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What is an equable climate?
Palaeogeography, Palaeoclimatology, Palaeoecology, 91 (1992): 1 12
1
Elsevier Science Publishers B.V., Amsterdam
What is an equable climate? D a n i e l I. A x e l r o d
Department of Botany, University of California, Davis, CA 95616, USA (Revised version accepted August 15, 199 !)
ABSTRACT Axelrod, D.I., 1992. What is an equable climate? Palaeogeogr., Palaeoclimatol., Palaeoecol., 91: 1-12. The ideal (theoretical) equable climate has a mean annual temperature of 14°C and a mean annual range of 0°C. Since 14°C is and has been the average mean temperature of the earth's surface, any departure from 14°C and 0°C results in a climate of lower temperateness or equability. Therefore, reference to an equable paleoclimate is meaningless unless quantified by a centrum from which equability is measured. Climates of low seasonality are not necessarily equable because such conditions occur from the inner wet tropics into glacial-border environments. Freezing often occurs in climates of moderate to high equability. Equable climates may support taxa from different vegetation zones, a relation also evident in the paleobotanical record.
Introduction At least three different meanings have been applied to the word equable in reference to climate, whether at present or in the past. First, the comm o n use of the word equable is that in Webster's Dictionary (1979, 2nd ed.): "equal and uniform (temperature) at all times." Table 1 shows that such conditions occur from the innermost tropics into polar climate (Grimsey, Iceland) and also in highland regions (e.g., Cotopaxi = paramo). Cotopaxi in Ecuador has a mean annual temperature (T) of 7.6°C and a mean annual range (A) of 0.9°C, Quito in Ecuador has a T of 13.5°C and an A of 0.6°C, and Madang in Papua has a T of 26.6°C and an A of 0.5°C. If all three examples are considered equable because of their "equal and uniform temperature", the word has no climatic significance because they are in very different clim a t e s - p o l a r , mild temperate, and inner tropical. Furthermore, while such thermal conditions may be termed aseasonal, or of low seasonality, Table 1 shows that these terms have no climatic significance because they occur in very different environments. Second, Sloan and Barron (1990) refer to an equable climate that has three elements: (1) a 0031-0182/92/$05.00
substantially reduced annual cycle of temperature, (2) a lack of extremes (not defined), and especially (3) a lack of subfreezing temperatures even at the poles. When their climatic model experiments show that the continental interiors and areas at high latitudes experience frost, even during the " w a r m " times in earth history, such climates are not considered equable. However, freezing conditions regularly occur in what is generally acknowledged to be the most equable area in the continental United S t a t e s - - t h e west coast of central California-and also the west coasts and in montane regions of other continents (see below). Further, Sloan and Barron do not indicate the limits of an equable climate, nor do they grade the degrees of equability, or define climates that are not equable other than having frosts. Third, Bailey (1960, 1964, 1966) proposed a concept of temperateness, or equability, that includes (a) the idea of reduced annual cycle of temperature (though with a small diurnal fluctuation), (b) a lack of high or low extremes as determined by a mean annual range < 20°C, and (c) freezing may occur in equable climates. In Bailey's model any departure from a mean annual temperature of 14°C and a mean annual range of
© 1992 - - Elsevier Science Publishers B.V.
2
D.I. AXELROD TABLE 1 Stations with "equal and uniform temperature" are distributed from polar (paramo) to inner tropical climate* Mean annual temperature
Mean annual range
Equability
3.2 7.6 9.4
8.9 0.9 1.2
44 58 66
11.7
1.4
82
13.5 16.7
0.6 1.1
94 83
19.0 21.3 24.1 25.9 26.6 26.7
0.8 0.7 1.0 1.1 0.5 1.4
65 55 48 44 43.5 43
(oc) Polar (paramo) Grimsey, Iceland Cotopaxi, Ecuador Papallacata, Ecuador
Microphyllforest E1 Angel, Ecuador
Montane evergreenforest Quito, Ecuador Kabale, Uganda
Tropical forest Tibacuy, Columbia Kibuye, Ruwanda Dundo, Angola Belem, Brazil Madang, Papau, New Guinea Pago Pago, Samoa
*Data from Wernstedt, 1972; World Weather Records, 1951-60, vol. 60.
O°C results in a climate of lower temperateness or equability. His idea is reviewed here because it clarifies the concept of equable climate and has considerable utility for interpreting paleoclimate and the biota that it supports.
Quantification of temperature The problem posed by the word equable in reference to climate is clarified by the nomograph (Fig. 1) constructed by Bailey. To quantify temperature, Bailey used two complementary measurements, Warmth (W) and Temperateness (M= moderation). As these have been applied previously to climatic problems (i.e., Axelrod, 1966, 1981, 1986; Axelrod and Bailey, 1969, 1976; Greller, 1989; Greller and Rachele, 1983), they are restated here briefly. Warmth (W) provides a measure of the growing season. It appears on the nomograph (Fig. 1) as a series of radiating lines that indicate the number of days (d) in which mean temperature reaches a specified level. Along, the radian W lO°C, no days have a mean temperature warmer than lO°C; this
is the edge of polar climate. The radian W 18°C has 365 days warmer than 18°C; this is the outer margin of tropical climate. The middle of the temperate zone is represented by W 14°C, with 183 days warmer than that. Other warmth lines and the duration of their growing seasons are indicated in Fig. 1. Emphasis must first be placed on the fact that in terrestrial environments mean annual temperature (T) alone has no climatic significance. Figure2 shows that T O°C occurs in glacial, tundra, taiga (= boreal forest), and conifer-hardwood forest environments. Localities with T 10°C range from close to tundra into mild temperate climates with mixed conifer and broadleaved deciduous hardwoods. Stations of T 20°C occur from the inner tropics (all days warmer than 20°C) well out into warm temperate regions, as at Mosul, Iraq, with 221 days warmer than W 15.3°C). Temperateness (M), or equability, is represented by a series of arcs nearly normal to the Warmth (W) lines (Fig. 1). The arcs are centered at a mean annual temperature T of 14°C and a mean annual range (A= amplitude) of O°C. This position,
3
WHAT IS AN EQUABLE CLIMATE?
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Fig. I. Nomogram showing the relation between warmth (W) and temperateness (M) or equability of climate (Bailey, 1960, 1964, 1966). The radiating lines indicate the number of days (d) warmer than the stipulated temperature. The arcs, which represent temperateness, are centered at T 14°C and A 0°C. Any departure from that point has a lower temperateness or equability rating which is graded from M 100 to M 0. In areas < T 14°C climate becomes warmer as range of temperature increases, whereas, in areas > T 14°C climate becomes cooler.
