Effects of different harvesting and site preparation operations on the peak flows of streams in Pinus Elliottii flatwoods forests

Forest Ecology and Management, 5 (1983) 77--86

77

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

EFFECTS OF D I F F E R E N T HAI~,VESTING A N D SITE P R E P A R A T I O N OPERATIONS ON THE PEAK FLOWS OF STREAm, S IN PINUS ELLIOTTII FLATWOODS FORESTS

BENEE F. SWINDEL l, C H A R L E S J. L A S S I T E R I and HANS R I E K E R K 2

USDA Forest Service, Gainesville, FL 32611 (U.S.A.) 2 School o f Forest Resources and Conservation, University o f Florida, Gainesville, FL 32611 (U.S.A.) (Accepted 15 April 1982)

ABSTRACT Swindel, B.F., Lassiter, C.J. and Riekerk, H., 1983. Effects of different harvesting and site preparation operations on the peak flows of streams in Pinus elliottii flatwoods forests. For. Ecol. Manage., 5: 77--86. Stream flows on three poorly drained and contiguous pine flatwoods catchments were monitored for 3.5 years. One was left untouched. Pine timber from another was manually harvested at the end of the 1st year, residues were chopped, terrain was bedded, and pine seedlings planted -- a minimum series of forest operations. The third was subjected to a maximum series of forest operations -- harvesting of tree-length logs with heavy equipment, lightwood stump removal, burning, windrowing, discing, bedding, and planting. Unlike the m i n i m u m series, the m a x i m u m series of forest operations significantly increased the peak outflows, particularly following windrowing. The increase was estimated to exceed sixfold immediately following windrowing and to decrease slowly and linearly with time thereafter (equation 3).

INTRODUCTION

Water yields from forested watersheds typically increase following tree harvests which reduce evapotranspiration ( Ellertsen, 1968; Brown et al., 1974; Kochenderfer and Aubertin, 1975; Lynch et al., 1975; Patric and Aubertin, 1977; Hewlett, 1979). There is the risk that increases following harvesting and site preparation for replanting will substantially increase the frequency, severity, and duration of downstream flooding -- thereby exacerbating soil erosion, nutrient losses, and n o n p o i n t source pollution. To ensure that the benefits obtained when managing forests are maximized, ,it is therefore desirable to have information on the effects o f forest operations on the timing of water flow as well as water yield. Hollis et al. (1978) and Swindel et al. {1982) described increases in water yields after harvesting of pine forests in coastal swamps and flatwoods, respectively, in the Lower Coastal Plain where surface water tends to be overabun-

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© 1983 Elsevier Scientific Publishing Company

78 dant. This paper is a sequel to the latter. It describes the effects of t w o forest harvesting and replanting regimes (minimum and maximum) on peak flow rates in managed slash pine (Pinus elliottii) flatwoods forests of north Florida. EXPERIMENTAL DESIGN Three contiguous watersheds of 67, 49, and 140 ha (thereafter denoted WS 1, WS 2, and WS 3, respectively), each surrounded b y a perimeter road (dike) with a single outlet for surface water, were established in naturally regenerated flatwoods forests in Bradford County, FL. T o p o g r a p h y o f the study site is typically flat ( b e t w e e n 43 and 45'~ m above sea level). The overstory is c o m p o s e d of open (about 16 m~/ha basal area) stands of slash pine with a midstory of c o m m o n gallberry (Ilex glabra) and saw-palmetto (Serenoa repens). Interspersed with these pinelands are small, w o o d e d swamps with cypress (Taxodium distichum) and other hardwoods. Swindel et al. (1982) described climate and soils and display an aerial photograph of the site (their fig. 1). On WS 1, pinelands comprising 49% of the watershed were clearcut harvested using labor intensive methods (Table I). Prior to machine planting seedlings (Pinus elliottii), logging debris and residual understory plants were double chopped with a roller drum chopper and the site was b e d d e d (Riekerk et al., 1980). The main drainage through WS 1 lies within cypress and h a r d w o o d stands which o c c u p y 51% of the watershed and which were not disturbed (Swindel et al., 1982, fig. 1). TABLE I Schedule of minimum and maximum forest operations when forest stands in watersheds (WS 1 and WS 2) in flatwoods forests of north Florida were harvested and replanted Date

