Forest Ecology and Management, 49 (1992) 87-99 87 Elsevier Science Publishers B.V., Amsterdam

Redistribution of soil nitrogen, carbon and organic matter by mechanical disturbance during

whole-tree harvesting in northern hardwoods

Douglas F. Ryan a, T h o m a s G. Hunt ing ton b and C. Wayne Mar t in c a Morris Arboretum, University o f Pennsylvania, 94 ! 4 Meadowbrook Avenue, Philadelpkia,

PA 19118, USA b USGS Water Resources Division, 6418 Peachtree Industrial Boulevard Suite B, Doraville,

GA 30360, USA c USDA Forest Service. Hubbard Brook Experimental Forest, Campion, NH 03223, USA

(Accepted 12 March 1991 )

ABSTRACT

Ryan, D.F., Huntington, T.G. and Martin, C.W., 1992. Redistribution of soil nitrogen, carbon and organic matter by mechanical diaturbance during whole-tree harvesting in northern hardwoods. For. Ecol. Manage., 49: 87-99.

To investigate whether mechanical mixing during harvesting could account for losses observed from forest floor, we measured surface disturbance on a 22 ha watershed that was whole-tree harvested. Surface soil on each 10 cm interval along 81, randomly placed transects was classified immediately after harvesting as mineral or organic, and as undisturbed, depressed, rutted, mounded, scarified, or scalped ( forest floor scraped away). We quantitatively sampled these surface categories to collect soil in which preharvest forest floor might reside after harvest. Mechanically mixed mineral and organic soil horizons were readily identified. Buried forest floor under mixed mineral soil occurred in 57% of mounds with mineral surface soil. Harvesting disturbed 65% of the watershed surface and removed forest floor from 25% of the area. Mechanically mixed soil under ruts with organic or mineral surface soil, and mounds with mineral surface soil contained organic carbon and nitrogen pools significantly greater than undisturbed forest floor. Mechanical mixing into underlying mineral soil could account for the loss of forest floor observed between the preharvest condition and the second growing season after whole-tree harvesting.

INTRODUCTION

T h e long- te rm ecologica l consequences for fores ts o f ha rves t i ng whole t rees ( r e m o v a l o f al l a b o v e - g r o u n d p a r t s ) has been the sub jec t o f r ecen t s tud ies ( J o h n s o n et al . , 1982; E d w a r d s a n d Ross -Todd , 1983; F r e e d m a n et al . , 1986; H o r n b e c k et al. , 1986; S m i t h et al . , 1986) . In n o r t h e r n h a r d w o o d forests , C o v i n g t o n ( 198 ! ) a n d F e d e r e r ( 1 9 8 4 ) f o u n d tha t a b o u t 50% o f c a r b o n a n d n i t rogen poo l s in p r e h a r v e s t fores t f loor was lost in a b o u t 15 yea r s a f t e r ha r -

© 1992 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$05.00

f38 D.F. RYAN ET AL.

vest by studying a sequence of stands of various ages following stem-only clear- cutting. Bormann and Likens (1979) have shown that streamwater export of nitrogen and uptake by regrowing vegetation account for less than 50% of nitrogen lost from forest floor in the first 5 years following cutting. A number of possible fates for this unaccounted for nitrogen have been hypothesized including gaseous loss through denitrification or recompartmentalization into mineral soil either through mechanical mixing by vehicle wheels and dragged trees or by transport in dissolved organics leaving the forest floor (Melillo, 1981 ). However, there is a paucity of evidence for the mechanisms causing the loss. To evaluate the availability of nitrogen for the next rotation of tree growth, it is critical to know whether these unaccounted for losses of nitrogen from the forest floor leave the ecosystem or are retained within it but in a different compartment. As part of a long-term study of how organic matter and nutrients are reorganized after whole-tree harvest (WTH) at the Hub- bard Brook Experimental Forest (HBEF), New Hampshire, USA, we inves- tigated the hypothesis that some carbon and nitrogen were lost from forest floor during the harvest process by being mixed with underlying mineral soil, rather than being lost from the ecosystem. Although chronosequences of many sites of different age since disturbance are useful to find broad trends, their resolution is often limited by intersite variability. Tile rationale of our overall study was to follow in detail many aspects of a single WTH site to investigate mechanisms causing broad trends. The work we report was meant to serve as a baseline for this long-term study as well as for comparison with other sites with less comprehensive data (e.g. Martin, 1988 ).