midway between polar and tropical climate, was selected because T 14°C is and has been essentially the mean temperature of the earth's surface. Furthermore, when polar and temperate regions were warmer, the tropical zone was cooler than at present. This is suggested by Miocene floras that represent a cooler (upland) vegetation zone than
that at the locality today. For example, the Miocene flora of Taiwan (Chaney and Chuang, 1968) is a broadleaved evergreen oak-laurel forest like that now at elevations above 500 m, yet the fossil locality is now in a tropical rainforest zone. Comparable relations are recorded in southern Japan (Tanai, 1961) where Miocene deciduous hardwood forest with montane conifers was replaced by a mixed broadleaved deciduous-evergreen forest on southern Honshu and by evergreen oak-laurel forest on Kyushu. The relation also appears to be represented in Vera Cruz province, Mexico. Earlier considered Miocene, the flora from Coatxacoalcos, in the Paraje Solo Formation, is now judged Pliocene (N20), according to ostracode stratigraphy in formations above and below the flora (Machain-Gastillo, 1985) and by planktic foraminifera (Akers, 1979, 1981, 1984), both kindly reported to me by Alan Graham (June, 1991). As polar regions became progressively colder in the later Paleogene and Neogene, the tropical region became warmer. Hence, it follows that new taxa not only originated north and south in the developing colder regions to form new communities, but also in the inner tropics as that area became warmer. On this basis, it follows that the more ancient "primitive" taxa would have survived chiefly in intermediate regions of mild to warm temperate, highly equable climate. This is shown by the occurrence of relict gymnosperms and ferns allied to those in Cretaceous floras (Fig. 3). They are preponderantly in mild to warm temperate ("subtropical") climates of high equability (M 60-70). It is this that has given rise to the notion that they indicate "tropical" climate at higher latitudes during the Cretaceous. Furthermore, relict forests of the Tertiary also occur in areas of high equability, as charted earlier (Axelrod, 1966, fig. 9). Examples include coastal California; the Canary Islands; montane Vera Cruz, Mexico; Szechuan-Hubei, China; Valdivia, Chile; New Zealand-SE Australia; Atherton Plateau, Queensland; montane Papua, New Guinea, vicinity of Buloio. As documented by Takhtajan (1969), there is a great concentration of ancient genera of several plant families in the eastern Himalayas, reaching into montane southern Yunnan, a region of high equability. Since no data have yet demonstrated
D.I.AXELROD
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unequivocally that the mean temperature of the earth's surface has been much colder or warmer than at present (Kennett, 1977; Shackleton, 1984; Adams et al., 1990; Barron, 1990; Kasting, 1989; Zachos et al., 1990), T 14°C evidently has had a central position since the Cretaceous and no doubt earlier. Any departure from the centrum of mean annual temperature of 14°C and a mean range of 0°C results in a climate of lower temperateness or
equability. Bailey recognized six broad categories (Table 2) of temperateness of the continents, as illustrated in Figs. 4 and 5. The degree of temperateness ( M = moderation) decreases with a greater range of temperature. For example, Oakland, California, and Des Moines, Iowa, both have a mean annual temperature of 14°C (Fig. 2). The mean annual range at Oakland is 9.5°C, but at Des Moines it is 30.4°C. Whereas light freezing may occur on 4-5 days/yr at Oakland, at Des Moines
WHAT IS AN EQUABLE CLIMATE?
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Fig. 3. Examples of equable climates in which relict conifers, cycads, and ferns occur that are allied to those of the Cretaceous. Costa Rico, "The richest tree-fern region in the world": 1 = Santario Duran, 2= Pacayas, Mexico: with cycads Zarnia, Dioon, Ceratozamia, tree ferns, Alsophila, Cibotium, Cyathea, 3 = Jalacingo, 4 = Chalcatongo, 5= Atzalan, 6 = Ayutla Mixe, 7--- Jalapa, 8 = Orizaba. Jamaica: Ferns, such as Marattia, Cyathea, Gleichenia, ,41sophila, Hymenophyllum, 9= Cinchonia. Malaya: 10 = Cameroon Highland, with ancient ferns, Matonia, Gleichenia, Dipteris, Cheiropleuria. New Zealand: with Agathis, Dicksonia, Cyathea, Podocarpus, 11 = Waipoa, 12= Te Paki, 13= Dargaville. Brazil: with Araucaria, numerous tree ferns, 14= Curybitana, 15= Curitiba. Australia: with numerous cycads, such as Macrozamia, Bowenia, Lepidozamia and tree ferns, 16= Albany, 17= Toowomba, 18 = Perth, 19= Norfolk Island, 20= Atherton. South Africa: with 25 species of the cycad Encephalartos, numerous podocarps, 21 = Port Elizabeth, 22 = Mabana, 23 = Pietermaritzburg, 24 = Graft Reinert. Chile: with araucarias, podocarps, tree ferns, 25 = Valdivia, 26 = Masatierra on Juan Fernandez Isl. (with the relict Thrysopteris). 1200 h r / y r r e g u l a r l y h a v e f r e e z i n g , s n o w , a n d ice. A s a result, t h e s e a r e a s h a v e v e r y d i f f e r e n t t e m p e r a t e n e s s o r e q u a b i l i t y r a t i n g s , M 64 a n d M 42, r e s p e c t i v e l y . T a b l e 1 also s h o w s t h a t t h e m e a n
a n n u a l r a n g e o f t e m p e r a t u r e is o n l y 0 . 5 ° C at M a d a n g , 0 . 6 ° C at Q u i t o . M e a n a n n u a l t e m p e r a t u r e a t M a d a n g is 2 6 . 6 ° C a n d 13.5°C at Q u i t o . T h e i r v e r y d i f f e r e n t t e m p e r a t e n e s s ratings, M 94
6
D.I. AXELROD
TABLE 2 Major categories of temperateness of the continents (Bailey, 1964). Category
M value
Examples
Supertemperate Very temperate Temperate Subtemperate Intemperate Extreme
100-80 80-65 65-50 50-35 35-20 20-0
Quito, Bogota, Equator (Kenya), Molo (Ecuador) Valpariso, Ensenada, Curitiba, Addis Ababa Norfolk, Paris, Capetown, Portland, Nairobi Juneau, Manaus, Rio de Janeiro, New York King Salmon (Alaska), Fairbanks, Aldan (Siberia) Verkhoyansk, Oymyakon, Yakutsk
Fig. 4. Temperateness of the contimerous United States. From Bailey, 1964. With permission of the American Geographical Society.
at Quito and M 43.5 at Madang, express their departure from the centrum of T 14°C and A O°C. Although some consider that a low range of mean annual temperature (i.e., low seasonality) indicates equable climate, such conditions occur from the inner tropics into polar regions (Table 1). Since all the stations in Table 1 cannot be equable, a centrum must be selected from which equability is measured. At present, T 14°C and A 0°C appear to be the best reference for estimating temperateness (Bailey, 1960, 1964). Figure 1 shows that the Warmth lines converge as the range of temperature decreases. With a
range of 5°C, polar (W 10°C) and tropical (W 18°C) climates are 13°C apart and separated in elevation by 2379 m, assuming a normal lapse rate (1°C/183 m). With a range of 20°C, they are separated by 26°C and 4758 m in elevation. Clearly, in a climate of high temperateness or equability, biota of different thermal zones may live in proximity. It is this that accounts for some of the "unusual" occurrences of fossil species in floras that lived under moderate to high temperateness. Examples are provided by the association of sclerophyllous live oak (Quercus), Catalina ironwood (Lyonothamnus), and mountain mahogany
WHAT
IS A N E Q U A B L E
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(Cereoearpus) with Sierra redwood (Sequoiadendron), fir (Abies), and spruce (Picea) in Nevada
superb photographs (19 x 12cm) of Sonoran Desert vegetation in Shreve's monograph. Finally, it is recalled that climates of high equability regularly harbor ancient genera and relict species. These occur in the temperate rainforests from Vera Cruz to Chiapas, eastern Mexico; in Yunnan, southern China; in the mixed mesophytic forests of Hubei-Szechuan, central China; in the sclerophyllous woodland and scrub of coastal California, Spain to Turkey, Chile, south Africa and South Australia (see Aschmann, 1973); in the laurel forest of the Canary Islands; in the temperate rainforests of southern Chile and New Zealandall areas with temperateness or equability ratings of M 60-70 +.
