MOISTURE IN THE FOREST FLOOR -

ITS DISTRIBUTION AND MOVEMENT

by B. J. Stocks

R6sumf en 'ransa;s

DEPARTMENT OF FISHERIES AND FORESTRY CANADIAN FORESTRY SERVICE

PUBLICATION NO. 1271 1970

ABSTRACT

The stratification and movement of moisture in the forest floor were examined in both the field and the laboratory.

Experimental results from forest duffs studied in California and Ontario show that after rain the highest moisture content in the forest floor is not at the duff-mineral soil interface but somewhat above it. Results indicate that this "inversion" is caused neither by a lack of water reaching this interface nor by the inability of the lowest duff layer to absorb water.

With increasing time after rain, the highest moisture content was found to be at a lower level in the duff layer until eventually it existed at the duff-mineral soil interface. Evaporation from the surface of the forest floor is the most important factor to be considered in determining this characteristic duff-drying pattern. The actual downward movement of moisture makes a negligible con­tribution to the pattern.

i

Published under the authority of the Minister of Fisheries and Forestry

Ottawa. 1970

QUEEN'S PRINTER FOR CANADA OTTAWA, 1970

Catalogue No. Fo. 47 - 1271

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Etude en 1aboratoire et sur 1e terrain de 1a stratification et du mouvement de l'humidite dans 1e sol forestier.

Les resu1tats d'experiences sur 1a 1itiere forestiere en Ca1ifornie et en Ontario demontrent qu'apres une p1uie, 1e contenu maximal en humidite dans 1e sol forestier n'est pas dans la zone de contact entre 1a 1itiere et 1e sol mineral, mais pas mal plus haut. et que cette "inversion" ne resu1te pas de l'insuffisance d'eau ni de 1a supposable faib1esse du pouvoir d'absorption in­herent a 1a section inferieure de la 1itiere.

Plus tard apres 1a p1uie. 1e contenu maximal se dep1a~a a 1a couche inferieure de 1a 1itiere puis eventue1lement a 1a zone de contact entre ce1le-ci et 1e sol mineral. L'evaporation est Ie facteur determinant de cette maniere caracteristique qu'a la 1itiere de secher; et 1e mouvement descendant de l'eau y contribue de fa~on bien negligeable.

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MOISTURE IN THE FOREST FLOOR -ITS DISTRIBUTION AND MOVEMENT

by

B.J. Stocks l

INTRODUCTION

It is generally accepted that the degree of decomposition of organic matter is the chief criterion for the separation of the layers of the forest floor into litter (L-layer), duff (F-layer), and humus (H-layer). The main processes of decomposition are oxidation of the organic matter and consequent progressive mineralization of the materials. Most studies dealing with the forest floor have analyzed this decomposition, being con­cerned either with the distribution of minerals and nutrients within the floor, or with the distribution of the fungi or fauna primarily causing the decomposition process.

Because the forest floor acts as a carrying agent for fire, and because the moisture content of this floor is a major factor in determining the intensity and rate of spread of a fire, fire researchers recognize the importance of water in the organic layers above the mineral soil. Most studies have dealt mainly with the L-layer, because it consists of undecom­posed materials, reacts most quickly to fluctuations in atmospheric con­ditions, and poses a serious problem with regard to fire ignition and rate of spread. Until recently, few studies have been made on the moisture content on the various strata of organic material in the forest floor, although it is well known that fire intensity is greatly increased when the duff and humus layers are dry. Lately, however, more work has been done with these layers, as it has become clear that drought indices should be based more on duff moisture content than they have been in the past. Also, forest-floor layers present a more natural forest fuel than do the fuel moisture sticks currently in widespread use in fire-danger-rating in the United States.

This project was undertaken with the following objectives: (1) to determine how moisture is stratified within the forest floor under different drought conditions and (2) to determine the pattern of the wetting and drying cycles of the organic layers following various amounts of rainfall.

In this study, the different forest-floor layers were determined in an arbitrary manner, as a function of depth or decomposition, and the term "duff" will refer to all organic matter present above the mineral soil.

lResearch Officer, Department of Fisheries and Forestry, Canadian Forestry Service, Sault Ste. Marie, Ontario.

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Although this study was carried out in California, the results are applicable in many areas of the world, including the upland pine regions of Ontario. The species involved may differ, but upland pine sites in Ontario have duff quite similar in thickness and composition to those studied.

REVIEW OF THE LITERATURE

Although no attempts have been made to study specifically the wet­ting and drying characteristics of the various layers within the duff, some researchers have mentioned the moisture content or rain-retention capacity of the forest floor.

