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Tree growth in the Mesozoic and Early Tertiary and the reconstruction of palaeoclimates
Abstract
Evidence from the distribution and characteristics of fossil wood in the Mesozoic and Early Tertiary indicates that a much warmer global climate prevailed in those times. There appears to have been a broad zone of largely non-seasonal climate stretching from about 32° N to 32° S (palaeolatitudes). In addition to this low-latitude zone, forest growth extended into very high palaeolatitudes where trees cannot grow at the present day. A number of theories have been proposed to account for the palaeoclimate responsible for this distribution of forests. Most notable have been those involving changes in the amount of carbon dioxide in the atmosphere, in the positions of the continents or in the obliquity of the earth's axis of rotation. Evidence from fossil forests indicates that a combination of the effects of an increased quantity of carbon dioxide in the atmosphere and the palaeopositions of the continents in the Mesozoic and Early Tertiary appears at the moment to be the simplest explanation for the climate of those geological times, without the need to invoke axial movement.
... Statistical characteristics of the growth rings have been used to determine the average annual radial growth and aspects of the paleoenvironment when the tree was growing, especially the mean ring width (e.g. Creber, 1977;Creber and Chaloner, 1985;Ammons, 1987;Morgans, 1999;Morgans and Hesselbo, 1999;Brison et al., 2001;Falcon-Lang et al, 2004;Taylor and Ryberg, 2007;Brea et al., 2015) and mean sensitivity, a measure of the relationship between tree growth and climate (Ammons, 1987;Morgans, 1999;Morgans and Hesselbo, 1999;Falcon-Lang, 2003;Falcon-Lang et al., 2004;Taylor and Ryberg, 2007;Davies-Vollum et al., 2011;Brea et al., 2015). ...
... The modern causes of ring formation are assumed to have held for the distant past, such that the regular formation of rings in petrified wood is interpreted as evidence of an annual cycle of climatic seasonality (e.g. Creber and Chaloner, 1985). Recognizing the presence of growth rings in petrified wood requires preservation of structural details across the tree"s radius, and the degree of preservation in petrified wood varies from almost perfect preservation of the cellular and subcellular features of the xylem to complete loss of all features. ...
... Accuracy in determining average tree growth characteristics from tree ring widths, and by extension environmental information related to the tree growth/climate relationship, requires estimation of the site mean ring width values (Fritts, 1976, p23), requiring at least some evidence of contemporaneous growth between the wood specimens (Creber and Chaloner, 1985). Further, short ring width time series are unlikely to provide an accurate estimate of the mean growth, because tree ring-width variability within individual trees varies through time. ...
... consiguiente prudencia, pero résulta de indudable interés para mejorar nuestro escaso conocimiento acerca de las condiciones y acontecimientos paleoclimâticos mesozoicos (AMMONS ef a<, 1987;CREBER, 1977;CREBER & CHALONER, 1985). ...
... actuales no permitirfan la instalaciôn de vida végétal. Existia una extensa zona de clima tropical, sin estaciones marcadas, que abarcaba aproximadamente desde los 321 de latitud N hasta los 321 de latitud S(CREBER & CHALONER, 1985).BATTEN (1984) considéra que las floras del Jurâsico y Cretâcico Inferior fueron, desde un punto de vista global, las mas uniformes que j amas hayan existido en la Ti erra.La penmsula Ibérica se encontraba en aquella época en las proximidades del limite climâtico de la banda tropical septentrional del Globo. SegùnPLATT (1989), el régimen hidrico en la région de Cameros fue, durante el Cretâcico Inferior, de carâcter semiârido a ârido, con variaciones hacia zonas de mayor humedad segùn la localidad.Las indicaciones sobre pautas paleoclimâticas que pueden obtenerse de la muestra estudiada son limitadas debido a la escasez del material intégrante de las mismas; al no presentar anillos de crecimiento tampoco Paleogeografîa de la cuenca de Cameros durante el Aptiense (Barremiense superior) con indicaciôn de la localizaciôn de la muestra (*) en la formaciôn "Enciso" (modificado de Paleogeography of thé Cameros Basin during thé Barremian-Early Aptian with indication of location of thé sample (*) on thé Enciso gr. ...
... CREBER (1977) senala que es posible obtener informaciôn procedentes de diversas localidades del Globo(SMITH et al., 1981) indica la existencia en aquella época de una amplia banda tropical que se extendfa hasta latitudes correspondientes hoy dfa a zonas templadas. el caso concreto de Europa y el Méditerranée Occidental, existen très datas que merece la pena destacarse: Portugal, Marruecos y Gran Bretana(SMITH et al., 1981;CREBER & CHALONER, 1985). Asf, tanto en Portugal como en Marruecos, los dendrolitos analizados presentan una ausencia de anillos de crecimiento marcados. ...
A study on three samples of fossil wood located in the Soto en Cameros' site (La Rioja, Spain) is presented. The analysis of the transverse, tangential and radial sections has able us to identify the fossil wood as Pinoxylon, being this one the first identification of this taxa in the Iberian's Lower Cretaceous. The growth characteristics observed in the sample have allowed us to make some considerations on the climatic conditions that encircled the site's sourroundings during the Lower Cretaceous. It was an homogeneous and regular climate with certain slower growth periods due, probably, to local or regional aridity conditions, as it occurs in many current subtropical climates.
... These plants are widely distributed from high latitudes across ancient northern Russia and in Xinjiang Uygur Autonomous Region to Eurasian mid-latitudes (Boucot et al. 2009;Oh et al. 2015), evidencing range extension southwards into humid regions (Philippe and Thévenard 1996). Petrified wood implies an unique palaeoenvironment and palaeoclimate (Falcon-Lang 2005), and samples are therefore of great significance for restorations (Creber 1977;Creber and Chaloner 1985;Martin-Closas and Gomez 2004;Uhl 2006;Wan et al. 2016). Thévenard (1996, 2009) reported that although the genus Xenoxylon had a circumpolar distribution, abundance decreased significantly in polar regions during global droughts (Philippe and Thévenard 1996;Philippe et al. 2009). ...
... The growth rings seen in these species provide data that can be used to reconstruct seasonal changes in palaeoclimate (Fritts 1976;Creber and Chaloner 1985). Distinct growth rings indicate that plant growth was significantly subject to seasonal climate changes. ...
Xenoxylon is an important boreal Mesozoic fossil wood genus and has been widely reported from sites across northern China. This paper reports well-preserved Xenoxylon wood from the Jurassic-Cretaceous boundary Tuchengzi Formation at Yanqing, Beijing, China. The fossil wood described here is ascribed to two species, i.e. Xenoxylon latiporosum and X. peidense. The present new finding enrichs our knowledge of the floral characteristics, especially the forest composition of the Tuchengzi Formation. These new records enlarge the fossil wood diversity of the genus Xenoxylon of the Tuchengzi Formation up to three species at Yanqing. Statistic results of the annual and mean sensitivity values of the X. latiporosum and X. peidense indicates that the trees inhabited under a sensitive climate with relatively stable interannual water supply. Additionally, some fungal remains represented by fungal hyphae were found with wood tissues of specimens of X. peidense. Typical wood rotting structures resembling those of the extant white rot are also found in X. latiporosum. This is the first record of fungal hyphae in Yanqing, Beijing, China, and provides us with new evidence that can be used to explore interactions between fungi and plants in the Late Jurassic.
Institutional abbreviations: YQ = Yanqing Silicified Wood National Geopark, Beijing, China
... The fossil wood described here is characterised by the absence of distinct growth rings (D or E of Brison et al., 2001), which is feature of trees growing in non-seasonal conditions (Fritts, 1976;Creber, Chaloner, 1985;Francis, Poole, 2002;Yang et al., 2013). However, it is important to note that in spite of the wide latitudinal distribution of the genus Agathoxylon, this taxon never displays rings of the (A, B, C of Brison et al., 2001) super-type, even when other woods from the same locality do (Brison et al., 2001). ...
... Similar trends can also be evidenced in the living Araucariaceae with Agathoxylon (Araucarioxylon)-type wood. This wood type also never displays the A, B, or C types (Seitz, Kanninen, 1989), even when growing in clearly seasonal climates (Creber, Chaloner, 1985). The other fossil woods reported from the Kota Formation show growth rings (Chinnappa, Rajanikanth, 2018). ...
The Kota Formation of the Pranhita-Godavari Basin is well known for its fossil fauna and flora especially for its silicified woods. However, the palaeoflora and its palaeoclimatic significance within the formation are poorly known. In spite of the fact that the formation yields a rich fauna and flora chronostratigraphic problems still exist. The present study aims to analyze the palaeofloras from the Kota Formation to understand their diversity and palaeoecological significance. We also describe a new species Agathoxylon kotaense belonging to the conifer family Araucariaceae. Our study shows that the flora was dominated by conifers and that it is comparable to that of the ?Late Jurassic-Lower Cretaceous Gangapur Formation, Pranhita-Godavari Basin and that of the Rajmahal Formation of the Rajmahal hills. The growth ring pattern and leaf fossil assemblage suggest that the growth conditions were seasonal, but mostly stressed.
... Fossilised plants offer valuable insights into plant diversity as well as palaeoclimatic, palae-oenvironmental and palaeoecological conditions of the past (Creber & Chaloner, 1985;Feng et al., 2013Feng et al., , 2017Feng et al., , 2019. Palynomorphs are frequently used as proxies in palaeoclimate studies due to their abundance in sedimentary deposits and their close correlation with vegetation composition and climatic/environmental changes (Traverse, 2007). ...
