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2 Forestry and Forest Products Research Institute, Tsukuba Norin, P.O. Box 16, Ibaraki 305–8687, Japan; and 3 National Herbarium of the Netherlands, P.O. Box 9514, 2300 RA Leiden, The Netherlands
Received for publication April 29, 1999. Accepted for publication January 4, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: altitude • climate • Cornaceae • Cornus • ecological wood anatomy • genus-level trends • latitude • species-level trends
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) showed the existence of clear latitudinal trends in three genera with a wide geographical distribution: Cornus s.l., Garrya, and Alangium excluding sect. Constigma. Vessel element length and fiber length of these three genera and tangential vessel diameter of Cornus s.l. showed a nearly linear decrease with increasing latitude with values at 
40° about half those of equatorial values. When Cornus species were roughly divided into large trees, medium-sized trees, small trees, and shrubs, latitudinal trends for vessel element length were apparent within each size group.
Latitudinal trends in several wood anatomical characters seem to be a general phenomenon for woody dicotyledonous genera with a wide geographical distribution. The trends have been detected in Ilex (Baas, 1973
), Symplocos (van den Oever, Baas, and Zandee, 1981
) and 17 smaller genera of 14 families (van der Graaff and Baas, 1974
). These studies only described correlations between latitude and wood anatomical characters and discussed the trends in terms of the tropical to the arctic climatic gradient. None has tried statistical comparison of wood anatomical variation with more precise climatic data.
At the species level, variation in wood structure has been studied more in relation to altitude than to latitude. Latitudinal and altitudinal trends in wood structure at the species level are either not detected, present but not pronounced, or obvious. Van der Graaff and Baas (1974)
found no altitudinal trends in six species nor latitudinal ones in one species. Sastrapradja and Lamoureux (1969)
did not find any altitudinal trends in Meterosideros polymorpha Gaud. in Hawaii either. On the contrary, Noshiro, Joshi, and Suzuki (1994)
showed in Alnus nepalensis D. Don that six characters were correlated with altitude and/or stem diameter. Among Rhododendron species in Nepal, Noshiro, Suzuki, and Ohba (1995)
and Noshiro and Suzuki (1995)
showed that 13 wood anatomical characters had a highly significant correlation with altitude and/or tree size at the genus level, but that only one to eight characters had a significant correlation with altitude and/or tree size within four studied species. Contradictory results have been obtained for intraspecific variation, probably influenced by the sample size and statistical analyses employed. A more detailed study on latitudinal and altitudinal trends at the species level is therefore necessary.
To evaluate the meaning of the latitudinal and altitudinal trends in wood anatomical characters at the species level, it is also necessary to consider the climatic conditions of sampling localities along the latitudinal and altitudinal gradients and the ontogenetic changes as reflected by growth ring numbers in the studied material. The sampling should be extensive and preferably cover the whole range of distribution of the selected species. Samples for the genus-level study should also be available from wide latitudinal ranges so that differences or similarities in the species- and genus-level trends are comparable. Cornus sensu lato (s.l.) is an ideal taxon for such a study.
Cornus s.l. shows wood anatomical variation that is correlated with latitude, and its trends were distinct from those of the other genera (Noshiro and Baas, 1998
). Wood anatomy supported the broad genus concept that Eyde (1987)
advocated. Analyses of rbcL sequences of Cornaceae and allies (Xiang et al., 1993
; Xiang and Soltis, 1998
), rbcL and matK sequences (Xiang, Soltis, and Soltis, 1998
), chloroplast DNA restriction sites (Xiang et al., 1996
), and morphological characters (Murrell, 1993
) also support the broad genus concept. Cornus s.l. consists of 65 species of deciduous trees or shrubs, rarely herbs, mostly in the mesic temperate zone of the northern hemisphere, with only two species in the equatorial part of the southern hemisphere.