M E T H O D S

We measured the proportion of the forest floor that was disturbed during harvesting and we sampled forest floor and mixed mineral soil in various dis- turbance categories. In addition, we estimated pool sizes of carbon and nitro- gen in forest floor and disturbed soil, permitting us to estimate how much forest floor was 'lost' by mechanical mixing and to compare these losses with those estimated by Covington (1981), Federer (1984), and Bormann and Likens (1979) on clear-cuts over longer time periods.

Sitedescription

We studied watershed W5, a 22 ha catchment in the HBEF dominated by beech (Fagus grandifolia), yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum) trees, about 65 years old before harvesting. All trees greater than 5 cm diameter at breast height (dbh) were cut in the fall and winter of 1983. This was a 'whole-tree' harvest in which both boles and branches were removed from the watershed with rubber-tired skidders. Fell-

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 89

3Om

Om

Im

® ~ -- - - WATERSHED BOUDARY

.---.._ ~ ELEVAT ION CONTOUR

. . . . STREAM

- - SK ID TRA ILS

"'~ ~ " sl om

Fig. I. Map of watershed W5 at Hubbard Brook Experimental Forest, NH, showing elevation, stream courses and temporary skid trails during harvesting in 1983-1985.

ing was done by a tracked feller buncher equipped with hydraulic shears on the less-steep, low-elevation portion of the watershed and with chain saws elsewhere. Trees were removed from 85% of the watershed between October 1983 and September 1984; trees were removed from 12% of the area in the summer of 1985 and felled trees were not removed on 3% of the area.

This watershed is generally well drained, moderately steep (20% slope),

90 D.F. RYAN ET AL.

southeasterly in aspect, and has an elevation range of 510-750 m (Fig. l ). The climate is humid continental, with a mean air temperature in July of 19 °C and in January of - 9 °C (Federer, 1973 ). Mean annual precipitation of 130 cm, is evenly distributed over the year (Likens et al., 1977 ). In winter, continuous snowcover develops to a depth of about 1.5 m which usually pre- vents surface soils from freezing even during the coldest months (Hart et al., 1962 ). Many aspects of the ecology and biogeochemistry of HBEF have been studied by numerous investigators (Likens et al., 1977; Bormann and Likens, 1979). The soils are acid, typic haplorthods of the Tunbridge-Lyman, Berk- shire and Skerry series. They are developed on stony, glacial till with a sandy- to silty-loam texture derived from highly metamorphosed mudstone and sandstone of the Littleton Formation. There is a continueus, well-developed forest floor (O horizon) of the mor type which ranged in depth from 3 to 15 cm (Huntington et al., 1988). Many properties of forest floor and mineral soil at HBEF have been previously studied (Gosz et al., t976; Bormann et al., 1977; Melilio, 1981; Huntington et ai., 1989; Huntington and Ryan, 1990).

Methods

To measure the microtopography of the soil surface before harvesting, we stretched tightly a tape between staples driven into the bases of pairs of trees approximately 10 m apart. Vertical distances were measured between the tape and the soil surface at 0. l m intervals along the tape. After harvesting, we repeated these measurements by stretching the tape between the same staples in the stumps of harvested trees (Fig. 2 ).

Microtopography was measured between 99 pairs of trees randomly lo- cated over the watershed before harvesting. After harvesting, 18 pairs could

B E F O R E C U T

. . . . . . . . . • " A F T E R C U T

H1 = VERTICAL DISTANCE MEASURED BEFORE CUFFING

H2 = VERTICAL DISTANCE MEASURED AFTER CUTTING

Fig. 2. Schematic of method used to measure soil surface microtopography showing tape stretched tightly between staples on stumps. The vertical distance from the soil surface to the tape was measured at 0.1 m intervals before and after harvesting.