The present study discusses the outcome of palynlogical and palaeobotanocal investigations of Middle Jurassic strata of the Tabas Block. The most commonly identified spore type is a trilete spore, Klukisporites, which accounts for 30 per cent, and the genus Ischyosporites which makes up 12 per cent of the spore collection. Plant fossils recovered from the borehole studied are indicative of a varied assemblage, starting with a diverse range of ferns and followed by Cycadophytes, Ginkgophytes and Coniferophytes. The predominance of the Lowland group in the Tabas Block during the Bajocian–Bathonian, as indicated by the Sporomorph EcoGroup (SEG) and Plant EcoGroup (PEG) models, suggests that the strata studied were laid down mainly in a lowland environment. Dinoflagellate cysts were found in locations that correspond to river and coastal ecogroups, hinting at marine influence.
... Permineralized plant stems are commonly found in sedimentary rocks from the Upper Paleozoic to the Quaternary and provide critical information for paleobotanical, paleoclimatic, paleoecological, and leaf phenological studies (Creber and Chaloner, 1984;Creber and Chaloner, 1985;Taylor and Taylor, 1993;Creber and Francis, 1999;Falcon-Lang, 2000a;Falcon-Lang, 2000b;Falcon-Lang and Bashforth, 2004;Feng et al., 2017;Feng et al., 2022). The Mesozoic is an important period for the diversification and radiation of gymnosperms, with a great variety of conifer and ginkgo fossil stems have been documented in this period (Bamford and Philippe, 2001;Philippe et al., 2004;Philippe et al., 2006;Gnaedinger, 2012;Tian et al., 2015;Jiang et al., 2016;Jiang et al., 2022;Vajda et al., 2016;Wang et al., 2017;Iamandei et al., 2018;Gnaedinger and Zavattieri, 2020;Afonin and Gromyko, 2021;Cheng et al., 2023). ...
... Pires and Guerra-Sommer (2011) compared the Jurassic monsoon climate to a hot, wet, and tropical climate, which they believe best represents the growth environment of the conifers in the MVF. Creber and Chaloner (1985) conducted a study on tree growth to reconstruct worldwide Mesozoic paleoclimate, hypothesizing non-seasonal climate below 32 • . This finding aligns with the hypothesis proposed by Lefranc and Guiraud (1990), who suggested semiarid-like climates as a plausible explanation for the remarkable preservation of fossilized wood through silicification. ...
... Growth ring sequences of in situ fossil stumps or logs may provide environmental information of the palaeolatitude where the tree grew. Earlier studies on growth ring features of fossil woods from different geological horizons (Chaloner & Creber, 1973;Creber & Chaloner, 1984, 1985Jefferson, 1982Jefferson, , 1983Francis, 1984Francis, , 1986Ash & Creber, 1992;Yadav & Bhattacharyya, 1994) have demonstrated their potential for palaeoenvironmental analysis. ...
Many of the fossil woods described from the Mesozoic of India possess distinct growth rings which provide valuable proxy data to decipher multitude of environmental information of the concurrent periods. Detailed growth ring study of Podocarpoxylon rajmahalense (Jain) Bose & Maheshwari from Rajmahal Hills, Bihar was conducted to understand the growth environment of the fossil wood. Growth ring features of the fossil wood indicate that it could either be derived from a branch or young stem of a tree growing in the exterior of the forest. Large amount of early wood with only 1-3 rowed latewood cells indicates good soil moisture availability during the growing season. Absence of false rings further adds to the conclusion that the moisture would not have been the limiting factor during the growing season. Warm temperate type of climate would have existed during the life span of the present fossil wood.
... Conifers are hypothesized to be crucial components on the Late Triassic terrestrial ecosystems in northern Xinjiang. Growth rings in fossil woods have been widely used to assess palaeoenvironmental conditions prevailing during the tree growth (Chaloner and Creber, 1973, 1988, 1990Creber and Chaloner, 1985;Wan et al., 2020a). In Megaporoxylon sinensis sp. ...
A coniferous trunk, Megaporoxylon sinensis sp. nov., is described from the Carnian–Norian (Triassic) Huangshanjie Formation in the Dalongkou section, Jimsar County, Xinjiang, northwestern China. The trunk is composed of pith, primary and secondary xylem. The pith is solid, circular, heterocellular, with pitted parenchyma and secretory cells and canals. The primary xylem is endarch, with spiral and scalariform thickenings on tracheidal walls. The secondary xylem is pycnoxylic, consisting of tracheids and rays. Radial tracheidal pitting is araucarian. Tangential pitting and axial parenchyma are absent. Uniseriate pits with oval apertures are commonly present on radial tracheid walls. Biseriate pitting occurs only on radial walls of the first to fifth tracheids outward the primary xylem. Rays are parenchymatous, uniseriate, and 1–7 cells high. Cross-field pitting is window-like or phyllocladoid. Previous records of species of Megaporoxylon Kräusel were limited to the Southern Hemisphere during the Carboniferous to Triassic. Its occurrence from the Upper Triassic of northern Xinjiang is the first report in the Northern Hemisphere and suggests a floristic exchange between Gondwana and Laurasia. The new trunk and two previously described fossil stems, including Medulloprotaxodioxylon triassicum Wan et al. and Xenoxylon junggarensis Wan et al., demonstrate that conifers were important elements of Carnian–Norian terrestrial ecosystems and had a higher diversity in northern China than previous thought. The growth ring pattern of the trunk and palaeogeographic reconstruction of the research area at high latitudes suggest that the growth of the trees was limited by the seasonality of the light regime.
... Fossil woods provide important clues in palaeoclimatic and palaeoenvironmental reconstruction (Wan et al., 2017b(Wan et al., , 2020a. Presence of growth rings in fossil woods is indicative of palaeoclimatic seasonality Creber, 1973, 1990;Creber and Chaloner, 1985;Wan et al., 2020a). On the other hand, absence of growth rings may indicate a non-to weakly seasonal climate with a short or no dry season (Falcon-Lang, 2003;Wan et al., 2014Wan et al., , 2017b. ...
Fifty-six cordaitalean trunks with anatomical features are discovered and described from the Moscovian (Pennsylvanian) Benxi Formation in Yangquan City, Shanxi Province, North China. They are allochthonously preserved in fluvial channel deposits and classified into two types based on the anatomy. The first type is characterized by a solid and heterocellular pith, endarch primary xylem and pycnoxylic secondary xylem with araucarian radial tracheidal pits and araucarioid cross-field pitting. The second type contains a septate pith and a pycnoxylic secondary xylem which is comparable to that of the first type. Primary xylem is absent in the second type. The diameter of these trunks ranges from 0.11 m to 0.55 m. The largest tree is estimated to be 29.96 m high using an allometric approach. The trunks represent the oldest and largest trees from the North China Block during the Pennsylvanian due to the absence of Silurian, Devonian, and Mississippian there. The fossil evidence shows that cordaitaleans were large arborescent trees growing on clastic substrates in the Cathaysia during the Pennsylvanian. Their occurrence from the upper Pennsylvanian in North China Block suggests that the previously reported diversity of cordaitaleans, which were estimated based on impressions, is a gross underestimate. The absence of growth rings in the trunks and co-occurrence of arborescent lycopsids in the same interval, together with coal seams and bauxite around the fossil-bearing horizons, indicate that the trees grew under perhumid tropical conditions.
... Importantly, the presence of a much larger proportion of earlywood than latewood has, so far, been noticed more often in Southern Hemisphere trees (Bamford and Corbett, 1994). In addition, the two wood specimens described in this paper show a growth ring type ʽʽAʼʼ (Creber and Chaloner, 1984), indicating that the plants thrived in an environment favoured by abundant water supply, and a growth controlled by climatic seasonal variation (Creber and Chaloner, 1985;Creber and Francis, 1999). The anatomy of our two species is similar to extant taxodiaceous Cupressaceae, as noted for most Protaxodioxylon species (Vozenin-Serra et al., 2011). ...
We describe for the first time in precise anatomical detail new occurrences of petrified woods in the K5 formation of the Metangula Graben (Niassa, northern Mozambique), pertaining to the Capitanian (mid Permian). The two new species of fossil wood are described from two very well-preserved specimens. The two new species, Protaxodioxylon verniersii sp. nov. and Protaxodioxylon metangulense sp. nov. are characterized by showing tracheids with mixed radial pitting and taxodioid pits in the cross-field, typical of the genus Protaxodioxylon. The specimens described are mostly distinguishable from other Protaxodioxylon species by the absence of axial parenchyma, the height of rays and the number of pits per cross-field. Protaxodioxylon species reported to date were mostly restricted to the Northern Hemisphere and to the late Mesozoic (Jurassic and Cretaceous). Surprisingly, however, this study infers that this fossil wood genus has a significantly longer time range than previously thought and underwent dispersal and diversification even earlier during the Palaeozoic. Additionally, both specimens display distinct growth rings indicating that the fossil woods grew in an environment controlled by seasonal variation with partial suspension of growth in times of less water availability and/or lower temperature. On the other hand, the occurrence of Protaxodioxylon suggest a humid temperate, warm subtropical climate condition during the Capitanian in the Metangula Graben.
... Fossilized plants, particularly well-preserved fossil woods, are helpful for assessing plant diversity and paleoclimatic, paleoenvironmental, and paleoecological conditions during the deep times (Creber and Chaloner, 1985;Taylor and Taylor, 1993;Poole and Ataabadi, 2005;Feng et al., 2010Feng et al., , 2013Feng et al., , 2017Feng et al., , 2019Wei et al., 2019;Wan et al., 2016aWan et al., , 2017aPhilippe et al., 2017;Iamandei et al., 2018;Sagasti et al., 2019;Ruiz et al., 2020aRuiz et al., , 2020bXia et al., 2020). A total of 19 species of Protophyllocladoxylon have been recorded from the Carboniferous to the Eocene (Philippe and Wilde, 2020;Zijlstra and Philippe, 2020), providing important information on the fossil floral compositions, and paleoclimatic and paleoenvironmental conditions (Zhang et al., 2010;Wan et al., 2019a). ...
