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B. Cascales-Miñana, P. Gerrienne, B. Sirjacq and P. Steemans

ABSTRACT

Most evolutionary innovations in plant vascular tissues, including secondary growth, occurred during the Devonian period (~420 to 360 million years ago) and had a major impact on land colonisation by plants and on their biodiversity. The hydraulic conductance of the secondary xylem of three shrubby or arborescent mid-late Givetian or earliest Frasnian plants (a probable new genus of Cladoxylopsida, the archaeopteridalean genus Callixylon and the stenokolean genus Brabantophyton) is examined for the first time. Plant fossils come from the Ronquières fossil site of Belgium. Results show that hydraulic conductivity of these early woody plants is more or less similar to that of modern gymnosperms, meaning water transport was already as efficient in Devonian plants as it is in living plants. Our results further suggest that tracheids with features helping for optimised water transport were quickly selected in the evolutionary history of vascular plants.

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D.W. Woodcock, H.W. Meyer and Y. Prado

ABSTRACT

This contribution presents descriptions of 14 fossil woods from the Piedra Chamana Fossil Forest in Peru, an assemblage of fossil woods and leaves dated at 39 Ma (late Middle Eocene). It is part two of the descriptions of the non-monocot angiosperm fossils from the site (see Woodcock et al. 2017). The woods are assigned to the subfamilies Bombacoideae, Bombacoideae/Malvoideae, Byttneroideae, Grewioideae, and Sterculioideae of Malvaceae and the families Melastomataceae, Muntingiaceae, Rubiaceae, Rutaceae, and Sapindaceae. Malvalean taxa make up around one-third of the wood types. Many of the woods are identifiable to modern-day genera or groups, including genera with species counted among the hyperdominant trees of the New World forests. Represented vegetation types include mixed freshwater swamp with Avicennia, seasonally flooded forest, and lowland tropical forest with a dry aspect. The assemblage shows floristic similarities to extant South American lowland tropical forest, particularly the seasonally flooded forests growing along white water rivers (várzea); however, the dry forest association has a less clear analog in the present-day tropics.

Fossil forest, Sexi, Peruvian Andes, coastal mangrove forest, seasonally flooded forest, dry tropical forest.

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Maya A. Bickner and Alexandru M.F. Tomescu

ABSTRACT

The Battery Point Formation of eastern Canada hosts an Emsian (c. 400–395 Ma) flora that marks one of the rare occurrences of anatomically-preserved Early Devonian plants. We describe four new euphyllophytes from small permineralized axes in this unit. Leptocentroxyla tetrarcha gen. et sp. nov. has a four-ribbed mesarch actinostele with Psilophyton-type (P-type) tracheids and a central area of scalariform tracheids. Stenoloboxyla ambigua gen. et sp. nov. has a bar-shaped to three-ribbed mesarch stele lacking central protoxylem, with one of the ribs less pronounced, P-type tracheids, and sclerenchyma forming a discontinuous layer in the cortex. Jowingera triloba gen. et sp. nov. has a three-ribbed mesarch actinostele with central protoxylem and P-type tracheids. Tainioxyla quebecana gen. et sp. nov. has bar-shaped xylem with mesarch protoxylem strands, P-type tracheids, and anatomy typical of cambial growth initiation. These new species raise the diversity of Battery Point Formation permineralized plants to nine genera, adding significantly to the diversity of Early Devonian plants characterized anatomically. The four species encompass structural diversity of unexpected breadth and novelty for their age. They are different from both older and coeval euphyllophytes and from younger euphyllophytes, exhibiting combinations of derived and plesiomorphic characters. Their mesarch actinosteles and barshaped protosteles, histological differentiation within metaxylem and cortex, and secondary growth, represent aspects of structural complexity common in more derived Middle-Late Devonian euphyllophytes. Concurrently, the four species share P-type tracheids typical of Early Devonian basal euphyllophytes with simpler anatomies. These new fossils offer a first glimpse of a plexus of plants representing a previously unsuspected stage of euphyllophyte morpho-anatomical evolution. They demonstrate significant euphyllophyte diversification and exploration of structural complexity under way during the Early Devonian, against a background of plesiomorphic-type tracheids. When more completely characterized, these Emsian plants will provide links for resolving phylogenetic relationships at the base of the euphyllophyte clade.

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Anna Dinella, Francesco Giammarchi and Giustino Tonon

ABSTRACT

Peatland ecosystems are an important archive of paleoclimatic information. Within this context, tree-ring data from trees growing in such ecosystems are extremely valuable resources, and subfossil trees from peat bogs have been widely employed in dendroclimatological studies. However, there are still gaps in our understanding of the relationships among tree growth, peatland hydrology and climate factors. Here, we summarize the principal studies on living peatland trees, with a particular focus on their use as a source of information on past climatic conditions. We discuss the main factors influencing tree growth in this environment, whether it is the local hydrological cycle or climate. We put a particular focus on the reliability of the climate signal recorded by living peatland trees, comparing it with that found in subfossil trees. Finally, we discuss the relevance of quantitative wood anatomy in the context of peatland ecosystems research.