Minimum practices WS 1

Maximum practices WS 2

1978 November--December 1979 January--February April May

harvest (shortwood)

harvest (long wood) lightwood stump removal

chop burn

June

August September--October November

windrow (KG-blade)

chop bed plant

disc bed plant

On WS 2, naturally regenerated pinelands comprising 74% of the watershed were clearcut harvested using machine intensive methods (Riekerk et al., 1980). Lightwood stumps were extracted from the soil (Table I), logging debris was burned and pushed into windrows (Morris et al., 1981), and interwindrow spaces were disced, bedded, and machine planted as in WS 1. (Morris et al. (1981) discovered that much of the litter layer and substantial amounts of

79 soil were inadvertently pushed into the windrows along with the logging debris.) The main drainage through WS 2 lies within or is b o u n d e d by site-prepared land so that interwindrow spaces open onto the main drainage (Swindel et al., 1982, fig. 1). Cypress and h a r d w o o d stands occupying 26% of WS 2 were n o t disturbed. None of the forests on WS 3 were disturbed (Swindel et al., 1982, fig. 1) -the untreated control site. METHODS Precipitation was measured with a total of seven standard rain gauges -- six along perimeter roads and one near the center of the three contiguous watersheds. Stream monitoring was accomplished by installing long-throated, critical flow flumes (Replogle, 1971) and analogue to digital water level recorders. Stream depths in the flumes have been recorded in p u n c h e d tapes at 15-min intervals since October 1977, except for brief periods of equipment failure. Depth-discharge rating equations (Replogle et al., 1978) allow c o m p u t a t i o n s of flow rates. From these continuously monitored flow rates, various storm hydrograph parameters are calculable. Individual storms were selected for further analysis if they produced a measurable hydrograph on the control watershed (WS 3) and at least one of Horves~

WATERSHED I

J Harvest

F-{[

WATERSHED 2

PRETREATMENT

J"

Chop

J

ChoP Bed

H

Stumping

H H Burn Windrow Disk Bed

I

IH

I H

TREATMENT

Plom

H Plant

I--I .L

POST-TREATMENT

20 Mean Rainfall (cm)

15

I0

5111, 1977

, 1978

1979 Year

and Julian Day

Fig. 1. Frequency and intensity (mean rainfall) of storms on Pinus elliottii flatwoods forests prior to and during treatment, and schedule of forest operations.