J.G. Wright (1935) found that the full duff layer (all organic matter above the mineral soil) had a higher saturation point than the top­layer duff (L-layer), and he correctly assumed that this was because the moisture-holding capacity of duff particles increased with increasing decom­position. Moisture capacities (expressed as percent of oven-dry weight) ranged from 150% to 220% for top-layer duff and from 200% to 300% for full­layer duff, depending on the species involved. Hardwood material studied had high moisture capacities, both top-layer and full-layer duff having values around 340%.

In Forest Influences, Kittredge (1948) listed field moisture capa­cities (of various forest floors), which ranged from 130% for manzanita (ArctostaphyLos manzanita Parry.) to 225% for red and white fir (Abies mag­nifica A. Murr. and Abies concoLor (Gord. and Glend.) Lindl.). He referred to French studies with Norway spruce (Picea abies (L.) Karst.), which showed that the litter could absorb 215% and the F- and H-layers (combined) could absorb almost 600% of their dry weight of water. He also cited a Minnesota study in which fresh pine litter absorbed from 150% to 350% of its dry weight of water.

The reasons for the large discrepancies in percent moisture content encountered in the literature are twofold. First, the number of different species involved: spruce forests characteristically have deep, well-decom­posed forest floors, which in many cases form peat bogs that will, therefore, hold more water than the forest floor of an upland pine stand where decom­position is generally much faster and more extensive. The thinner duff layers on upland pine sites indicate a fast rate of decomposition, although their thickness also varies with microclimate and amount of litter fall. Deciduous material decomposes more rapidly, forming a mull profile with large propor­tions of decomposing humus; this greatly increases its moisture-holding capacity. Secondly, the methods used to determine water-holding capacities listed in these studies were not always the same. Some gravitational water may sometimes have been included; this would give somewhat high values.

Other research pertinent to this study has dealt with the ability of the forest floor to retain rainfall. In a Wisconsin study, W.R. Curtis (1960) found that hardwood and pine forest floors held the same amounts of rainfall after rainstorms of different intensities. The percent of total rainfall held was over 90% for storms of 0.25 inch or less, and decreased

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with increase in rainfall until only 10% of the rain in storms of 1.00 inch or more was retained in the forest floor itself. The rest passed through into the mineral soil.

In a somewhat similar experiment conducted in the New Jersey Pine Barrens, J.M. Bernard (1963) found the water-retaining capacity of the litter layer to be equivalent to approximately 0.05 inch of rainfall and of the l­inch layer immediately below it to be 0.45 inch. From its oven-dry weight, he concluded that the forest floor could thus hold 0.5 inch of water.

L.J. Metz (1958) studied the water-retaining capacity of 2-inches­deep, undecomposed loblolly pine (Pinus taeda L.) litter in the southeastern United States. He found that the litter held a minimum of 0.01 inch and a max­imum of 0.09 inch of water depending on the amount of rainfall. The maximum re­tention of 0.09 inch (220% moisture content) occurred only after prolonged rains.

A.J. Simard (1968) studied the moisture-holding capacity of old and new undecomposed conifer needles and found that initially older needles absorb more water, but that after prolonged soaking the differences in moisture re­tention were slight.

In Canada, work is now under way on a revised drought index that will reflect the state of dryness of the full duff layer as a measure of the buildup of fire hazard. In some cases duff moisture content is determined by destruc­tive sampling on different days and in others intact duff samples are placed in nylon mesh trays in the forest floor and are wei~hed daily throughout the fire season. Preliminary results (Van Wagner, 1965 ) have shown in a few cases that the wettability of duff seems low when the duff has a low or very high initial moisture content.

In the summer of 1967, I took part in a brief study of duff moisture content profiles at the Petawawa Forest Experiment Station in Ontario (Van Wagner, 1968 3). The data were taken from two pine stands and an aspen stand, but only six sets of samples were taken throughout the summer. The few results obtained showed that after rainfall the part of the forest floor having the highest moisture content was not at the mineral soil interface but somewhat above it. Only after a substantial drought did the layer immediately above the mineral soil have the highest moisture content.

This report contains the results of further investigations of the problem in California and examines in greater detail the movement of moisture in forest duff, the moisture profiles established, and the causes of stratifica­tion in the forest floor. The results will be compared with those obtained in the Canadian study, carried out at Petawawa, to see if similarities exist.

2Van Wagner, C.E. 1965. Drying and wetting rates of duff layers of various thicknesses. Can. Dep. Forest., Petawawa Forest Exp. Sta., Progr. Rep., Project P-6l8 (unpublished).

3Van Wagner, C.E. 1968. Duff moisture content profiles at Petawawa. Can. Dep. Forest., Petawawa Forest Exp. Sta., Intern. Rep. P.S.-8. 6 p.

