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Keywords:

  • bound water;
  • cellulose;
  • fibre saturation point (FSP);
  • hydration;
  • partial specific volume;
  • plant–water relations;
  • thermodynamics;
  • wood

Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

Contents

  • Summary 1

  • I. 
    Introduction 2
  • II. 
    The FSP: an overview 2
  • III. 
    Equilibrium thermodynamics and the FSP 5
  • IV. 
    Inside the FSP 7
  • V.  
    Living trees and the FSP 10
  • VI.  
    Conclusions 11
  • Acknowledgements 12

  • References 12

Summary

This review is about the behaviour of water in cell walls. The aim is to introduce to biologists the concept of the fibre saturation point (FSP), and the related research of material scientists and engineers on the thermodynamics and chemistry of water in timber and wood. In the review, we first summarise what the FSP is, why it is important and how the FSP is routinely used by engineers and material scientists to estimate the volume fractions of solid, liquid and gas phases in bulk timber. We then show that the FSP can be intuitively understood using equilibrium thermodynamics. That analysis shows that the FSP is based on the concept that a certain (and repeatable) amount of water is chemically bound to cellulose and other substances in wood. That water, sometimes called bound water, exists in a water–cell wall mixture. The noted physical chemist and wood scientist, A. J. Stamm, called this mixture a ‘solid solution’. In timber, the ‘solid solution’ is considered a separate phase from adjacent water in either a pure liquid phase or a vapour phase. Following that, we examine the FSP and wood–water dynamics at the molecular and cellular level. Despite differences between timber and living trees, we conclude that the FSP-based framework long used by material scientists and engineers is likely to be useful to biologists.


Abbreviations
[D]

mass concentration of dry matter, also called ‘basic density’

FSP

fibre saturation point.

I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

Plant cells are mostly made of water. Hence it is no surprise that, at the cellular level, water is widely recognised as being essential for plant functions from several points of view – solvent, reactant, cell size, etc. Similarly, in a broader ecological context, the availability of water has long been recognised as a very important determinant of the type, amount and productivity of vegetation (Woodward, 1987). These and similar observations underlie much of the biological research into plant–water relations.

Biologists are not the only scientists interested in water and plant material. For example, the interaction between timber (i.e. dead plant material) and water has long been studied by engineers and material scientists. Their interest is that timber has long been, and remains, an important construction material. The engineering-orientated research has been primarily motivated by the observations that (1) dry timber swells when exposed to water, and that (2) dry timber is much stronger (i.e. it has increased strength at elastic limits in endwise compression and cross-bending, and has an increased modulus of rupture in bending; Skaar, 1972) than wet timber. The early research was focussed on understanding these two observations, and from that beginning a vast literature has developed about the interaction between timber and water. For example, the transport of water through timber has been intensively studied (e.g. Booker, 1977; Siau, 1984) because this gives important insights for designing industrial processes to impregnate timber with preservatives or other materials. Similarly, the chemistry of wood has been intensively studied in the context of paper and pulp production.

Going back 100 years or so, biologists were often aware of the engineering-orientated research (e.g. Büsgen & Münch, 1929). However, the situation is quite different now, perhaps because of the inevitable specialisation in the separate disciplines. For example, the basic concept which underlies the engineering–materials science approach to the relationship between timber and water is the fibre saturation point (FSP), which has been in routine practical use for at least the last 50 years. A description may be found in any wood science text (e.g. Stamm, 1964; Skaar, 1972, 1988; Siau, 1984). What is surprising is that the FSP is more or less absent from the biological literature about plant–water relations. For example, we are only aware of three papers where the FSP has been mentioned or applied in the biological literature. The first of those was an important paper from over 20 years ago that (implicitly) used the FSP to estimate the amount of water stored in tree stems and related that to the amount of water in the transpiration stream (Waring et al., 1979). The second was a paper that looked at water transport from a mechanical perspective (Roderick & Berry, 2001). This theoretically orientated research has prompted new observations and experiments investigating the structure–function relationships of trees (Atwell et al., 2003; Barbour & Whitehead, 2003; Thomas et al., 2004; Barbour et al., 2005). The third was a recent and rigorous thermodynamic treatment of the transport mechanisms of water through living trees (Lampinen & Noponen, 2003). Their use of the FSP is not surprising because the authors came from an engineering background. There may be more papers that we are not aware of, but the point remains that the FSP is conspicuous by its absence from the biological literature.