8O the other watersheds -- measurable taken to mean a storm flow volume ~>0.0025 a r e a cm. F r o m D e c e m b e r 1 9 7 7 t h r o u g h J u n e 1 9 8 1 , 55 i n d i v i d u a l s t o r m s w e r e selected b y t h e s e criteria: 23 p r i o r t o i n i t i a t i o n o f f o r e s t o p e r a tions, 19 d u r i n g t h e p e r i o d o f t r e a t m e n t , a n d 13 s u b s e q u e n t t o t r e a t m e n t (Fig. 1). A l t h o u g h s t o r m s in 1 9 7 9 ( t h e t r e a t m e n t y e a r ) w e r e as n u m e r o u s as t h o s e in 1 9 7 8 ( t h e p r e t r e a t m e n t y e a r ) , t h e y were less intense. I n 1 9 8 0 a n d 1981 ( f o r c o n v e n i e n c e - n o t d e p i c t e d ) s t o r m intensities r e m a i n e d r e l a t i v e l y l o w and f r e q u e n c y d i m i n i s h e d s h a r p l y , so t h a t o n l y 12 s t o r m s c o n f o r m e d t o t h e selection criteria d u r i n g 1 9 8 0 a n d t h e first h a l f o f 1 9 8 1 . This p a p e r deals o n l y w i t h peak flow rates f o r individual s t o r m s d e f i n e d as t h e increase in d i s c h a r g e r a t e a t t r i b u t a b l e t o t h e s t o r m : Thus, p e a k f l o w r a t e is c a l c u l a t e d as t h e m a x i m u m d i s c h a r g e r a t e (in m 3 rain -1 k m -2, or M M K ) r e a c h e d d u r i n g or f o l l o w i n g a s t o r m m i n u s t h e initial d i s c h a r g e r a t e (in M M K ) o b s e r v e d i m m e d i a t e l y p r i o r t o t h e s t o r m . T a b l e II s h o w s d a t e s o f o c c u r r e n c e , rainfall, a n d p e a k f l o w r a t e s m e a s u r e d o n each o f t h e t h r e e w a t e r s h e d s f o r all s t o r m s in t h e a n a l y s e s t h a t follow. D a t a available f r o m t h e 23 s t o r m s o c c u r r i n g p r i o r t o i n i t i a t i o n o f f o r e s t ope r a t i o n s w e r e used t o e s t a b l i s h p e a k f l o w r e l a t i o n s h i p s b e t w e e n WS 1 a n d WS 3 t o e v a l u a t e e f f e c t s o f m i n i m u m o p e r a t i o n s , a n d b e t w e e n WS 2 a n d WS 3 t o evaluate effects of m a x i m u m operations. Prior to the onset of forest operations, d a t a f o r WS 2 - - t o r e c e i v e m a x i m u m f o r e s t o p e r a t i o n s - - were l o s t o n 11 occasions. T h u s , in e s t a b l i s h i n g baseline c o m p a r i s o n s d u r i n g t h e p r e t r e a t m e n t phase, t h e r e w e r e 23 p a i r e d o b s e r v a t i o n s o f WS 1 a n d WS 3 b u t o n l y 12 o f WS 2 a n d WS 3. TABLE II Individual storm rainfall (r.) and peak flow rates woods forest of north Florida

(pi) from WS 1, WS 2, and

Year

WS 1 (rain)

WS 2 (max)

WS 3 (control)

rx (cm)

Pl (MMK)

r2 (era)

P~ (MMK)

r3 (cra)

P3 (MMK)

1.42 1.37 4.88 2.29 0.48 5.28 0.58 4.65 2.57 1.04 7.01 15.09 3.73 3.15

0.94 0.48 7.72 2.83 0.45 15.57 3.08 9.37 3.36 1.10 5.30 135.87 2.70 1.17

1.45 1.35 4.47 2.26 0.48 5.69 0.64 4.39 2.36 0.81 7.16 14.78 3.81 3.89

1.74 1.79 14.06 4.24 0.87 29.09 0.87 14.19 4.94 1.31 9.57 155.31 ---

1.50 1.32 4.55 2.31 0.48 5.89 0.64 4.27 2.29 0.84 7.06 14.96 3.61 4.37

0.70 0.51 4.45 1.50 0.25 7.47 1.02 2.22 1.62 0.33 4.27 62.93 1.07 0.95

Month

1977

12

1978

1

3 4 5 7

Day

17 25 8 13 17 19 25 3 8 14 19 4 8 12

W$ 3 in flat-

81

Year

Month

8

Day

13 16 17 1 5 6 11 17 18

W S 1 (rain)

WS 2 ( m a x )

WS 3 ( c o n t r o l )

r~ (cm)

r2 (era)

r3 (cm)

p~ (MMK)

6.73 1.19 2.51 19.86 2.16 0.30 1.78 1.04 2.11

19.52 1.34 6.14 85.52 3.66 0.35 1.38 0.55 2.06

7.39 1.07 2.36 19.76 2.13 0.30 1.70 1.09 2.54

5.23 1.98 2.39 2.36 2.26 3.86 3.18 3.35 3.51 1.57 1.07 1.63 5.18 2.77 2.92 2.69 1.68 2.79 2.13