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EXPERIMENTAL PROCEDURE

The experimental procedure and the discussion sections are each in two parts: the field investigation of duff moisture and the subsequent laboratory analysis.

Field Study

All forest floor material studied came from Blodgett Experimental Forest, an area of some 2,600 acres managed by the School of Forestry and Conservation of the University of California and located on the west slope of the Sierra Nevada in Eldorado County at an elevation ranging between 4,100 and 4,600 feet. The forest consists of young-growth, mixed conifer­type stands; the topography is moderate and the soil is reddish brown and loamy (Holland Series).

During the spring of 1968, weekly sampling trips were made to Blodgett Forest. Ten trips were made in all, from snowmelt to substantial drought.

Duff samples were taken from five different areas, each with a different stand composition. The main species of each sampling site are listed as follows:

Location: I ponderosa pine (Pinus ponderosa Laws.) white fir (Abies concoZor (Gord. and Glend.) Lindl.) sugar pine (Pinus Zambertiana Dougl.)

II ponderosa pine

III ponderosa pine sugar pine Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) incense cedar (Libocedrus decurrens Torr.)

IV incense cedar black oak (Quercus keZZoggii Newb.) ponderosa pine

V black oak

Each week a l-square-foot sample was taken from each location, the duff layer being removed down to the mineral soil. Each sample was then separated into five successive horizontal layers (subsamples), the basis for this separation being the degree of decomposition of the duff with increasing depth. For example, first-year needles were considered as the first, or upper, subsample; 2-year-old needles were the second subsample, and so on. Needles start to decompose in their second year on the forest floor and this process continues with time; so the third, fourth, and fifth subsamples are made up of increasingly decomposed forest material. In many forested areas this fifth layer would be almost entirely humus, but the duff

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samples taken at Blodgett Forest showed only a negligible amount of humus above the mineral soil, decomposed needles being still recognizable as such in the fifth layer.

Each sample was separated into its five subsamples in the field, and the subsamples were then placed in bags and weighed. The moisture content and dry weight of each layer were then determined by oven-drying at 100 C for 24 hours.

Moisture content profiles were drawn in histogram form for each sample for each day. The profiles were qualitatively similar for all five sites, and were therefore combined into a composite graph (Figure 1)

loyer Date Buildue Inde!

April 6 6

April 9 11

April 16 21

April 23 34

May 1 S4

May 7 6S

May 21 18

May 28 31

June 4 48

June 11 28

% MOISTURE CONTENT (dry wt.)

Figupe 1. Composite duff moisture content profiles (of the five California sites studied) for a 10-week period.

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representing all five sites on each of the 10 sampling days. The U.S. Buildup Index for the day of collection is also shown on this graph, since it indirectly correlates fire hazard with fuel moisture, through the measurement of certain weather variables.

As a basis for comparison with other forested areas in Canada and the United States, the average depth and weight (oven-dry) of each of the five duff types sampled are listed in Table 1.

Table 1. Average depths and weights of duff types studied

Duff type*

I - pP,wF,sP II - pP

III - pP,sP,Df,iC IV - iC,bO,pP V - bO

*NOTE: pP - ponderosa pine sP - sugar pine bO - black oak

Average depth (inches) (cm)

3.4 3.4 3.3 3.5 2.1

8.6 8.6 8.4 8.9 5.3

wF - white fir

Average dry weight (lb/sq ft) (g/m2)

0.96 1.15 1.06 1.46 0.70

4,689.6 5,617.8 5,178.1 7,132.1 3,419.5

Df - Douglas-fir iC - incense cedar

Additional samples were taken from each site to determine the maximum moisture-holding capacity of each of the five component layers in each duff sample. Kittredge (1948) defines the "field moisture capacity" of the forest floor as the maximum amount of water that the duff can retain against the force of gravity. In this study an attempt has been made to determine field moisture capacity according to the foregoing definition.

Material from each duff layer was placed loosely in a screened cube and soaked for 48 hours in a cold-water bath. Each cube was then removed from the bath and allowed to drain until water ceased to drip from it (5 to 10 minutes) before it was weighed and the moisture content values were calculated. The process was repeated three times for each layer (with different samples) in each duff type and the average taken as the maximum moisture content value for that particular layer.

Histograms of these maximum values (for each of the five sampling sites) appear in Figure 2.

For comparison, Figure 3 shows moisture-content profiles and maximum moisture-ho1ding-capacity values obtained in the Canadian study carried out at Petawawa. Moisture profiles from pure red pine (Pinus resinosa Ait.), jack pine (pinus banksiana Lamb.), and trembling aspen (Populus tremuloides Michx.) stands were qualitatively similar in shape and were thus combined to give part (a) of this composite graph, showing data from three of the six sampling days.