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Stéphanie C. Bodin, Rita Scheel-Ybert, Jacques Beauchêne, Jean-François Molino and Laurent Bremond

ABSTRACT

Tropical tree floras are highly diverse and many genera and species share similar anatomical patterns, making the identification of tropical wood charcoal very difficult. Appropriate tools to characterize charcoal anatomy are thus needed to facilitate and improve identification in such species-rich areas. This paper presents the first computer-aided identification key designed for charcoals from French Guiana, based on the wood anatomy of 507 species belonging to 274 genera and 71 families, which covers respectively 28%, 67% and 86% of the tree species, genera and families currently listed in this part of Amazonia. Species of the same genus are recorded together except those described under a synonym genus in Détienne et al. (1982) that were kept separately. As a result, the key contains 289 ‘items’ and mostly aims to identify charcoals at the genus level. It records 26 anatomical features leading to 112 feature states, almost all of which are illustrated by SEM photographs of charcoal. The descriptions were mostly taken from Détienne et al.’s guidebook on tropical woods of French Guiana (1982) and follow the IAWA list of microscopic features for hardwood identification (Wheeler et al. 1989). Some adjustments were made to a few features and those that are unrelated to charcoal identification were excluded. The whole tool, named CharKey, contains the key itself and the associated database including photographs. It can be downloaded on Figshare at https://figshare.com/s/d7d40060b53d2ad60389 (doi: 10.6084/m9.figshare.6396005). CharKey is accessible using the free software Xper2, specifically conceived for taxonomic description and computer aided-identification.

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Anna L. Jacobsen, R. Brandon Pratt, Martin D. Venturas and Uwe G. Hacke

ABSTRACT

Xylem vessels interconnect to form the vessel network that is responsible for long-distance water transport through the plant. As plants dehydrate, the water column within vessels cavitates and gas emboli form, which block transport through embolized vessels. The impact of vessel blockages on transport through the xylem tissue depends upon vessel size and the arrangement and connections between vessels in the network. We examined if there was a correlation between vessel length and diameter within poplar stem xylem tissue using both silicone-injection and analysis of tissue volumes scanned using high-resolution computed tomography (microCT). We then used microCT to scan intact stems sampled over varying water potentials to examine if larger vessels, which would have the greatest impact on hydraulic transport, were more vulnerable to cavitation and embolism than smaller vessels. Within the xylem tissue, larger diameter vessels tended to be longer than narrow diameter vessels. Vessel size distributions indicated that most vessels were narrow and short, with fewer large vessels. Larger volume vessels tended to embolize at higher water potentials and the mean vessel volume of embolized vessels declined as water potentials declined. Hydraulic transport through the xylem tissue was near zero when about 40% of the vessels within the xylem tissue volume were embolized, suggesting important vessel network effects occur as water moves through a three-dimensional (3D) tissue. The structure of the vessel network is important in understanding the impact of emboli within vessels on the overall hydraulic function of xylem tissue.

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Andrea Cecilia Acosta-Hernández, J. Julio Camarero and Marín Pompa-García

ABSTRACT

Warming-amplified drought stress may decrease productivity and growth in both wet and dry conifer forest ecosystems. However, the seasonal radial-growth responses to climate, drought and related climate atmospheric patterns have not been compared in detail in wet and dry sites. We focus on drought-prone northern Mexico conifer forests and compare growth responses in tree species from wet (Pseudotsuga menziesii) and dry sites (Pinus leiophylla). To characterize the responsiveness to inter- and intra-annual changes in water availability we used dendrochronology and measured tree-ring (TRW), earlywood (EW) and latewood (LW) widths. We calculated adjusted LW (LWadj) by removing the influence of EW on LW. We identified E- (narrow tracheids within the early-wood) and L-type (wide tracheids within the latewood) intra-annual density fluctuations (IADFs) and related their frequencies to seasonal climate and drought. We also related growth to atmospheric patterns related to the El Niño-Southern Oscillation (ENSO) which drives precipitation patterns in the studied region. Wet-cool conditions during the prior winter and current spring linked to El Niño events enhanced TRW and EW, particularly in P. menziesii, whereas wet summer conditions enhanced LWadj. The formation of E- (P. leiophylla) and L-type (P. menziesii) IADFs was associated with seasonal fluctuations in precipitation and temperature, suggesting different strategies to withstand drought. The sensitive P. menziesii strongly responded to short spring droughts, whereas the tolerant P. leiophylla responded to longer spring droughts. Seasonal wood measures (EW, LWadj) and IADFs are proxies of intra-annual fluctuations in water availability in similar conifer forests.