2.14 0.47 0.64 1.44 3.12 2.70 1.91 5.81 0.52 0.80 0.39 2.03 18.65 6.78 8.35 9.17 0.75 -0.64

5.69 1.98 2.39 2.44 2.21 3.86 2.51 6.02 4.27 1.88 0.89 1.50 5.28 3.15 2.64 2.21 1.68 3.48 2.62

8.10 1.88 1.68 1.80 4.39 5.59 3.15 4.88 5.59 3.02 3.33 3.05 5.61

12.60 0.61 3.77 0.58 0.30 0.52 1.80 1.00 4.70 1.36 0.52 0.71 0.37

9.19 2.03 1.65 1.85 4.52 5.64 3.35 4.62 5.66 2.87 3.56 3.12 5.51

P2 (MMK)

----------

P3 (MMK)

8.05 0.99 2.79 19.05 2.29 0.33 1.93 0.74 2.82

8.30 0.76 2.16 58.88 4.89 0.45 1.45 0.77 2.54

3.02 0.79 3.52 4.41 7.48 6.53 1.33 21.52 0.44 1.24 ---28.81 23.44 17.10 -0.14 1.02

5.59 2.01 2.39 2.41 2.34 3.84 1.57 5.59 4.72 2.08 0.79 1.52 5.11 3.18 2.92 1.83 1.78 4.37 3.40

1.41 0.40 0.49 0.46 0.31 0.91 0.28 1.92 0.10 0.12 0.27 0.83 1.78 1.13 2.11 1.58 0.34 0.48 0.66

62.16 --1.33 3.41 5.71 8.04 4.45 20.63 3.82 1.94 4.24 0.94

9.19 2.08 1.60 1.98 4.65 5.36 3.53 4.70 5.79 3.28 3.63 3.07 5.84

3.65 0.36 1.07 0.54 0.30 0.84 0.70 0.68 0.83 0.78 0.25 0.33 0.73

(Initiation of treatments) 1979

1

2 4 5 8

9

10 11

20 23 30 6 24 5 31 . 5 23 24 12 14 15 24 27 30 23 2 11

(Conclusion of treatments) 1980

12 1 5 6 7

1981

2 3

6

6 12 14 25 19 25 29 11 18 5 22 30 12

82 RESULTS

D i f f e r e n c e s a t t r i b u t a b l e to t w o r e g i m e s - - m i n i m u m and m a x i m u m f o r e s t operations

During the period prior to treatment, a good predictor of the peak flow rates on WS 1 and WS 2 was f o u n d to be the simple linear regression model: Pi = bo + b~ps + e

where Pi is peak flow rate from a to-be-treated watershed (i = 1 or 2); ~ is peak flow rate from the control watershed; e is residual error; and b0 and b~ are regression coefficients estimable from p r e t r e a t m e n t data. Fitted least squares regressions, coefficients o f determination (r~), and standard errors of regression (s) were: ~5~ = - 0 . 0 0 1 + 1.832p~ ;

r: = 0.955;

s = 6.99 MMK

(la)

r ~ = 0.992;

s = 4.01 MMK

(lb)

and ~ = 1.997 + 2 . 4 5 2 p a ;

w h e r e / ~ and fi~ are estimated peak flow rates from WS 1 and WS 2, respectively. There was no significant departure from linearity in these relations. Figs. 2 and 3a show residuals from the preceeding regressions for each individual storm for which peak flow data were available through June 1981. Fig. 2 is a storm-by-storm plotting of WS 1 residuals, F~, given by: ~ = p~ -- ~ =p~ + 0.001 -- 1.832p~

(2a)

HARVEST

CHOP CHOP

I

H

I

BED

I

1

"

~

~

~

n~

_~

J_

o

T.

Dote of

Storm

Fig. 2. Excess o f observed over expected peak flow (MMK) on WS 1 before, during, and

after m i n i m u m forest operations based on pretreatment period regression (equation la).

