3-1

TABLE 9. Seasonal means ( + S.E.) of percentage moisture of Cornus floridalitterbags at the time of collection on uncut WS 2 and clearcut WS 7, with six bags

per watershed and six collection dates per season (n = 36)

Season Uncut WS 2

Winter 40-44 ±7-91Spring 35-18 ±27-93Summer 70-69+6-30Autumn 46-27 ±6-48

Clearcut WS 734-17±7-4527-93 ±6-4267-28 ±5-837-49 ±6-24

Various taxa of microarthropods responded differently to clearcutting, changing therelative abundances of major groups (Table 7). Similar patterns were observed inlitterbags of all species. The groups most reduced by clearcutting, on a percentage basis,were mesostigmatid and oribatid mites (Table 6). Mean annual densities of mesostigmatidand oribatid mites in all litterbags on WS 7 were 50 and 54% lower, respectively, than onWS 2. Mean annual densities of prostigmatid mites and collembola in all litterbags on WS7 were 20 and 24% lower, respectively, than on WS 2. The greater reduction inpopulations of oribatid and mesostigmatid mites increased the relative abundance ofprostigmatid mites and collembola on WS 7 (Table 7). The mean annual density of the'other' group, which included insects, spiders, chilopods, diplopods and other assortedinvertebrates was higher on WS 7 by an average of 34%.

DISCUSSION

Clearcutting appeared to slow litter decomposition rates on WS 7 at Coweeta. Decreaseddecomposition rates on the xeric portion of WS 7 remained apparent 8 years later.


However, the degree of difference between clearcut and control plots had decreased for C.fiorida and A. rubrum litter, suggesting that decay rates are returning to pre-cut levels.Decreased decomposition rates may contribute to increased surface organic matterstorage following clearcutting. There were significant increases in surface organic massfollowing clearcutting of WS 7 (Mattson 1986). Pre-cut surface organic mass (L and Flayers) was estimated at 816 g m~2 and in the first year after clearcutting increased to970 gm~2 (Seastedt & Crossley 1981). By the sixth year surface organic mass hadincreased to 1158 g m~2 (Mattson 1986). Increased forest floor mass is partly due to inputsof debris during clearcutting,! but reduced decomposition rates may also be important. Ingeneral, two factors—changes in microclimate and in litter quality—may potentiallyreduce litter decomposition rates.

Microclimatic changes included increased temperature extremes and more frequentand intense wetting and drying cycles. Both temperature extremes and variabilityrecorded at the soil-litter interface on WS 7 were greater than on WS 2 (Table 8).Litterbags collected from WS 7 were also generally drier (Table 9). This may be due toslower recovery of canopy cover on the xeric southwest-facing slopes (Boring & Swank1986). Although increased temperatures and actual evapotranspiration (AET) followingclearcutting should increase decomposition rates (Meentemeyer 1978), this was not thecase (Whitford et al. 1981). The combination of greater temperature extremes andmoisture variability created a microclimate that was suboptimal for the decomposercommunity, affecting both primary (bacteria and fungi) and secondary (microarthro-pods, nematodes, etc.) decomposers. Forest floor microbial biomass, estimated fromATP measurements, decreased in the first year after cutting (Waide et al. 1987) and bothshorter-term (1-3 years, Waide et al. 1987) and longer-term (6 year, Mattson 1986)decreases in COa evolution from the forest floor occurred. Maximum temperatures near50 °C recorded at the soil-litter interface after clearcutting (Abbott, Seastedt & Crossley1980) were sufficient to kill oribatid mites or inhibit their reproduction (Seastedt &Crossley 1981). Rapid canopy development may ameliorate the harsher microclimate, butit may take considerably longer for the decomposer community to recover completely.

Although the soil-litter fauna play an important role in decomposition and nutrientcycling, few North American studies have considered the effects of clearcutting on thefauna. Our results indicated reduced total microarthropod densities and changes in therelative abundance of microarthropod groups on WS 7. Similar results were reportedfollowing cutting of a mixed conifer-hardwood forest in Canada (Bird & Chatarpaul1986) and both cutting alone and cutting followed by slash burning in a British Columbiaforest (Vlug & Borden 1973). Microarthropods can affect decomposition processesdirectly and indirectly (Seastedt 1984). These effects include comminution and modifica-tion of the substrate, selective grazing and microbial stimulation (Crossley 1977).Exclusion or reduction of microarthropods usually decreases litter decay rates (see reviewby Seastedt 1984). Previous studies at Coweeta indicated a 33% reduction in annual decayrates of C.florida litter treated with naphthalene to reduce microarthropods (Seastedt &Crossley 1983). Lower microarthropod densities on WS 7 may contribute to reduceddecomposition rates. Decay rates were most reduced for Q. prinus litter, whichjvas theslowest decomposing species. This agrees with the observation that microarthropodshave a relatively greater effect on the decomposition of more recalcitrant litter (Seastedt1984).

Changes in relative abundance of microarthropod groups may also be important due todifferences in the trophic/functional roles of these groups. The Prostigmata include a


variety of forms, although most associated with the forest floor are piercing fungivores,predators or generalized saprovores. The Mesostigmata found in this study were mostlypredacious forms. Many Mesostigmata are predacious on nematodes (Mankau &Imbriani 1978) and may affect populations of microbial-feeding nematodes which may, inturn, affect decomposition. Both the Oribatei and Collembola are primarily fungivorous.Reduced oribatid densities may be particularly important in lowering decompositionrates. Oribatid mites are fungivores with chelate mouthparts capable of fragmenting andcomminuting litter (Seastedt 1984; Norton 1985). Oribatids showed the most extremeimmediate response to the cleareut (Abbott, Seastedt & Crossley 1980; Seastedt &Crossley 1981) and they remain most reduced (Fig. 4). Oribatid populations may be moresusceptible to microclimatic changes and may also take longest to recover due to lowfecundity and long generation times (Norton 1985).

