Bulletin


BULLETIN 

OF THE 

ILLINOIS STATE LABORATORY 

OF 

NATURAL HISTORY 


URBANA, ILLINOIS, U . S. A. 


STEPHEN A. FORBES, PH.D., L.L.D., 


DIRECTOR 

VOL. XII. SEPTEMBER, 1915 ARTICLE T. 

THE RELATION OF EVAPORATION AND SOIL MOISTURE 

TO PLANT Sl CCESSION IN A RAVINE 


BY 

FRED THEODORE ULLRICH 

M~{UR~l ".,,101\1 "IlK~t~ 

i9L .j SEP 



ERRATA AND ADDENDA

Page 5, first column; line 2, for Sheperdia read Shepherdia; line 11, for

amcricana read americanus.

Page 9, line 5 from bottom, for XYII read X.

Page 42, line 4 from bottom of key, for Pyromorphidae read Eiicleidae.

Page 73, line 7 from bottom of key, for or read atid seldom.

Page 100, just below key, insert as follows:—
The following species were examined:

LopUoptilus cloisclla Clemens

Laverna brevivittella Clemens

Page 107, lines 8 and 9 from bottom, page lOS, line 10 from bottonr, and page

110, line 10, for Cucullianae read Cuculliinae.

Page 110, line 8 from bottom, dele Polia.

Page 112, line 19 from bottom, for Metathoracic read Mesothoracic.

Page 129, line S, for never read sometimes.

Page 131, at end of second line insert Paleacrita Riley.

Page 15S: first column, after Paleacrita, 127, add 131; second column, after

Polia dele 110.

Page 170, line 4, for Strayiomyiidae read Stratiomyiidae.

Page 243, line 2, for alternata read alternatus.

Page 307: line 5 from bottom, for with read and; line 16 from bottom, for

Homeodactyla read Homoeodactyla.

Page 314, line 15 from bottom, for Cecidomyiidae read Coenomyiidae.
Page 321, line 12 from bottom, for Stratomyia read Stratiomyia.

Page 324, line 6, for pantherina read pantherinus.

Page 370, line IS, for Empidoidea read Empididoidea.
Page 428, line 8 from bottom, for ilesnottim read Mcsonottun.
Page 461, line 10 from bottom, for Aeshnide read Aesluiidae.

Page 478, line 13 from bottom, for vigilas read vigilax.

Page 519, line 2, dele side.

Page 528, line 10, for caruncluatum read caruncvlatum.



(;LliNt;OK, ILLINOIS, AND VICINITY



Article I.

—

The Relation of Evaporation and Soil Moisture to

Plant Succession in a Ravine. By Fred Theodore Uelrich.*

Introduction

For some time geologists have surmised that differences in the

evaporative power of the air and the soil-moisture content account

for the succession of vegetation that accompanies the physiographic

changes in the development of a ravine. In order to determine

whether or not this surmise can be supported by experimental data,

this study was made during the summer of 1913. The ravine selected

for investigation was the McLeish ravine, which lies in the south-

eastern part of the beautiful village of Glencoe, about sixteen miles

north of Chicago. It is mapped in the Chicago folio of the United

States Geological Survey as a part of the Evanston-Waukegan re-

gion. The general geographic features of this region are the moraine

plain or rolling upland, the present shore, and the lake plain with its

associated beach ridges. The rolling upland which constitutes the

larger part of this area is glacial in origin, consisting of the usual

glacial drift, clay, sand, gravel, and boulders, and rises more than

sixty feet above the level of the lake. Since the ice sheet retreated,

some of the glacial material has been reworked by rivers, waves, and

winds, and thus is stratified. The heterogeneity in the composition

of the drift and the resulting differences in the resistance to the forces

of corrasion, and the great elevation of the upland above the level

of the lake have furnished and are furnishing excellent conditions

for the development of ravines. Of the many that have been carved

in this upland, the McLeish ravine, although one of the smaller, may
be considered a good representative.

Physiography of the McLeish Ravine

(Plate XVIU)

It would be foreign to this study to give a complete exposition of

the geology and physiography of the ravines of this upland region.

Such a presentation is given by Atwood and Goldthwait ('08). How-
ever, an intelligent appreciation of the method and results of this

study requires a brief description of the location and direction of the

•Accepted by the University of Chicaffo for the degree of Master of Arts in

Botany.



courses of the ravine and its tributaries, the probable origin of the

ravine, and its present physiography. The McLeish ravine is formed
by the confluence of two gulHes, the heads of which, if measured in

straight distance, are each about 2000 feet inland. From the junc-

tion of the two gullies the ravine has an almost easterly direction for

about 1500 feet, until it reaches its main tributary from the south,

v.'hen it continues in a northeasterly direction for about 800 feet to

the point where it empties into the lake. From the south, in addition

to the main tributary, the ravine receives a number of small gullies.

