john l. roth & david l. dilch

Cour.Yon;

h.-lnE:,t.S~mc1:enbcr,:,,30:1G5-171.Fra~.;:furt

am J,;ain,1.3.1

:""7-:)

Some Considerations in Leaf Si ze and Leaf Margin ====~==========================================

Analysis of Fossil Leaves -----~------------------­ -------------------------

JOHN L. ROTH & DAVID L.

DILCH~

Bloomington I n t rod u c t i o n

BAILEY & SINNOTT published a series of papers on their obse~­ vations of the relationship between leaf margin and climate in 1915 and 1916. They analyzed published reports and h~rbarium collections of various floras of the world and noted that the percentage of dicotyledonous species of a flora with entire mar­ gined leaves is distinctively high in tropical and physiological­ ly dry areas while it is chRrart~riptically low in temperate re­ gions. They suggested that this relationship might be very use­ ful to paleobotanists in their analysis of the paleoclimatic conditior.s of the Cretaceous and Tertiary (BAILEY & SINNOTT 1915). Likewise, RAUNKJAER (1934) noted a positive correlation between leaf size and precipitation. He observed that as available soil moisture increases the percentage of species with large leaves in a flora also increases. He also proposed a system of classi­ fying leaves by size and correlating them with climateo These pa­ pers received little attcIltion from paleobotanists who at that time were content with their own method of corrclating fossil species with the favored cli~ates of their nearest living rela­ tives. Nevertheless. during the 1960's and 1970's. it became apparent that many of the taxonomic assignments of fossil leaf remains were incorrect, and therefore, the paleoclimatic inter­ pretations based upon these misidentified fossils were invalid (WOLFE & HOPKINS 1967, DILCHER 1971, 1973). Paleobotanists soon turned to leaf margin and leaf size ana­ lysis as an alternative ~ethod of investiGating paleoclimatic conditions which is independent of taxonomic consideration.s.

- 166 ­

Charts were constructed illustrating the correlation of leaf margin and leaf size of modern vegetation with climate, elevati­ on and latitude. The data used to construct these charts was compiled from herbarium collections and from published floris­ tic reports much as BAILEY & SINNOTT did for their original pa­ pers. The fossil floras were compared to these charts to find the closest match in percentage of entire margined species and in leaf size profile. The paleoclimate was then interpreted to be roughly equivalent to any modern flora with similar leaf size and margin type. This method assumes that the foliar physiognomy of a fossil deposit accurately reflects the physiognomy of the living community from which it is derived. However, recent fin­ dings show that depositional systems are selective and tend to distort the physiognomic composition of fossil floras. WOLFE (1969, 1971) pointed out that large leaves tend to be fragmented during transport and, therefore, underrepresented in fossil de­ posits. This would lead to a cooler or dryer interpretation of the paleoclimate than actually existed. MACGINITIE (1953) noted that fossil floras represent riparian vegetation almost exclusi­ vely. In some enviro!l!!lents ripari~n vegetation tends to produce large leaves with entire margins. The overrepresentation of such leaves in a fossil deposit would lead to a wetter or warmer in­ terpretation of the paleoclimate. In other environments RICHARDS (1952) observed that streamside vegetation is often stenophyl­ lous with small, 1anceolate, entire margined leaves. The over­ representation of stenophyllou9 leaves in a fossil deposit would yield a cooler or dryer paleoclimatic analysis. SPICER (1975) discussed the sorting of plant remains in a modern depositional site. He noted that the sun leaves, those leaves on the perifery of the tree crown which are exposed to greatest solar radiation, tend to be smaller and thicker than shade leaves. Since such lea­ ves are exposed to higher wind energies they have the greatest potential for aerial transport and tend to be overrepresented in Bome fossil deposits. Furthermore, shade leaves tend to be trap­ ped withi~ the trunk space of a forest which restricts their dispersal and chances for fossilization. Selection favoring the deposition of sun leaves and restricting the deposition of shade leaves would result in a much cooler or dryer representation in the fossil deposit. Several workers have eluded to the importan­

ce of the seasonality of leaf fall in evergreen and deciduous spe­ cies in the fossil record (CHANEY 1924; FERGUSON 1971; SPICER 1975). Evergreen species usually shed their leaves in the spring when new growth begins. Thi often coincides with an increased rate of deposition due to increased precipitation. Evergreen lea­ ves tend to be thick and entire margined. The overrepresentation of such leaves in a fossil deposit would lead to a wetter or war­ mer paleoclimatic interpretation. FERGUSON (1971) discussed the influences of climate on leaf dispersal and noted that high humi­ dity increases the weight per unit area of leaves and limitg ae­ rial transport. He also pointed out. however. that high humidity favors the development of extensive river systems which would com­ pensate somewhat for the lost aerial transport. Climate affects the physiognomy. dispersal and sedimentation of leaves. Met hod s a n d