A new permineralized stem, Protophyllocladoxylon yiwuense Gou et Feng sp. nov., is described from the Middle Jurassic Xishanyao Formation in Naomaohu Town of Yiwu County, Hami City, Xinjiang Uygur Autonomous Region, Northwest China. The stem is only preserved with secondary xylem, which is pycnoxylic and exclusively composed of tracheids and parenchymatous rays. The tracheids have uniseriate or biseriate, alternately arranged bordered pits on their radial walls, and uniseriate bordered pits on the tangential walls. The rays are uniseriate or partially biseriate, up to 34 cells high. There are one or two large, simple pits in each cross field. A branch trace horizontally passes through the secondary xylem showing anatomical structures identical to the secondary xylem. Analyses of the growth-ring width and mean sensitivity values indicate that P. yiwuense had long growing seasons with readily available water supplies and favorable paleoclimatic conditions. White-rot decay features are commonly recognized in the stem, including pocket-like cavities, complete removal of the middle lamellae, thickened corners and separation of the secondary walls of tracheids, as well as septa-like structures in the tracheid lumens due to the cell's reactions to fungal activity. Our study sheds new light into the plant diversity and paleoenvironmental and paleoecological conditions of the Middle Jurassic.
... Silicified woody trunks are playing an important role in the palaeobotanic (Falcon-Lang & Bashforth, 2004;Falcon-Lang & Scott, 2000;, palaeoecological (Creber & Ash, 1990;Feng, Schneider, Labandeira, Kretzschmar, & Röβler, 2015;Feng, Wang, & Liu, 2010;Feng, Wang, Rößler, Ślipi nski, & Labandeira, 2017;García Massini, Falaschi, & Zamuner, 2012;Wan, Yang, Liu, & Wang, 2016;Wan, Yang, & Wang, 2014;Wei, Gou, Yang, & Feng, 2019) and palaeoclimatic (Creber & Chaloner, 1984, 1985Creber & Francis, 1999;Falcon-Lang, 1999;Taylor & Taylor, 1993;Wan, Yang, He, Zhou, et al., 2017;Wan, Zhou, Yang, & Wang, 2016;Zhang, Wang, Liu, & Li, 2010) research. Their anatomical characteristics provide unique information for the systematics and physiology of gymnosperms and other plant groups (Falcon-Lang, Kurzawe, & Lucas, 2014;Wan, Yang, Tang, Liu & Wang, 2017), which are beneficial for realizing plant diversity in the geological periods. ...
A silicified trunk, Zhuotingoxylon liaoi Wan, Yang, Wang, Liu et Wang gen. et sp. nov., is described from the uppermost part of Guodikeng Formation in South Taodonggou section, Turpan–Hami Basin, Xinjiang Uygur Autonomous Region, northwestern China. It is characterized by a solid pith, endarch primary xylem and pycnoxylic wood. The pith is composed of parenchyma and sclereids. Radial walls of primary xylem tracheids have spiral and scalariform thickenings. Secondary xylem consists of thick‐walled tracheids and parenchymatous rays. Uniseriate rounded pits with oval apertures are distributed on radial tracheidal walls separately. Cell walls of rays are homogeneous and smooth. Rays are 1–10 cells high in tangential section. Cross‐field pits are cupressoid. There are 1–4 bordered pits with slit‐like to oval apertures in each cross‐field. Based on the anatomical features of the pith and xylems, it is proposed that the new stem has a coniferous affinity. The new fossil stem adds to the knowledge of vascular plant diversity close to the Permian–Triassic boundary.
... Thus, quantitative analysis of growth rings has been regarded as an important method in modern forestry and chronological and climatological studies (Creber and Chaloner, 1984;Schweingruber, 1996;Esper et al., 2012;Mbow et al., 2013;Schollän et al., 2015;Huang et al., 2020). Growth-ring analysis has also been proven as a powerful tool to assess the physiological features of ancient plants and the paleoclimate and paleoenvironment through geological history (Chapman, 1994;Creber and Chaloner, 1985;Creber and Francis, 1999;Falcon-Lang, 2000a,b;Keller and Hendrix, 1997;Taylor and Taylor, 1993;Wan et al., 2016Wan et al., , 2020Philippe et al., 2017). ...
Fossil wood serves as a natural data source for deciphering plant physiological features and paleoclimatic and paleoenvironmental conditions experienced during its lifespan. Although permineralized conifer wood commonly occurs in the Jurassic of Xinjiang Uygur Autonomous Region (Xinjiang), Northwest China, their physiological features and paleoclimatic and paleoenvironmental significance have been rarely studied. Here, we quantitatively analyzed the anatomical characteristics of a permineralized conifer stem (Agathoxylon sp.) from the Middle Jurassic Xishanyao Formation in Naomaohu Town of Yiwu County, Hami City, Xinjiang. The fossil stem is only preserved with secondary xylem. The secondary xylem is pycnoxylic and exclusively composed of tracheids and parenchymatous rays. Tracheids have mostly uniseriate or biseriate, partially triseriate or tetraseriate, alternately or sub-oppositely arranged bordered pits on their radial walls. Rays are mostly uniseriate, partially biseriate, and are up to 27 cells high. There are 4 to 11 cupressoid pits in each cross field, arranged in the araucarioid type. Growth rings are distinct and wide. The mean sensitivity and ring markedness index analyses of growth rings suggest that a stable paleoclimate with regular seasonal fructifications could have prevailed in the Santanghu Basin during the Middle Jurassic. Statistical analyses of cell diameters in 11 successive growth rings indicate that this fossil plant was deciduous. Our study sheds new light on leaf phenology and paleoclimatic and paleoenvironmental conditions of Middle Jurassic conifer plants.
... The Agathoxylon santacruzense described here show distinct growth rings and they are of type A or B as classified by Creber & Chaloner (1984) and illustrated by Brison et al. (2001) reflecting paleoclimate of this area did have marked seasons during the tree growth (Fritts, 1976;Creber & Chaloner, 1985;Francis & Poole, 2002;Yang et al., 2013). The other wood taxa Cupressinoxylon, Ginkgoxylon, Podocarpoxylon, Prototaxoxylon and Taxaceoxylon with distinct growth rings reported from the Kota Formation also favors such interpretations (Chinnappa & Rajanikanth, 2018). ...
Agathoxylon santacruzense (Araucariaceae) is reported here from the Lower–Middle Jurassic Kota Formation of Pranhita-Godavari Basin, India. The taxon was originally described from the Middle Jurassic sediments of La Matilde Formation, Santa Cruz Province, Argentina. The wood has low percentage of latewood, and abrupt transitions of early-latewood reflects periods of intense growth. The growth ring parameters indicate that the growth conditions were seasonal but mostly stressful, and in some periods presented an erratic interruption. These growth rings show close similarity to those of modern tropical to subtropical conifers growing in the southern hemisphere. These results are concurrent with the broad subtropical belt that existed in the Jurassic. Keywords: Agathoxylon, Jurassic, palaeoenvironments, Kota Formation, India. RESUMO – Agathoxylon santacruzense (Araucariaceae) é aqui descrita para a Formação Kota do Jurássico Inferior–Médio da Bacia de Pranhita-Godavari, Índia. O táxon foi originalmente descrito a partir do Jurássico Médio da Formação La Matilde, Província de Santa Cruz, Argentina. A madeira com baixo percentual de lenho tardio e transição abrupta de lenho tardio reflete períodos de intenso crescimento. Os parâmetros do anel de crescimento indicam que as condições de crescimento foram sazonais, mas principalmente estressantes e em alguns períodos apresentaram uma interrupção errática. Esses anéis de crescimento apresentam semelhanças mais próximas com os das coníferas tropicais atuais e subtropicais que crescem no hemisfério sul. Esses resultados são compatíveis com o amplo cinturão subtropical que existia no Jurássico. Palavras-chave: Agathoxylon, Jurassic, paleoambientes, Formação Kota, Índia.
... The evidence from fossil wood (Creber and Chaloner, 1985) clearly indicates an element of seasonality for the Catlins Jurassic, although exactly what contributed to this is less clear. Two of the fern taxa in the Catlins Jurassic help to very broadly constrain local temperatures. ...
... Based on Creber and Chaloner (1985), who established six categories of tree rings, according to the type of earlywood/latewood transitions and the earlywood/latewood quantities, Gerards et al. (2008) defined three larger categories (A, DE, O). Material in hand determined as Podocarpoxylon sp. ...
Three types of charcoalified fossil homoxylous woods are described from the Upper Cretaceous of the Bohemian Cretaceous Basin: Podocarpoxylon sp.,?Podocarpoxylon vel Taxodioxylon sp. and Protocupressinoxylon sp. They come from the uppermost part of the Peruc-Korycany Formation, which is mid-Cenomanian in age.
Podocarpoxylon sp. shows distinct growth rings, uniseriate rays of medium height, 1–2 podocarpoid cross-field pits, and abietinean pitting on radial tracheid walls.
Wood of ?Podocarpoxylon vel Taxodioxylon sp. differs in its indistinct growth rings, its rays which are uniseriate with medium height to multiseriate with horizontal resin duct, 1–2 podocarpoid cross-field pits per field, and mixed type of pitting on its radial tracheid walls.