Three Japanese species of Cornus s.l. are deciduous trees covering wide latitudinal and altitudinal ranges throughout Japan and are appropriate materials for the study of species-level latitudinal trends in wood structure along altitudinal and ontogenetic gradients. Climate data are available throughout Japan for the last 30 yr, so trends in wood structure can also be assessed critically along climatic gradients. Cornus controversa Hemsl. ex Prain (Swida controversa (Hemsl.) Soják) of sect. Mesomora is a tree up to 24 m tall and 70 cm in diameter, which grows widely in warm to cool temperate forests from Hokkaido to Kyushu in Japan, and in Korea, Taiwan, central and south China, and in the Himalayas. Cornus kousa (Buerg.) Nakai (Benthamidia japonica (Sieb. et Zucc.) Hara) of sect. Syncarpea is a medium-sized tree up to 15 m tall and 50 cm in diameter, in montane forests from Honshu to Kyushu, and in subtropical forests of Ishigaki Island of the Ryukyu Islands in Japan, and in Korea. Cornus kousa var. chinensis occurs widely in central and south China, and individuals on Ishigaki Island growing in subtropical forests may be included in this variety. Cornus macrophylla Wall. (Swida macrophylla (Wall.) Soják) of sect. Kraniopsis is a tree up to 20 m tall and 50 cm in diameter, in warm temperate forests from Honshu to Kyushu in Japan, and in Korea, Taiwan, central China to the Himalayas and Afghanistan. Wood structure of the three species is similar except for the vessel grouping (Figs. 1–5; Noshiro and Baas, 1998
). All have diffuse porous wood, heterocellular multiseriate rays, diffuse-in-aggregates axial parenchyma, and scalariform perforation plates. Vessels often form short radial multiples in C. controversa, but are exclusively solitary in the other two species.
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), the genus-level trends in Cornus s.l. will be analyzed against climatic factors, and their influence on wood structure will be evaluated. Finally, species-level trends will be compared with latitudinal trends at the genus level, and the evolutionary implication of wood structural diversification at the genus and species levels will be discussed. Because of the difficulty in quantifying ray characters, we studied only vessel and fiber characters. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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15° and an altitudinal range of over 1500 m, representing the distribution ranges of the three species in Japan (Table 1). All the specimens of C. controversa were collected in Japan between Hokkaido and Kyushu from a continuous distribution range. All the specimens of C. kousa were from the montane forest of Japan, but three samples were from the subtropical forest on Ishigaki Island with a disjunct distribution. Most specimens of C. macrophylla were collected in Japan between Honshu and Kyushu up to 1350 m, but five specimens were collected in Nepal between 2250 and 2630 m.
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). The list of materials with their localities and specimen numbers is given in the Appendix, and ranges of tree size, altitude, latitude, and estimated values for climatic factors of habitats are listed in Table 1.
Wood blocks were sectioned and macerated according to the standard techniques for light microscopy. Quantitative characters of vessel elements in cross sections were measured with an image analysis system. Two to three areas, 1.28 x 0.96 mm2, were selected for the measurement of specimens, and extremely narrow rings were excluded whenever possible. Thirty elements per sample for vessel element length and fiber length were measured from macerations. Samples for maceration were collected from outermost growth rings. Vessel grouping was measured only for C. controversa and is expressed by the index of Carlquist (1988)
where the total number of vessels is divided by the total number of vessel groups. All the studied preparations were deposited at TWTw, Tsukuba, Japan. For the genus-level trends of Cornus s.l., the same specimens as cited in Noshiro and Baas (1998)
were used, but additional data for vessel element and fiber lengths are presented.
To assess ontogenetic changes in quantitative characters, vessel element length, fiber length, and tangential vessel diameter were studied in one tree of Cornus controversa from the data of Sugawa (1979
; and unpublished data). The sample tree was 60 yr old and 32 cm in stem diameter at breast height and was collected at Shizuoka, Japan. To assess expected fluctuation in vessel element and fiber lengths from random samples within a growth ring, three blocks of 400–500 µm in radial thickness were collected and compared within a growth ring of one specimen each of the three Cornus species.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. In our material the 10th growth ring nearly equalled 5 cm in diameter at breast height. Thus specimens over 5 cm diameter were provisionally treated as mature wood in this study. Among the specimens used for the species-level study, a gradual increase of cell dimensions was observed as stem diameter increased, but the difference in cell dimensions between stem diameter of 10 and 60 cm was within the variation range at stem diameter of 30 cm (Fig. 7). Ontogenetic trends in mature wood were therefore negligible in the following species-level analyses.
Within a growth ring, fiber length of woody dicotyledons can fluctuate up to five times the minimum value at about the growth ring boundary (Bisset and Dadswell, 1950
; Hejnowicz and Hejnowicz, 1958
; Swamy, Parameswaran and Govindarajalu, 1960
; Süss, 1967
). Vessel element length is more or less constant in diffuse-porous wood (Hejnowicz and Hejnowicz, 1958
; Swamy, Prameswaran and Govindarajalu, 1960
; Süss, 1967
). In the three Cornus species, both vessel element and fiber lengths were fairly constant throughout a growth ring and fluctuated <10% of the minimum value (Fig. 6). Thus our sample of 500 µm in radial thickness could be considered to yield the average value for the whole growth ring for vessel element and fiber lengths.