Miocene floras (i.e., Axelrod, 1956, 1985, 1991). Spruce and fir are also recorded with lauraceous genera (Machilis, Phoebe), magnolia, red gum (Liquidambar), elm (Ulmus), and broadleaved deciduous oaks in the Miocene of Oregon, Washington and Idaho (Chaney and Axelrod, 1959; Smiley and Rember, 1985). Local climate in a forest zone may harbor a more diverse (richer) flora than that in the adjacent area, providing conditions are more equable. For example, in the Salmon Mountains, northwestern California, the valleys of Sugar, Duck Lake and Horse Range creeks have as many as 10-12 conifers associated within a radius of scarcely 100 m. These represent the richest association of conifers on this continent. Because all these valleys have a northeast orientation, they are less influenced by the hot summer sun, evaporation is lower, and there is cool-air drainage from higher elevation. They are, therefore, more equable than valleys in the adjacent region that are oriented east, west, or south, and whether they are broad or narrow. In those valleys, the same species that are associated in the valleys of Sugar and adjacent creeks are separated by fully 600 m (2000 ft.) in three regional forest zones-mixed conifer, fir, and subalpine. A different example is provided by the flora of the Sonoran Desert (Shreve, 1951). The lowland flora that lives under a hotter desert climate is not so diverse in composition and forms more open communities than those in the uplands. Temperature in the latter areas is regularly more temperate, the differences being well expressed in Table 3. They can especially be grasped by examining the
Discussion
An increase in the range of temperature (A) increases the amount of freezing. A small increase in A is more significant in bringing freezing conditions to cool regions than to warmer ones. In this and other examples (Axelrod, 1984, 1986), estimates of the frequency of freezing are in reference to "normal" frequencies as developed by Bailey (1966). Figure 6 shows that at T 10°C, if the range of temperature is 8°C, about 1% hr/yr would experience freezing (Loc. A), but with a range of 10.5°C, 3% hr/yr freezing would be expected (Loc. B). At T 15°C, with a range of temperature of 14°C, 1% hr/yr would have freezing (Loc. C), whereas with a range of 18°C, 3% hr/yr would have freezing (Loc. D). At T 20°C and A 19.5°C, freezing would be expected on 1% hr/yr (Loc. E), but at A 25°C, 3% hr/yr would have freezing (Loc. F).
TABLE 3 Comparison o f climatic data in lowland and upland Sonoran Desert vegetation, Baja California Sur* Station (elev.)
Position
Mean annual temperature (°C)
Mean annual range
Temperateness or equability
Loreto (12 m) C o m m o n d u (400 m)
26 ° 1', 111 ° 21' 26 ° 2', 111 ° 49 °
24.8 22.5
14.1 11.7
M 43 M 48
Santa Rosalia (26 m) San Ignacio (150 m)
27 ° 19', 112 ° 21' 27 ° 25', 112 ° 52'
23.6 21.8
15.1 12.0
M 44 M 49.5
*Climatic data from Garcia, 1973.
9
W H A T IS AN E Q U A B L E C L I M A T E ?
TABLE 4
MEAN ANNUAL RANGE OF TEMPERATURE
5
10
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25
Hours of frost in zonal forest types, southeastern United States*
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Tropical semi-deciduous Key West, FL Homestead, FL
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Fig. 6. Illustrating the relation between mean annual temperature and frost frequency as the range of temperature increases.
These estimates of the approximate frequency of freezing raise the question of its occurrence in forests of broadleaved evergreens or forests with mixtures of broadleaved evergreens and deciduous hardwoods. Although broadleaved evergreens have been considered by some as "frost-free" indicators, they are not. Such mixtures of species regularly occur where freezing is of frequent occurrence, as on the Atlantic coastal plain from northern Florida to coastal North Carolina where fully 5 % hr/yr have freezing (Table 4). The mixed mesophytic forest of eastern Asia, which ranges from the mountains in central China to southern Korea and central Japan (Wang, 1961), represents the transition between broadleaved evergreen and mixed deciduous hardwood forests. It has a frost frequency of 5-10% hr/yr (Wang, 1961, table 14; Bartholomew, et al., 1983, table 1). These periods of freezing are of brief duration on each daily occurrence. Approximately 2-3 days with continuous subfreezing temperature probably would eliminate most broadleaved evergreens. Obviously, the present frequencies of freezing cannot be translated directly into conditions of the Tertiary because global climates were not so severe as at present. The oceans were warmer at middle and high latitudes, ice caps and frigid cold waves were
Temperate broadleaved evergreen forest Ft. Meyers, FL Coco Beach, FL Tampa, FL Orlando, FL Panama City, FL Jacksonville, FL Pensacola, FL Valpariso, FL
1 3 6 14 53 81 100 125
Southern mixed hardwood (Canopy of bd-lvd, evergreen and bd-lvd, decid, trees) Savanna, GA Ft. Rucker, AL Charleston, SC Montgomery, AL Myrtle Beach, SC Augusta, GA Moorehead City, NC
152 197 209 250 325 349 370
Deciduous forest (Oak-Pine Hickory) Atlanta, GA Greenville-Spart'nbrg, SC Elizabeth City, NC Richmond, VA
371 375 375 795
*Kindly assembled by Andrew M. Greller, 1990. **Data from U.S. Dep. Air Force, Army, and Navy, Engineering Weather Data, 1978.