Protocupressinoxylon sp. also displays indistinct growth rings, and its rays are either uniseriate of medium height or multiseriate with horizontal resin ducts, but it has 1, occasionally 2–3 cupressoid cross-field pits per field, and abietinean to mixed pitting on radial tracheid walls.
One of the three woods shows wood with growth rings, suggesting seasonally dry climate. The composition of the wood assemblage and its comparison with plant megafossils, sporomorphs and sedimentology suggest that there are two paleoenvironments in Unit 5 of the Peruc-Korycany Formation in the Pecínov quarry: a swampy coniferous forest in the coastal lowlands, and a mesophytic forest in the uplands.
... However, Daley (1972) pointed out that given frost-free conditions and high rainfall levels, tropical plants may have become established in low-lying damp areas even though temperatures were not truly tropical and seasonal variations were taking place. Nevertheless, the picture of early and middle Eocene warmth was confirmed by later studies (Creber and Chaloner 1985;Collinson and Hooker 1987). Further confirmation of the existence of Eocene warmth came from the isotopic evidence obtained from cores in the North Sea and elsewhere (Buchardt 1978). ...
The pre-Quaternary period in the geomorphological evolution of England and Wales is a paradox. It is within this time interval that the main traits of topography originated, including the configuration of highlands and lowlands, the presence of extensive surfaces of low relief and the drainage pattern. However, very little about the pre-Quaternary landscape development can be said with confidence, especially regarding the timing, and rather few geomorphic features can be convincingly demonstrated to date back to the Palaeogene or Neogene. In addition, several key issues such as the magnitude of erosion and the spatiotemporal relationships between geomorphic surfaces remain controversial. This chapter provides an overview of pre-Quaternary landscape evolution, combining classic themes such as planation surfaces and weathering mantles with more recent approaches such as calculation of uplift rates and depth of erosion. The Palaeogene and Neogene are better considered as periods of ongoing change during which the landscape gradually took its shape in response to tectonic and climatic forcings rather than definite ages of certain geomorphic features.
... Palaeoecological implications. Fossil woods are an important component of the Mesozoic fossil flora and they are the one of the best source to understand the palaeoecology and palaeogeography of the geological past (Fritts, 1976;Creber and Chaloner, 1985;Falcon-Lang, 1999;Francis and Poole, 2002;Philippe et al., 2004Philippe et al., , 2009Oh et al., 2011;Yang et al., 2013;Chinnappa and Rajanikanth, 2018). It is known that the Jurassic climate was considerably more equable than that of present day, with tropical-subtropical conditions. ...
... It represents a unique paleoenvironmental and paleoclimatic dataset (Falcon-Lang, 2005), providing a continuous record of what growing conditions were like in the vicinity of trees over a period of decades to centuries (Creber, 1977;Creber and Francis, 1999;Falcon-Lang, 2005). Growth rings in fossil wood provide the most sensitive geological data for assessing paleoclimatic seasonality prevailed during the tree growth Creber, 1973, 1990;Creber and Chaloner, 1985;Wan et al., 2016). Distinct growth rings present in our wood specimen indicates a marked seasonality of the environment where it grew. ...
For the first time, a silicified wood with well-preserved inner structures is reported from the Kungurian (Early Permian) Snapper Point Formation in the southern Sydney Basin, southeastern Australia. It is characterized by uni- to tetraseriate araucarian radial tracheidal pits and large, simple cross-field pitting. This wood is attributed to Protophyllocladoxylon dolianitii Mussa 1958, and it is assumed to be of cordaitalean affinity, which is one of the dominant and arborescent element of the early Permian Gangamopteris Taiga. Evidence of growth patterns of the fossil wood, and the paleogeographical reconstruction shows that the host tree of the fossil wood was growing in a strongly seasonal polar light regime. The wide growth rings (5.315–12.316 mm) and the large number of tracheids per ring (99–288) indicate the tree flourished under favorable conditions during the growing season, with ample water and clement temperatures, enhanced by the extended daylight hours of the Antarctic-type summer. The occurrence of frost rings in the wood, corroborated by additional sedimentological information suggest that unusual climatic events (aperiodic cold and freezing events) occurred in terrestrial ecosystems of the southern Sydney Basin during the Kungurian. The study demonstrates that fossil wood of exceptional preservation, like the one reported here, can offer valuable and potentially unique information crucial for deep-time paleoclimatic reconstruction.
... The interplay of internal factors such as the action of plant growth regulators and photosynthate supply, together with external (environmental) phenomena, was thoroughly assessed. Creber and Chaloner (1985) focussed on tree growth in the Mesozoic and Paleogene as evidenced from tree ring morphology. It was concluded that the high-latitude polar forests, mainly of conifers and supporting relatively diverse faunas, in this extensive greenhouse interval were able to survive the seasonal light regime due to the relatively equable palaeotemperatures and the continental configurations. ...
William G. (‘Bill’) Chaloner FRS (1928–2016) was one of the world’s leading palaeobotanists and palynologists. He developed a love of natural science at school which led to a penchant for palaeobotany at university. Bill graduated in 1950 from the University of Reading, and remained there for his PhD, supervised by Tom Harris, on the spores of Carboniferous lycopods. After completing his PhD in 1953, Bill undertook a postdoctoral fellowship in the USA. He returned to the UK and, in 1956, began a long and distinguished academic career at four colleges of the University of London. His first position was at University College London, where he continued to work on Paleozoic palaeobotany and palynology. His 1958 paper on the effects of fluctuating sea levels on Carboniferous pollen-spore assemblages proved highly influential. Bill moved to a Chair at Birkbeck College in 1972, began to use the scanning electron microscope and was elected a Fellow of The Royal Society in 1976. He is the only pre-Quaternary palynologist to have been given the latter honour. In 1979, Bill was appointed to the Chair of Botany at Bedford College where he began to apply plant fossil evidence to general scientific problems. He began to work on arthropod–plant interactions, fossil charcoal and growth rings in wood. Bill was awarded the Medal for Scientific Excellence by the American Association of Stratigraphic Palynologists in 1984. Bedford College and Royal Holloway College merged in 1985, and Bill moved to the amalgamated institution. Bill continued to investigate very diverse topics, and added the analysis of leaf stomata, global environmental change and molecular palaeontology to his portfolio. Following retirement in 1994, Bill continued his research and teaching at Royal Holloway, University of London. His final paper was published in 2016, bringing to an end a research career of 66 years.
... They range from the Carboniferous to Quaternary, and are found either alone or associated with other fossils in the strata of various depositional facies and palaeoecosystems. Well preserved fossil woods with detailed anatomical features are particularly valuable in reconstructing palaeovegetation and palaeoclimate (Chaloner and Creber, 1973;Fritts, 1976;Creber and Chaloner, 1985;Yadav and Bhattacharyya, 1994;Falcon Lang, 2000;Francis and Poole, 2002;Yang et al., 2013). Fossil woods represent a relatively plant ecosystem existed on land and Manik and Srivastava, 1991;Muralidhar Rao, 1991;Jeyasingh and Kumarasamy, 1994;Prakash and Rajanikanth, 2004;Ram-Awatar and Rajanikanth, 2007;Kumarasamy and Jeyasingh, 2007;Bonde, 2010;Kumarasamy, 2013aKumarasamy, , 2013bChinnappa and Rajanikanth, 2016). ...
Nomenclature reappraisal, diversity pattern and palaeoclimatic implications of Jurassic, Triassic and Early Cretaceous pycnoxylic woods in India are undertaken in the present study. Among the fourteen generic names published previously, only eight are validly published and the rest are nomenclaturally illegitimate. About 51 species were reported under these genera to date. There is a gradual increase of species diversity of fossil wood from the Triassic to Early Cretaceous. The nature of the growth rings was applied to understand the palaeoclimate. The lack of distinct growth rings in the Triassic woods suggests absence of seasonality. The Jurassic woods with an inconsistency in growth rings and presence of growth interruptions suggest climate was seasonal and turbulent. During the Early Cretaceous, conifer dominated vegetation and with wider growth rings and gradual transition suggests warm environments with pronounced seasonality. The general increase in mean ring width from the Triassic to Early Cretaceous indicates ameliorating climatic conditions, particularly benign summer conditions. © 2018 Elsevier Ireland Ltd Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS.
... Die Bildung jährlicher Zuwachszonen in Bäumen tropischer Regionen stellte man sogar lange Zeit in Frage. Seit dem letzten Jahrhundert wurden jedoch in heutigen tropischen trockenen und feuchten Wäldern mehrere Studien unternommen, welche das Phänomen der Jahrringbildung belegen, wenngleich sich diese signifikant von jener in temperierten Zonen unterscheidet (coster 1927, 1928 Francis 1984, creber & chaloner 1985, ash & creber 1992, Falcon-lang 1999, brison et al. 2001, Falcon-lang et al. 2001, taylor & ryberg 2007, pires & guerra-soMMer 2011, gulbranson et al. 2014, benício et al. 2015, Fletcher et al. 2015. In der Summe werden auch hier standortspezifische lokale Eigenheiten und paläoklimatische Bedingungen und Variationen in den Zuwachszonen abgebildet (Montañez et al. 2016, lutharDt et al. 2016 ...