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 9 !

not be found because the staples or, in some cases, the stumps had been re- moved. At the 81 pairs that could be relocated, the soil surface along each O.1 m interval of the transects was visually classified as organic or mineral. Each interval was also classified as 'depressed' if the organic soil was intact but compressed as by a single pass of equipment or a felled tree, as 'scarified' if there was minor mixing of forest floor with mineral soil, as "scalped' if the forest floor had been sheared away exposing but not mixing the mineral soil, and as "rut" or "mound' if the postharvest soil surface was lower or higher than the preharvest surface. In addition, we recorded areas with surface rock or where soil surface was inaccessible due to vegetation (e.g. tree stumps, large surface roots or fallen logs).

We sampled soils in the second growing season after cutting (August 1985) along 17 transects previously classified for surface disturbance from three el- evation bands (510-620, 620-710, 710-750 m), and from areas where soil sampling would not interfere with other research. The length of each transect within each disturbance category was treated as a stratum, and each point 0.1 m apart within each stratum was a potential location from which to sample soil randomly.

For soil sampling the "organic depressed' stratum was combined with 'or- ganic ruts'. "Scalped', 'surface rock' and "vegetation' categories were not sam- pled because forest floor or mixed soil were inaccessible or not present at their surface. Sampling was designed to place about eight sampling points in each of the six disturbance strata and in each of the three elevational strata. At each sampling point a 15 cm × 15 cm wooden template was fastened to the soil surface with l0 cm steel nails. We cut the soil around the edges of the template with a keyhole saw to sever all roots or buried branches. We trenched around the outside of the template to the depth of undisturbed mineral soil. Modify- ing the technique of Covington ( 1981 ), we collected the pedestal of soil un- der the template and above the undisturbed mineral soil. These samples con- sisted of forest floor or mechanically mixed mineral soil. Where the former forest floor was buried by mixed mineral soil, the samples also included this buried organic matter. We rejected sampling points less than 30 cm from a previously chosen sampling point and/or those at which the wooden template overlapped more than one stratum.

We investigated whether mechanically mixed and undisturbed soil profiles could be distinguished in the field by digging a series of test pits in obviously disturbed and undisturbed areas of the watershed. Mechanically mixed soils consistently had recognizable fragments of above-ground plant parts such as leaves, needles, twigs, branches, or boles (on heavily traveled skid trails) at depth in the profile. In undisturbed soils these fragments occurred only in Oi and Oe horizons near the surface. Mixed soils also lacked structure and root networks observed in undisturbed horizons. Whether mixed or undisturbed, we distinguished organic from mineral soil in the field visually using the cri-

92 D.E RYAN ET AL.

terion of 20% organic carbon (Soil Survey Staff, 1975 ). Although making this separation visually is subjective (Federer, 1982), we chose to use it because it was practical for handling a large number of samples in the field.

Laborawry analysis

Samples were air-dried, sieved to 6 mm (1 /4 inch) and weighed. We ~e- lected this sieve size because we knew of no standard for organic soil and the 2 mm standard for mineral soil (Soil Survey Staff, 1975 ) was too fine to be practical for sieving organic soils. A subsample was oven-dried at 105 °C to determine moisture content. A further oven-dried subsample was pulverized to a powder in mortar and pestle or Wiley mill. The powder subsample was analyzed for total nitrogen and carbon in a Carlo Erba Model 1500 C-N ana- lyzer which combusted the sample in pure 02, reduced all nitrogen to N2, oxidized all carbon to CO2 and quantitatively separated gasses by chromato- graphy. Atropine and citrus leaves (NBS 1572) were used as standards. Soils found to have less than 20% carbon were resieved to 2 mm and -eanalyzed for carbon and nitrogen to conform with SCS standards for sieving mineral soil (Soil Survey Staff, 1975 ). Significance of differences in means of carbon and nitrogen soil concentrations and pool sizes was determined at the 95% level using a one-way ANOVA and separation of means was tested using the least significant difference method (Statistical Analysis Systems Institute, 1982).

