Variation Between Tree Species

Durability of heartwood is dependent on the kinds and concentration of extractives present, their chemical stability, and their resistance to microbial inactivation. Many tree species (e.g., Madurapomifera or Robiniapseudoacacia) contain only one or two toxic compounds, often in very small quantities, yet the wood is extremely durable (45, 54). In other species a number of similar toxic compounds may occur together (Libocedrus decurrens (5)), or toxic compounds of a very different chemical nature may be present in the heartwood of a single species (stilbenes and ellagitannins in Eucalyptus sideroxylon (23). In either case, the combination of compounds acts additively or perhaps even synergistically to produce an inhibitory environment. 

The comparative decay resistance of the more common North American woods (Table 9.2.2) has frequently been published (37, 44, 45). This list has not been revised since 1961 and new information would change the decay rating of several species (e.g., Juglans nigra from resistant to moderately resistant (21)). Other lists (Table 9.2.3) are available that give the relative durability of species not native to North America (44, 45). 

Resistant or very resistant Moderately resistant Slightly or nonresistant 

Baldcypress (old growth) Baldcypress (young growth) Alder 

Cypress, Arizona Pine, eastern white Birches 

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Wood is a natural material, and as such it exhibits great variability in its properties. Variations exist between species, between trees and within the tree itself. This can cause difficulties for any study of the material, making it difficult to compare results. In any review of the wood science literature (or indeed any branch of science), it is readily apparent that there are sometimes disagreements between different studies, and it is not... 

Individual trees of the same species may vary considerably in decay resistance (45). In a rangewide study of the decay resistance of Juglans nigra, individual trees from the same site varied significantly in decay resistance (21). This variation is attributed mainly to genetic variability. Similarities in heartwood decay resistance between trees collected at different sites suggest that site is not a major fac-... 

Regardless of the species or where the forests are established, the variation in wood properties between trees is very great and can be great even in boards sawn from the same tree. In particular, the social status of the tree in the stand (whether dominated or dominant trees) has a great effect on the growth rate in diameter and the occurrence of reaction wood. Table 36.10 lists some of the characteristics of... 

Between various wood species great differences can occur in pH as well as in the buffer capacity. Even within the same wood species, differences might occur due to seasonal variations, portion of the wood substance under investigation, pH of the soil, age of the tree, time span after cutting, drying and processing parameters. 

Molecular phylogeny is a discipline that studies species differences between DNA or protein sequences. Its basic tenet is that during evolution, the sequences have drifted apart by mutation and selection as well as by random drift and fixation of variants in certain positions. The earlier two species separated the more differences became fixed. Phylogenetic trees are constructed on the basis of mutual differences of protein and/or DNA sequence. Comparison of intraspecies variation with between-species variation may in the future yield information on the neutralist/selectionist alternative. McDonald and Kreitman (1991) devised an interesting test against neutrality that compared the ratio of silent/replacement mutation of a given locus within a species with the same ratio between two related species. Under the neutral theory this should be equal (corrected for sample size), but in fact it is not (see Li, 1997, and Hudson, 1993, for a discussion). 

A problem always faced in designing an evolutionary investigation is judging how many individuals need to be analyzed from a particular species or population. This problem manifests itself across the range of questions, from inferring species trees from gene trees to comparing variation within and between populations. 

Figure 5.11. Mean whole-tree values of stiffness and density for hardwoods and softwoods. Superimposed is the range in values expected of lumber from Firms radiata and Eucalyptus nitens. Variations within species are greater than differences between mean values for species.

Figure 10 A rooted universal tree of life, showing the three domains of life. The tree is based on sequence comparisons of ribosomal RNA, analysed by Carl Woese and his colleagues. The order and length of branches are proportional to the sequence similarities within and between the domains and the kingdoms of life — in other words, they are directly proportional to the genetic similarities between species. It is humbling to note that the animals, plants and fungi account for just a small comer of the Eucarya domain, and that there is less variation in ribosomal RNA sequences within the entire animal kingdom than there is between different groups of methanogen bacteria.

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