Physiographers have summarized under three heads the succes-

sive stages in the cycle of the development of such a ravine as the

one here considered : the V-shaped valley, with slopes normally con-

vex; the U-shaped valley, a stage of development where detritus

descending the slopes is not all carried away by the stream, and con-

sequently the valley is being widened faster than it is deepened ; and,

finally, the valley with a broad bottom, in which transformation is

affected partly by erosion and partly by deposition in the ravine. The
tributaries of the McLeish ravine are mostly in the first stage of the

cycle, while the main course of the ravine shows that it is passing

from the V-shaped stage into the U-shaped stage.

It is altogether likely that this ravine had its infant development

accelerated by swampy depressions in the upland near to the lake.

Such swamps or marshes within reasonable distance from the head

of the cliff of the lake shore would, through seepage, make materials

between them and the lake shore so mobile that the head of the gully

would work inland with unusual rapidity. The influence of such de-

pressions in ravine formation can at present be seen in the vicinity

of Glencoe.

In the weekly visits to this ravine, the physiographic changes due

to weathering, wash, and lateral corrasion were decidedly noticeable.

Some of the records made at the time of observation read as follows

:

( I
)—during a heavy rain—small streams of clay are flowing leisurely

to the bottom of the stream; (2) it will be only a matter of a rela-

tively short time when two large hard-maple trees will become so

tmdermined by the streams caused by heavy rains as to fall across the

channel; (3) a large slump of clay has been deposited at the bottom

of the ravine during the preceding week, due to the undermining

action of the stream and to the percolating of the water through the

soil above the cavity; (4) in following the ravine, from the point

where the main tributary enters, to its mouth, a gradation of detritus

ranging from boulders and cobblestones to sand is passed (PI. I, Fig.

t ) ; ( 5 ) the waves of the lake usually maintain a bar of shingle and

sand across the mouth of the ravine, which results in the formation



of a pool of water which gradually filters into the lake as more water

is brought down the ravine; (6) after heavy rains the channel is

invariably reopened by sweeping the materials which form the barrier

into the lake. This gives some conception and appreciation of the

force of the current as an active agent in modifying the physiography

of the ravine. These statements indicate the dynamic character of

the McLeish ravine.

Succession of Vegetation

After this brief consideration of the geographical and physio-

graphical features, we may now turn our attention to the vegetation.

Cowles ('oi), in his "Plant Societies of Chicago and Vicinity," calls

attention to the fact that the slopes of the embryonic V-shaped ravine,

after they attain sufficient stability, develop a carpet of luxuriant

vegetation. He says : "In a comparatively few years the vegetation

leaps, as it were, by bounds through the herbaceous and shrubby

stages into a mesophytic forest, and that, too, a maple forest, the

highest type found in our region." This- cpotation describes admi-

rably what has taken place in the greater portion of this ravine and its

tributaries. A somewhat detailed examination of the summer and
fall flora of the ravine showed that the slopes of the gullies leading

into the ravine (PI. I, Fig. 2) were nearly or quite devoid of vege-

tation, with the exception of some mosses on the north-facing slopes.

A wash or gully near the mouth of the ravine, comparable to the

head of a ravine working inland, had rather severe conditions for

plant growth in the upper half of its slope. Owing to the extreme
exposure to light, wind, and heat, and to the absence of humus in the

soil, only a few of the xerophytic pioneers succeeded in establishing

themselves. Near the foot of this wash a more or less stable minia-

ture plateau (PI. II, Fig. 3) was formed. On this, during the latter

part of the summer, there was a great variety and profusion of

asters,—one of the most pleasing sights in the ravine at that time. The
slopes with mesophytic forest vegetation, with its usual undergrowth,

constitute the most general feature of the ravine. This mesophytism
in a ravine is regarded by ecologists as only temporary. After the

vertical cutting has reached base-level, and as lateral cutting reduces

the inclination of the slope, removing the humus and increasing the

exposure, there is retrogression from mesophytism to xerophytism.

This tendency towards xerophytism is checked through the develop-

ment of vegetation, which finally results in the establishment of the

permanent mesophytic climax of the region. In the ravine under con-

sideration one of the strongest features showing the significance of

exposure in the gradual retrogression from mesophytism to xero-



phytism was the paucit}' of tender mesophytic forms and the less

luxuriant vegetation in general on the south-facing slopes. In the

early spring the north-facing slopes of the ravine were covered more
abundantly with the various plants of the vernal flora and certain

mosses than the south-facing slopes.