RESULTS

Area of surface disturbance

We observed signs of mechanical disturbance of surface soil on 65% of the watershed after logging (Table 1 ). No mechanical disturbance to surface soil was detected on 30% of the area and no surface soil was observed on the re- maining 5% because of surface rock, stumps, roots or logs. Ruts with surface mineral soil occupied 18% of the area and were the category of disturbed sur- face soil with the greatest area as well as the greatest change in surface micro- topography. In extreme cases, we observed mineral ruts 80 cm deep, although nearly half the mineral ruts were less than l0 cm deep. After harvesting, 25% of the watershed had mineral soil at the surface. This area was determined by summing scalped, mineral mound, and mineral rut categories.

Mechanically mixed, mineral horizons were observed in 90% of the min- eral mound and 73% of the mineral rut samples (Table 2), By comparison, mechanically mixed organic horizons were observed relatively infrequently, with mineral ruts paradoxically having the highest frequency (20%). 'Min- eral ruts' were classified based on a visual inspection of the surface soil only;

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 93

TABLE 1

Percentages of area disturbed by type and height or depth of disturbance from 81 transects measured betbre and after whole-tree harvesting on wate~hed W5

Type Depths or heights (cm)

0 1-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 Total SE

Undisturbed 30.3 30.3 2.8 Depressed 3~7 0.1 3.8 0.8 Scarified 5.6 7.4 13.0 1.3 Scalped 0.8 0.8 0.8 Organic mounds ! !.4 I. I 0.2 0.1 i 2.8 i,6 Mineral mounds 2.7 !.5 !.0 0.3 0.1 5.6 !,1 Organic ruts 10.2 0.4 10.6 1,4 Mineral ruts 8.9 4.3 2.5 !.i 0.5 0.5 0.2 0.1 18.1 2,7 Vegetation 1.8 1.8 0,4 Bare rocks 3.2 3.2 0.7

Total 40.9 45.1 7.4 3.7 1.5 0.6 0.5 0.2 0.1 100.0

TABLE 2

Percentage of soil samples containing mechanically disturbed mineral and organic soil horizons and buried organic horizons in the various soil surface disturbance categories following whole-tree harvesting

Surface disturbance category

Percentages of samples containing disturbed horizons

Mechanically disturbed Buried

Organic Mineral Organic horizons horizons horizons

Undisturbed - - - Scarified - - 8 (2/25) Organic mounds 8 (2/24) - 4 (1/24) Mineral mounds 3 ( l /30) 90 (27/30) 57 (17/30) Organic ruts 10 (2/20) 15 (3/20) 15 (3/20) Mineral ruts 20 (6/30) 73 (22/30) 20 (6/30)

Number of samples observed containing horizon/total number of samples in category in parentheses. Percentages in soil surface disturbance categories do not sum to 100% because some samples con- tained unmixed soil while others contained more than one of the horizons described.

w h e n t h e e n t i r e l a y e r o v e r l y i n g u n m i x e d m i n e r a l so i l w a s s a m p l e d a n d a n a -

l y z e d f o r c a r b o n , 2 0 % o f t h e s a m p l e s t u r n e d o u t t o h a v e g r e a t e r t h a n 2 0 %

o r g a n i c c a r b o n . F o r e s t f l o o r b u r i e d u n d e r m i n e r a l so i l o c c u r r e d i n 5 7 % o f m i n e r a l m o u n d s i t es , i n 2 0 % o f m i n e r a l r u t s a n d 1 5 % o f o r g a n i c r u t s .

Concentrations and pools of carbon and nitrogen

I n u n d i s t u r b e d , p o s t h a r v e s t f o r e s t f l o o r , t h e o r g a n i c m a t t e r p o o l , ( 7 6 _+ 16

94 D.F. RYAN ET AL.

TABLE3

Carbon and nitrogen concentration (%) and pool size (t ha- ~ ) of forest floor or mechanically mixed soils above the depth of undisturbed mineral soil by surface disturbance categories and whole watershed in sec- ond growing season after whole-tree harvest ( _+ standard error of the mean )

Surface Area Concentration (%) Pools (t ha- ~ ) Number of disturbance (% of) soil samples category Nitrogen Carbon Nitrogen Carbon