not so developed as today, nor did mountainous tracts and high plateaus have their present elevation-all of which contributed to lower ranges of temperature and hence to greater equability in all vegetation zones. As for the fossil record, Paleocene floras of the Great Plains show a transition from a dominantly broadleaved evergreen to mixed evergreen-deciduous hardwood forest in proceeding northward from the Raton basin, New Mexico, to southern Canada (Brown, 1962). The area of the northern plains (Dakota, Montana, Wyoming) was covered with a rich mixed mesophytic forest. Deciduous hardwoods were dominant though mild climate is implied by such genera as Zamia, Araucaria, Sabal,
10
Thrinax, Ficus, Cinnamomum and Bauhinia. The assemblage suggests a mean annual temperature of about 14°C and a mean annual range of 12-14°C. These estimates give an equability rating of M 60-62 and a frost frequency of 1.0-1.5% hr/ yr with freezing. The percentage of hours with freezing increased northward as broadleaved evergreens were replaced by more numerous deciduous hardwoods and temperate conifers (spruce, pine, dawn redwood, etc.). Conditions farther north in the interior were more severe, as suggested by Sloan and Barron (1990) on the basis of their model studies. By middle Eocene time (45-47 Ma) the volcanic uplands (Challis episode) that stretched from northern Nevada (Bull Run flora) through Idaho (Hailey, Thunder Mountain floras) and northward were covered with montane conifer forests at elevations near 1500-1200 m, decreasing northward (Axelrod, 1968). In that area, mean annual temperature at the lower margin of the montane conifer forest in the transition to mixed conifer-deciduous hardwood forest was near 8°C and the range of temperature was about 10-1 I°C. The forests in that ecotone probably were subject to at least 8% hr/yr (c. 700 hr) with freezing, and temperateness, or equability, was about M 55 (Axelrod, 1990). Finally, attention is directed to the strength of the thermal relations advocated by Bailey (1960, 1964, 1966), for they provide the most reliable and reproducible method yet devised for estimating the climatic field of a modern or fossil flora, or of a taxon. From a single point on the nomograph (Fig. 1), determined from the mean annual temperature (T) and the mean annual range of temperature (A = amplitude), the following can readily be determined (also see Axelrod, 1981; Axelrod and Bailey, 1976): (a) the mean temperature of the warm month and cold month, (b) the march of mean monthly temperature, (c) the approximate amount of freezing, if any, (d) the duration of the growing season (W= warmth) in terms of the number of days warmer than a given temperature, (e) the temperateness or equability rating of the assemblage, (f) the water-need (N) indicated for the thermal regime, and (g) the approximate elevation of a fossil flora in an interior area by compari-
D.I. AXELROD
son with temperature inferred for a flora of similar age at or near sea level. Some investigators have reconstructed paleoclimate using only the mean temperature of the warm month and cold month. This gives an incomplete climatic analysis because it does not provide an indication of the duration of the growing season, nor the degree of frost frequency, of the equability, or of water-need. Furthermore, zonal vegetation (i.e., rainforest, deciduous forest, etc.) ranges across latitude and altitude (i.e., Greller, 1989; Kfichler, 1964; Webb, 1959) and hence into zones of different warmth and temperateness (Axelrod, 1981, fig. 10). These are not determinable from warm and cold month temperature alone. For these reasons, the method has regularly overestimated the degree of paleotemperature change suggested by a sequence of fossil floras and has also led to erroneous results in analyzing the probable elevation of a fossil flora in the interior. Conclusion
The usual concept of an equable climate ("equal and uniform at all times") has no climatic significance. Such conditions occur from the inner tropics to high latitudes and differ greatly in climate, in growing season, and biota. Those who have referred to equable paleoclimates, or to those of low seasonality, have not quantified their definition, have not indicated how it was determined, nor have they presented evidence of the frequency of freezing or when a climate is not equable. Following Bailey, I suggest that the words equable, equability and temperateness be used in reference to the departure of estimated terrestrial paleotemperature from T 14°C and A 0°C. This thermal level, midway between polar (W 10°C) and tropical (W 18°C) climate, is essentially the mean temperature of the earth's surface and has been reasonably stable since at least the Cretaceous. Any departure from it defines a climate of lower temperateness as measured on a scale of M 100 to M 0. Manifestly, reference to an equable climate, either today or in the past, is essentially meaningless unless an approximate figure (i.e., M 55-60) is indicated for the degree of temperateness, as classified in Table 2 after Bailey (1964).
WHAT IS AN EQUABLE CLIMATE?
Acknowledgements This article was reviewed by Homer H. Aschmann, Andrew M. Greller and Lisa C. Sloan. Their critical comments have improved the paper and are greatly appreciated. This contribution is an outgrowth of research on Tertiary floras supported by National Science Foundation, most recently under Grant BSR 8817805.
References Adams, G.G., Lee, D.E. and Rosen, B.R., 1990. Conflicting isotopic and biotic evidence for tropical sea-surface temperatures during the Tertiary. Palaeogeogr., Palaeoclimatol., Palaeoecol., 77: 289-313. Akers, W.H., 1979. Planktic foraminifera and calcareous nannoplankton biostratigraphy of the Neogene of Mexico. Tulane Stud. Geol. Paleontol., 15: 1-32. Akers, W.H., 1981. Planktic foraminifera and calcareous nannoplankton biostratigraphy of the Neogene of Mexico. Tulane Stud. Geol. Paleontol., 16: 145-148. Akers, W.H., 1984. Planktic foraminifera and calcareous nannoplankton biostratigraphy of the Neogene of Mexico. Tulane Stud. Geol. Paleontol., 18: 21-36. Aschmann, H., 1973. Distribution and peculiarity of Mediterranean ecosystems. In: F. de Castri and H.A. Mooney (Editors), Mediterranean Type Ecosystems. Springer, New York, pp. 11-19. Axelrod, D.I., 1956, Mio-Pliocene floras from west-central Nevada. Univ. Calif. Publ. Geol. Sci., 33: 1-322. Axelrod, D.I., 1966. A method for determining the altitude of Tertiary floras. Palaeobotanist, 14: 144-171. Axelrod, D.I., 1968. Tertiary floras and the topographic history of the Snake River basin. Geol. Soc. Am. Bull., 79: 713-734. Axelrod, D.I., 1981. Altitudes of Tertiary forests estimated from paleotemperature.ln : Proc. Symp. Quinghai-Xizang (Tibet) Plateau, Beijing. Science Press, Beijing, pp.131-137. Axelrod, D.I., 1984. An interpretation of Cretaceous and Tertiary biota in polar regions. Palaeogeogr., Palaeoclimatol., Palaeoecol., 45:105 147. Axelrod, D.I., 1985. Miocene floras from the Middlegate basin, west-central Nevada. Univ. Calif. Publ. Geol. Sci., 129: l 279. Axelrod, D.I., 1986. Analysis of some palaeogeographic and palaeoecologic problems of palaeobotany. Palaeobotanist, 35: 115-129. Axelrod, D.I., 1990a. Environment of the middle Eocene (45 Ma) Thunder Mountain flora, central Idaho. Natl. Geogr. Res., 6: 355-361. Axelrod, D.I., 1991. The Early Miocene Buffalo Canyon Flora of Western Nevada. Univ. Calif, Publ. Geol. Sci., 135: 1-180. Axelrod, D.I. and Bailey, H.P., 1969. Paleotemperature analysis of Tertiary floras. Palaeogeogr., Palaeoclimatol., Palaeoecol., 6: 163-195.