res u 1 t

8

In this study the foliar hysiognomy of recent sediments in a small lake near Bloomingtcn, Indiana, U.S.A. was compared to the foliar physiognomy of the forest, surrounding the lake. In analy­ zing the lake bottom sediments 34 sacples 1/2 m2 and 10-15 cm thick were taken at 1/2 m intervals along a 10 m transect from the bank to the middle of the lake. Leaves were washed from each sample in a wooden frame with a screen bo tom and the percentage of large leaves, those over 20 cm 2 in area, in each sample was calculated. A correlation was observed between leaf size and dis­ tance from shore (Fig. 1). Leaves from the riparian vegetation were not transported far from shore and the deposits in the mid­ dle of the lake were composed almost exclusivel JT of small, thick. sun leaves. The 534 leaves recovered were identified to species. The average leaf size for each species was calCUlated and a leaf size profile (DILCHER 1973) was constructed. The percentage of entire margined species was also calculated. In the forest all the trees, shrubs and woody vines were sampled and identified. The percentage of entire margined species was calculated and a leaf size profile constructed. The results are compared in Fig. 2. Fewer than half of the living species of woody plants were re­ presented in the lake bottom sediments. No woody vines were re­ presented, although two species grow along the shoreline. Most of the tree forms were represented but several understory shrubs

were not. If the current method of foliar physiognomic analysis were applied, both the leaf margin and leaf size of the leaves on the lake bottom would yield a colder or dryer climatic inter­ pretation than actually exists in the area. Con c 1 u s ion s Each depositional environment operates under specific selecti­ ve pressures which favor the deposition of some leaf types while restricting the deposition of others. Therefore, the foliar phy­ siognomy of fossil deposits cannot be compared directly to the foliar physiognomy of living plant ommunities. Before foliar physiognomy can be utilized as an index to paleoclimatic conditi­ ons, we must understand the relationships between leaf form and climate and between living plant cODmunities and the leaf depo­ sits derived from them. Many modern leaf deposits should be stu­ died to determinate how the foliar physiognomy of living plant communities is affected in various depositional environments. The d.ata from these modern depositional sites should be carefully correlated with climatic conditions an ataloged for use as com­ parative refel.'f'nce d2ta. sample ca telog sheet is presented in Fi~. 3 using data from this partiCUlar study. The climatic diagram was prepared according to the method outlined by WALTER (1973). In ad­ dition, a short description of the depositional site, the date it wes sampled and its geographic position are given. On the reverse side of the sheet, a species ist of the living vegetation is given with the average leaf length, margin type and size class of each species. Those represented in the lake bottom sediments are indicated with an asterisk. F ssil leaf deposits can then be com­ pared to this catalog of modern depositional ites eliminating much of the variation caused by depositional systems. Furthemore, this catalog may provide us with the data for future paleoecologi­ cal investigations. In addition to increased understanding of leaf deposition and foliar physiognomy, we may find new correlations of leaf form with elevation, latitude or other environmental fac­ tors.

- 169 ­

LAKE

r-. N

o

~100 fI

'--' 90 (I)

~ 80 1 AND LIVING FLORAS

SN,lPLES*

•

I-l

::l '6

BOTTO~1

Leaf Size Profile

•

• •

•

•

• • •• • 6

7

8

9

• 10

Distance [rom Shore (H) *34 samples 1/2 m. sq. , 537 leaves

Fig. 1.

Fig. 2.

Ref e r e n c e s BAILEY, 1.\'l. & SINNOTT, E.YI., 1915: A botanical index of Cretace­ ous and Tertie.ry climates. -- Science, 41: 831-834; r,:ashing­ ton, l-;'ew Xork. BAILEY, I.W. & SINNOTT, B.W., 1916: The climatic distribution of certain types of angiosperm leaves. -- Am. J. Bot., 1: 24-39; Washington. CHANEY, R.W., 1924: Quantitative studies of the Bridgecreek Flo­ ra. -- Am. J. Sci., §: 127-144; New Haven. DILCHER, D.L., 1971: A revision of the Eocene flora of southeas­ tern North America. -- Paleobotanist, 20: 7-18; Lucknow. DILCHER, D.L., 1973: A paleoclimatic interpretation of the Eocene floras of southeastern North Anerica. In: GR~~, A., Vegete­ tion and vegetaticnal history of northern Latin America, pp. 39-59, ~lsevier Pub.Co.; New York. Ferguson, D.K., 1971: The Miocene flora of Kreuzau, West Germany 1: the leaf remains. -- pp. 297, Nc:.:,th Holland Pub.Co.; Amsterd2Jn. MACGINITIE, H.D., 1953: Fossil plants of the Florissant beds, Cc­ lorado. -- Carnegie Insto Wash. Publ. 2.22: 198 pp; Washing­ ton .. RJ1.UNKIAER, C' t 1934: The life forms of plants and statistical plant geography. -- pp. 632, Oxford Univ.Press.j Lcn60n.

- 170 ­ A SAMPLE CATALOG SHEET DESCRIPTION: Small lake, (100 x 20 11), no tributaries, fed by run off and springs, surrounded by Western Meso­ phytic forest.

CLU1ATIC DIAGRAH Bloomington, In. (250 M) [79-91) HAT

HAP 12.6 C 1064 rnm

MAX

45

50

100

40

80 E

39° 10' 'N, 86° 31' W, NE 1/4, Sec. B, T 9 N, R 1 W, Bloomington Quadrangle.

z

COLLECTION DATE:

LOCATION: ,...... ~

60

30

'-'

o

May 1977

H

40

20

E H

P-