Undisturbed 30.3 (2.8) 1.50 (0.11)a 32.2 (2.5)d 1.89 (0.39)g 41.9 (9.0)i 25 Scarified 13.0 (0.8) 1.12 (0.10)b 23.6 (2.2)e 1.38 (0.26)g 29 .5 (5.7)i 25 Organic mound 12.8(I.6) 1.19(0.11)b 27.4(2.8)d,e 2.09(0.25)g 48.6 (6.4)i,j 24 Mineral mound 5.6 ( I . I ) 0.59 (0.047)c 13.3 (I .3)f 3.03 (0.38)h 68.9 (8.8)k 30 Organicrut 14.4(2.2) 1.32(0.11)a,b 31.5(3.0)d 2.95(0.33)h 72.1 (9.7)k 20 Mineral rut 18.1 (2.7) 0.63 (0.066)c 13.4 (l .4)f 3.16 (0.50)g,h 79.0 (13.6)j,k 30 Scalped 0.8 (0.3) . . . . . Vegetation 1.8 (0.4) . . . . . Rock 3.2 (0.7) . . . . . Whole watershed 100 - - 2.19 5 !.3 154

(area weighted )

Means followed by the same letter were not significantly different (P< 0.05 ).

t ha -~ ), carbon pool (41.9_+9 t ha -~ ) and nitrogen pool (1.9_+0.4 t ha -~ ) were not significantly different from preharvest forest floor surveys (Hun- tington et al., 1988 ).

Carbon and nitrogen concentrations in mixed soil were lower where min- eral soil rather than organic soil was at the surface (Table 3). The pools of soil carbon and nitrogen in the scarified and organic mound categories were not significantly different from the undisturbed category. Mineral mounds and mineral ruts were not significantly different from one another in either concentration or pool sizes of carbon and nitrogen. Carbon and nitrogen pools in mineral mounds and organic ruts were significantly greater than undis- turbed scarified or organic mound categories. Mineral ruts had carbon pools that were greater than undisturbed and scarified categories but their nitrogen pools were not different from any of the other disturbance categories primar- ily because of the high variability observed in mineral ruts. Organic ruts had carbon concentrations that were not different from undisturbed forest floor but their pool of organic carbon was greater than forest floor.

DISCUSSION

Our methods allowed us to examine the effects of WTH on forest floor me- chanical disruption both from the perspective of surface area involved and of nitrogen and carbon pools that were involved. In order to combine and com- pare various categories of soil pools we had to make several assumptions which precluded statistical tests of these comparisons. We included discussion of

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 95

these comparisons nevertheless because they suggest the likely fate of forest floor disturbed by harvesting.

The fraction of surface area that was mechanically disturbed by WIIT can be compared with several other WTH sites in New England (Martin, 1988). The fraction of surface area without mechanical disturbance at this site (30%) was comparable with a central hardwood site in Connecticut (29%) but greater than another northern hardwood in New Hampshire (7%) or a spruce-fir site in Maine (8%). However, the fraction of the area with exposed mineral soil, was greater at this site (25% ) than at the other northern hardwood site (11%), the central hardwood (8%) or the spruce-fir site (18%). Restriction of ve- hicle access to one end of the long, narrow watershed with limited stream crossings (Fig. 1 ) caused a denser network of skid trails than on previously studied whole-tree harvests. Steepness of the slope probably also required a denser network of trails. The densest part of the trail network on W5 lies be- tween 610 and 710 m elevation, the portion of the watershed with highest slope (Fig. 1 ). Skidder movement at this site was more localized to trails, which reduced the area that was mechanically disturbed, but at the same time caused more intensive disruption of surface soil on skid trails themselves and thus the higher proportion of surface mineral soil observed. Obviously, dis- ruption of surface soil during WTH is highly site-specific.