[1 Axelrod, D.I. and Bailey, H.P., 1976. Tertiary vegetation, climate and altitude of the Rio Grande depression, New Mexico-Colorado. Paleobiology, 2: 235-254. Bailey, H.P., 1960. A method of determining the warmth and temperateness of climate. Geogr. Ann., 42: 1-16. Bailey, H.P., 1964. Toward a unified concept of the temperate climate. Geogr. Rev., 54: 516-545. Bailey, H.P., 1966. The mean annual range and standard deviation as measures of dispersion of temperature around the annual mean, Geogr. Ann,, 48A: 183-194. Barron, E.J., 1990. Tropical temperature stability and the implications for the distribution of life. In: Geol. Soc. Am. Abstr. Annu. Meet., p. A76. Bartholomew, B., Boufford, D.E. and Sponberg, S.A., 1983. Metasequoia glyptostroboides--its present status in central China. J. Arnold Arbor., 64: 105-128. Brown, R.W., 1962. Paleocene flora of the Rocky Mountains and Great Plains. U.S. Geol. Surv. Prof. Pap., 375: 1-119. Chaney, R.W. and Axelrod, D.I., 1959. Miocene floras of the Columbia Plateau Carnegie Inst. Washington Publ., 617: 1 229. Chaney, R.W. and C.-C., Chuang, 1968. An oak-laurel forest in the Miocene of Taiwan. Proc. Geol. Soc. China, ll: 3-18. Cheng, W.C., 1939. Les for&s du Se-Tchouan et du Si-Kiang Oriental. Trav. Lab. For. Toulouse, 5, 1 (2): 1-233. Garcia, E., 1973. Modifications al Sistema de CIassificaci6n Climfitica de K6ppen (Para adaptario a Las Condiciones de lat Repflblica Mexicana). Univ. Nac. Auton. Mex., 20, 207 pp. Graham, A., 1976. Studies in Neotropical paleobotany. 2. The Miocene communities of Vera Cruz. Ann. Mo. Bot. Gard., 63: 787-842. Greller, A.M., 1989. Correlation of warmth and temperateness with the distributional limits of zonal forests in eastern North America. Torrey Bot. Club Bull., 116: 145-163. Greller, A.M. and Rachele, L.D., 1983, Climatic limits of exotic genera in the Legler palynoflora, Miocene, New Jersey, U.S.A. Rev. Palaeobot. Palynol., 40:149 163. Kasting, J.F., 1989. Long term stability of the Earth's climate. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 75:83-95. Kennett, J.P., 1977. Cenozoic evolution of Antarctic circulation, the circum-Antarctic Ocean, and their impact on global paleoceanography. J. Geogr. Res., 82:3843 3860. K/ichler, A.W., 1964. Potential natural vegetation of the contimerous United States. Am. Geogr. Soc. Spec. Publ., 36, 38 PP. Machain-Castillo, M., 1985. Ostracode biostratigraphy and paleoecology of the Pliocene of the Isthmian salt basin, Veracruz, Mexico. Tulane Stud. Geol. Paleontol., 19: 123-139. Shackleton, N.J., 1984. Oxygen-isotope evidence for Cenozoic climate change. In: R. Brenchley (Editor), Fossils and Climate. Wiley, New York, pp. 27-34. Shreve, F. and Wiggins, I.L., 1951. Vegetation and Flora of the Sonoran Desert. Carnegie Inst. Wash. Publ., 591, 1, 192 PP. Sloan, L.C. and Barron, E.J., 1990. "Equable" climates during Earth history? Geology, 18: 498-492.
12 Smiley, C.J. and Rember, W.C., 1985. Composition of the Miocene Clarkia flora. In: C.J. Smiley (Editor, Late Cenozoic History of the Pacific Northwest. Am. Assoc. Adv. Sci. Pac. Div., San Francisco, CA, pp. 95-112 Takhtajan, A., 1969. Flowering Plants: Origin and Dispersal. Smithsonian Inst. Press, Washington, DC, 310 pp. Tanai, T., 1961. Neogene floral change in Japan (Geol. Mineral., 11). J. Fac. Sci., Hokkaido Univ., pp. 119-398. Wang, Chi-Wu, 1961. The forests of China with a survey of grassland and desert vegetation. Maria Moors Cabot Found. Publ., 5, Harvard Univ., Cambridge, 313 pp.
D.I. AXELROD Webb, L.J., 1959. Physiognomic classification of Australian rain forests. J. Ecol., 47: 551-570. Wernstedt, F.L., 1972. World Climatic Data. Climatic Data Press, Lemont, PA, 522 pp. World Weather Records, 1951-1960. United States Department of Commerce, Environmental Sciences Services Administration, v.6. Zachos, J.C., Lohmann, K.C. and Stott, L., 1990. Late Cretaceous to Early Oligocene evolution of Southern Hemisphere meridional temperature gradients. Geol. Soc. Am. Abstr. Annu. Meet., pp. A171-172.
1
Elsevier Science Publishers B.V., Amsterdam
What is an equable climate? D a n i e l I. A x e l r o d
Department of Botany, University of California, Davis, CA 95616, USA (Revised version accepted August 15, 199 !)
ABSTRACT Axelrod, D.I., 1992. What is an equable climate? Palaeogeogr., Palaeoclimatol., Palaeoecol., 91: 1-12. The ideal (theoretical) equable climate has a mean annual temperature of 14°C and a mean annual range of 0°C. Since 14°C is and has been the average mean temperature of the earth's surface, any departure from 14°C and 0°C results in a climate of lower temperateness or equability. Therefore, reference to an equable paleoclimate is meaningless unless quantified by a centrum from which equability is measured. Climates of low seasonality are not necessarily equable because such conditions occur from the inner wet tropics into glacial-border environments. Freezing often occurs in climates of moderate to high equability. Equable climates may support taxa from different vegetation zones, a relation also evident in the paleobotanical record.
Introduction At least three different meanings have been applied to the word equable in reference to climate, whether at present or in the past. First, the comm o n use of the word equable is that in Webster's Dictionary (1979, 2nd ed.): "equal and uniform (temperature) at all times." Table 1 shows that such conditions occur from the innermost tropics into polar climate (Grimsey, Iceland) and also in highland regions (e.g., Cotopaxi = paramo). Cotopaxi in Ecuador has a mean annual temperature (T) of 7.6°C and a mean annual range (A) of 0.9°C, Quito in Ecuador has a T of 13.5°C and an A of 0.6°C, and Madang in Papua has a T of 26.6°C and an A of 0.5°C. If all three examples are considered equable because of their "equal and uniform temperature", the word has no climatic significance because they are in very different clim a t e s - p o l a r , mild temperate, and inner tropical. Furthermore, while such thermal conditions may be termed aseasonal, or of low seasonality, Table 1 shows that these terms have no climatic significance because they occur in very different environments. Second, Sloan and Barron (1990) refer to an equable climate that has three elements: (1) a 0031-0182/92/$05.00
substantially reduced annual cycle of temperature, (2) a lack of extremes (not defined), and especially (3) a lack of subfreezing temperatures even at the poles. When their climatic model experiments show that the continental interiors and areas at high latitudes experience frost, even during the " w a r m " times in earth history, such climates are not considered equable. However, freezing conditions regularly occur in what is generally acknowledged to be the most equable area in the continental United S t a t e s - - t h e west coast of central California-and also the west coasts and in montane regions of other continents (see below). Further, Sloan and Barron do not indicate the limits of an equable climate, nor do they grade the degrees of equability, or define climates that are not equable other than having frosts. Third, Bailey (1960, 1964, 1966) proposed a concept of temperateness, or equability, that includes (a) the idea of reduced annual cycle of temperature (though with a small diurnal fluctuation), (b) a lack of high or low extremes as determined by a mean annual range < 20°C, and (c) freezing may occur in equable climates. In Bailey's model any departure from a mean annual temperature of 14°C and a mean annual range of
© 1992 - - Elsevier Science Publishers B.V.
2
D.I. AXELROD TABLE 1 Stations with "equal and uniform temperature" are distributed from polar (paramo) to inner tropical climate* Mean annual temperature
Mean annual range
Equability
3.2 7.6 9.4
8.9 0.9 1.2
44 58 66
11.7
1.4
82
13.5 16.7
0.6 1.1
94 83
19.0 21.3 24.1 25.9 26.6 26.7
0.8 0.7 1.0 1.1 0.5 1.4
65 55 48 44 43.5 43
(oc) Polar (paramo) Grimsey, Iceland Cotopaxi, Ecuador Papallacata, Ecuador
Microphyllforest E1 Angel, Ecuador
Montane evergreenforest Quito, Ecuador Kabale, Uganda
Tropical forest Tibacuy, Columbia Kibuye, Ruwanda Dundo, Angola Belem, Brazil Madang, Papau, New Guinea Pago Pago, Samoa
*Data from Wernstedt, 1972; World Weather Records, 1951-60, vol. 60.