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layer LOCATION

l-wF,sP,pP

lI-pP

III-pP,sP,Df,iC

IV-iC,pP,bO

V-bO

o 100 200 300 400 500

% MOISTURE CONTENT (drywtJ

Date

June 2

Days since O.S in. of rgjn

June 8 o

June19 2

Species

Red Pine

Jack Pine

Aspen

100 200 300 400

% MOISTURE CONTENT (dry wt.l

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Figure 2. Maximum mois­ture-holding-capacity values (by component layers) for the five duff types studied.

Figure 3. The Canadian (Petawawa) study: (a) Composite moisture­content profiles of three sites for 3 days. (b) Maximum moisture­hoZding-capacity values for the three sites.

Laboratory Study

Causal factors for the observed duff drying pattern were further investigated in the laboratory. Although wind, insolation, relative humid­ity, and temperature are major variables in the forest and may confuse results obtained in the field, they can be easily controlled or eliminated indoors. Temperature remained between 71 F and 73 F and relative humidity between 53% and 57%, although no effort was made to control either variable.

The forest duff to be studied was removed from a second-growth stand of ponderosa pine, Douglas-fir, and incense cedar in Blodgett Exper­imental Forest. Ponderosa pine was the major species in the stand, and its needles made up approximately 90% (by weight and volume) of the duff samples studied.

Sections of the forest floor, 1.5 feet x 2 feet, were cut and removed intact by sliding a piece of sheet metal under them. About 1 inch of mineral soil was removed from under ea~ sample. Care was taken to select duff samples which were as uniform as possible. Each sample was approximately 3 inches (7.6 cm) thick, and the average oven-dry weight of the whole duff layer was 1.09 lb/sq ft (5324.7 g/m2).

Each section was then placed in a plywood box about 7 inches deep and with about 4 inches of mineral soil in the bottom of it to simulate field conditions as closely as possible. The boxes fitted snugly on all sides of the duff sections.

From each duff section, two samples were cut out, each approx­imately 6 x 6 inches. These small samples were then removed from the box and cut horizontally into five subsamples, each of approximately the same thickness (0.6 inch). Each subsample was then placed on a small piece of nylon mesh, and the five layers were then replaced in their original vertical sequence, the fifth layer being directly above the mineral soil. The 6- x 6-inch sections were then replaced in their original position in the larger sample, nylon mesh protruding up from under each layer to facilitate removal (see Figures 4(a) and 4(b». Layers of equal thickness were selected arbitrarily, since it appeared that separating duff layers on the basis of decomposition (as was done in the field study) was based entirely on personal opinion, and thus layer distinction might vary with the individual performing the experiment.

Four duff samples were obtained and prepared in the manner just described, and different amounts of water as simulated rainfall were added to them. The first box of duff had 0.5 inch of water added to it, the second had 1.0 inch, the third 1.5 inches, and the fourth 2.0 inches. Each of the five layers in each 6- x 6-inch sample was removed and weighed immediately before and after the simulated rainfall and then returned to its original position in the larger sample. As the duff layers dried, weighings were made every 12 hours for the first week and daily thereafter for 30 days. The nylon mesh used was fine enough to hold the layer it supported intact, but it still permitted water to pass through.

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Figure 4(a).

Figure 4(bJ.

The five individual duff layeps with the nylon mesh used to sepapate them. Each layep is 0.6 inch thick and 6 x 6 inches in apea.

A 1.5- x 2.0-foot duff sample with two 6- x 6-inch samples pemoved. This exposes the undep­lying minepal soil.

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The device used to simulate the rainfall was a sheet-metal tray of the same dimensions as each duff box. Small holes of equal diameter were punched on a I-inch grid pattern over the entire bottom of the tray. Layers of cloth and newspaper were placed inside the tray and, when the prescribed amount of water was added, this retarded water flow through the holes sufficiently to establish a regular and uniform "rainfall" pattern with equal amounts of water coming through each hole. The droplets were approximately 0.2 inch in diameter and the "rainfall" rate was about 1 inch/hour. Since the "rainfall" distribution was uniform, it can be assumed that each part of the duff box received the same amount of water (0.5, 1.0, 1.5, or 2.0 inches). The tray was suspended 4 feet above the duff boxes during wetting and was moved back and forth slightly so that "rain" fell on all parts of the duff samples.