The criteria that we developed to distinguish disturbed and undisturbed soil were useful because they allowed us to identify in the second growing season after harvest the volume of soil that had been mechanically mixed during harvesting. Federer (1982) has suggested that the Ap and Op horizon definitions may apply to soil horizons mixed during harvesting. The Soil Con- servation Service (SCS) (1981 ) designated horizons with a "p' to indicate disturbance of the surface layer by cultivation, pasturing or similar use. A mixed, organic, surface horizon was designated Op. A disturbed mineral ho- rizon, even though clearly once an E, B, or C horizon was designated Ap. An organic horizon buried under mineral soil was an Ob horizon. These desig- nations seemed to b~ applicable to our mixed mineral, mixed organic and buried organic horizons in the second season after cutting. However, we would expect the diagnostic signs that we used to become less recognizable over time. As root networks redevelop, as buried fragments of surface litter decompose, and as horizons reform on disturbed areas, we think that the difference be- tween disturbed and undisturbed soils would become progressively harder to distinguish at longer intervals after harvesting. At present, we do not know how long signs of disturbance persist in forest soils.

Our observations indicated the nature of some types of surface disturbance. Only a small fraction of organic mounds or organic ruts had churned soils below their surface (Table 2), indicating that these were probably areas that received relatively light traffic. In organic ruts, the similar concentration of carbon and nitrogen but larger pool size compared with undisturbed forest

96 D.F. RYAN ETAL.

floor (Table 3 ) probably indicates that organic ruts occurred on areas of the watershed that had thicker accumulation of forest floor before cutting. From this we concluded that disturbed areas after WTH with surface organic soil either received relatively light traffic or were in areas with exceptionally thick forest floor.

What became of the forest floor that had previously occupied the 25% of the watershed which had surface mineral soil after harvesting? Could the WTH operation have swept this forest floor into piles or could it have mixed it into underlying mineral soil? Summing the carbon pools in all postharvest cate- gories with surface' organic soil (undisturbed, scarified, organic mound and organic rut) yielded 46 t C ha- ~ which exceeded 41.9 t ha- ' observed in un- disturbed forest floor. Thus after WTH, the forest floor carbon pool did ap- pear to be larger by about 10% indicating that perhaps some 'windrowing' of forest floor occurred but probably not enough to account for the 25% of the area from which forest floor was 'lost' during WTH.

We investigated whether the missing tbrest floor had been mixed with min- eral soil to the extent that it no longer met the criteria for organic soil of greater than 20% carbon (Soil Survey Staff, 1975 ). The concentration of carbon and nitrogen in the mineral rut and mineral mound soil was intermediate between that in lbrest floor and the top 10 cm of mineral soil collected by Huntington et al. ( 1988 ) before harvesting (Table 4). In that study, mineral soil was sam- pled in 10-cm-thick strata from 60 pits randomly distributed over watershed W5. In the current study, concentrations of carbon and nitrogen in mixed mineral horizons were consistent with a mixture of forest floor and shallow mineral soil (Table 4). To achieve the concentrations of carbon and nitrogen which we observed in mixed mineral soil would require between 300 and 330 t ha- t of shallow mineral soil (a layer 6-7 cm deep) to be mingled with the forest floor on the area of the watershed that had surface mineral soil after

TABLE 4

A comparison of carbon and nitrogen concentrations in preharvest forest floor and 0-10 cm depth mineral soil (Huntington et al., 1988) with mixed mineral horizons with surface mineral soil from the current study

% Carbon % Nitrogen

Before harvesting Forest floor 34 1.5 0-10 cm mineral soil 6.5 0.33 Combined forest floor 10.7 0.5

and 0-10 cm mineral

.4fter harvesting Mineral mounds 13.3 0.59 Mineral ruts 13.4 0.63

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 97

WTH. On the basis of the whole watershed this would amount to 76-83 t ha- ~ of mineral soil mixed with forest floor. Area-weighted pools of nitrogen and carbon in mechanically mixed mineral soil horizons exceeded those in undisturbed forest floor by 9.4 t C ha-~ and 0.3 t N ha-~. This 'excess" of carbon and nitrogen in mixed mineral soil pools over that in forest floor was the contents of the shallow mineral soil that was mixed with forest floor dur- ing WTH. Thus some forest floor from areas with surface mineral soil after WTH may have been scraped into piles, but probably the majority was mixed with, or buried under, shallow mineral soil to the extent that organic soil was no longer recognizable at the surface.