Trends in the three Cornus species
In contrast to the latitudinal trends for Cornus s.l., latitudinal trends at the species level were negligible. In Cornus controversa, vessel element length, fiber length, vessel frequency, and tangential vessel diameter all had nearly horizontal trends along latitude with only one significant correlation at the 1% level (Fig. 8). Fiber length showed a very weak, but recognizable, trend when compared with the other three characters, but the decrease from 31.5° N to 45.3° N was only 7.1% according to the linear regression, which was statistically significant here. All the other characters had <3.3% increase or decrease within the distribution range. Variation ranges for each character were constant throughout the distribution range of this species, except for the narrower range for tangential vessel diameter in the specimens of Hokkaido above 42.6° N.
Cornus kousa showed more marked latitudinal trends in the four wood anatomical characters than C. controversa, but this was likely because of the three samples from Ishigaki Island at 24° N (Fig. 9). These three specimens had a disjunct distribution at low altitudes in a subtropical forest and tended to have longer vessel elements and fibers, lower vessel frequency, and wider vessels than the specimens from Kyushu to northern Honshu. If we exclude these specimens, three wood anatomical characters showed no latitudinal trends without any significant correlation. From 30.3° N to 40.5° N, vessel element length, fiber length, and vessel frequency decreased 5.2, 7.8, and 6.6%, respectively, and tangential vessel diameter increased 8.3%, according to the linear regression. Because of the large number of specimens at 35° N, variation ranges for each character tended to be largest around this latitude, almost equal to the total variation for each character.
In Cornus macrophylla, vessel element length and fiber length were more constant than in C. controversa, but vessel frequency and tangential vessel diameter, respectively, showed a medium decrease and increase, with vessel frequency having the only significant correlation (Fig. 10). From 27.8° N to 41.0° N, vessel frequency decreased 25.7%, and tangential vessel diameter increased 12.6%, according to the linear regression. Variation range was nearly constant throughout the distribution range of this species, but seemed to be displaced upward between 30° and 35° N. The specimens from Nepal at 28° N that grew at very high altitudes between 2250 and 2630 m tended to have slightly shorter vessel elements and fibers and smaller vessels than the Japanese specimens. If we exclude the Nepalese specimens, latitudinal trends were more marked for vessel element and fiber lengths and less so for vessel frequency and tangential vessel diameter. Thus within the three Cornus species, latitudinal trends were almost negligible, except for Cornus kousa when the specimens on Ishigaki Island were included.
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and seemed to be intrinsic trends in wood structure. Thus, except for a minor ontogenetic influence of stem diameter, nonanatomical factors did not influence wood structure of the three Cornus species at latitudes higher than 30° N.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Latitudinally, most species of woody plants grow within a range of 15°, except for tropical coastal species, and an infraspecific study covering temperate to tropical areas is usually difficult. One exception is Symplocos cochinchinensis (Lour.) S. Moore studied by van den Oever, Baas, and Zandee (1981)
. Among its 22 specimens growing between 23° S and 33° N, however, no significant latitudinal trends existed either in vessel diameter, vessel element length, vessel frequency, bar number of perforation plates, or fiber length (according to a new simple correlation analysis of van den Oever et al.'s data). Though existence of latitudinal trends was not detected in this widely distributed species, further studies may uncover a threshold latitude for the expression of latitudinal trends.
Altitudinally, there may be a threshold altitude at 2000 m. Alnus nepalensis is the only species thus far known to have altitudinal trends conforming with the genus-level altitudinal or latitudinal ones (Noshiro, Joshi, and Suzuki, 1994
). In this species, the population above 2000 m had more vessels, shorter vessel elements, and shorter fibers than that below 2000 m. Within either population, however, no altitudinal trends were detected except for vessel element length in the upper population. Thus difference in the population average above and below 2000 m mainly defined the altitudinal trends in this species. In Symplocos cochinchinensis specimens growing between 0 and 3900 m, vessel element and fiber lengths tended to be shorter in six specimens above 2000 m than in 12 specimens below 2000 m, but with a low significance level of 5%, according to the data of van den Oever, Baas, and Zandee (1981)
. Such a threshold altitude at 2000 m may also explain distinctive character values of Nepalese specimens of C. macrophylla.
Altitudinal trends in these species are, however, exceptional. The almost complete lack of latitudinal or altitudinal trends in wood structure of the three Cornus species throughout their distribution range agrees with the results of van der Graaff and Baas (1974)
and Sastrapradja and Lamoureux (1969)
and conforms with the results for four individual Rhododendron species (Noshiro and Suzuki, 1995
). Thus, the wood structure of these species is not affected by either latitudinal or altitudinal gradients.