The Chemnitz Fossil Forest represents a diverse T0 assemblage of a fossil forest ecosystem of early Permian
age (Sakmarian, 290.6 ± 1.8 Ma), which was preserved by a series of volcanic events. The hygrophilous plant
community grew on the mineral substrate of an alluvial plain under a sub-humid local climate. Strong seasonality triggered the formation of growth rings in arborecent woody plants, such as cordaitaleans, conifers, medullosan seed ferns and calamitaleans. From several hundred fossil trees 53 specimens were selected to measure more than 2,000 growth rings in individual sequences of up to 77 rings. Ring sequences were statistically analysed by standard dendrochronological methods to determine cell parameters, growth rates and mean sensitivity, as well as to correlate the tree’s growth increments. Based on morphological and statistical analyses on cells in single tree-rings, different tree-ring types were classified. Cordaitaleans and conifers have distinct regular tree-rings (type 1), whereas calamitaleans and medullosans show indistinct regular tree-rings (type 2). Growth rings of type 1 and 2 are evidenced to represent true annual tree rings. In contrast, so called event rings (type 3) occur solitary in ring sequences and represent severe damage to trees, especially in calamitaleans and medullosans. Results further reveal differences between plant groups regarding their physiological reactions or adaptations to seasonal fluctuations. Cordaitaleans and conifers were well-adapted to seasonal drought showing relatively high growth rates and low mean sensitivity, but no event rings of type 3b/c. In contrast, calamitaleans and medullosans grew slower and reacted more sensitive to environmental perturbances or water deficiency, thus indicating severe environmental events by type 3b/c rings. Type 3a event rings represent stem injuries which resulted from variable causes. Many of the investigated specimens serve as reliable short-term environmental archives up to 79 years back in time from the T0 volcanic event. Several ring sequences show a distinct cyclic pattern caused by wide and narrow rings with an average periodicity of 10.6 years, which was used for correlation. The distinct regular cyclicity most likely seems to reflect a climatic signal caused by the 11-year solar cycle. If so, this would be the first evidence of sunspot activity in the Paleozoic from a biological climate archive. The study shows that tree rings as natural data archives offer the opportunity to investigate the fourth dimension in the three-dimensionally preserved Chemnitz Fossil Forest ecosystem.
... Due to their high potential for providing palaeoecological information, dendroecological methods have been applied to fossil wood, mainly petrified or charcoalified remains (e.g. Francis, 1984;Creber and Chaloner, 1985;Ash and Creber, 1992;Falcon-Lang, 1999;Brison et al., 2001;Falcon-Lang et al., 2001;Taylor and Ryberg, 2007;Pires and Guerra-Sommer, 2011;Gulbranson et al., 2014;Benício et al., 2015;Fletcher et al., 2015). However, application is generally limited due to comparatively small data bases, caused by multiple taphonomic biases such as inadequate cell preservation or fragmentation during various transport processes. ...
The early Permian Chemnitz Fossil Lagerstätte (Leukersdorf Formation, Chemnitz Basin, SE Germany) represents a diverse T⁰ assemblage of a fossil forest ecosystem around the Sakmarian-Artinskian transition (290.6 ± 1.8 Ma), which was preserved by pyroclastic deposits of a multi-phased volcanic eruption. The multi-aged plant community consists of predominantly hygrophilous elements, which grew on an alluvial plain mineral substrate under sub-humid conditions, representing a wet spot environment. Strong seasonality triggered the formation of annual tree rings in arborescent woody plants, such as pycnoxylic gymnosperms, medullosan seed ferns and calamitaleans. From several hundred fossil trees, the 53 best-preserved specimens were selected and investigated in detail by measuring 2,081 tree rings in individual sequences of up to 77 rings. Ring sequences were analysed by standard dendrochronological methods to determine both annual growth rates and mean sensitivity. Morphological and statistical analyses on single tree rings reveal different tree-ring types according to the different plant groups. Pycnoxylic gymnosperms have distinct and regular tree rings, whereas medullosan seed ferns and calamitaleans show indistinct and regular tree rings as well as so called event rings. Results reveal differences between plant groups regarding their physiological reactions or adaptations to seasonal fluctuations. In comparison to pycnoxylic gymnosperms, both medullosan seed ferns and calamitaleans exhibit reduced growth rates and more sensitive reaction to environmental perturbances as water deficiency pointing to comparably lower adaptation to seasonally dry palaeoclimate. In this context, event rings are in many cases traced back to plant physiological stress during particularly severe drought periods. Altogether, these fossil trees serve as sensitive environmental archives, which shed light on growth conditions several decades back in time from the entombing eruption.
... If this was also the case in the Mesozoic, then the entomophilous pollination strategy of Bennettitales may have been less effective at high southern latitudes (60°-70°S) then occupied by Australia. At these latitudes, low-diversity forests of open structure experiencing short growth seasons may have been the norm through most of the Mesozoic (Creber and Chaloner 1985;Hill and Scriven 1995). Williamsoniaceae (of which all Australian Bennettitales appear to be members) had slender, regularly branched, divaricating stems (Watson and Sincock 1992;Pott and McLoughlin 2014; in contrast to the stocky, cycad-like, monopodial architecture of North American and Eurasian Cycadeoidaceae axes (Wieland 1906;Delevoryas and Hope 1976). ...
Several dispersed reproductive organs of bennettitopsid gymnosperms are described and illustrated from Triassic to Cretaceous strata of Australia: Williamsonia eskensis sp. nov. (Middle Triassic), Williamsonia ipsvicensis sp. nov. (Upper Triassic), Williamsonia durikaiensis sp. nov. (Lower Jurassic), Williamsonia sp. (Lower Jurassic), Williamsonia rugosa sp. nov. (Middle Jurassic), Williamsonia gracilis sp. nov. (Lower Cretaceous), Cycadolepis ferrugineus sp. nov. (Lower Jurassic), Cycadolepis sp. (Lower Cretaceous), and Fredlindia moretonensis Shirley 1898 comb. nov. (Upper Triassic). Among these, W. eskensis appears to represent the oldest bennettitalean reproductive structure yet identified. Although global floras expressed less provincialism during the Mesozoic and many genera are cosmopolitan, Australian bennettopsid species appear to have been endemic based on the morphological characters of the reproductive structures. Bennettopsids have a stratigraphic range of around 210 million years in Australia and are widely and abundantly represented by leaf fossils, but only around 20 specimens of reproductive structures, of which half are attributed to Fredlindia, have been recovered from that continent’s geological archive. The extremely low representation of reproductive organs vis-à-vis foliage is interpreted to reflect a combination of physical disintegration of the seed-bearing units while attached to the host axis and, potentially, extensive vegetative reproduction in bennettopsids growing at high southern latitudes during the Mesozoic.
... Moreover, the variability in latewood development contrasts with the consistently abrupt termination of the main growth season reflected in woods from high-latitude trees (Creber & Chaloner 1984). Growth rings from trees of the Jurassic -Palaeogene palaeoequatorial belt of north Africa and SW USA tend to have modest annual growth increments (1-3 mm) but indistinct (or absent) ring boundaries (Creber & Chaloner 1985;Bamford et al. 2002;Parrish & Falcon-Lang 2007). Some woods from Jurassic warm middle latitudes (Neuquen, Argentina) have ill-defined growth rings (Morgans-Bell & McIlroy 2005). ...
Central Skåne (Scania) in southern Sweden hosts evidence of extensive Jurassic volcanism in the form of mafic volcanic plugs and associated volcaniclastic deposits that entomb well-preserved macro-plant and spore–pollen assemblages. Palynological assemblages recovered from the Höör Sandstone are of Hettangian–Pliensbachian age and those from the overlying lahar deposits are dated as Pliensbachian–early Toarcian (?). Palynomorph assemblages from these units reveal significantly different ecosystems, particularly with respect to the gymnospermous components that represented the main canopy plants. Both palynofloras are dominated by osmundacean, marattiacean and cyatheacean fern spore taxa but, whereas the Höör Sandstone hosts abundant Chasmatosporites spp. pollen produced by plants related to cycadophytes, the volcanogenic deposits are dominated by cypress family pollen (Perinopollenites) with an understorey component rich in putative Erdtmanithecales (or possibly Gnetales), and collectively representing vegetation of disturbed habitats. Permineralized conifer wood attributed to Protophyllocladoxylon sp., belonging to plants that probably produced the abundant Perinopollenites grains, is abundant in the volcanigenic strata, and shows sporadic intraseasonal and multi-year episodes of growth disruption. Together with the relatively narrow but marked annual growth rings, and the annual and mean sensitivity values that span the complacent–sensitive domains, these features suggest growth within Mediterranean-type biomes subject to episodic disturbance.
... Moreover, the variability in latewood development contrasts with the consistently abrupt termination of the main growth season reflected in woods from high-latitude trees (Creber & Chaloner 1984). Growth rings from trees of the Jurassic -Palaeogene palaeoequatorial belt of north Africa and SW USA tend to have modest annual growth increments (1-3 mm) but indistinct (or absent) ring boundaries (Creber & Chaloner 1985;Bamford et al. 2002;Parrish & Falcon-Lang 2007). Some woods from Jurassic warm middle latitudes (Neuquen, Argentina) have ill-defined growth rings (Morgans-Bell & McIlroy 2005). ...