Covington ( 1981 ) (corroborated by Federer, 1984), found a decrease in forest floor organic matter of 31 t ha- ~ ( 17 t C ha- ~ ) in 15 years after clear- cutting. Both investigators studied the same collection of sites that had been clear-cut at various times in the past. Covington ( 1981 ) only sampled soils showing no sign of mechanical disturbance. Federer (1984) sampled forest floor without regard to disturbance but found nearly identical results to Cov- ington ( 1981 ). Bormann and Likens (1979) analyzed Covington's ( 1981 ) data and estimated that 0.35 t N ha-~ were lost from forest floor in 4 years after clear-cutting. Thus forest floor loss we observed during the whokc.tree harvest operation was comparable with that estimated for the first 4 years following a clear-cut and about one-third of the maximum loss which has been observed 15 years after clear-cutting. We suspect that some of the decrease in forest floor observed by Covington (1981) and Federer (1984) was due to mechanical disturbance. The amount of mechanical disturbance at their sites was probably less than at our site, both because these were commercial oper- ations with traffic patterns more like the sites studied by Martin (1988) and because the trees were skidded from their sites without branches. Both Fed- erer (1984) and Covington (1981) estimated forest floor loss from a smoothed curve of forest floor organic matter pools at their several sites so that any mechanical disturbance effect would have been included in their es- timate of forest floor loss. Other processes such as accelerated decomposition, diminished woody inputs or movement of dissolved organics into mineral soil also probably acted at Covington's and Federer's sites because they ob- served organic matte~ ~ pools in forest floor to continue to decrease for 15 years after cutting and not just to decrease immediately after harvest a~ one would expect if only mechanical disturbance had occurred.

Our study suggests that caution is needed in interpreting studies such as Covington ( 1981 ) and Federer (1984). The portion of the area of a site that is disturbed during harvesting can vary depending on site characteristics (slope, stream network, soil texture, and drainage) and vehicle traffic pat- tern. The ability to recognize areas in which forest floor has been disturbed or removed by harvesting may decrease with time. Furthermore, as the tech- nology and methods of harvesting change over time, the pattern of soil distur-

98 D.F. RYAN ET AL.

bance occurring during harvesting may change in a systematic way with the risk that, after the fact, studies of a collection of sites harvested at different times could be complicated by a pattern of initial soil disturbance that also changed over time. All of these factors underscore the importance of meas- uring the amount of mechanical disturbance that occurs during harvesting in studies of the reorganization in forest floor after harvesting.

CONCLUSIONS

We have found that after WTH at our site, both forest floor with no surface mechanical disturbance and soil with severe surface disturbance were equal to, or greater than, values observed on previously studied WTHs. These dif- ferences were probably explained by site-specific skidder traffic patterns.

After the harvest, surface mineral soil occupied 25% of the area covered by forest floor preharvest because forest floor had been mechanically mixed with mineral soil to the extent that it was unrecognizable as organic soil and, to a lesser degree, forest floor had been scraped into piles. Thus carbon and nitro- gen "loss' from forest floor by mechanical disturbance was a matter of defini- tion rather than a loss from the ecosystem. These elements were recompart- mentalized into mineral soil where they could be reached by the roots of regrowing vegetation. Studies of W5 forest floor will be difficult to compare with the previous findings of Covington ( 1981 ) unless areas with disturbed and intact forest floor are considered separately. Future research should in- vestigate how quickly forest floor is restored after it has been removed from the surface by mechanical disturbance as well as to what degree availability of nutrients for regrowing vegetation is changed when forest floor is mechan- ically mixed with mineral soil.

ACKNOWLEDGMENTS

We wish to thank G. Miller and J. Berino for help in the field and laboratory and A. Federer, R. Yanai, T. Fahey and J. Hughes for comments on drafts of this manuscript. This research was supported by grant BSR8316953 from the National Science Foundation and was part of the Hubbard Brook Ecosystem Study.

REFERENCES

Bormann, F.H. and Likens, G.E., 1979. Pattern and Process in a Forested Ecosystem. Springer, New York, 253 pp.

Bormann, F.H., Likens, G.E. and Melillo, J.M., 1977. Nitrogen budget for an aggrading north- ern hardwood forest ecosystem. Science, 196: 981-983.