Genus-level trends in Cornus s.l. and comparison with nonanatomical factors
Although latitudinal trends were clear within Cornus s.l. as a whole (Noshiro and Baas, 1998), trends can differ in the two species groups in the Old and New Worlds, which diverged from each other in the early Tertiary or before (Tiffney, 1985
; Rogers, 1993
). Latitudinal trends for the Old and New World specimens, depicted using the data of Noshiro and Baas (1998), were similar; both have a significant correlation at the 0.5% level (Figs. 11–13). According to the regression lines, from 0° to 50° latitude vessel element length decreases 52 and 46%, fiber length 45 and 53%, and tangential vessel diameter 47 and 52% in the Old and New Worlds, respectively. Thus, the Old World and New World species groups of Cornus s.l. have similar latitudinal trends in wood anatomical features, though it is not clear whether the separation of the two groups happened before the establishment of latitudinal trends or vice versa.
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; Crowley and North, 1991
) and is not correlated linearly with latitude. A stronger correlation of wood anatomical characters with latitude than with climatic factors seems to indicate a direct influence of more latitudinally linear geophysical factors on wood structure, such as gravity, intensity of solar radiation, or length of photoperiod. Another possibility is that some unknown ecological parameters resulting from a combination of climatic and/or geophysical factors may have latitudinally linear trends and may affect wood anatomy in ways not clarified by simple correlation analysis of individual parameters. One or several of these parameters must have influenced wood structure along with the adaptive radiation of constituent species within Cornus s.l.
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, in floristic wood anatomical studies such as Baas, Werker, and Fahn (1983)
, Baas and Schweingruber (1987)
, Carlquist (1975a)
, and Carlquist and Hoekman (1985)
, or in other systematic works such as Zhang, Baas, and Zandee (1992)
. This comparison is well illustrated in the diagram of Dickison (1989)
, where decrease in vessel element length and vessel diameter and increase in vessel frequency commonly occur in gradients toward the cool temperate or montane or xeric habitats from the tropical mesic one. In two phylogenetically divergent genera, Hibbertia (Dilleniaceae) and Erythroxylum (Erythroxylaceae), Rury and Dickison (1984)
showed that not just wood anatomy, but plant stature and leaf morphology and anatomy were clearly correlated with a gradient of water availability. The adaptive merits of latitudinal or ecological trends in wood structure are obscure at present, especially for vessel element and fiber lengths (Baas, 1982
). The similarity in latitudinal and altitudinal trends to mesic to xeric trends must be related with physiology and ecology of respective species, and their relationship should be clarified in relation to the phylogenetic diversification of species in respective genera.
Comparison of the genus- and the species-level trends in Cornus s.l
Cornus s.l. as a whole has more marked latitudinal trends and its characters vary more than the three Asiatic species. Between 30° and 40° N, a latitudinal range common to the three species, vessel element length, fiber length, and tangential vessel diameter for the whole genus decreased 14.5% from 1149 µm, 15.6% from 1680 µm, and 14.8% from 55 µm, respectively, according to the linear regression for Cornus s.l. and using data from additional specimens to the materials studied by Noshiro and Baas (1998)
. Within the same latitudinal range, vessel element length, fiber length, and tangential vessel diameter decreased 2.5, 5.3, and 2.4% in Cornus controversa, 5.3, 7.8, and -8.33% in C. kousa excluding samples of the Ishigaki Island, and 4.3, 1.8 and -6.0% in C. macrophylla excluding Nepalese samples, respectively. For vessel frequency, trends differed between species and genus levels, and species-level trends had a linear increase or decrease, whereas the genus-level trend had a more or less curvilinear increase (Noshiro and Baas, 1998
). The relative amount of variation at the species and genus levels can be compared by employing coefficients of variation, i.e., the standard deviation expressed as a percentage of the average corrected to exclude the effect of sample size. The coefficients for vessel element length, fiber length, vessel frequency, and tangential vessel diameter range from 9.1 to 11.7%, 8.5 to 10.6%, 19.7 to 24.8%, and 9.2 to 12.0%, respectively, for the three species, but these values are twice as large, 21.4%, 23.1%, 51.7%, and 24.4%, respectively, for Cornus s.l.