Central Skåne (Scania) in southern Sweden hosts evidence of extensive Jurassic volcanism in the form of mafic volcanic plugs and associated volcaniclastic deposits that entomb wellpreserved macro-plant and spore-pollen assemblages. Palynological assemblages recovered from the Höör Sandstone are of Hettangian-Pliensbachian age and those from the overlying lahar deposits are dated as Pliensbachian-early Toarcian (?). Palynomorph assemblages from these units reveal significantly different ecosystems, particularly with respect to the gymnospermous components that represented the main canopy plants. Both palynofloras are dominated by osmundacean, marattiacean and cyatheacean fern spore taxa but, whereas the Höö r Sandstone hosts abundant Chasmatosporites spp. pollen produced by plants related to cycadophytes, the volcanogenic deposits are dominated by cypress family pollen (Perinopollenites) with an understorey component rich in putative Erdtmanithecales (or possibly Gnetales), and collectively representing vegetation of disturbed habitats. Permineralized conifer wood attributed to Protophyllocladoxylon sp., belonging to plants that probably produced the abundant Perinopollenites grains, is abundant in the volcanigenic strata, and shows sporadic intraseasonal and multi-year episodes of growth disruption. Together with the relatively narrow but marked annual growth rings, and the annual and mean sensitivity values that span the complacent-sensitive domains, these features suggest growth within Mediterranean-type biomes subject to episodic disturbance.
... The climate consideration, mainly the seasonal variations in temperate conditions applied in the geologically older horizons of Palaeozoic eras also. Dendroclimatology, Palaeodendroclimatology have thus equally gained importance in Palaeoclimate research (Brown, 1925;Chaloner & Creber, 1973;Creber & Chaloner, 1984, 1985Jefferson, 1982Jefferson, , 1983Francis, 1984Francis, , 1986Ash & Creber, 1992;Taylor et al., 1992;Tidwell & Medlyn, 1993;Chapman, 1994;Yadav & Bhattacharyya, 1994, 1996aCreber & Francis, 1999). ...
Tree rings revealed in a newly described gymnosperm wood Araucarioxylon ghoshii sp. nov. from Raniganj Formation, Raniganj Coal field, India are similar to the tree rings described in the in situ gymnosperm fossil woods from the Late Permian of Antarctica. The rings of the fossil woods from Antarctica and India have higher proportion of early woods compared to little amount of late wood. Characteristic zigzag pattern of early wood tracheids formed due to collapsing and crushing of tracheid files and false rings occur in the woods recorded from two distant areas. The characters suggest occurrence of rapidly growing young forest in both the continents during Late Permian. Quantitative analysis of growth rings in A. ghoshii reveals a CSDM curve with right skewedness +19% suggesting its possible evergreen nature with small to moderate leaf retention time.
William G. Chaloner, widely known as Bill, was a world leader in the study of plant fossils. He was a pioneer in the development of palaeopalynology and helped integrate studies of macroscopic plant fossils with investigations of fossil pollen and spores. His early work expanded our understanding of Carboniferous coal-forming plants and vegetation and his investigations on the changing distributions of fossil plants contributed to improved knowledge of biogeographic patterns during the late Paleozoic, including the concept of continental drift before it was widely accepted. Bill's relentless creativity demonstrated how the fossil record of plants could be exploited to reconstruct ancient climates. He also recognized that because certain structural features of fossil leaves directly record levels of the greenhouse gas carbon dioxide they are valuable ‘biosensors’ of ancient atmospheric composition allowing investigation of the link between the greenhouse effect and past warm climates. With his clear and critical mind and grasp of diverse subject matter, Bill was adept at distilling disparate information into a coherent and understandable whole. An engaging communicator, and an unfailingly supportive mentor, he inspired many young scientists during his five decades of service to colleges of the University of London.
Fossil wood is a significant proxy for terrestrial palaeoclimate and palaeogeography. Abundant and diversified Mesozoic fossil wood has been documented in China. However, there are limited reports of Jurassic fossil wood records from northwest China. Herein, several calcified wood fossils are described in anatomical detail, from the Middle Jurassic (Bajocian to Callovian) Xinhe Formation in Gansu Province. They were identified as two species of the fossil wood genus Xenoxylon Gothan of Coniferales, namely X. latiporosum (Cramer) Gothan and X. peidense Zheng et Zhang. The newly collected material represents the first record of Jurassic petrified wood from Gansu Province. Based on the palaeoclimatic implication of Xenoxylon, we propose that the Baojishan Basin may have experienced a distinct short-term cooling during the Middle Jurassic. We analysed the fossil diversity and spatial-temporal distribution of Xenoxylon species in China.
A total of 15 Xenoxylon species have been documented in China. These occurred from the Late Triassic to Late Cretaceous, but flourished from the Jurassic to Cretaceous intervals. Geographically, they are distributed across 13 provinces, mainly in northeast China. Significantly, this record expands the distribution of this important climate indicator taxon.
Trees faithfully record the physiological responses to climatic and environmental changes during their life spans, which can be obtained by using quantitative analysis of the anatomical characteristics of growth rings. Thus, quantitative analysis of growth rings is widely used in modern forestry, geography, palaeoclimatology and palaeontology. In this paper, we systematically compared different parameters and methods of quantitative analysis of growth rings, including the identification of Growth Ring Type, Growth-ring Width, Annual Sensitivity, Mean Sensitivity, Ring Markedness Index, Percentage Latewood, Percentage Diminution, Vulnerability Index, Mesomorphy Index, and the cumulative algebraic sum of each cell's deviation from the mean of the radial diameters (CSDM). The application of these parameters and methods on fossil conifer wood not only provides an unique opportunity to assess the leaf longevity and phenology of ancient conifer plants, but also provides comprehensive understandings of the palaeoenvironmental and palaeoclimatic conditions during the geological past. Therefore, quantitative analysis of growth rings has irreplaceable advantages in the study of fossil conifer wood.
Jurassic flora of the Tibetan Plateau is poorly known, with limited fossil records. The paleobiogeography of terrestrial biota is, however, one of the major evidences to reconstruct the intricate paleogeography of an area. Well-preserved fossil wood is described from the Middle Jurassic Xiali Formation (Callovian) in the South Qiangtang terrane of the Tibetan Plateau, SW China. The wood specimen is a well-preserved secondary xylem without growth rings. Bordered pits on the radial tracheid walls are mostly uniseriate distant, locally contiguous. Cross-fields have 1–2 large oopores. This systematic study indicates that the current fossil wood represents a new species, Circoporoxylon tibetense sp. nov. This is the earliest Laurasian data for the genus Circoporoxylon, which has a Gondwanan origin and subsequently became common in Laurasia during the Cretaceous. The present new finding indicates that Circoporoxylon might have reached the Laurasia via a Tibetan terrane. The presence of fungal remains and wood rotting structures resembling the extant white-rot testifies for the plant–fungal interaction in the Callovian. The absence of growth rings in Circoporoxylon wood and the absence of gypsum layers in the South Qiangtang terrane, together with the widely occurrence of gypsum layers in the North Qiangtang terrane, suggests the South Qiangtang terrane might be wetter than the latter during the Callovian age. The paleogeomorphological highlands of Central Uplift were most likely the boundary between the two. This paleoclimate zonation implies that the Permian to Mesozoic megamonsoon of the supercontinent Pangaea might have terminated in the northern of Paleo-Tethys before the Callovian.
Trees grow towards the sunlight via a process of phototropism. The trunk phototropism processes are frequently observed in Northern Hemisphere from high latitude to at least the Tropic of Cancer region, and also occur in some in situ preserved vertical petrified woods in various geological ages. However, such evidence is still very limited and poorly known in fossil record; and the relationship between tree ring phototropism and rotation of tectonic blocks is unclear. Here we report the eccentricities of living and fossil trees as a proxy to determine geological block rotation at the same latitudes within the North China Block. The dominant eccentricity of living trees is southwest 219° ± 5°. By contrast, standing in situ fossil trunks in the Mid-Late Jurassic Tiaojishan Formation and the Late Jurassic Tuchengzi Formation had average eccentricities of 237° and 233.5°, respectively. These differences shed light on the palaeogeographical changes, indicating that the North China Block rotated clockwise from the Late Jurassic to the present day. This result is largely coincident with the palaeomagnetic results, indicating that the North China Block rotated clockwise by 26.5° ± 5.5° since the Middle to Late Jurassic transition.
A silicified gymnospermous wood, Yangquanoxylon miscellum gen. nov. et sp. nov., is described from the Upper Pennsylvanian-lower Permian Taiyuan Formation, Yangquan City, Shanxi Province, North China. The pycnoxylic wood is composed of thick-walled tracheids and thin-walled rays. It is characterized by mixed radial tracheid pits, uniseriate and homogeneous rays, occurrence of scalariform radial pitting, and various types of cross-field pitting. There are four types of cross-field pitting in Y. miscellum gen. nov. et sp. nov., including simple, taxodioid, phyllocladoid, and araucarioid. The absence of growth rings within fossil woods described here and reported previously from North China suggests a humid climate with minimal precipitation variations during the Late Pennsylvanian-early Permian.
The conventional approach to Tertiary palaeobotany and the analysis of the climatic implications of floristic change is based on an assumption of relative stability of the taxon/climate relationship. The observed pattern of migration of land plant taxa has largely overshadowed the recognition of changes which may have accompanied migration, in species tolerance and in the phenotypic response of plant structure to environmental constraints. Some features of plants are tightly controlled by the genotype (i.e. are conservative), while others are less so, and in that sense are designated as showing phenotypic plasticity. A series of morphological features of plants (the width and character of growth rings in wood, leaf physiognomy and stomatal density) range from being conservative to plastic in the response that they show to environmental change. The nature of the response (both immediate, phenotypic and gradual genotypic evolutionary adaptation) is reviewed.
The data base on which the Cretaceous and Tertiary vegetation history of Antarctica can be built regains very slender; it consists of a few sequences onshore that have been sampled for their spore and pollen content, and an equally limited number of drillsites offshore, on the continental shelf, that have provided palynological data (Fig. 7.1). To these records of in situ pollen and spores, there can be added information available from the palynomorphs that are recycled by glacial processes and incorporated into surficial sediments on the seafloor around Antarctica.