83 Figure 3a is a storm-by-storm plotting of WS 2 residuals, ~2, given by: r2 = P 2 - - P 2 =p2 -- 1.997 -- 2.452p3

(2b)

Fig. 2 shows that m i n i m u m forest operations did n o t increase the frequency or magnitude of discrepancies between actual and expected peak flows. Fig. 3a shows that m a x i m u m practices greatly increased frequency and magnitude of such discrepancies. The remainder of this report shows that statistical tests of such treatment effects are indeed significant, and attempts to relate the peak flow responses to individual operations. °

"~

WINDROW

40

HARVEST

[

BURN IDIISC BED

i ,,

I

;

->

~ 20

_~ 20E

L ~ J

Date of Storm

Fig. 3. Excess of observed over expected peak flow rate (MMK) on WS 2 before, during, and after maximum forest operations based on: (a) pretreatment period regression (equation lb); and (b) regression with windrowing component (equation 3). Peak flow response to m a x i m u m operations A test for the statistical significance of the apparent change in peak flows as a result of maximum operations is provided by the principle of conditional error. The test requires only the pooled error sums of squares (SSE) and error degrees of freedom (f) from separate regressions pre- and post-treatment, and the error sums of squares (SSE*) and error degrees of freedom (f*) from a c o m m o n regression fitted to all the data. The test statistic is distributed as Snedecor's F with f* -- f and [ degrees of freedom (Swindel, 1970). Partition-

84 ing the peak flow data at harvest, these statistics are: Error SS Preharvest 82 = 1 . 9 9 7 + 2.452~)3 Post-harvest ~2 = - 3 . 9 9 1 + 15.513 p3

160.6336 962.6495 1123.2831 +

Common

3887.6699

fi2 =

5.514 +

X

SSE* -- SSE SSE

f

F~4

2.433p3

-

- f* --

-

34 3 8 8 7 . 6 6 9 9 - 1123.2831 X = 41.84 36 -- 34 1123.2831

f

d.f. 10 24 -~+ 36

Snedecor's F of 41.84 on 2 and 34 d.f. is significant at any conventional level of probability (tabular F at the 0.0001 level is less than 13). Thus, peak flow surely increased following harvest on WS 2. (The same test on WS 1 produced F520 = 2.86 which is n o t significant at the 0.05 level.) Efforts to isolate which of the maximum operations (Table I} was mainly responsible for increasing peak flows was s o m e w h a t frustrated b y the l o w frequency of storms following the treatment year (Table II). And the effects are confounded. Nevertheless, isolation of the dominant operation(s) was att e m p t e d based on the data available. Various regressions were fitted in which the coefficients were allowed to change at the time of harvesting, windrowing, discing, and bedding. All these analyses pointed to windrowing as the dominant operation altering the pretreatment period relationship -- and they suggested that the principal effect was in altering the slope coefficient. Thus, the simplest equation deemed satisfactory for describing all the data from WS 2 was:

/~2 = 0.419 + (2.488 + 12.597 w 0 - 0.00769 WoW) P3

(3)

where w0 is an indicator ( d u m m y ) variable taking the value 0 prior to windrowing and 1 afterwards; w is days since windrowing; and/~2 and P3 are as before. Statistics of fit were r 2 = 0.960 and s = 5.58 MMK. Residuals from equation (3) are plotted in Fig. 3b. Neither visual inspection of the residuals nor any analyses we c o n d u c t e d suggested any important explanatory variable was o m i t t e d from equation (3). Thus, equation (3) is empirically judged to be sufficient to describe the observed response. Any effects of harvesting, discing, and bedding were necessarily c o n f o u n d e d with the effects of windrowing. Independent tests o f these effects in the presence of more frequent storms could show that they t o o affect peak flows.