Pi^ANTs OF the; Ravine and the Oak Uplands

The plants of the ravine are recorded under two more or less

distinct physiographic areas; the xeromesophytic slopes of the gul-

lies (Region i of the table) and the mesophytic slopes of the ravine

(Region 2). Some of the plants of the oak uplands (Region 3) are

listed, to emphasize the differences between it and the ravine. The
vegetation of the oak uplands may be thought of as either antedating

or succeeding that of the ravine. The vegetation in these three suc-

cessional regions is not entirely distinct, but even a superficial ex-

amination of the following list shows striking differences. As this

study covers only the period from June 21 to October 18, 191 3, much
of the vernal vegetation is omitted.

In the examination of this list of plants it should be borne in mind
that the vegetation of the slopes of the gullies and ravine is noted

much more completely than that of the oak uplands ; that the in-

dividuals of certain species were generally more numerous on the

slopes of the ravine than on the slopes of the gullies ; and that in

certain localities of the ravine such plants as Osuioiiiisa longistylis

(PI. Ill, Fig. 5), Amphicarpa monoica (PI. II, Fig. 4), and Aralia

nudicaiilis (PI. IV, Fig. 6) had developed pure growths to the ex-

clusion of all other species. Furthermore, and most important of all

from the standpoint of this study, the list of plants for any of the

three regions must not be considered as static but as dynamic in char-

acter. As already emphasized in an earlier part of this paper, with

the changes in the physiography of the ravine the vegetation of its

slopes tends to assume a xerophytic aspect, but only until stability of

topography favors the succession of forest types, whose climax in

the Evanston-Waukegan region is essentially the same as that of the

mesophytic slopes of the ravine, the hard-maple type.

An investigation which is to determine whether or not the differ-

ences in the evaporative power of the air and in the soil-moisture

content account for these successions and the minor differences within

each of the areas where plants have been listed, involves, as the first

step, the selection of certain typical stations—stations significant from

the standpoint of exposure and ravine development ; as the second

step, the measurement, at regular intervals, of the evaporative power





6

of the air and the soil-moisture content at the stations selected ; and
finally, as the third and last step, the interpretation of results.

Location and Description of the Stations

In this study seventeen different stations were established; fifteen

in the ravine, one in the oak uplands, station i6, and still another in

the open uplands, station 17 (PI. IX, Fig. 15), the vegetable garden
of the McLeish estate. The fifteen stations in the ravine were lo-

cated, speaking generally, as follows: Station i (PI. I, Fig. 2) in an

embryonic ravine; station 2, in a portion of a ravine, with lateral

cutting delayed, forming a miniature clav cafion; stations 3 (PI. I.

Fig. I), 4 (PI. ni. Fig. 5), 5 (PI. IV, Fig. 6), 6 (PI. V, Fig. 8).

and 7 (PI. VI, Fig. 9), in the part of the ravine with the mesophytic
forest vegetation on the slopes; stations 8 (PI. IV, Fig. 7), 9 (PI.

VII, Fig. 10), 10 (PI. VII, Fig. ri), 11 (PI. II, Fig. 4), and. 12

(PI. VIII, Fig. 12) in a portion slightly less mesophytic than the

preceding; and stations 13 (PI. VIII, Fig. 13), 14 (PI. II, Fig. 3),
and 15 (PI. IX, Fig. 14), on the slopes of a wash or gully near the

mouth of the ravine. The specific geographical location of each of

these stations in the ravine and its tributaries and also of those in the

oak and open uplands is shown on the surface map (PI. XVIII), while

the angle of slope, direction of exposure, and soil and vegetation char-

acteristics are enumerated in the following tabular summary.
The "wilting coefficient"^—a term used by Briggs and Shantz

('12:9) to represent the moisture content of a soil corresponding to

the wilting point of a plant—for various soils in the McLeish ravine

was correlated with the soil-moisture eciuivalent by an indirect method
devised by these authors ('12:56-57). This coefficient indicates

the limit of soil-water content above which growth must occur, al-

though plants will live and continue to draw water below this point.

In listing the dominant vegetation of each station the emphasis
was placed on the herbaceous forms and the seedlings of shrubs and
trees because they are a much better index of present ecological con-

ditions than the mature trees.