The aim of this paper is to introduce the concept of the FSP to biologists. We first review the general principles on which the fibre saturation concept is based. We then show that it has a straightforward thermodynamic explanation in terms of bound and free water. After that, we describe what the engineering–materials science literature has reported about the dynamics of water in wood. Finally, we give some brief notes about extending the concepts to living trees.

II. The FSP: an overview

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

1. What is the FSP?

When a small amount of liquid water is poured onto oven dried timber, a new equilibrium will eventually be established. Observations show that when equilibrium is attained, the added water is not visible in the voids. Instead the water must, by default, be located inside the cell wall matrix. Hence, the mass of the cell walls increases, as does the volume. This is observed macroscopically as swelling of the timber sample. Concurrently, the strength of the timber progressively declines as the moisture content increases. As water continues to be added, and the moisture content increases further, the system eventually reaches an equilibrium state where liquid water begins to accumulate in the voids. When that occurs, the cell walls cease swelling and the strength of the timber no longer changes with the moisture content. The moisture content (defined by wood scientists as the ratio of liquid mass to dry mass) at which this occurs is called the FSP. According to Skaar (1988), p. 35, the FSP was originally described by the American forester, H. D. Tiemann, as ‘the moisture content at which the cell cavities contained no water, but the cell walls were fully saturated with liquid moisture.’ Precise definition of the FSP has proved elusive possibly because different measurement techniques can give slightly different estimates of the FSP (Stamm, 1971). Despite that difficulty, the basic ideas underlying the FSP have been repeatedly confirmed over the last 100 years.

2. The FSP as a chemical phenomenon

What is the nature of the forces that hold the water preferentially in the cell walls? Obvious candidates are chemical bonding and/or capillary (i.e. surface) forces. This question can be addressed by first determining the FSP of intact timber samples. After that, the samples are pulverised to create a larger surface area (but the total mass is kept constant), and the FSP determined again. The results from those experiments show that the FSP remains more or less the same, regardless of whether the timber sample is intact or pulverised (Stamm, 1964). This implies that there is a distinct number of binding sites for water in the timber, and that the number of binding sites is (more or less) independent of the macroscopic surface area of the sample. On those observations, the forces are interpreted as being primarily chemical forces. Further support for that conclusion comes from the fact that heat (known as the heat of wetting) evolves when water is added to dry wood, which also implies the existence of chemical reactions (Stamm, 1964). In essence, the phenomenon can be thought of as one of hydration. (Later, we show that this conclusion is consistent with a whole range of thermodynamic and chemical evidence.)

This does not mean that engineers and material scientists do not recognise capillary forces. Rather, they recognise that for the typical dimensions of conduits in timber (of the order of ≈1–100 µm), the reduction in vapour pressure estimated using the Kelvin capillary equation is very modest (relative humidity ≈99%) (Browning, 1963; Stamm, 1964). Cell walls are assumed to be porous at atomic scales, but at these scales it is assumed that water makes its own space by virtue of the large chemical forces involved (Stamm & Hansen, 1937; Stamm, 1964). Consequently, wood scientists have not applied the Kelvin capillary equation for spaces smaller than about 5 nm (e.g. Browning, 1963). That view is consistent with recent molecular dynamics simulations of water in small pores showing a ‘switching’ behaviour between liquid and vapour phases (Beckstein & Sansom, 2003).

In summary, in the wood and fibre science literature, water is considered to exist in timber in three states: (1) as vapour in the gas-filled voids; (2) as ‘free’ or ‘bulk’ liquid water in the voids; and (3) as ‘bound’ water in the cell wall matrix (Stamm, 1967a). At the FSP, all of the liquid water is bound water, as this represents the maximum amount of water that can be taken up from the vapour phase by a unit mass of timber at a given temperature (Browning, 1963). (More details of the temperature dependence are given in Section III.2.) The integrated mixture of cell wall material and ‘bound’ water is recognised as a distinct phase and has been called a ‘solid solution’ by Stamm (1964). An alternative and perhaps more informative term for the ‘solid solution’ is a ‘gel’ (Skaar, 1988).