85 CONCLUSIONS

Storm peak flow rates may or may not increase following clearcutting, site preparation, and planting in flatwoods forests -- depending on the specific forest operations imposed. In watersheds where a substantial proportion is not harvested, and where harvested portions do not open onto drainage ways, forest operations that only subdue residual understory and do not displace the litter and soil (such as labor intensive logging and chopping) may produce no discernable increase in storm peak flow rates. More intensive operations may produce very substantial increases in storm peak flows. Windrowing appeared to produce the dominant increase in storm peak flow rates of the major operations (harvesting, windrowing, discing, bedding) we imposed on 74% of a 49-ha watershed. A simple empirical model (equation 3, above) suggests that peak flow following windrowing exceeds by sixfold the peak flow in the absence of disturbance, and the increase diminishes slowly with time following disturbance. If prevention of downstream flooding is a priority of forest management, windrowing of lands adjacent to major drainages should be avoided. Further data from these watersheds into the next rotation may confirm the lack of an affect on peak flow rates following minimum operations, and establish the rate of return to pretreatment response following maximum operations. ACKNOWLEDGEMENT

Container Corporation of America furnished the land, isolated the watersheds, and imposed the prescribed treatments.

REFERENCES Brown, H.E., Baker, M.B., Rogers, J.J., Clary, W.P., Kovner, J.L., Larson, F.R., Avery, C.C. and Campbell, R.E., 1974. Opportunities for increasing water yields and other multiple use values on ponderosa pine forest lands. USDA For. Serv. Res. Pap. RM-129, 36 pp. Ellertsen, B.W., 1968. Forest hydrologic research conducted by the Tennessee Valley Authority. Water Resour. Bull., 4(2): 25--33. Hewlett, J.D., 1979. Forest water quality: an experiment in harvesting and regenerating piedmont forests. Georgia Forest Research Paper, School of Forest Resources, University of Georgia, Athens, GA, 22 pp. Hollis, C.A., Fisher, R.F. and Pritchett, W.L., 1978. Effects of some silvicultural practices on soil-site properties in the Lower Coastal Plains. In: C.T. Youngberg (Editor). Forest Soils and Land Use, Colorado State University, Fort Collins, CO, 585--606. Kochenderfer, J.N. and Aubertin, G.M., 1975. Effects of management practices on water quality and quantity: Fernow Experimental Forest, West Virginia. In: Municipal Watershed Management Symposium Proceedings. USDA For. Serv. Gen. Tech. Rep. NE-13, pp.14--24. Lynch, J.A., Sopper, W.E., Corbett, E.S. and Aurand, D.W., 1975. Effects of management practices on water quality and quantity: The Pennsylvania State Experimental Watersheds. In: Municipal Watershed Management Symposium Proceedings. USDA For. Serv. Gen. Tech. Rep. NE-13, pp. 32--46.

86 Morris, L.A., Pritchett, W.L. and Swindel, B.F., 1981. Windrow composition. IMPAC Rep o r t 6(3). University of Florida, Gainesville, FL, 22 pp. Patric, J.H. and Aubertin, G.M., 1977. Long term effects of repeated logging on an Appalachian stream. J. For., 75: 492--494. Replogle, J.A., 1971. Critical-depth flumes for determining flow in canals and natural channels. Trans. Am. Soc. Agric. Eng., 14: 128--136. Replogle, J.A., Riekerk, H. and Swindel, B.F., 1978. Flow metering flumes m o n i t o r water in coastal forest watershed. Water and Sewage Works, 125: 64--67. Riekerk, H., Swindel, B.F. and Replogle, J.A., 1980. Initial hydrologic effects of forestry practices in Florida flatwoods watersheds. IMPAC Report 5(4). University of Florida, Gainesville, FL, 24 pp. Swindel, B.F., 1970. Some applications o f the principle of conditional error. US For. Serv. Res. Pap. SE-59, 48 pp. Swindel, B.F., Lassiter, C.J. and Riekerk, H., 1982. Effects o f clearcutting and site preparation on water yields from slash pine forests. For. Ecol. Manage., 4: 101--113.