Measurement of the Evaporative Power of the Air

The instrument that was used to measure the evaporative power
of the air in the different stations was the Livingston ('10) porous

cup. The structure and operation of this simple instrument have
been so well set forth by the inventor and others that have used it for

scientific purposes that an explanation of its management would be



0
)

Qn<OSu

34



8

superfluous. The writer is aware that the instrument does not register

with complete accuracy the evaporative power of the air when there

has been precipitation during the period for which the measurement
is made. During a rain, indeed, there may be a flow of water into

the bottle. DrizzHng rain for a long period will cause more water

to enter the cup than will the same aggregate precipitation if due to

a heavy rain extending over a sh(\)rt period. Since the results in this

study have only a comparative value, the entrance of water into the

bottles does not vitiate the results. This fact was first pointed out

by Brown ('lo) and later emphasized by Fuller ('ii). In the

handling of the instruments for scientific accuracy, in the selection

of the intervals of time for reading, and in plotting the results, the

writer had the constant advice and suggestion of Dr. Fuller, of the

Department of Botany of the University of Chicago. Without his

help many of the difficvdties would not have been so readily over-

come. All the instruments were standardized before being placed in

the respective stations, and restandardized at intervals of six to eight

weeks. By the coefficients thus obtained all the readings were re-

duced to a common unit. The first reading was made on July 5,

1913, and weekly readings were made thereafter until October 18,

when the last record was made. Thus the investigation extended over

a period of 112 days. Very few of the instruments were molested

during the entire season, and hence the regularity of the results for

each of the stations is rarelv interrupted. These results are expressed

as the average daily loss for the weekly interval between the readings.

These results are graphically represented with the weekly intervals

as abscissje. and the amount of daily loss by the standard atmometer
in cubic centimeters as ordinates.

INTERPRETATION AND DISCUSSION OF RESULTS

I. The graphs in Figure 16 (PI. X) show that the average daily

rate of evaporation for the weekly intervals was almost uniformly

greater in the embryonic ravine than in either the narrow or broad

portions, and greater in the broad than in the narrow. The average

amounts of evaporation per day for the entire season at stations i,

2, and 3 were 5.44 c.c, 3.27 c.c, and 4.75 c.c. respectively; that is

2.17 c.c. per day greater in the embryonic ravine than in the narrow
portion, .69 c.c. greater in the embryonic ravine than in the broad

portion, and 1.48 c.c. greater in the broad than in the narrow portion.

In terms of percentage the rate of evaporation was 66.3% greater

in the embrvonic than in the narrow portion, 14.5;% greater in the

embryonic than in the broad, and 45-2% greater in the broad than



9

in the narrow.* Since all of these stations are located near the floor

and represent successional stages in ravine development, the data

suggest a movement from moderate moisture conditions in the em-
bryonic ravine to greater moisture in the narrow portion, with a

reversion to slightly drier conditions as the ravine changes from the

V-shaped to the U-shaped type.

2. The graphs in Figure 17 (PI. X) show that the rate of evapo-

ration was only about two-thirds as great at the foot of the northwest-

facing slope as near the top of the same slope, and that there was
little difference in the rate of evaporation for the greater part of the

season between the foot and the middle of this slope. The average

amounts of evaporation per day at the foot, middle, and near the top

of this slope were 4.75 c.c, 4.28 c.c, and 6.67 c.c. respectively.

The graphs in Figure 22 (PI. XIII) show similar results;

the amount of evaporation per day was 2.33 c.c, or 63.4%,
greater near the top than at the foot of the north-facing slope.

The rate of evaporation at the middle of this slope was, for

about the first two-thirds of the season, nearly the same as

near the top. The graphs in Figure 19 (PI. XI) show simi-

lar results for the opposing slope, the average amount of evapo-

ration per day being 4.75 c.c, 5.51 c.c, and 7.6 c.c. respectively for the

bottom, middle, and top of the southeast-facing slope—2.85 c.c, or

60%, greater near its top than at its foot. A similar series of stations

in a less mesophytic portion of the ravine exhibit a similar set of con-

ditions to those found in the more mesophytic portion. Here the

rates of evaporation at the bottom, middle, and top of the north-facing

slope (PI. XIII, Fig. 22) were respectively 3.67 c.c, 6.36 c.c, and 6.0

c.c, while for the south-facing slope (PI. XIV", Fig. 24) they were 3.67
c.c, 5.74 c.c, and 7.93 c.c. ; that is, the average amount of evaporation

per day was 4.26 c.c, or 11.6% greater near the top of the south-

facing slope than at its foot. The graph for the daily rate of evapo-

ration for weekly intervals at the middle of the southeast-facing slope

(PI. XI, Fig. 19) is diagrammatically between the graphs for the evap-

oration at the foot of the northwest- and near the top of the south-

east-facing slopes, and likewise the graph for the rate of evaporation

of the middle of the south-facing slope (PI. XIV, Fig. 24) is between
the graphs for the foot of the north- and near the top of the south-

facing slopes. As already pointed out, the rates of evaporation for

the middle of the northwest- and north-facing slopes (PI. XVII, Fig.