At the end of the 3~-day drying period, each subsample layer was oven-dried and its moisture content value for each day determined. The wetting effects of each of the four different simulated amounts of rain are shown in Figures 5 to 8. In each case, graph (a) shows the moisture content (as a percent of the oven-dry weight) for the five separate duff layers, plotted against the time (in days) after "rain," whereas the smaller graph (b) shows the moisture content of the five different layers on a specific number of days after wetting. Figure 9 shows the drying curves of the whole duff layers (one for each "rainfall" amount).

The duff used in these laboratory studies had been indoors for a number of days before wetting, and as a result the moisture-content values of the different duff layers were quite low--a situation encountered in the forest after a prolonged drought. Moisture-content values ranged from a minimum of about 15% in the first (or uppermost) layer to about 30% in the fifth (or lowest) layer in the laboratory samples before wetting. Since dry duff might not absorb water as well as duff in a slightly moist con­dition, the same wetting approach was applied to moist duff to determine if its wetting and drying pattern was the same as that of the dry duff. Four duff samples were selected as before and placed on 4 inches of mineral soil, and 1 inch of simulated rainfall was added to each sample. After the samples had been allowed to dry for 1 week, the prescribed "rainfall" amounts were added as before. The amount of water retained was measured, and the results, which appear in Table 2, will be discussed later in this report.

It was felt that perhaps the 4 inches of mineral soil in the bottom of each duff box might not be sufficient, and that a deeper soil layer might have been needed under the duff to prevent water that had passed through the forest floor from accumulating in the mineral soil to such a degree that it backed up and affected the moisture-content readings for the lower duff layers. To test this, a duplicate study was run in which the same amounts of "rain" were added to duff samples of similar thickness, which had been placed in deep plywood boxes containing 2 feet of mineral soil to allow very free drainage. The results of this part of the study are discussed in the following section.

10

.... .....

0.5- RAIN

180. 7. MOISTURE CONTENT

160.

2

lAO I '" ~3 0( 5 (b) -

1120~ 4

5

w lit

5 (0) ::> 11'1 .... <II

(5 ~

N

. \.~ \. ~·;:., .... -----------~~t

". '':'::::':'==: --~......:... .....::~ '-- .. _ .. _ .. _ .. __ .. _-, 20

o 10 20 30

TIME (days)

Figure 5. ~ying rates for the five separate duff lay­ers after 0.5 inah of simulated rainfall.

180,

I 16'

lAO

120

i i 1001\

'" 60 III: ::>

1.0"RAIN

7. MOISTURE CONTENT 0 ~~ UUI

II It' ...........

2

'" ~3 0( ....

4

5

6(0)

~ Aot,: -r-\ -, ............ La~er ~ \ "-. ...-... ---------1 ,., '" '- _., . "'-..._._._. -, . -0,- "---- --------=-.. -1 201 ~ ... __ •• _ •• _ ••

6 10 2 TIME (day.)

30

I~I! 200 I

Figure 6. Drying rates for the five separate duff layers after 1.0 inah of simulated rainfall.

..... N

1.5" RAIN

7. MOISTU

I~I 0

I

160 2

150 l '"

140' ~3 « ~ 7(b)

'" ...: 120 • t

~ " 5 ~

20

7(a) \\~, -,

W 1 \ \ ' ~: " ; 60 \ \ • '"

o . \ " :( .\ '" Layer ~ 40 • \," ''''_._._ • .::::;

.'-.::""---- - - -::=:=: _.e_ -.--.. - ...

o 10 20 30 TIME (days)

Figure 7. DTying rates for the five separate duff Lay­ers after 1.5 inches of simuLated rainfaLL.

..2.:Q."RAIN

180

~ 0

I

~UOI \ 2 _I \\" ~" 1

i )..

.=!!120 .. z w Z10' o U _\.

~j~ '\"'" o \ . :(60: \

~ \ -', S(a)

\ \. "Layer " ........... --5 401 '\ .......... ---_A ~ '-'-'-=--=l

20

(j

", -~--.'-.. -, "--.. """"-- ... _ .. 10 20

TIME (days) 30

SIb)

Figure 8. DTying rates for the five separate duff Lay­ers after 2.0 inches of simuLated rainfaZL.

w a:

120

;:, 40 .... V)

o ~ ~ 20

o

2.0"rain

1.0"rain

O.S"rain

10 20 TIME (days)

30

FiguPe 9. Drying aurves for the duff layers as a whole, not separated into layers as in Figures 5 to 8.

Table 2. Comparison of total retention capabilities of dry and moist duff

Moist duff Dry duff

Rainfall Percent Amount Percent Amount

(inches) retained retained retained retained (inches) (inches)

0.5 10.24 0.051 19.52 0.098 1.0 9.00 0.090 12.73 0.127 1.5 7.24 0.109 10.92 0.164 2.0 5.09 0.112 10.83 0.217

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100

I- 75 Z w I-Z o u w 50

'" ::> t-Il)

o ~ N

2

o

__ ...... Loyer .......... --...: ------5

~----::...:~::-::a

._.-'--'-3 .. ---..... ..