The solar energy input into the very high southern latitudes determines the maximum productivity level that could have been achièyed by Antarctic ecosystems in the geological past when there was not a major glaciation. The input of energy must supply all that is needed the primary producers (the green plants) to carry out photosynthesis.Finally, the carnivorous animals feed either on the herbivorous ones or on other carnivorous ones. Parasites feed either on living plants or animals and scavengers feed on the latter2019s dead remains. Every organism in the ecosystem is thus dependent on the energy input either directly or indirectly. The solar input of about 3500 megajoules/m2/yr (Farman and Hamilton 1987; LaGrange 1963) for high latitudes in Antartica will determine for the continent a temperature ecosystem.
Growth rings are described in gymnosperm woods of the Victor Szlápelis Forest, Salamanca Formation, Lower Paleocene of the Chubut province, Argentina. This forest is composed by a large number of petrified trunks of parautocthonous provenance. The presence of trees with more than 1 m in diameter suggests an exuberant growth and good environmental conditions for tree develoment. Associated sediments suggests that fossil trees were developed in a stable environment related with fluvial to deltaic characteristics. Tree-ring methods are used to study the growth characteristics of these gymnosperms. Wood anatomy and characters associated to the growth records indicate that climate was warm-temperate and humid, with a dry summer and a wet spring.
Polar climate exerts a strong influence on global atmospheric and oceanic conditions, and therefore a sound knowledge of the thermal regime, precipitation/evaporation, and the presence òr amount of ice at high latitudes is essential for understanding a wide range of global systems. Probably the most sensitive indicator of atmospheric conditions, and one that bequeaths an extensive and abundant fossil record, is land vegetation. Accurate reconstructions of polar vegetation through time, and appropriate interpretations of the climatic signal contained in the resultant plant fossil record, is therefore critical to our understanding of global paleoclimate, the evolution and distribution of global biotas, and successful retrodiction of sources of fossil fuels and climate-related mineral deposits.
The terminology used in the paleoanatomical study of gymnospermous woods, proceeding from Carboniferous and Permian of Gondwana, is characterized by vagueness of criteria regarding the selection of the characteristics of taxonomic value. In this regard, a greater negligence is comparatively emphasized in relation to the pith than the other units that composed the stem and the lack of uniformity in the nomenclature used in primary and secondary xylem paleoanatomy. Such procedures have led to elaboration of incomplete generic and specific descriptions, to the difficulty of comparison between the specimens and, often, to the inadequate taxonomic. Thus, aiming at the definition of the features of taxonomic value and the standardization of terminology, a methodology based on proposition and discussion of the anatomical characteristics of the pith and primary and secondary xylem used to inter-taxa differentiation of the Gondwana gymnospermous woods is presented. Notes about the anatomy of the other units of stem, neighboring the secondary xylem, and illustrations of taxa that exhibit the characters considered are also presented.
The recent discovery of diverse fossil flowers and floral organs in Cretaceous strata has revealed astonishing details about the structural and systematic diversity of early angiosperms. Exploring the rich fossil record that has accumulated over the last three decades, this is a unique study of the evolutionary history of flowering plants from their earliest phases in obscurity to their dominance in modern vegetation. The discussion provides comprehensive biological and geological background information, before moving on to summarise the fossil record in detail. Including previously unpublished results based on research into Early and Late Cretaceous fossil floras from Europe and North America, the authors draw on direct palaeontological evidence of the pattern of angiosperm evolution through time. Synthesising palaeobotanical data with information from living plants, this unique book explores the latest research in the field, highlighting connections with phylogenetic systematics, structure and the biology of extant angiosperms.
The conclusion is made that present-day climate appears to have changed as a result of man's inadvertent impact and this change may be considerably increased in the nearest decades. The article considers a possibility of using the numerical models of climatic theory to study future climatic changes under the conditions of increasing influence on climate of man's economic activity.
A study of the dry weights and ages of 57 Salix arctica plants growing in a higharctic region shows that their annual relative growth-rate, which decreases with age, is about one-fifth of the annual relative growth-rate of comparable plants under temperate conditions. Data on the thickness of annual rings indicate that there is a large and progressive increase in annual growth from high-arctic to sub-arctic regions.
Three Jurassic conifer woods from the Rajmahal hills, Bihar are described as Circoporoxylon amarjolense n. sp., Dadoxylon agathioides n. sp., and Taxaceoxylon n. gen. Taxaceoxylon sp. cf. rajmahalense (BHARDWAJ) Krausel & Jain.
The relative growth-rate of plants grown on a vermiculite culture medium in an arctic climate during the growing season was about a quarter of that of comparable plants on the same medium in a temperate climate. In both climates the relative growth-rate was lower on natural soils than on vermiculite. Net assimilation rates and, to a lesser extent, leaf-area ratios were depressed by arctic climates and soils.
Net assimilation rates of seven species in various habitats in two arctic regions were about 0.1–0.3g dm⁻²wk⁻¹. Previous suggestions that net assimilation rates in arctic regions equal or exceed those in temperate regions are attributed to misinterpretation of data or to inadequate methods.
There is evidence that the depression of net assimilation rates in arctic regions is due to the low temperatures, which, especially when associated with soil nitrogen deficiency, reduce the rate at which assimilates are used in respiration and new growth; this causes sugars to accumulate to levels at which they depress assimilation.
Increasing the concentration of CO2 in the air from the usual 300 ppm to 1, 000 ppm in growth rooms with temperatures of 20° C during the 16-h light period and 15° C during the 8-h dark period increased the total dry weight of sugar-beet, barley, and kale by about 5o per cent. A further increase in CO, concentration to 3, 300 ppm increased dry weight slightly more. These effects occurred with light intensities ranging from 3.7 to II.6 cal dm−² min−¹ of visible radiation supplied by a mixture of fluorescent and tungsten lamps, and were only slightly greater with the brighter light. Extra CO2 also increased leaf area, though relatively less than dry weight, and the number of barley shoots but not of sugar-beet or kale leaves; it decreased leaf-area ratio, specific leaf area, and the ratio of tops to roots. Maize was taller with extra CO2.
Net assimilation rates in 1, 000 and 3, 300 ppm CO2 were about 20 and 30 per cent respectively greater than in 300 ppm. Uptake of CO2 in the light by complete tops and single leaves also increased with increase in CO2 concentration. Photosynthesis of leaves of plants recently transferred to a new CO2 concentration depended only on that concentration and not on the original one.
Doubling the light intensity from 3.7 to 7.7 cal dm−² min−¹ affected dry weight, leaf area, net assimilation rate, etc., similarly to a tenfold increase in CO2 concentration.
Calorific values are given for the different types of plant material in an age series of plantations of Pinus sylvestris and the amounts of energy in the organic matter present per hectare of woodland are calculated. Following afforestation with Scots pine a progressive accumulation of energy occurs up to 35 years of age when the organic matter present per hectare contains about 10, 000×108 calories. Values are given for the energy content of the primary production by the trees and ground vegetation, of energy accumulation in the tree stock, understory vegetation, and litter, of energy removed in the harvested trees and of energy released by decomposition. The photosynthetic efficiency of the forest community is calculated and shown to be comparable to that of high producing agricultural crops so that it is suggested that the forest community makes very full use of site conditions.
Carbon dioxide enrichment to 600 ppm increased the amount of dry matter produced by Callistephus chinensis plants in growth cabinets with negligible mutual shading over a period of 18 weeks. Further enrichment to 900 ppm showed smaller and more variable increases. These effects were the result of a higher unit leaf rate of the treated plants. The direct effect on unit leaf rate was partly offset by a reduction in leaf-area ratio, and this was due almost entirely to the effect on specific leaf area with hardly any effect on leaf-weight ratio. Carbon dioxide accelerated flower development by about a week at 600 ppm and somewhat less at 900 ppm. The proportion of the total plant weight in the form of flowers showed a similar trend with time in all treatments and the relationship between flower-weight ratio and dry-matter content of flowers was likewise similar for all treatments, with the highest dry-matter contents of about 19 per cent associated with the highest flower-weight ratios of about 0.44 for mature flowers. Carbon dioxide enrichment significantly increased the dry-matter content of leaves. The efficiency of energy conversion based on incident light and a twenty-four-hour cycle of 8 h light and 16 h dark for small plants of 140-300 mg total dry weight (leaf areas of 50-120 cm 2) was about 4.7 per cent for the 325 ppm treatment, 6.3 per cent for 600 ppm, and 5.5 per cent for 900 ppm. By reference to some further experiments on the growth of C. chinensis cultivar Johannistag in glasshouse conditions, considerable adaptive response to high and low light intensity was also demonstrated.
Nitrogen deficiency was evaluated as an ecological factor affecting CO 2 assimilation in sugar beet ( Beta vulgaris L.) leaves. Nitrogen deficiency reduced the chlorophyll concentration and photochemical capacity of the older leaves but not of young and recently mature leaves. The maximum rate of photosynthesis was reduced in leaves of all ages. This was correlated with a smaller leaf protein content and with an increase in CO 2 diffusion resistances. Although the rate of photosynthesis was reduced as much as 40% when plants were grown for 14 days without nitrogen, the effects were reversible, and activity could be restored to the normal rate within 4 days by adding nitrate nitrogen to the deficient nutrient solution.