17, and PI. XIII, Fig. 22) were not intermediary between those at

the foot and near the top of these slopes; the coincidences in the

•In the computation of the percentage rates the smaller of the two numbers is

always used as the base.



10

rates of evaporation at the middle and foot of the northwest-facing

slope (Fig. 17) during the early part of the season might be attrib-

uted to the fact that the instrument at the middle of the slope was
almost surrounded by a dense growth of Osmorhiza longistylis (PI.

Ill, Fig. 5) ; and the slightly higher rate of evaporation at the middle
than at the top of the north-facing slope during the latter part of the

season (PI. XIII, Fig. 22) might have been due to the unusual expos-

ure of the middle of the slope compared with that at the top ( PI. VII,

Figs. 10 and 11). Further, the differences in evaporation on different

parts of the northwest- and north-facing slopes (Figs. 17 and 22),
are less than on corresponding parts on the opposite slopes, and thus a

station between two other stations would give less striking differ-

ences. In summary, the graphs in figures 17, 22, 19, and 24 show
that as the northwest-, southeast-, and south-facing slopes are as-

cended there is a graded rise in the evaporative power of the air, the

southeast- and south-facing slopes showing this more decidedly than

the northwest- and north-facing slopes.

3. The graphs in Figure 18 (PI. XI) show that the rate of evap-

oration was generally greater near the top of the southeast-facing

slope than near the top of the opposite slope, the average amount per

day for the entire series being nearly i c.c, or 13.9%, greater; the

graphs in Figure 20 (PI. XII) show that the rate of evaporation was
almost unifonnly greater at the middle of the southeast-facing slope

than at the middle of the northwest-facing one, the average amount
of evaporation per day being 1.23 c.c, or 28.7%, greater; the graphs

in Figure 23 (PI. XIII) show that the rate of evaporation for every

weekly interval was greater near the top of the south-facing slope

than near the top of the opposite north-facing one, being 1.93 c.c, or

32.1%, greater; while the graphs in Figure 25 (PI. XIV) show re-

sults at variance with those in Figure 20 (PI. XII)—a greater rate of

evaporation at the middle of the north-facing slope than at the middle

of the opposite south-facing one. This was undoubtedly due, as be-

fore suggested, to the unusual exposure of the middle of the north-

facing slope, which was subject to less shade than any of the other

stations in the immediate vicinity excepting the one near the floor of

the ravine. The results shown by figures 18, 20, and 23 indicate a

greater loss of water and consequently drier conditions on the south-

east-facing slope than on the northwest-facing slope, and likewise

greater loss of water and drier conditions on the south-facing slope

than on the north-facing one. This difference in evaporation is un-

doubtedly due to the fact that the incident rays of the sun strike the

southeast- and south-facing slopes more nearly at right angles, which

means greater absorption and radiation of energy on the southeast-



11

and south-facing slopes than on the northwest- and north-facing

slopes. Furthermore, this difference tends to explain the presence of

tender mesophytic forms on the north-facing slopes of a ravine dur-

ing the early part of spring, before the development of leaves on the

trees, and their absence on the south-facing slopes.

4. The graphs in Figure 21 (PI. XII) show that the rate of

evaporation was greater throughout the season in the oak vtplands

than near the top of the northwest- and of the southeast-facing

slopes, being 2. 11 c.c, or 31.6%, greater per day for the entire season

in the oak uplands than near the top of the northwest-facing slope,

and 1. 18 c.c, or 15.5%, greater than near the top of the southeast-

facing slope. The graphs in Figure 26 (PI. XV) show similarly that

the rate of evaporation in the oak uplands was generally greater than

near the top of the north- and south-facing slopes, the average daily

evaporation for the entire period being 2.78 c.c, or 46.3%, greater in

the oak uplands than near the top of the north-facing slope, and .85

c.c. per day, or about 10%, greater than near the top of the south-

facing slope. This indicates that the upper portions of the south-

facing ravine slopes have an evaporation very close to that of the oak
uplands. Indeed one might expect a greater evaporation near the top

of the south-facing slopes than in the oak uplands, due to the in-

fluence of slope exposure. Further, the differences in the average
amount of evaporation per day between the oak uplands and near the

floor of portions of the ravine were 4.03 c.c. and 5. 11 c.c. In summary,
all of the preceding statements show greater evaporation in the oak
uplands than in any part of the mesophytic portion of the ravine.