Litter nitrogen dynamics were altered on WS 7. Nitrogen concentrations in the residuallitter increased at a slower rate per unit mass lost and this resulted in lower levels of netnitrogen immobilization. This suggests that clearcutting affects the underlying processescontrolling nitrogen fluxes in forest litter and that the effects may be relatively longlasting.

Several hypotheses may account for the slower increases in nitrogeiLconcentration perunit mass lost on WS 7. (i) Differences in availability of exogenous nitrogen (i.e.throughfall, litter leachate, etc.) between the watersheds may mean that more carbonmust be respired to incorporate a given amount of nitrogen in microbial biomass.(ii) Differences in microclimate and resource quality may affect the composition of theprimary decomposers community. Different decomposers have different protoplasmicC:N ratios and assimilate carbon and nitrogen with different efficiencies (Alexander1977). (iii) The microclimate on WS 7 may result in alternating periods of microbialgrowth, during which carbon is respired and nitrogen immobilized, and microbialsenescence when nitrogen that was incorporated in microbial biomass can be leached.Alternating wet and dry periods increased nutrient leaching in laboratory microcosms(Witkamp & Frank (1970). (iv) Lower microarthropod densities may cause lowernitrogen immobilization. Microarthropods can influence microbial activity and affectnutrient immobilization and release in litter (Seastedt 1984).

The presence of microarthropods generally increases nitrogen concentrations indecaying litter (Seastedt 1984). Microarthropod effects on net nitrogen fluxes depend oneffects on both concentration and mass loss. In this study, slower decay rates, slowerincreases in nitrogen concentration and lower levels of net immobilization were observedon WS 7, where microarthropod densities were also significantly lower. This suggests thatreduced microarthropod densities may contribute to changes in decomposition processesfollowing clearcutting. However, caution must be used in implying a cause and effectrelationship. Lower microarthropod densities may simply be correlated with the samefactors that result in lower decay rates and may not themselves be a significant causalfactor. Further studies are needed to clarify the relative importance of microarthropodreductions in relation to abiotic factors.

Site-specific effects of clearcutting

Forests in the south-eastern U.S., and elsewhere, may respond differently toclearcutting than north-eastern U.S. forests. Clearcutting in north-eastern U.S. forestsincreased decomposition rates and caused long-term decreases in surface organic matter(Bormann & Likens 1981; Covington 1981; Federer 1984). Clearcutting also increased

696 Decomposition following clearcutting

litter decay rates in a Ponderosa pine forest in central Arizona (Klemmedson, Meier &Campbell 1985). Effects on decomposition of cellulose squares at three elevations inVancouver, B.C., Canada varied among sites and with depth in the forest floor profile(Binkley 1984). Clearcutting reduced decay rates of Pinus radiata litter in New Zealand(Will, Hodgekiss & Madgwick 1983). Our results indicate reduced decomposition rates onsouth-facing slopes in the southern Appalachians. Litter decay rates in the mesic areas ofWS 7 were reduced for at least the first 3 years after clearcutting (J. B. Waide, R. L. Todd& D. A. Crossley Jr, unpublished data) and decay rates remain lower 8 years later on thexeric sites. i

The different responses of'southern Appalachian and north-eastern hardwood foreststo clearcutting are partly due to pre-cut differences in forest floor processes. Baselinedecomposition rates are apparently lower in north-eastern hardwood forests. Standingmass of surface organic matter (L, F and H layers) at Hubbard Brook in the WhiteMountains of New Hampshire was estimated to be 4680 g m~2 (Gosz, Likens & Bormann1976) as opposed to 1144 g m~2 on WS 2 at Coweeta (Berish & Ragsdale 1986). Estimatesof standing stocks of nutrients in the forest floor at Coweeta were also much lower than atHubbard Brook (Swank & Waide 1980). Slower decomposition rates and largeraccumulations of organic matter and nutrients are thought to be due to periods ofunfavourable climate (i.e. long cold winters with a heavy snow cover) (Swank & Waide1980). Canopy removal at these sites may increase solar insolation and lead to moreoptimal conditions for decomposition. Forests at more southern latitudes, such asCoweeta, do not experience harsh winters and have adequate precipitation throughoutthe year. Here, conditions for decomposition may be more favourable under a closedcanopy. Canopy removal may decrease decomposition rates by increasing temperaturesand reducing moisture levels. In addition to these regional effects, changes induced byclearcutting may also depend on such site-specific attributes as aspect, harvesting and sitepreparation techniques. Therefore we suggest that the effects of forest disturbance dependnot only on the disturbance itself, but on the nature of pre-disturbance processes and on'site-specific' effects of the processes studied.

ACKNOWLEDGMENTS

We thank L. R. Boring, B. L. Haines, R. W. Parmelee, T. R. Seastedt, W. T. Swank, J. B.Waide and two anonymous reviewers for criticism and comments on earlier drafts of thismanuscript. Thanks to the U.S. Forest Service for their cooperation in the use of theCoweeta facilities and to J. B. Waide for allowing us to use some of his unpublished data.This research was supported in part by NSF grant BSR-8012093 to the University ofGeorgia Research Foundation.

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