3. Estimating the bulk phases in wood

There are important practical advantages of separating the water in timber into the three above-mentioned phases. These arise from the observations that (1) the FSP is an estimate of the maximum amount of water that can be ‘bound’ to the cell wall matrix, and that (2) the density of dry cell wall material is very conservative at ≈1.5 g cm−3 (Stamm, 1964). Hence, given an estimate of the fresh volume, fresh mass, dry mass and FSP of a piece of timber, it is straightforward to estimate the volume fractions of the three bulk phases (gas, liquid, solid) in a timber sample, and this is done routinely by engineers (e.g. Rijsdijk & Laming, 1994) and by material scientists. In routine applications, the default value of the FSP is often assumed to be 30% (i.e. the ratio of liquid mass to dry mass is 0.30). At the outset, we emphasise that a universal FSP at 30% is, at best, an approximation, and in later sections we examine this approximation in greater detail. Before that, we show how this framework is routinely used to estimate the volume occupied by the three bulk phases in timber.

The calculations used to estimate the three phases in timber have remained more or less the same over the years, but widely different terminology has been used (e.g. Stamm, 1964; Siau, 1984; Skaar, 1988). The a-u-s-V scheme (Roderick et al., 1999a; Roderick & Berry, 2001), as summarised below, uses exactly the same concept, and calculations, but presents a simplified terminology for use in estimating the three bulk phases. The details follow.

The total volume of a timber sample (V, m3) is the sum of the volumes of the three phases:

  • image(Eqn 1 )

where Va (subscript ‘a’ for air, the ‘gas’ phase), Vu (subscript ‘u’ for solution, the ‘liquid’ phase) and Vs (subscript ‘s’ for structure, the ‘solid solution’ phase, or, more simply, the ‘solid’ phase) are the three phases. The mass (m, kg) of the sample is the sum of the dry mass (md) and liquid mass (mq):

  • image(Eqn 2 )

where the mass of gas is ignored. The FSP is used to allocate the water between the solid and liquid phases. To do that for timber, it is usually assumed that all of the dry mass is in cell walls, and hence is part of what we have called the solid phase. (This could be modified for use in living trees by accounting for the small amount of dry matter in the vacuoles and cytoplasm of cells, i.e. the liquid phase.) The FSP (αf) is defined as:

  • image(Eqn 3 )

where mq,x is the maximum possible mass of liquid that can be located in the ‘solid’ phase of timber. The mass of liquid in the ‘solid’ phase (mq,s) is estimated as:

  • image(Eqn 4 )

and the mass of the liquid phase (mu) is then:

  • image(Eqn 5 )

The volume of the liquid phase is given by:

  • image(Eqn 6 )

where ρ (kg m−3) is the density. Similarly, the mass and volume of the ‘solid’ phase (ms, Vs) are:

  • image(Eqn 7 )

and:

  • image(Eqn 8a )

In routine applications the density of the ‘solid’ phase (ρs) is unknown and is usually estimated using the linear mixture approximation:

  • image(Eqn 8b )

where the density of the dry matter (ρd) is assumed constant at ≈1.5 g cm−3, and the density of water bound to the cell wall material is assumed to be equal to the density of pure liquid water (ρw, 1 g cm−3). From a mechanical and thermodynamic perspective, the linear mixture assumption is wrong and leads to a flawed model and hence flawed understanding, because one should use the relevant partial specific (or molar) volumes for the calculations, and because the partial specific volume of water varies with the moisture content of the timber (see Section III.3). However, when used to estimate the volume fractions of the bulk phases, the approximation would lead to errors of perhaps a few per cent at most (see Section III.3). This is adequate for many practical purposes. Finally, the volume of the gas phase is then estimated by difference (Eqn 1). Although the mathematics appears tortuous, in practice the calculations are very simple. See Fig. 1 for an example.

image

Figure 1. Example calculations for estimating the volume of the three bulk phases (gas, liquid, solid) in a timber sample given measurements of the fresh volume (1.0 cm3), fresh mass (0.80 g), dry mass (0.35 g) and fibre saturation point (FSP; 0.3).