These differences may account for the more xerophytic aspect of the

vegetation in the oak uplands than of that in the ravine (see p.

5). The graphs in Figure 21 (PI. XII) and Figure 26 (PI.

XV) further show that the rate of evaporation in the open up-

lands, the strawberry bed. was nearly twice as great as in the oak up-

lands or in the most xerophj^tic portion of the ravine, the average

daily rate for the entire period near the top of the northwest- south-

east-, north-, and south-facing slopes, the oak uplands, and the open
uplands being 6.67 c.c, 7.6 c.c, 6 c.c, 7.93 c.c, 8.78 c.c, and 16.39 c.c.

respectively. The high rate of evaporation in open uplands clearly

shows the effect of shade in retarding evaporation. This gives some
conception of the relatively high rate of transpiration of the plants

that constitute the various crops as compared with the vegetation that

springs up in the forests, it having been clearly shown by Livingston

('10) that there is a definite relation between rate of evaporation of

water from a free surface and the rate of transpiration in a leaf.



12

5- The graphs in Figure 27 (PI. XV) show relatively high rates

of evaporation when compared with the rates in other portions of the

ravine. The average amounts of evaporation per day at the foot,

middle, and head of the gully, or wash, were 8.93 c.c, 8.31 c.c, and

9.32 c.c. respectively. The evaporation on the gully slope shows less-

gradation than on the mesophytic slopes of the ravine, the dififer-

ence between the average at the foot and the head of the gully being

only .39 c.c. The evaporation on the gully slope was about the same
as that in the oak uplands, which was 8.78 c.c. per day for the season.

There is no question that during certain parts of the day the evapo-

ration was greater on the gully slope than in the oak uplands. These

Iiigh rates of evaporation on this slope suggest a vegetation with a

xeropjiytic aspect (see p- 5). In the early part of the summer the

gully slope was nearer devoid of vegetation than during the latter

part of the season. Reference has been made in an earlier part of

tiiis paper to the luxuriance of the late-summer flora of the

slope (PI. II, Fig. 3). These changes in the vegetative conditions

would tend to account for the special variations in the graphs for the

gully slope, the presence of abundant vegetation about the atmom-
eter tending to check the evaporation and its absence tending to in-

crease the evaporation relatively.

6. A comparison of all the graphs, with the exception of those

for the station in the open uplands and the stations on the gully

slope, shows a remarkable similarity in the variation of the average

daily loss for the weekly intervals, the minimum evaporation occur-

ring during the week preceding August 16 and the maximum during

the week preceding September 12. The graphs for the station in the

open uplands and the stations on the gully slope show that the lowest

rate of evaporation was during the same week as for the other sta-

tions, while the highest rate for the open uplands was during the

week preceding August 2, and for the stations on the gully slope

liigh rates occurred during the weeks preceding July 12 and 26, and

August 2 and 31, as well as during the week preceding September 12.

SUMMARY AND CONCLUSION

The average daily evaporation rate for any weekly period was
greater in the embryonic ravine than that for the corresponding

weekly period in the narrow ravine; almost always greater in any

portion of the broad ravine with mesophytic slopes than in the nar-

row ravine
;
greater near the top of slopes of mesophytic portions of

ravine than near the floor; generally greater at the middle of the

southeast-facing slopes than at the middle of the opposite slope; al-



13

most uniformly greater near the top of the southeast- and south-

facing slopes than near the top of the opposite northwest- and north-

facing slopes ; usually greater in the oak uplands than in any meso-
phytic portion of the ravine ; and, finally, the greatest average daily

evaporation for any weekly period for any of the stations was in the

open and exposed upland, as noted for the strawberry bed. (PI. XVI,
Fig. 28.)

If the differences in the evaporative power of the air at the dif-

ferent stations are taken as an index of the relative xerophytism and
mesophytism of the vegetation, the preceding summary permits the

following ranking of the stations, based upon their progressive xero-

phytism : the embryonic ravine, the narrow ravine, the foot of the

mesophytic slopes, near the top of the northwest- and north-facing

slopes, near the top of the southeast- and south-facing slopes, the

oak uplands, and, the most xerophytic of all, the open uplands, repre-

sented by the strawberry bed. In general the movement is from a

moderate rate of evaporation in the embryonic ravine to a lower rate

in the narrow portion, and then a reversion to a higher rate, which
gradually increases until it reaches its climax for the ravine near the

top of the southeast- and south-facing slopes, and the climax for all

of the stations in the open uplands.