10(b)

1.0"Rgin

---··-1

6 100

I- 75 z .... I­Z o u 50 w

'" ::> l-II)

o ~ 25

N

o

10 (0) O.S"Roin

layer

··,~ __ --__ 5 ----, , ":::::-;._-3 .-'-' ··......::::--2

•• .. 1

2 4 TIME (days)

6

Figure 10. Drying rates of duff layers when evap­oration is retarded.

A final experiment was conducted in the laboratory to determine whether a downward movement of water in the duff with time after wetting was being masked by evaporation from above to such an extent that it was not detectable unless evaporation was retarded. Duff samples 6 x 6 inches were cut out, sectioned, and wet as before. They were then covered completely with metal trays which prevented any evaporation from the duff layer. The samples were uncovered and weighed every 12 hours for a week, and the drying curves for each layer appear in Figure 10. Only results from the 0.5- and 1.0-inch "rains" are given, since the drying rates for the samples receiving 1.5- and 2.0-inch rains were generally similar to those shown.

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DISCUSSION

Field Study

Figure 1 indicates the manner in which the forest floor dries over a period of time. Obviously, in the absence of rain, the forest floor is continually drying. This rate of drying is, of course, dependent on many factors, weather and site conditions being the most important. Of the five sample locations, some dried out more quickly than others, because of differences in duff thickness and canopy density; however, all sites showed the characteristic drying pattern that appears in this composite graph.

A striking feature of duff drying that this graph shows is the formation of a characteristic moisture-content profile within the duff layer. Immediately after snowmelt (April 6), the highest moisture content is not found at the mineral soil interface, i.e., the fifth layer, but in the third layer. As drying progresses the highest moisture content is found at in­creasingly greater depths within the duff, until finally, after a prolonged drought, the fifth layer contains the highest amount of water. Actually, the moisture content of all layers is decreasing with increasing time after rain, but the moisture content of each layer decreases more than that of the layer directly below it. Thus the whole duff layer appears to dry out from the top downward.

Since each duff type studied (the types studied at Petawawa included) showed this inversion as a characteristic of its moisture content, perhaps some of the possible reasons for this should now be discussed.

The f1rst possible reason for the inversion may be that, because of some physical or chemical property, the fifth (lowest) layer is incapable of holding as much water as the layers above it. However, the full satura­tion tests show that, with the exception of Location IV, where the fourth layer had a higher maximum moisture content than the fifth layer, the maximum moisture-holding capacity of duff increases with depth (Figure 2). Each successive layer is capable of holding more water than the layers above it, although the relationship is not linear. In the Canadian study (Figure 3), however, for all three duff types studied, the lowest duff layer could not hold as much total moisture as the layer above it. This is probably because, in the Petawawa study, the fifth layer was mainly humus, whereas no humus was found in the California duff studied. Thus, it would appear that where humus is present, it reduces the amount of water that the lowest duff layer is capable of holding, and this contributes to the moisture content inversion. Figure 2 shows that the full saturation value for the litter (uppermost) layer in the deciduous stand (bO) is much higher than the litter values obtained on the coniferous sites. Litter values for all sites studied here correspond well with values found in the literature. The range in maximum moisture content between the five layers is much narrower for the deciduous site than for any of the coniferous areas.

Other possible reasons for the inversion are: (1) that during rain, not enough water reaches the fifth layer to raise its moisture content

15

to the level of those layers above it; (2) that the mineral soil under the duff draws the water in the lower duff layers downward, and that this makes the moisture content in the fifth layer lower than in those above it; or (3) that a combination of both these effects is in operation. These possibilities were investigated and will be discussed later with the laboratory results.

On May 14, 1.5 inches of rain fell on the study areas, and on June 7 another 0.5 inch fell. Although these rains served to decrease the Buildup Index considerably, they did not increase the duff moisture content sufficient­ly for significant increases to be measurable a few days later. It appears that prolonged soaking under winter snow increased the moisture contents of the various duff layers much more than subsequent rainfalls. This point will also be discussed further when the laboratory study is dealt with.

Laboratory Study

Although it may appear somewhat artificial, the indoors study of duff drying and wetting under constant laboratory conditions gave results that were more easily interpreted than had the study been done in the forest.