O°C results in a climate of lower temperateness or equability. His idea is reviewed here because it clarifies the concept of equable climate and has considerable utility for interpreting paleoclimate and the biota that it supports.
Quantification of temperature The problem posed by the word equable in reference to climate is clarified by the nomograph (Fig. 1) constructed by Bailey. To quantify temperature, Bailey used two complementary measurements, Warmth (W) and Temperateness (M= moderation). As these have been applied previously to climatic problems (i.e., Axelrod, 1966, 1981, 1986; Axelrod and Bailey, 1969, 1976; Greller, 1989; Greller and Rachele, 1983), they are restated here briefly. Warmth (W) provides a measure of the growing season. It appears on the nomograph (Fig. 1) as a series of radiating lines that indicate the number of days (d) in which mean temperature reaches a specified level. Along, the radian W lO°C, no days have a mean temperature warmer than lO°C; this
is the edge of polar climate. The radian W 18°C has 365 days warmer than 18°C; this is the outer margin of tropical climate. The middle of the temperate zone is represented by W 14°C, with 183 days warmer than that. Other warmth lines and the duration of their growing seasons are indicated in Fig. 1. Emphasis must first be placed on the fact that in terrestrial environments mean annual temperature (T) alone has no climatic significance. Figure2 shows that T O°C occurs in glacial, tundra, taiga (= boreal forest), and conifer-hardwood forest environments. Localities with T 10°C range from close to tundra into mild temperate climates with mixed conifer and broadleaved deciduous hardwoods. Stations of T 20°C occur from the inner tropics (all days warmer than 20°C) well out into warm temperate regions, as at Mosul, Iraq, with 221 days warmer than W 15.3°C). Temperateness (M), or equability, is represented by a series of arcs nearly normal to the Warmth (W) lines (Fig. 1). The arcs are centered at a mean annual temperature T of 14°C and a mean annual range (A= amplitude) of O°C. This position,
3
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Fig. I. Nomogram showing the relation between warmth (W) and temperateness (M) or equability of climate (Bailey, 1960, 1964, 1966). The radiating lines indicate the number of days (d) warmer than the stipulated temperature. The arcs, which represent temperateness, are centered at T 14°C and A 0°C. Any departure from that point has a lower temperateness or equability rating which is graded from M 100 to M 0. In areas < T 14°C climate becomes warmer as range of temperature increases, whereas, in areas > T 14°C climate becomes cooler.
midway between polar and tropical climate, was selected because T 14°C is and has been essentially the mean temperature of the earth's surface. Furthermore, when polar and temperate regions were warmer, the tropical zone was cooler than at present. This is suggested by Miocene floras that represent a cooler (upland) vegetation zone than
that at the locality today. For example, the Miocene flora of Taiwan (Chaney and Chuang, 1968) is a broadleaved evergreen oak-laurel forest like that now at elevations above 500 m, yet the fossil locality is now in a tropical rainforest zone. Comparable relations are recorded in southern Japan (Tanai, 1961) where Miocene deciduous hardwood forest with montane conifers was replaced by a mixed broadleaved deciduous-evergreen forest on southern Honshu and by evergreen oak-laurel forest on Kyushu. The relation also appears to be represented in Vera Cruz province, Mexico. Earlier considered Miocene, the flora from Coatxacoalcos, in the Paraje Solo Formation, is now judged Pliocene (N20), according to ostracode stratigraphy in formations above and below the flora (Machain-Gastillo, 1985) and by planktic foraminifera (Akers, 1979, 1981, 1984), both kindly reported to me by Alan Graham (June, 1991). As polar regions became progressively colder in the later Paleogene and Neogene, the tropical region became warmer. Hence, it follows that new taxa not only originated north and south in the developing colder regions to form new communities, but also in the inner tropics as that area became warmer. On this basis, it follows that the more ancient "primitive" taxa would have survived chiefly in intermediate regions of mild to warm temperate, highly equable climate. This is shown by the occurrence of relict gymnosperms and ferns allied to those in Cretaceous floras (Fig. 3). They are preponderantly in mild to warm temperate ("subtropical") climates of high equability (M 60-70). It is this that has given rise to the notion that they indicate "tropical" climate at higher latitudes during the Cretaceous. Furthermore, relict forests of the Tertiary also occur in areas of high equability, as charted earlier (Axelrod, 1966, fig. 9). Examples include coastal California; the Canary Islands; montane Vera Cruz, Mexico; Szechuan-Hubei, China; Valdivia, Chile; New Zealand-SE Australia; Atherton Plateau, Queensland; montane Papua, New Guinea, vicinity of Buloio. As documented by Takhtajan (1969), there is a great concentration of ancient genera of several plant families in the eastern Himalayas, reaching into montane southern Yunnan, a region of high equability. Since no data have yet demonstrated
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unequivocally that the mean temperature of the earth's surface has been much colder or warmer than at present (Kennett, 1977; Shackleton, 1984; Adams et al., 1990; Barron, 1990; Kasting, 1989; Zachos et al., 1990), T 14°C evidently has had a central position since the Cretaceous and no doubt earlier. Any departure from the centrum of mean annual temperature of 14°C and a mean range of 0°C results in a climate of lower temperateness or
equability. Bailey recognized six broad categories (Table 2) of temperateness of the continents, as illustrated in Figs. 4 and 5. The degree of temperateness ( M = moderation) decreases with a greater range of temperature. For example, Oakland, California, and Des Moines, Iowa, both have a mean annual temperature of 14°C (Fig. 2). The mean annual range at Oakland is 9.5°C, but at Des Moines it is 30.4°C. Whereas light freezing may occur on 4-5 days/yr at Oakland, at Des Moines
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Fig. 3. Examples of equable climates in which relict conifers, cycads, and ferns occur that are allied to those of the Cretaceous. Costa Rico, "The richest tree-fern region in the world": 1 = Santario Duran, 2= Pacayas, Mexico: with cycads Zarnia, Dioon, Ceratozamia, tree ferns, Alsophila, Cibotium, Cyathea, 3 = Jalacingo, 4 = Chalcatongo, 5= Atzalan, 6 = Ayutla Mixe, 7--- Jalapa, 8 = Orizaba. Jamaica: Ferns, such as Marattia, Cyathea, Gleichenia, ,41sophila, Hymenophyllum, 9= Cinchonia. Malaya: 10 = Cameroon Highland, with ancient ferns, Matonia, Gleichenia, Dipteris, Cheiropleuria. New Zealand: with Agathis, Dicksonia, Cyathea, Podocarpus, 11 = Waipoa, 12= Te Paki, 13= Dargaville. Brazil: with Araucaria, numerous tree ferns, 14= Curybitana, 15= Curitiba. Australia: with numerous cycads, such as Macrozamia, Bowenia, Lepidozamia and tree ferns, 16= Albany, 17= Toowomba, 18 = Perth, 19= Norfolk Island, 20= Atherton. South Africa: with 25 species of the cycad Encephalartos, numerous podocarps, 21 = Port Elizabeth, 22 = Mabana, 23 = Pietermaritzburg, 24 = Graft Reinert. Chile: with araucarias, podocarps, tree ferns, 25 = Valdivia, 26 = Masatierra on Juan Fernandez Isl. (with the relict Thrysopteris). 1200 h r / y r r e g u l a r l y h a v e f r e e z i n g , s n o w , a n d ice. A s a result, t h e s e a r e a s h a v e v e r y d i f f e r e n t t e m p e r a t e n e s s o r e q u a b i l i t y r a t i n g s , M 64 a n d M 42, r e s p e c t i v e l y . T a b l e 1 also s h o w s t h a t t h e m e a n
a n n u a l r a n g e o f t e m p e r a t u r e is o n l y 0 . 5 ° C at M a d a n g , 0 . 6 ° C at Q u i t o . M e a n a n n u a l t e m p e r a t u r e a t M a d a n g is 2 6 . 6 ° C a n d 13.5°C at Q u i t o . T h e i r v e r y d i f f e r e n t t e m p e r a t e n e s s ratings, M 94
6
D.I. AXELROD
TABLE 2 Major categories of temperateness of the continents (Bailey, 1964). Category
M value
Examples
Supertemperate Very temperate Temperate Subtemperate Intemperate Extreme
100-80 80-65 65-50 50-35 35-20 20-0
Quito, Bogota, Equator (Kenya), Molo (Ecuador) Valpariso, Ensenada, Curitiba, Addis Ababa Norfolk, Paris, Capetown, Portland, Nairobi Juneau, Manaus, Rio de Janeiro, New York King Salmon (Alaska), Fairbanks, Aldan (Siberia) Verkhoyansk, Oymyakon, Yakutsk
Fig. 4. Temperateness of the contimerous United States. From Bailey, 1964. With permission of the American Geographical Society.