The average amount of evaporation for each of the stations, as

shown in Figure 28 (PI. XVI), strongly reinforces diagrammatic-

ally the preceding conclusion.

The data clearly show that the differences in the rates of evapora-

tion at the various stations are sufficient to indicate that the atmos-
pheric conditions are effective factors in causing plant succession in a

ravine.

Measurement of Soil Moisture

Samples of soil were taken weekly at depths of 7.5 cm. and 15 cm.

in the narrow ravine, station 2 ; at the foot of the most mesophytic
slope of the broad ravine, station 3 ; and at the foot of the gully slope,

station 13. Each sample was placed in an air-tight soil jar and
weighed when brought to the laboratory, and then placed in a drying

oven at a temperature of 100° C. for one week, or until it ceased to

lose weight. It was then weighed to determine the loss in weight

resulting from the removal of the soil moisture, and the amount of

such moisture was calculated in terms of percentage of the dry weiglit

of the soil. The results have been plotted with the weekly intervals

as abscissae and the percentages of moisture as ordinates (PI. XVI,
Fig. 29, and PI. XVII, Fig. 30 and 31).



14

It had long been thought that there could be no direct relation

established between the percentage of water in a soil and the amount
that is available for plant growth. Recently, however, a standard

has been discovered by Briggs and Shantz ('12) by which the water
content of the soil may be directly related to continuity of plant vital-

ity. They have termed this standard or constant the "wilting co-

efficient" of the soil, and defined it (p. 6) as "The moisture content

of the soil (expressed as a percentage of the dry weight) at the time

when the leaves of the plant growing in that soil first undergo a per-

manent reduction in their moisture content as the result of a de-

ficiency in the soil-moisture supply." Only the soil moisture over and
above the wilting coefficient is available for plant growth, and hence

this surplus has been termed "growth w'ater." In this investigation

the wilting coefficients of the various soils were found by the "indi-

rect method" devised by Briggs and Shantz ('12:56-57) which is

based on the relationship of the wilting coefficient to the moisture

equivalent as determined by physical methods. These coefficients

have been represented (in figures 29, 30, and 31, plates XVI and
XVII) by transverse broken lines, hence the growth water, or that

portion of the soil moisture available for the growth of plants, is

denoted by the interval between the lines representing the total per-

centage of moisture present and those showing the amount of the

wilting coefficients.

Interpretation and discussion of results .

1. The graphs in Figures 29 (PI. XVI) and 30 (PI. XVII)
show that the percentage of moisture at stations 2 and 3 at a depth

of 7.5 cm. was greater than at a depth of 25 cm. in each of the

weekly examinations that was made. (The results are graphically

represented, with the weekly intervals as abscissse, and the percent-

ages of soil moisture at the time of each examination as ordinates.

)

No such regularity in the per cent, of moisture content at the differ-

ent depths was found at station 13 (PI. XVII, Fig. 31).

2. The samples taken at a depth of 7.5 cm. show in a general

way that the per cent, of moisture content in the narrow ravine at

station 2 was less than in the broad ravine, station 3, and that the

percentage of moisture content at the foot of the gully slope at sta-

tion 13 was less than at either of the other two stations examined.

The samples from a depth of 25 cm. did not show any such varia-

tions. From this we must not hastily conclude that conditions were
more favorable for plant growth from the standpoint of soil mois-

ture in the broad ravine than in the narrow ravine, or more favorable



15

at either of these two stations than at the foot of the gully slope, but
further examine our data. What is really important in soil-moisture

comparisons is the relative amount of growth water, or the differ-

ence between the actual water content of the soil and the water con-
tent not available for growth in the plant. The average percent-

ages of moisture at a depth of 7.5 cm. for broad ravine, narrow ra-

vine, and gully slope were, in the order named, 28.3, 24.1, and 17.7.

The respective .wilting coefficients for the same were 18, 15.1, and
10.2. This indicates that the average percentage of growth water in

the broad ravine was 10.3, in the narrow ravine 9, and for the foot

of the gully slope 7.5. Thus, from the standpoint of soil moisture,

supplies were slightly more abundant in the broad than in the narrow
ravine, and least abundant at the foot of the gully slope.

If the wilting coefficient may be regarded as indicating a condi-

tion in which the water supply of plants is reduced to such a degree

as to cause permanent wilting, a recovery from which is only possible

by an increase of the soil moisture, the graphs clearly show that at

no time did the plants wilt permanently from the lack of water in

the soil. One of the plants that wilted regularly during midday on
sunshiny days was the jewel weed. This must not be attributed to

the lack of water in the soil, but to a more rapid rate of evaporation

from the portion of the plant above ground—a more rapid rate than
that at which the water was taken in by the root of the plant. Such
wilting would not be permanent wilting, which would eventually

mean the death of the plant, but a temporary wilting, which would
mean the restoration of the turgidity of the plant with a decrease of

evaporation such as is caitsed by the coming of nightfall.