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In Fig. 1, the FSP was assumed to be 0.30, and this is a good starting approximation for most timber and other cellulose-based materials (Stamm, 1964). However, wood scientists have found that the FSP does vary. Typically, the FSP decreases in denser wood (Skaar, 1988). The relation between wood density and the FSP is convenient because in practical forestry applications, the most commonly used intensive variable is the mass concentration of dry matter ([D], kg m−3):

  • image(Eqn 9 )

Foresters (and wood scientists) call [D] the basic density. The underlying significance of [D] for foresters is that many of the mechanical properties and commercial attributes of timber vary directly with [D] (Desch, 1973; Zobel & van Buijtenen, 1989). This is easy to understand, because if the density of the dry cell wall material is approximately constant (≈1.5 g cm−3), then it follows that, as [D] increases, the volume fraction of the ‘solid’ phase increases and the volume fraction of the remaining phases (i.e. gas and liquid) must therefore decline. It is also easy to show that [D] should be significant from a biological perspective, because if a plant has a low value of [D], a given increment of dry mass will produce a larger change in volume than for a plant with a high value of [D]. Hence, plants with a low value of [D] should, all else being equal, increase their volume faster than those with a high value of [D] (Roderick, 2000).

Using the above knowledge, it is straightforward to develop a simple overall understanding of how the volume fractions of the phases, and the amount of bound water, varies in timber with different [D] (Fig. 2). It is interesting that engineers and biologists might view Fig. 2 with possibly different perspectives. The fundamental starting point for biologists is the liquid phase because that is the ultimate source of the biochemical activity in living plants. However, the fundamental starting point for engineers interested in the strength of wood is the ‘solid’ phase because that is the principal source of the mechanical strength. Ecologists are often simultaneously interested in both aspects because they both impose constraints on plant architecture and function (Niklas, 1992; Roderick & Berry, 2001).

image

Figure 2. Variations in the volume fractions of free water, bound water and dry matter as a function of [D] in timber samples that are assumed to be saturated with liquid water (i.e. no gas in the voids). The total space occupied by the bound water and dry matter is the ‘solid’ phase (Vs). In this example, liquid water occupies all the remaining fluid space (Va + Vu) because the sample is assumed to be saturated throughout. The calculations assume a linear mixture (Eqn 8b) with the density of dry cell walls set at 1.54 g cm−3 and the density of liquid water set at 1 g cm−3. The fibre saturation point (FSP) was estimated using an empirical relation, inline image, where ρw is the density of liquid water (for derivation, see eqn C5 in Roderick & Berry, 2001), that describes the general trend of decreasing FSP as [D] increases (Skaar, 1988, fig. 1.32).

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III. Equilibrium thermodynamics and the FSP

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

In the previous section, the basic observations underlying the FSP were described along with an important practical application of the concept – estimating the volume fractions of the phases. In this section, we examine the FSP from the point of view of equilibrium thermodynamics. We do not attempt to give the underlying mathematics – rather, the thrust of this section is to look at the FSP from an intuitive viewpoint.

1. FSP and relative humidity

A typical example of an adsorption–desorption experiment that is routinely reported in the wood science literature is shown in Fig. 3. In this example, the relative humidity of ambient air approaches 100% when the moisture content of the timber is about 30%. For moisture contents greater than 30%, the relative humidity of ambient air would remain near 100%. For this example, we can assume that the FSP is about 30%. However, because of the asymptotic nature of the relation, one could equally argue that the FSP is at 28% or 29% or 31%, etc. One pragmatic approach to obtaining (more) consistent estimates of FSP is to define it as the equilibrium moisture content when the relative humidity is arbitrarily close to 100%, e.g. say 99.5% (Browning, 1963; Stamm, 1964).

image

Figure 3. Relationship between relative humidity of ambient air and the equilibrium moisture content (mass of liquid per unit dry mass as a percentage) for wetting (adsorption) and drying (desorption) cycles of Sitka spruce (Picea sitchensis) at 25°C and atmospheric pressure. Redrawn from Fig. 1 in Stamm & Loughborough (1935).

