Elisabeth A. Wheeler

Evolution of xylem physiology

Publisher Summary This chapter summarizes the evolution of wood anatomical traits in woody angiosperms and the recognition of ecological preferences from past geological records for a number of arbitrarily defined wood functional types. Xylem evolution can be viewed in the context of a “trade-off” triangle, with different adaptive solutions to the structure/function problems depending on environmental demands as well as phylogenetic constraints (taxonomic history). Evolution of xylem physiology is complicated by the fact that, ever since the early evolution of land plants, xylem has simultaneously performed multiple functions. Regression analysis of various physiological and anatomical characters is used in the chapter to look for trends and possible “trade-offs” between various extant species. The temporal and spatial distribution of the wood types is largely explained by the “tradeoffs” between xylem conductive efficiency and vulnerability to embolism. The two major causes of embolism in plants appear to be freezing and water stress, but the mechanism for embolism formation differs in these two cases. Regression analysis results provide a basis for understanding the variations in the incidences of vessel diameters as related to freezing and drought. However, additional studies are needed to elucidate the physiological significance of variations in vessel wall thickness, vessel perforation type, wood density and parenchyma distribution. The chapter concludes with a discussion on the ecological patterns in xylem anatomy in terms of their functional significance.

Inside Wood – A Web resource for hardwood anatomy

Inside Wood is an Internet-accessible wood anatomy reference, research, and teaching tool. The InsideWood database has coded wood anatomical descriptions based on the IAWA List of Microscopic Features for Hardwood Identification and is accompanied by a collection of photomicrographs. As of November 2010 there were over 5,800 descriptions and 36,000 images of modern woods, and over 1,600 descriptions and 2,000 images of fossil woods. CITES-listed timber species and other endangered woody plants are included in this digital collection hosted by North Carolina State University’s library. This web site has value in helping with wood identification because it has a multiple entry key that allows searching by presence or absence of IAWA features and it serves as a virtual reference collection whereby descriptions and images can be retrieved by searching by scientific or common name or other keywords.

Estimation of temperature and precipitation from morphological characters of dicotyledonous leaves

The utility of regression and correspondence models for deducing climate from leaf physiognomy was evaluated by the comparative application of different predictive models to the same three leaf assemblages. Mean annual temperature (MAT), mean annual precipitation (MAP), and growing season precipitation (GSP) were estimated from the morphological characteristics of samples of living leaves from two extant forests and an assemblage of fossil leaves. The extant forests are located near Gainesville, Florida, and in the Florida Keys; the fossils were collected from the Eocene Clarno Nut Beds, Oregon. Simple linear regression (SLR), multiple linear regression (MLR), and canonical correspondence analysis (CCA) were used to estimate temperature and precipitation. The SLR models used only the percentage of species having entire leaf margins as a predictor for MAT and leaf size as a predictor for MAP. The MLR models used from two to six leaf characters as predictors, and the CCA used 31 characters. In comparisons between actual and predicted values for the extant forests, errors in prediction of MAT were 0.6°-5.7°C, and errors in prediction of precipitation were 6-89 cm (=6-66%). At the Gainesville site, seven models underestimated MAT and only one overestimated it, whereas at the Keys site, all eight models overestimated MAT. Precipitation was overestimated by all four models at Gainesville, and by three of them at the Keys. The MAT estimates from the Clarno leaf assemblage ranged from 14.3° to 18.8°C, and the precipitation estimates from 227 to 363 cm for MAP and from 195 to 295 cm for GSP.

Variations in dicot wood anatomy: a global analysis based on the insidewood database

Information from the Inside Wood database (5,663 descriptions) was used to determine the relative abundance of selected IAWA Hardwood List Features, for the whole world and for the broad geographic regions used in the IAWA List. Features that occur in more than 75 % of the records are: growth ring boundaries indistinct or absent, diffuse porosity, exclusively simple perforation plates, alternate intervessel pitting, and non-septate fibers. The geographic distribution of vessel element features found in this study is consistent with previous studies: ring porosity is a Northern Hemisphere adaptation; numerous, narrow, short vessel elements are more common in temperate regions than in tropical regions. Element size is related to habit, with few wide vessels being a syndrome that is virtually absent from shrubs and small trees. The co-occurrence of selected features, ones that earlier have been suggested to be correlated, was examined; e.g., tangential vessel arrangement and ring porosity, rare axial parenchyma and septate fibers, tracheids and exclusively solitary vessels that are of medium to wide diameter. Axial parenchyma features show geographic variation, with aliform to confluent parenchyma and bands more than 3 cells wide being primarily tropical in occurrence. Storied rays, crystals, and silica bodies are more common in the tropics than in the temperate Northern Hemisphere. For ray features, geographic patterns are less apparent. In Australia, incidences of some features (vestured pits, solitary vessels, radial/diagonal vessel arrangement) are influenced by the Myrtaceae being a major component of the flora. This paper is but a general overview. Information from the Inside Wood database when combined with detailed information on ecological and geographical distributions of species, and subjected to more robust statistical analyses can be used to address a variety of questions on the evolution of wood structure and the ecological and phylogenetic significance of suites of features.