4. These data show in general so generous a supply of soil mois-

ture at the various stations within the ravine that it may be safely

assumed that the soil moisture is a factor contributing to the rapid

advance of succession in such situations, and should have consider-

ation in explaining the early attainment of advanced mesophytism.

5. These data also show that while the variations in soil mois-

ture are not great at the different stations, the differences that do
exist correspond in a general way to those of evaporation and may
indicate a contributing factor in causing plant succession in a ravine.

In order to make these soil-moisture studies more convincing, weekly

examinations should have been made at the other fourteen stations.

Conclusion

The close relationship of evaporation and soil moisture to plant

succession in a ravine is clearly evident from this study. It would
have been more convincing if it had extended over a longer period;



16

but the writer does not believe that in case the period had been longer

the general results would have been different, since the investigation

extended over the most critical season of the year, namely, the part of

the summer when the evaporation rates are relatively high and soil

moisture relatively low; and these are the most critical conditions,

for the development and survival of plants.

Acknowledgments

Grateful acknowledgment is here extended to Dr. H. C. Cowles,

of the University of Chicago, for suggesting the problem and site for

study; to Mr. and Mrs. McLeish for their kindness in permitting

weekly visits to their estate, on which the ravine is located ; to H.
DeForest for photographs of vegetation in vicinity of stations, and to

Dr. Geo. D. Fuller, also of the University of Chicago, for his helpful

suggestions in overcoming the difficulties that arose in the course of

tlie investigation.

Literature cited

Atwood, Wallace W., and Goldthwait, James W.
'08. Physical geography of the Evanston-Waukegan region.

Bull. 7, State Geol. Surv. 111.

Briggs, L. J., and Shantz, B. L.

'12. The wilting coefficient for different plants and its indirect

determination. Bull. No. 230, Bur. of Plant Industry, U. S.

Dept. Agr.

Brown, Wm. H.

'10. Evaporation and plant habitats in Jamaica. Plant World,

13: 268-272.

Cowles, H. C.

'01. Plant societies of Chicago and vicinity. Bull. No. 2, Geoj,r.

Soc. Chicago.

Fuller, Geo. D.

'11. Evaporation and plant succession. Bot. Gaz., 52: 193-208.

Livingston, B. E.

'01. Operation of the porous-cup atmometer. Plant World,

18:11 1-119.

'13. The resistance offered by leaves to transpirational water loss.

Plant World, 16: 1-33.



Plate I

Fii*". 1- Stitioii 3, at the foot of the main ravine. Hard maple fallea across the channel as
the result of lateral corrasion.

Fig-. 2 Station 1, in an embryonic V-shaped ravine working its way back into the oak uplands.



Platk II

^^S^4L' "



Plate TIT

Fig-. 5. Station 4. Tlie lower half of the picture shows an
almost pure g-rowlh of Osntorhiza lougistyltSy on northwesl-facinp
slope.



Plate IV

Fig-, b. Station 5, near the top of the uortliwest-faciug slope. Aralia midicaulis is the domi-
naut plant of the undergrowth.

Pijf. 7. Station 8, on the Hoor of the nifsophytic portion of the ravine.



Plate V

Fig-. 8. Station 6, near the middle of the southeast-facing
slope. Hard maple and ashes are dominant.



Plate VI

Figr- *'. Station 7. near tlie top of the south-east facine" slope.



Plate VII

^^Hf^i^^lllilr^



Plate VIII

i'Mg. 12. Station 12, near the top of the south-facing- slope among- red oaks and witch hazel.

Fig. 13. Station 13, at the foot of the wash near the mouth of the ravine. The only place in the
ravine showing Jumperus communis.



Plate IX

Fig. 14. Station 15, ai the head of the wash near the mouth of the ravine.

Fig-. 15. Station 17, on the open upland. The strawberry bed on the McLeish estate.



Platd X

JULY AUGUST SEPTEMBER OCTOBER



Plate XI

JULY



Plate XII

JULY



Plate XIII

JULY



Plate XIV



Plate XV

JULY



Plate XVI

17

16

15

14

13

1*^



Plate XVII

JULY



Pirate XVIII

Map of McLeish Ravine, Glencoe, Illinois. Scale about ,>, miles to the inch.