at Quito and M 43.5 at Madang, express their departure from the centrum of T 14°C and A O°C. Although some consider that a low range of mean annual temperature (i.e., low seasonality) indicates equable climate, such conditions occur from the inner tropics into polar regions (Table 1). Since all the stations in Table 1 cannot be equable, a centrum must be selected from which equability is measured. At present, T 14°C and A 0°C appear to be the best reference for estimating temperateness (Bailey, 1960, 1964). Figure 1 shows that the Warmth lines converge as the range of temperature decreases. With a
range of 5°C, polar (W 10°C) and tropical (W 18°C) climates are 13°C apart and separated in elevation by 2379 m, assuming a normal lapse rate (1°C/183 m). With a range of 20°C, they are separated by 26°C and 4758 m in elevation. Clearly, in a climate of high temperateness or equability, biota of different thermal zones may live in proximity. It is this that accounts for some of the "unusual" occurrences of fossil species in floras that lived under moderate to high temperateness. Examples are provided by the association of sclerophyllous live oak (Quercus), Catalina ironwood (Lyonothamnus), and mountain mahogany
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D.I. AXELROD
(Cereoearpus) with Sierra redwood (Sequoiadendron), fir (Abies), and spruce (Picea) in Nevada
superb photographs (19 x 12cm) of Sonoran Desert vegetation in Shreve's monograph. Finally, it is recalled that climates of high equability regularly harbor ancient genera and relict species. These occur in the temperate rainforests from Vera Cruz to Chiapas, eastern Mexico; in Yunnan, southern China; in the mixed mesophytic forests of Hubei-Szechuan, central China; in the sclerophyllous woodland and scrub of coastal California, Spain to Turkey, Chile, south Africa and South Australia (see Aschmann, 1973); in the laurel forest of the Canary Islands; in the temperate rainforests of southern Chile and New Zealandall areas with temperateness or equability ratings of M 60-70 +.
Miocene floras (i.e., Axelrod, 1956, 1985, 1991). Spruce and fir are also recorded with lauraceous genera (Machilis, Phoebe), magnolia, red gum (Liquidambar), elm (Ulmus), and broadleaved deciduous oaks in the Miocene of Oregon, Washington and Idaho (Chaney and Axelrod, 1959; Smiley and Rember, 1985). Local climate in a forest zone may harbor a more diverse (richer) flora than that in the adjacent area, providing conditions are more equable. For example, in the Salmon Mountains, northwestern California, the valleys of Sugar, Duck Lake and Horse Range creeks have as many as 10-12 conifers associated within a radius of scarcely 100 m. These represent the richest association of conifers on this continent. Because all these valleys have a northeast orientation, they are less influenced by the hot summer sun, evaporation is lower, and there is cool-air drainage from higher elevation. They are, therefore, more equable than valleys in the adjacent region that are oriented east, west, or south, and whether they are broad or narrow. In those valleys, the same species that are associated in the valleys of Sugar and adjacent creeks are separated by fully 600 m (2000 ft.) in three regional forest zones-mixed conifer, fir, and subalpine. A different example is provided by the flora of the Sonoran Desert (Shreve, 1951). The lowland flora that lives under a hotter desert climate is not so diverse in composition and forms more open communities than those in the uplands. Temperature in the latter areas is regularly more temperate, the differences being well expressed in Table 3. They can especially be grasped by examining the
Discussion
An increase in the range of temperature (A) increases the amount of freezing. A small increase in A is more significant in bringing freezing conditions to cool regions than to warmer ones. In this and other examples (Axelrod, 1984, 1986), estimates of the frequency of freezing are in reference to "normal" frequencies as developed by Bailey (1966). Figure 6 shows that at T 10°C, if the range of temperature is 8°C, about 1% hr/yr would experience freezing (Loc. A), but with a range of 10.5°C, 3% hr/yr freezing would be expected (Loc. B). At T 15°C, with a range of temperature of 14°C, 1% hr/yr would have freezing (Loc. C), whereas with a range of 18°C, 3% hr/yr would have freezing (Loc. D). At T 20°C and A 19.5°C, freezing would be expected on 1% hr/yr (Loc. E), but at A 25°C, 3% hr/yr would have freezing (Loc. F).
TABLE 3 Comparison o f climatic data in lowland and upland Sonoran Desert vegetation, Baja California Sur* Station (elev.)
Position
Mean annual temperature (°C)
Mean annual range
Temperateness or equability
Loreto (12 m) C o m m o n d u (400 m)
26 ° 1', 111 ° 21' 26 ° 2', 111 ° 49 °
24.8 22.5
14.1 11.7
M 43 M 48
Santa Rosalia (26 m) San Ignacio (150 m)
27 ° 19', 112 ° 21' 27 ° 25', 112 ° 52'
23.6 21.8
15.1 12.0
M 44 M 49.5
*Climatic data from Garcia, 1973.
9
W H A T IS AN E Q U A B L E C L I M A T E ?
TABLE 4
MEAN ANNUAL RANGE OF TEMPERATURE
5
10
15
20
25
Hours of frost in zonal forest types, southeastern United States*
10
Zonal Forest Type, Station** w
Tropical semi-deciduous Key West, FL Homestead, FL
er U.I
~1~
-
c
".•
Av. hrs/yr <0°C
0 0
- -
2G
Fig. 6. Illustrating the relation between mean annual temperature and frost frequency as the range of temperature increases.