The potentials and limitations of dicotyledonous wood anatomy for climatic reconstructions

The incidences of selected features of dicotyledonous wood that are believed to be of ecologic and/or phylogenetic significance (distinct growth rings, narrow and wide vessel diameter, high and low vessel frequencies, scalariform perforations, tangential vessel arrangement, ring porosity, and helical wall thickenings) were plotted through time (Cretaceous-Recent). There are marked differences between the Cretaceous and Tertiary in the frequency of all wood anatomical features. Incidences of features that are associated with markedly seasonal climates in extant floras do not approach modern levels until the Neogene. Correlations of wood anatomical features with ecology do not appear to have been constant through time, because in the Cretaceous different features provide conflicting information about the climate. Throughout the Tertiary the southern hemisphere/tropical and the northern hemisphere/temperate regions differed in the incidences of ecologically significant features and these differences are similar to those in the Recent flora. Pos- sibilities for reliably using dicotyledonous wood for climatic reconstructions appear restricted to the Tertiary and Quaternary. However, at present the fossil wood record for most epochs and regions is too limited to permit detailed reconstructions of their past climate.

Phylogeny of Robinioid Legumes (Fabaceae) Revisited: Coursetia and Gliricidia Recircumscribed, and a Biogeographical Appraisal of the Caribbean Endemics

Abstract Morphological data and sequences from the nuclear ribosomal ITS region, and the chloroplast trnL intron and matK locus were sampled from robinioid legumes to infer phylogenetic relationships. The monophyletic robinioid clade includes 11 genetically and often morphologically distinct subclades ranked as genera with the following well supported higher level relationships: ((Hebestigma, Lennea), ((Gliricidia, Poitea), (Olneya, Robinia, Poissonia, Coursetia, Peteria, Genistidium, and Sphinctospermum))). In order to render all 11 robinioid genera monophyletic, the genus Hybosema is synonymized with Gliricidia, and the genus Poissonia is resurrected to accommodate four morphologically disparate species previously classified in Coursetia. Three new combinations are required to accommodate these two generic recircumscriptions: Gliricidia robustum, Poissonia heterantha, and Poissonia weberbaueri. Ages of clades and evolutionary substitution rates are derived from a rate-smoothed Bayesian likelihood approach on sequences from the ITS region and the matK locus. Time constraints are derived from the Tertiary fossil wood species Robinia zirkelii, which shares apomorphic wood characters with the Robinia stem clade. The Cuban endemic Hebestigma is estimated to have diverged at least 38 Ma from its Mesoamerican sister genus Lennea, whereas the Greater Antillean Poitea is estimated to have diverged at least 16 Ma from its continental sister Gliricidia. This study reveals that sequences from the ITS region are amenable to exhaustive taxon sampling because of the high levels of variation at and below the species level. The evolutionary substitution rate for the ITS region is estimated at 3.1–3.5 x 10–9 substitutions/site/year, approximately an order of magnitude faster than that estimated for the matK locus.

Dicotyledonous wood anatomical characters as predictors of climate

Abstract The relationships among wood anatomical characters and climate are examined for 50 wood anatomical features in floras from 37 regions in North America, South America, Africa and Malaysia. Correlations, simple regressions and multiple regressions, were used to develop models for the prediction of climate from wood anatomy. The climate variables considered were: mean annual temperature, mean annual range in temperature, cold month mean temperature, mean annual precipitation, precipitation of the driest month and length of the dry season. Good correlations were found with temperature; poorer correlations with precipitation. The climate variables, especially the temperature-related ones, were best predicted by two or more wood anatomical characters considered together. Characters that we selected to calculate climate variables are: vessels with multiple perforations; spiral thickenings present in the vessels; vessel mean tangential diameter less than 100 μm; fibers septate; rays commonly more than 10 cells wide; rays heterocellular with four or more rows of upright cells; rays storied; axial parenchyma absent or rare; marginal parenchyma present; and wood ring-porous. Models were validated on seven temperate and six tropical sites. Based on these results, a method for determining paleoclimate from fossil wood assemblages is suggested.

A global analysis of parenchyma tissue fractions in secondary xylem of seed plants

Summary Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large‐scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions. Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP). We found a 29‐fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction. Our results illustrate that both temperature and growth form are important drivers of RAP fractions. These findings should help pave the way to better understand the various functions of RAP in plants.

Wood Identification -A Review

Wood identification is of value in a variety of contexts - commercial, forensic, archaeological and paleontological. This paper reviews the basics of wood identification, including the problems associated with different types of materials, lists commonly used microscopic and macroscopic features and recent wood anatomical atlases, discusses types ofkeys (synoptic, dichotomous, and multiple entry), and outlines some work on computer-assisted wood identification.

Parallelism and Reversibility in Xylem Evolution a Review

The irreversibility of the major trends of xylem evolution, such as the origin of vessels in primitive angiosperms with long fusiform initials, and the shifts from scalariform to simple perforations and from tracheids to libriform fibres, has long been accepted by wood anatomists. Parallel development of these and other xylem features is generally accepted, and is suggested by the distribution patterns of the fibre and perforation plate type. Some recent phylogenetic analyses of seed plants suggest that there also have been some reversals in these general trends. The likelihood and extent of parallel origins and reversions of the major trends in xylem specialization are explored here by analysing a number of published hypotheses on the phylogenetic relationships within wood anatomically diverse major clades of angiosperms, and within some individual families. On the basis of these analyses, it appears that for these major Baileyan transformation series, parallelisms were more than twice as common as reversals. Functional adaptations to increased efficiency and safety of hydraulic architecture can largely explain the high incidence of parallelisms in xylem evolution.