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Kelly Cristina Moreira dos Santos, Gabriel Uriel Cruz Araújo dos Santos, Claudia Franca Barros, Haroldo Cavalcante de Lima and Cátia Henriques Callado

ABSTRACT

Stryphnodendron Mart. is a widespread genus in the Neotropics and its species are widely used for their timber, in popular medicine, and for tanning. The similarities in their external morphology make species identification difficult in this genus. This study describes and compares the wood anatomy of the seven species of Stryphnodendron most frequently found in Brazilian forest remnants, in order to identify which anatomical features can be used in their segregation. From seven species 31 samples of Stryphnodendron were studied. Principal Component Analysis was used to evaluate wood anatomical characters. The species were separated into two main groups, congruent with the division into multifoliolate and paucifoliolate species, due to the presence of diffuse, lozenge-aliform and confluent axial parenchyma. In the multifoliolate group, although two subgroups were formed due to ray width in number of cells, none of the species were individualised, which corroborates previous findings of high morphological and anatomical similarities of the multifoliolate species.

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Elisabeth A. Wheeler and Pieter Baas

ABSTRACT

We revisited questions about changes in the incidences of functional wood anatomical traits through geologic time and compared the incidences of these traits in the fossil record with modern wood anatomical diversity patterns in order to test classical (“Baileyan”) and more recent ecophyletic hypotheses of xylem evolution. We contrast patterns through time for tropical and higher (paleo)latitudes. Data are from the InsideWood database. There are striking differences between woods from high and mid latitudes versus tropical (paleo)-latitudes. At temperate and subtropical latitudes (Laurasia and high latitude Gondwana), the epoch by epoch time series supports the Baileyan transformation series of vessel-bearing woody angiosperms (basal woody angiosperms and eudicots): “primitive” features such as scalariform perforations, exclusively solitary vessels, apotracheal diffuse parenchyma and heterocellular rays abound in the Cretaceous and become much less frequent in the Cenozoic, especially post-Eocene. In contrast, in the paleotropics hardly any changes occur in the incidences - each epoch has an equally “modern” spectrum of wood anatomical attributes. Although climatic gradients from the poles to the equator were less steep in the Cretaceous than in the late Cenozoic, the wood anatomical differences between mid-high latitude woods and tropical woods were much more pronounced in the Cretaceous than in later epochs. This seeming paradox is discussed but we cannot resolve it.

We suggest that tropical conditions have accelerated xylem evolution towards greater hydraulic efficiency (simple perforations), biological defense and hydraulic repair (elaborate paratracheal parenchyma patterns) as evidenced by late Cretaceous tropical latitude woods having near modern incidences of almost all traits. At higher paleolatitudes of both the Northern and Southern Hemisphere “ancestral” features such as scalariform perforations were retained in cooler and frost-prone regions where they were not selected against in mesic habitats because of lower demands on conductive efficiency, and could even be advantageous in trapping freeze-thaw embolisms. The fossil wood record remains too incomplete for testing hypotheses on the wood anatomy of the earliest angiosperms. The low incidence of so-called “xerophobic” woods sensu Feild with scalariform perforations with numerous (over 40) closely spaced bars in the Cretaceous tropical fossil record is puzzling. At higher paleolatitudes such woods are common in the Cretaceous.

Ring porosity, an indicator of seasonal climates and deciduousness, occurs at low levels in the Cretaceous and Paleogene at higher paleolatitudes only, and reaches modern levels in the Miocene. In Neogene and Recent temperate Northern Hemisphere, wide vessels are virtually restricted to ring-porous woods. In the tropics, there is a low incidence of ring porosity throughout all epochs.

The fossil record indicates that ecophysiological adaptation to tropical or temperate conditions was already evident in the Cretaceous with considerable latitudinal differences.

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Brett A. Bergman, Edward G. Bobich, Stephen D. Davis, Yasuhiro Utsumi and Frank W. Ewers

ABSTRACT

A node is the point of attachment of the leaf to the stem of a plant; gaps associated with nodes have been viewed as discontinuities of the stem vascular system. We tested the hypothesis that the node/gap is a spring-like joint that impacts stem flexibility even well after the leaves have been shed, with some stems specialized for elongation and others for flexibility. Four-point bending tests were done using an Instron Mechanical Testing Device with the independent variable being the number of nodes in the stem segment and dependent variables being Modulus of Elasticity (MOE), Modulus of Rupture (MOR), and xylem density. Node anatomy was examined microscopically to assess structure and function. The stiffness of the stem was inversely proportional to the frequency of leaf nodes. Surprisingly, xylem density was inversely proportional to the frequency of leaf nodes in stems of adult trees. The tissue around nodes/gaps consisted of twisted and contorted cells that may be effective at absorbing compressive and tensile stresses. Because nodes behave as spring-like joints, the frequency of nodes relates to function, with some stems specialized for vertical expansion and others for light capture and damping of wind stress. The ultimate stems on a tree are the most bendable, which may allow the trees to avoid breakage.