WHOLE-TREE HARVESTING IN NORTHERN HARDWOODS 99

Covington, W.W., 1981. Change~ in forest floor organic matter and nutrient content following clear cutting in northern hardwoods. Ecology, 62:41-48.

Edwards, N.T. and Ross-Todd, B.M., 1983. Soil carbon dynamics in a mixed deciduous forest following clear cutting with and without residue removal. Soil Sci. SOc. Am. J., 47: 1014- 1021.

Federer, C.A., 1973. Annual cycles of soil and water temperature at Hubbard Brook. USDA For. Serv. Res. Note NE-167, Upper Darby, PA, 7 pp.

Federer, C.A., i 982. Subjectivity in the separation of organic horizons of the forest floor. Soil Sci. SOc. Am. J., 46: 1090-1093.

Federer, C.A., 1984. Organic matter and nitrogen content of the forest floor in even-aged north- ern hardwoods. Can. J. For. Res., 14: 763-767.

Freedman, B., Duinker, P.N. and Morash, R., 1986. Biomass and nutrients in Nova Scotian forests and implications of intensive harvesting for future site productivity, For. Ecol. Man- age., 15: 103-127.

Gosz, J.R., Likens, G.E. and Bormann, F.H., 1976. Organic matter and nutrient dynamics of the forest floor in the Hubbard Brook Forest. Oecologia (Berlin), 22: 305-320.

Hart, G.E., Jr., Leonard, R.E. and Pierce, R.S., 1962. Leaf fall, humus depth, and soil frost in a northeast hardwood forest. USDA For. Serv., Northeast. For. Exp. Sin. Rcs. Note 131, 3 pp.

Hornbeck, J.W., Martin, C.W., Pierce, R.S., Bormann, F.H., Likens, G.E. and Eaton, J.S., 1986. Clearcutting northern hardwoods: Effects on hydrologic and nutrient ion budgets. For. Sci., 32: 667-686.

Huntington, T.G. and Ryan, D.F., 1990. Whole-tree-harvesting effects on soil nitrogen and car- bon. For. Ecol. Manage., 31: 193-204.

Huntington, T.G., Ryan, D.F. and Hamburg, S.P., 1988. Soil nitrogen and carbon pools in a northern hardwood forest ecosystem. Soil Sci. SOc. Am. J., 52:1162-1167.

Huntington, T.G., Johnson, C.E., Johnson, A.H., Siccama, T.G. and Ryan, D.F., 1989. Carbon, organic matter and bulk density relationships in a forested spodosol. Soil Sci., 148: 380-386.

Johnson, D.W., West, D.C., Todd, D.E. and Mann, L.K., 1982. Effects of sawlog vs. whole-tree harvesting on the nitrogen, phosphorus, potassium, and calcium budgets of an upland mixed oak forest. Soil Sci. SOc. Am. J., 46: 1304-1309.

Likens, G.E., Pierce, R.S., Eaton, J.S. and Johnsor,, N.M., 1977. Biogcochemistry of a Forested Ecosystem. Springer, New York, 146 pp.

Martin, C.W., 1988. Soil disturbance by logging in New England--review and management rec- ommendations. North. J. Appl. For., 5: 30-34.

Melillo, J.M., 1981. Nitrogen cycling in deciduous forests. In: Nitrogen cycling in terrestrial ecosystems. Process, ecosystem strategies and management impacts. Ecol. Bull. (Stock- holm), 33: 427-442.

Smith, C.T., Martin, C.W. and Tritton, L.M., (Eds.), 1986. Proc. 1986 Symp. on the Productiv- ity of Northern Forests Following Biomass Harvesting. USDA For. Serv., Northeast. For. Exp. Sin., Broomall, PA, NE-GTR-115, 104 pp.

Statistical Analysis Systems Institute, 1982. SAS User's Guide: Statistics. SAS Institute, Cary, NC, 584 pp.

Soil Conservation Service (SCS), 1981. Examination and description of soils in the field, chap- ter 4 (revised). Soil Survey Manual. SCS Directive 430-V.

Soil Survey Staff, 1975. Soil Taxonomy, Agric. Handb. No. 436, USDA. U.S. Government Printing Office, Washington, DC.