Figures 5 to 8 show the drying characteristics of the five duff layers for 30 days after "rainfall." Graph (a) in each case shows that, under the four different amounts of "rainfall," the drying curves for the top two layers follow a characteristic smooth, fairly exponential pattern-­a fast initial drying rate, followed in turn by a period of moderate drying and a prolonged period of only very slight drying. That the third, fourth, and fifth layers dry in varying manners is probably due both to the insul­ating effect of the layers above them, and to the influence that the amount of water added has on their moisture contents. For example, 0.5 inch of "rain" raises the moisture content of the first and second layers to almost the same level as 2.0 inches do, whereas 2.0 inches of "rain" raise the moisture level of the third, fourth, and fifth layers much more than 0.5 inch does. Each layer takes longer to dry to a fairly constant moisture content than does the layer above it because of the previously mentioned insulating effect. In no case does the fifth layer absorb more than the fourth, and as a result the inversion appears between these layers for all "rainfall" amounts. It was not present_before wetting. The length of time this inversion is present depends on how long it takes for the fourth layer to dry to a lower moisture content than the fifth layer, as well as on the difference in moisture content between the fourth and fifth layers. In each case the inversion disappears at the point on the graphs where the drying curves for the fourth and fifth layers cross. Graph (b) in each case shows that the highest moisture content level moves downward with time after wetting, each duff layer drying at a faster rate than the layers below it.

Figure 9, which is fairly self-explanatory, shows that the greater the "rainfall," the more slowly the duff layer as a whole dries out.

After the calculation of maximum moisture-holding capacities of the duff layers studied in the laboratory, it was found that there was a marked decrease in percentage of full saturation with increasing depth. In the

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field study, the moisture values on April 6 were closer to full saturation in the third and fourth layers than in the upper layers because of the soaking effect over winter, whereas after laboratory wetting the upper two layers were closest to full saturation. However, field measurements were not made directly after snowmelt and, as a result, the two upper layers had dried out somewhat. Right after snowmelt, the top four layers in the field study would probably be nearly saturated. Why then is this not the case after laboratory "rainfalls" of up to 2.0 inches? The answer can be seen by observing the data given for dry duff in Table 2.

This table shows that only a very small percentage of the water added is actually absorbed by the duff; the rest passes through to the underlying mineral soil. From the data shown, it is obvious that the more "rainfall" added (up to 2.0 inches), the more is retained by weight, and the less is retained by percentage of total "rain" added. For example, a higher percentage of water is retained after the 0.5-inch "rainfall" (19.52%) than after a "rainfall" of 2.0 inches (10.83%); however, more water is actually retained after a 2.0-inch "rain" (0.217 inch compared with 0.098 inch).

The reason for the low rate of retention appears to be threefold: (1) the rate of "rainfall" was quite fast (1 inch/hour); (2) the "raindrops" were of large diameter; and (3) the duff was quite dry before wetting took place.

An attempt was then made to compare the retention capability of a moist duff with that of the dry samples already studied. It was considered that moist duff (wetted with 1 inch of "rain" the previous week) would be more capable of retaining water than dry duff, but Table 2 shows that for each of the four amounts of "rainfall," the dry duff absorbed more of the water added than did the moist duff. In other studies (Van Wagner, 1965~) it has been reported that duff at a low moisture-content level does not absorb as much rain as duff that is somewhat wet. Although this was not the case in this study, it should be noted that neither duff absorbed much water at all--certainly not when the amount absorbed is compared with the amount added. The drying curves followed the same pattern as those for the dry-duff study and hence are not shown here.

In the duplicate study, in which the duff was on 2 feet of mineral soil, there were no appreciable differences in duff wetting and drying characteristics from the study carried out on 4 inches of mineral soil. The 4-inch-deep mineral soil layers reached higher moisture contents after each of the four "rainfalls" than did the 2-foot-deep soils, and the shallow soil layers dried out more quickly. However, since the wetting and drying patterns of the duff layers were the same over both soil depths, it can be concluded that depth of mineral soil had no significant effect on the drying of duff. With the 2-foot soil samples the inversion in moisture content

~Van Wagner, C.E. 1965. Drying and wetting rates of duff layers of various thicknesses. Can. Dep. Forest., Petawawa Forest Exp. Sta., Progr. Rep., Project P-61B (unpublished).

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between the fourth and fifth duff layers was still present after all "rain­falls", and it was generally no more pronounced than with the shallower soil. While the 2-foot mineral soil layer simulated a deep forest soil, the 4-inch soil could be considered representative of a shallow soil over bedrock.