Three phases are identified in tracheid production by the main stem of a 12th year forest-grown tree of Picea sitchensis: cell production, radial cell expansion and cell wall thickening. For 15 days during formation of early wood daily cell production was positively correlated (P = 0.01) with daily solar radiation. Increases in cell production appear related to time-lagged increases in numbers of cells undergoing expansion and then wall thickening. Radial diameter and wall thickness were measured for each individual cell along the post wall thickened section of radial files of tracheids. The radial diameters of neighbouring cells were positively correlated, groups of 4-5 large cells tending to alternate with groups of small cells. Furthermore, large cells had thicker walls than small cells. It is suggested that this distinctive structure is the result of fluctuations in the environment which induce changes in the rates of cell production. An increase in cell production increases the length of the queue of cells waiting to pass successively out of the zones of cell expansion and wall thickening. Consequently cells spend longer in the development stages, they expand to a greater diameter and their walls thicken to a greater thickness than when fewer cells are produced and the queue is shorter. We suggest that to investigate the influence which weather may have on the dynamics of tracheid production it is necessary to sample developing wood with a frequency twice as great as that of the frequency of change of the weather variable under consideration.
Reducing light intensity to 1800 ft-c was more than compensated by raising CO2 concentration about 50% above current ambient levels. Since ambient CO2 may rise by 50% within 50 years, while light intensities are not likely to drop to 1800 ft-c, the combined effect will permit rates of net photosynthesis to rise above present levels.
Measurements of biomass and estimates of net primary productivity were made on a stand of birch (Betula pubescens) of 330 m2 at Eqaluit Bay (latitude 61⚬ 06' N; longitude 45⚬ 58' W) in south-western Greenland, about 30-40 km south of the northern limit of the species. It forms scrub-woodland c. 2.5 m high on sheltered slopes. Individual trees lack trunks, their branches ascending from a horizontal stem or `caudex'. A partially destructive sampling technique was used. Total biomass was estimated at 54.9 t/ha, with 1.2 in leaves, 35.8 in branch wood and bark, 11.5 in caudices and 6.4 in roots. Net primary productivity of above-ground parts was estimated at 1.98 t ha-1 yr-1 with only 0.64 in the branch wood and bark and 0.14 in the caudices, compared with 1.20 in the leaves. Net primary productivity of roots was estimated at 0.09 t ha-1 yr-1. The results probably represent nearly minimal values for deciduous woodland.
Productivity studies have shown that plants with marked vertical extension of photosynthetic crown can be more productive per unit area of land or water occupied than plants whose photosynthetic surface is spread in a thin horizontal sheet on the earth's surface if environmental factors are not otherwise limiting. Geometric models including a flat disc and cones of several heights but constant base radius show that heightening cones intercept progressively more light. Amount of chlorophyll displayed per unit area of earth's surface can also increase greatly with vertical extension of aerial crown. These observations suggest that thickness, geometric configuration, and chlorophyll content of the photosynthetic portion of the vegetation per unit area of earth's surface, and light intensity incident on surfaces at right angles to sun's rays should be measured and described as basic data in primary productivity studies.
Photosynthesis of leafy shoots was determined under a range of CO2 concentrations that spanned past and likely future levels of atmospheric CO2. The temperature regime was also manipulated. Response of photosynthesis to CO2 was pronounced, and differed markedly among species. A substantial rise in atmospheric CO2 is likely to produce shifts in species composition of southern California mountain and foothill vegetation, with several conifer species being disadvantaged.
Encelia farinosa is one of a number of species of desert perennial shrubs in which individuals exhibit considerable seasonal variation in the amount and structure of leaf tissue. The function of seasonal leaf variability in adapting this species to the desert environment was investigated. Field observations demonstrated that leaf quantity and structure are controlled by the moisture status of the environment. The field observations also established the influence which leaf quantity and structure have on the CO"2 exchange capacity and water status of the shrub. These observations were corroborated by laboratory experiments. The laboratory investigations also indicated that the influences of leaf structure are brought about by alterations in the resistances to CO"2 and water vapor diffusion.
Net photosynthesis of Pinus attenuata and Pinus coulteri seedlings was determined by CO2 analysis under limiting conditions of soil moisture and atmospheric CO2. As the CO2 limiting factor was removed by enhancement of atmospheric CO2, net photosynthesis increased even when soil water potential was below -- 15 bars. This indicates that net photosynthesis of vegetation in dry climates will respond to increasing atmospheric CO2 in future years.
PREFACE TO THE SECOND EDITION LIST OF SYMBOLS 1. SCOPE OF ENVIRONMENTAL PHYSICS 2. GAS LAWS Pressure, volume and temperature Specific heats Lapse rate Water and water vapour Other gases 3. TRANSPORT LAWS General transfer equation Molecular transfer processes Diffusion coefficients Radiation laws 4. RADIATION ENVIRONMENT Solar radiation Terrestrial radiation Net radiation 5. MICROCLIMATOLOGY OF RADIATION (i) Interception Direct solar radiation Diffuse radiation Radiation in crop canopies 6. MICROCLIMATOLOGY OF RADIATION (ii) Absorption and reflection Radiative properties of natural materials Net radiation 7. MOMENTUM TRANSFER Boundary layers Wind profiles and drag on uniform surfaces Lodging and windthrow 8. HEAT TRANSFER Convection Non-dimensional groups Measurements of convection Conduction Insulation of animals 9. MASS TRANSFER (i) Gases and water vapour Non-dimensional groups Measurement of mass transfer Ventilation Mass transfer through pores Coats and clothing 10.MASS TRANSFER (ii) Particles Steady motion 11.STEADY STATE HEAT BALANCE (i) Water surfaces and vegetation Heat balance equation Heat balance of thermometers Heat balance of surfaces Developments from the Penman Equation 12.STEADY STATE HEAT BALANCE (ii) Animals Heat balance components The thermo-neutral diagram Specification of the environment Case studies 13.TRANSIENT HEAT BALANCE Time constant General cases Heat flow in soil 14.CROP MICROMETEOROLOGY (i) Profiles and fluxes Profiles Profile equations and stability Measurement of flux above the canopy 15.CROP MICROMETEOROLOGY (ii) Interpretation of measurements Resistance analogues Case studies: Water vapour and transpiration Carbon dioxide and growth Sulphur dioxide and pollutant fluxes to crops Transport within canopies APPENDIX BIBLIOGRAPHY REFERENCES INDEX
This chapter reviews some of the carbon dioxide–climate sensitivity studies to analyze effect of carbon dioxide on climate. The carbon dioxide-induced change of the atmospheric temperature is analyzed based upon the results from radiative, convective models of the atmospheric-surface system. A general circulation model of climate is elaborated as a prognostic system of equations representing the physical and dynamical processes, which control climate. A brief description of climate as simulated by the Manabe–Stouffer model is presented. The climatic effect of the increase in carbon dioxide concentration is discussed based upon analysis of the differences between the two statistically stationary states which emerge from the 1×CO2 and 4×CO2 experiments. The distribution of the carbon dioxide-induced change in a global model climate is discussed. The latitudinal and seasonal variation of the signal-to-noise ratio for the carbon dioxide-induced change of zonal mean surface air temperature over continents is also illustrated.
The material consists of a single block of silicified Gymnosperm wood collected by Francis Rodd on his last journey in the Central Sahara. The block measures 10 × 8.5 cm. (radial) × 8 cm. (tangential). It consists entirely of secondary wood, there being no trace of pith, primary xylem, or of any tissues outside the xylem. The preservation of the tissues is, for the greater part, poor, but certain areas are relatively well preserved, and permit of the following description.
Transverse sections (P1. XL, fig. 1) do not give any clear indication of the presence of ammal rings. There are, however, certain regions where, for a depth of two tracheids, the size of the latter is small compared with those on either side. The xylem appears to consist entirely of tracheids, there being no xylem parenchyma present. The tracheids themselves are of the Araucarian type, showing a single row of large contiguous pits which are flattened above and below (P1. XL, fig. 2). Very rarely the pits are in two rows, and, in these examples, the pits are placed alternately. The pits are confined to the radial walls. In transverse section many of the tmcheids show dark-brown contents, while in longitudinal section (P1. XL, figs. 3 & 4) the tracheids appear to be septate. These appearances are probably due to the presence of resin-plates.
The medullary rays (P1. XL, figs. 3 & 4) are invariably uniselriate, and are from 2 to 18 cells deep, the majority being from 5 to
In a prior paper (Part I), the point was made that, assuming an unchanged climate, water use efficiency in agricultural crop production will likely be favored by the increase in CO2 concentration projected to occur within the next half century. Since climatic change is likely to result from the CO2 concentration increase, its possible impacts on agricultural productivity must also be considered. An attempt to do so, using the Great Plains region of North America as a case study, is reported in this paper (Part II).
A number of climatic models predict significant increases in surface temperature. Manabe and Wetherald's (1980) model provides the most specific projections for a hypothetical Northern hemisphere continent. That model also predicts an intensification of the hydrologic cycle with rainfall distribution altered so that some zones will receive more and some less as a result of a doubling in the atmospheric CO2 concentration. The zone between 37 and 47° N latitude will suffer a reduction in availability of soil moisture.
A number of regression models of grain yield as a function of temperature and precipitation have been used to anticipate the impacts of the projected climatic changes. The value of this approach is questioned. An alternative approach - the study of the migration of major agricultural crops across strong climatic gradients - is proposed. Changes in the geographical distribution of the hard red winter wheat zone in North America provide an example. The point is also made that factors currently limiting food production must be considered in order to predict the possible impacts of any given climatic change. In the central Great Plains today, the energy consumed by evapotranspiration often exceeds that supplied by net radiation since sensible heat advection from dryer regions to the south and west provides a major additional input of energy. If, as models project, the excess of precipitation over evaporation increases south of 37° N, the advection of sensible heat and, hence, the rates of evapotranspiration and degree of water stress on growing plants could be reduced in the adjacent regions to the north.