In the latitudinal plots of wood anatomical features, the values for the three Asiatic species are relatively high, compared with the regression lines for Cornus s.l. (Figs. 11–13). This trend is especially marked in the tangential vessel diameter of Cornus controversa and C. macrophylla. This difference probably derives from the small size of trees from southern localities used for the systematic study of Cornaceae and allies (Noshiro and Baas, 1998). In the present study, most samples were obtained from large mature individuals, whereas xylarium specimens from small individuals were used in the systematic study. Thus, among the samples for the species-level studies, xylarium specimens used in the previous systematic study are scattered at the lower end of the ontogenetic trends (Fig. 7). Moreover, the localities of the xylarium specimens were confined to the southern part of the specific distribution ranges, 31°–36° N for C. controversa specimens, 31°–36° N for C. kousa specimens, and 27°–36° N for C. macrophylla specimens. Even if such bias exists in xylarium specimens, the general latitudinal trends in wood anatomical characters are manifest. If we could have used only fully mature wood samples in the systematic study, the regression lines for the genus may have even been steeper than those shown in the systematic study.
Evolutionary scenario
Cornus s.l. has clear latitudinal trends in wood structure, mostly a highly significant linear decrease in fusiform cell size along the whole distribution range, whereas each species has narrower variation without any latitudinal or altitudinal gradients. Differences in latitudinal trends at the genus and species levels in Cornus s.l. remain to be explained.
Though at present climatic factors show a weaker correlation with wood anatomical features than latitude, historically climate must have influenced wood structure as a water transporting system. Paleoclimatological data indicate that in the mid-Cretaceous a warmer climate prevailed, especially at high latitudes, with a gentler latitudinal gradient than at present (Crowley and North, 1991
). With a major climatic degradation at the end of Eocene, climate became cooler and seasonality became more manifest (Wolfe, 1978, 1992, 1994
). Wood anatomical features of dicotyledons were affected by these changes in climate since the Cretaceous, and percentages of short vessel elements increased over time together with an increased incidence of distinct growth rings, implying increased seasonality (Wheeler and Baas, 1991, 1993
). Quantitative wood anatomical ranges within a genus and its constituent species may have been narrow when the genus became a distinct entity in the late Cretaceous or early Tertiary, when the warmer climate prevailed and the latitudinal climatic gradient was gentle. With the development of more variable climate with distinct seasonality in the late Tertiary, these quantitative features likely became more variable with the adaptive radiation of various species.
Paleobotanical evidence shows diversification of Cornus s.l. in the Tertiary at least since the Eocene, but there seems to be no concrete Cretaceous record of this genus so far (Suzuki, 1982
; Eyde, 1988
; Taylor, 1990
; Taylor and Taylor, 1993
; Mai, 1995
). Eyde (1988)
reviewed reports of Cornus fossils and regarded Eocene fruit stones as the oldest authentic record of this genus. At present it is not clear when in the Tertiary diversification and adaptive radiation in Cornus s.l. was most active, and how these correlated with climatic degradation and steepening of latitudinal gradients.
Vessel elements and fibers in the secondary xylem of woody dicotyledons derive from fusiform initials in the cambium. Vessel element length nearly equals the length of fusiform initials in the cambium (Bailey, 1920
). Bailey and Tupper (1918)
and Bailey's students clarified evolution of various wood structural features of angiosperms in line with the reduction in tracheary element length from gymnosperms tracheids to angiosperms vessels. The present study suggests that, beside this large-scale evolutionary trend in the whole angiosperms, length of vessel elements or fusiform initials and other related quantitative features also have a distinct evolutionary history for individual genera involving ecological (latitudinal) adaptation.
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FOOTNOTES |
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4 Author for reprint requests (e-mail: noshiro{at}ffpri.affrc.go.jp
).
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  H. Motomura, S. Noshiro, and M. Mikage Variable Wood Formation and Adaptation to the Alpine Environment of Ephedra pachyclada (Gnetales: Ephedraceae) in the Mustang District, Western Nepal Ann. Bot., August 1, 2007; 100(2): 315 - 324. [Abstract] [Full Text] [PDF]
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 J. B. Fisher, G. Goldstein, T. J. Jones, and S. Cordell Wood vessel diameter is related to elevation and genotype in the Hawaiian tree Metrosideros polymorpha (Myrtaceae) Am. J. Botany, May 1, 2007; 94(5): 709 - 715. [Abstract] [Full Text] [PDF]
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F. LENS, P. GASSON, E. SMETS, and S. JANSEN Comparative Wood Anatomy of Epacrids (Styphelioideae, Ericaceae s.l.) Ann. Bot., June 1, 2003; 91(7): 835 - 856. [Abstract] [Full Text] [PDF]
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J. Liu and S. Noshiro Lack of latitudinal trends in wood anatomy of Dodonaea viscosa (Sapindaceae), a species with a worldwide distribution Am. J. Botany, April 1, 2003; 90(4): 532 - 539. [Abstract] [Full Text] [PDF]
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