These estimates of the approximate frequency of freezing raise the question of its occurrence in forests of broadleaved evergreens or forests with mixtures of broadleaved evergreens and deciduous hardwoods. Although broadleaved evergreens have been considered by some as "frost-free" indicators, they are not. Such mixtures of species regularly occur where freezing is of frequent occurrence, as on the Atlantic coastal plain from northern Florida to coastal North Carolina where fully 5 % hr/yr have freezing (Table 4). The mixed mesophytic forest of eastern Asia, which ranges from the mountains in central China to southern Korea and central Japan (Wang, 1961), represents the transition between broadleaved evergreen and mixed deciduous hardwood forests. It has a frost frequency of 5-10% hr/yr (Wang, 1961, table 14; Bartholomew, et al., 1983, table 1). These periods of freezing are of brief duration on each daily occurrence. Approximately 2-3 days with continuous subfreezing temperature probably would eliminate most broadleaved evergreens. Obviously, the present frequencies of freezing cannot be translated directly into conditions of the Tertiary because global climates were not so severe as at present. The oceans were warmer at middle and high latitudes, ice caps and frigid cold waves were
Temperate broadleaved evergreen forest Ft. Meyers, FL Coco Beach, FL Tampa, FL Orlando, FL Panama City, FL Jacksonville, FL Pensacola, FL Valpariso, FL
1 3 6 14 53 81 100 125
Southern mixed hardwood (Canopy of bd-lvd, evergreen and bd-lvd, decid, trees) Savanna, GA Ft. Rucker, AL Charleston, SC Montgomery, AL Myrtle Beach, SC Augusta, GA Moorehead City, NC
152 197 209 250 325 349 370
Deciduous forest (Oak-Pine Hickory) Atlanta, GA Greenville-Spart'nbrg, SC Elizabeth City, NC Richmond, VA
371 375 375 795
*Kindly assembled by Andrew M. Greller, 1990. **Data from U.S. Dep. Air Force, Army, and Navy, Engineering Weather Data, 1978.
not so developed as today, nor did mountainous tracts and high plateaus have their present elevation-all of which contributed to lower ranges of temperature and hence to greater equability in all vegetation zones. As for the fossil record, Paleocene floras of the Great Plains show a transition from a dominantly broadleaved evergreen to mixed evergreen-deciduous hardwood forest in proceeding northward from the Raton basin, New Mexico, to southern Canada (Brown, 1962). The area of the northern plains (Dakota, Montana, Wyoming) was covered with a rich mixed mesophytic forest. Deciduous hardwoods were dominant though mild climate is implied by such genera as Zamia, Araucaria, Sabal,
10
Thrinax, Ficus, Cinnamomum and Bauhinia. The assemblage suggests a mean annual temperature of about 14°C and a mean annual range of 12-14°C. These estimates give an equability rating of M 60-62 and a frost frequency of 1.0-1.5% hr/ yr with freezing. The percentage of hours with freezing increased northward as broadleaved evergreens were replaced by more numerous deciduous hardwoods and temperate conifers (spruce, pine, dawn redwood, etc.). Conditions farther north in the interior were more severe, as suggested by Sloan and Barron (1990) on the basis of their model studies. By middle Eocene time (45-47 Ma) the volcanic uplands (Challis episode) that stretched from northern Nevada (Bull Run flora) through Idaho (Hailey, Thunder Mountain floras) and northward were covered with montane conifer forests at elevations near 1500-1200 m, decreasing northward (Axelrod, 1968). In that area, mean annual temperature at the lower margin of the montane conifer forest in the transition to mixed conifer-deciduous hardwood forest was near 8°C and the range of temperature was about 10-1 I°C. The forests in that ecotone probably were subject to at least 8% hr/yr (c. 700 hr) with freezing, and temperateness, or equability, was about M 55 (Axelrod, 1990). Finally, attention is directed to the strength of the thermal relations advocated by Bailey (1960, 1964, 1966), for they provide the most reliable and reproducible method yet devised for estimating the climatic field of a modern or fossil flora, or of a taxon. From a single point on the nomograph (Fig. 1), determined from the mean annual temperature (T) and the mean annual range of temperature (A = amplitude), the following can readily be determined (also see Axelrod, 1981; Axelrod and Bailey, 1976): (a) the mean temperature of the warm month and cold month, (b) the march of mean monthly temperature, (c) the approximate amount of freezing, if any, (d) the duration of the growing season (W= warmth) in terms of the number of days warmer than a given temperature, (e) the temperateness or equability rating of the assemblage, (f) the water-need (N) indicated for the thermal regime, and (g) the approximate elevation of a fossil flora in an interior area by compari-
D.I. AXELROD
son with temperature inferred for a flora of similar age at or near sea level. Some investigators have reconstructed paleoclimate using only the mean temperature of the warm month and cold month. This gives an incomplete climatic analysis because it does not provide an indication of the duration of the growing season, nor the degree of frost frequency, of the equability, or of water-need. Furthermore, zonal vegetation (i.e., rainforest, deciduous forest, etc.) ranges across latitude and altitude (i.e., Greller, 1989; Kfichler, 1964; Webb, 1959) and hence into zones of different warmth and temperateness (Axelrod, 1981, fig. 10). These are not determinable from warm and cold month temperature alone. For these reasons, the method has regularly overestimated the degree of paleotemperature change suggested by a sequence of fossil floras and has also led to erroneous results in analyzing the probable elevation of a fossil flora in the interior. Conclusion
The usual concept of an equable climate ("equal and uniform at all times") has no climatic significance. Such conditions occur from the inner tropics to high latitudes and differ greatly in climate, in growing season, and biota. Those who have referred to equable paleoclimates, or to those of low seasonality, have not quantified their definition, have not indicated how it was determined, nor have they presented evidence of the frequency of freezing or when a climate is not equable. Following Bailey, I suggest that the words equable, equability and temperateness be used in reference to the departure of estimated terrestrial paleotemperature from T 14°C and A 0°C. This thermal level, midway between polar (W 10°C) and tropical (W 18°C) climate, is essentially the mean temperature of the earth's surface and has been reasonably stable since at least the Cretaceous. Any departure from it defines a climate of lower temperateness as measured on a scale of M 100 to M 0. Manifestly, reference to an equable climate, either today or in the past, is essentially meaningless unless an approximate figure (i.e., M 55-60) is indicated for the degree of temperateness, as classified in Table 2 after Bailey (1964).
WHAT IS AN EQUABLE CLIMATE?
Acknowledgements This article was reviewed by Homer H. Aschmann, Andrew M. Greller and Lisa C. Sloan. Their critical comments have improved the paper and are greatly appreciated. This contribution is an outgrowth of research on Tertiary floras supported by National Science Foundation, most recently under Grant BSR 8817805.
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12 Smiley, C.J. and Rember, W.C., 1985. Composition of the Miocene Clarkia flora. In: C.J. Smiley (Editor, Late Cenozoic History of the Pacific Northwest. Am. Assoc. Adv. Sci. Pac. Div., San Francisco, CA, pp. 95-112 Takhtajan, A., 1969. Flowering Plants: Origin and Dispersal. Smithsonian Inst. Press, Washington, DC, 310 pp. Tanai, T., 1961. Neogene floral change in Japan (Geol. Mineral., 11). J. Fac. Sci., Hokkaido Univ., pp. 119-398. Wang, Chi-Wu, 1961. The forests of China with a survey of grassland and desert vegetation. Maria Moors Cabot Found. Publ., 5, Harvard Univ., Cambridge, 313 pp.
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