Both the field and the laboratory studies show clearly that, as the time since wetting increases, the level of highest moisture content in the duff moves downward. This could be due to evaporation from above drying out each duff layer more than the one below it, or to the slow movement of duff moisture downward with time, or to a combination of both of these factors. The retarded-evaporation test was carried out to see which of these possibilities is most likely. Figure 10 shows that there is a slight increase in the moisture content of the lower duff layers with time. This increase is most noticeable with the first few measurements after wetting and then it diminishes. Despite some evaporation during weighing, all samples showed this small downward movement in the lower layers. The upper duff layers continue to dry with time, but at a very slow rate since evap­oration from above is greatly retarded. Thus it is possible for some downward moisture movement to occur in the lower duff layers where evapora­tion is almost totally prevented.

It is probable then, that there is a downward movement of water in the lower duff layers under normal field conditions, since the upper duff layers effectively retard evaporation in the lower layers. Any down­ward movement of water in the upper two layers would be more than offset by evaporation. This slight water movement would only be noticeable soon after wetting and would disappear quickly as the effect of evaporation from above was felt in the lower layers. It seems safe to conclude that this very slight water redistribution contributes a negligible amount to the changing of the duff moisture-content profile with time.

CONCLUSIONS

A number of conclusions can be drawn from this study.

(1) After a rainfall (or snowmelt) the forest floor has a characteristic moisture-content profile, in which the highest moisture-content level is not at the mineral soil interface but somewhat above it. This inversion is common to all duff types studied.

(2) As time after wetting increases, the level of highest moisture content moves downward through the duff until, after a number of days without rain, the highest moisture level in the forest floor is at the duff­mineral soil interface. The whole duff layer is drying, but the duff at lower levels is drying at a slower rate than the duff above it, since it is more insulated from the air above the forest floor.

(3) Evaporation from the forest floor is by far the most important factor in determining this characteristic duff drying pattern. Each component layer in the duff is more affected by weather variables (wind, relative

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humidity, insolation) than the layers beneath it and thus dries more quickly. Actual downward movement of duff moisture makes a negligible contribution to the duff drying pattern. Slight water movement in the lower duff may be noticeable shortly after wetting, but this is quickly offset by evaporation from above.

(4) The characteristic moisture-content "inversion" evident in this study was caused neither by a lack of water reaching the fifth duff layer nor by the inability of this layer to hold water: the fifth layer proved capable of holding more moisture than any layer above it. A high percentage of the water actually reached this layer, and considerable amounts went through into the mineral soil. It is most likely, although unproven, that the inversion is caused by the mineral soil drawing moisture in the lower duff layers downward. However, the hygroscopicity of the mineral soil was not mentioned and could have been important. The presence of humus under the duff layer also appears to contribute to the inversion, because the humus material seems incapable of holding as much moisture as the duff above it.

(5) The forest floor did not approach full saturation in this study. Highest duff moisture values occur after prolonged soaking under winter snow, and fast rains on dry duff result in comparatively low moisture-content values (owing to the extremely low retention capability of dry duff).

(6) The wetting ot the forest floor does not occur in a uniform manner during fast rainfalls. The upper two duff layers were wet uniformly in the laboratory samples studied, but wetting of the lower duff layers was irregular, the water moving downward only in certain parts of the duff, probably because of channelling. With a finer, more prolonged rain, the whole forest floor may be wetted in a uniform manner through­out.

It is important to realize the limitations of the laboratory section of this study: only one duff type was studied and only one rate of "rainfall" used (with only one raindrop size). If the rate of "rainfall" could be slowed, and the rain particles made finer, this would probably result in a much higher percentage retention and perhaps a different moisture-content profile in the duff layer.

ACKNOWLEDGMENTS

The author wishes to thank C.E. Van Wagner and Gy Pech of the canada Department of Fisheries and Forestry for their help in the formula­tion of this study.

The assistance of Dr. P.J. Zinke, Dr. P.R. Day and Paul Casamajor of the University of California at Berkeley is also gratefully acknowledged.

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BERNARD, J .M. Barrens.

REFERENCES

1963. Forest floor moisture capacity of the New Jersey Pine Ecology 44(3):574-576.

CURTIS, W.R. 1960. Moisture storage by leaf litter. U.S.F.S. Lake States Forest Exp. Sta., Tech. Note 577. 4 p.

KITTREDGE, J. 1948. Forest influences. Amer. Forest. Ser. McGraw-Hill Book Company.

METZ, L.J. 1958. Moisture held in pine litter. J. Forest. 56(1):36.

SIMARD, A.J. 1968. The moisture content of forest fuels. I, II, and III. Can. Dep. Forest. Rural Develop., Forest. Br., Forest Fire Res. Inst. Inform. Rep. FF-X-14, 15, and 16.

WRIGHT, J.G. 1935. Research in forest protection. Can. Dep. Interior, Forest-Fire Res. Note 1 (reprinted 1967).

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