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The relation in Fig. 3 makes sense because it shows that when the equilibrium moisture content is less than the FSP, not all of the water molecules are ‘free’ to enter the adjacent gas phase. In other words, some of the water molecules have reduced mobility because of chemical interactions within the ‘solid’ phase.

2. Temperature dependence of the FSP

Following from the previous section, we would expect that the FSP should also vary with the temperature at which the measurements are made. Specifically, at higher temperatures, a larger proportion of the water molecules should have sufficient kinetic energy to enter an adjacent gas phase. Hence, the FSP should decline as the temperature increases. That is precisely what has been observed (Fig. 4). In summary, the FSP decreases at higher temperatures, although at typical environmental temperatures of 5–35°C, say, the FSP is still roughly equal to the widely assumed (default) value of 30% (Fig. 4).

image

Figure 4. Change in fibre saturation point (FSP) with temperature for Sitka spruce (Picea sitchensis). Redrawn from Fig. 3 in Stamm & Loughborough (1935).

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3. Partial specific volume of water and the FSP

Building on the relative humidity and temperature dependence, we now examine the changes in the partial specific volume of the ‘bound’ water as the moisture content is varied. The ‘bound’ water is considered part of the ‘solid solution’. This in turn means that below the FSP, as the moisture content is varied, the mass of the ‘solid’ phase changes and the volume of the phase also changes. To examine that change, we can use the partial specific volume (denoted here as ∂Vs/∂mq,s), which is the ratio of the change in volume of the whole phase to the change in the mass of water in the phase, while everything else is held constant. For comparative purposes, it is convenient to reference this quantity to the specific volume of pure liquid water (νw) at the same temperature and pressure, in other words:

  • image(Eqn 10 )

where C has the units m3 kg−1 and can be called an effective compression. We use the term ‘effective compression’ to emphasise that the changes happen at constant temperature and with the pressure in the surroundings (i.e. atmosphere) constant.

When using the linear mixture approximation (Eqn 8b; Fig. 2), it is assumed that the effective compression (C, Eqn 10) is zero. For example, if the specific volume of pure liquid water was 1 cm3 g−1 at the temperature and pressure of the experiment, then the linear mixture approximation assumes that this is also the partial specific volume. On that assumption, as the moisture content is varied, the volume of the ‘solid’ phase would increase by 1 cm3 for every gram of water that becomes part of the ‘solid’ phase. Determining the value of the effective compression (C, Eqn 10) was a matter of considerable controversy amongst wood and material scientists in the early 1900s. After many years, the controversy was eventually resolved by a series of careful experiments conducted during the 1930s (Stamm & Loughborough, 1935; Stamm & Seborg, 1935; Stamm & Hansen, 1937). In those experiments, the partial specific volume of water in the ‘solid solution’ was estimated by weighing timber samples having different moisture contents in different fluids (i.e. different gases and liquids). The experimental results are summarised in Fig. 5 and discussed below.

image

Figure 5. Variation in the effective compression (C in Eqn 10) at different moisture contents. Moisture content expressed as mass of water per unit dry mass. Data are for Sitka spruce (Picea sitchensis) and are redrawn from Fig. 1 in Stamm & Seborg (1935).

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The data in Fig. 5 are interpreted as follows. When water is initially added to dry timber, the moisture content of that timber increases, as does the mass of the ‘solid solution’ (i.e. cell wall–water mixture). Hence, the volume of that phase also increases. In Fig. 5, the effective compression is ≈0.05 cm3 g−1 when the timber is dry. This means that the initial ‘bound’ water occupies less space than pure liquid water (at the same temperature and pressure). Assuming that the specific volume of pure liquid water was 1 cm3 g−1, the above-noted effective compression means that the volume of the ‘solid’ phase of dry timber increased by about 0.95 cm3 for every gram of water that became part of the ‘solid’ phase. As the moisture content increased, the effective compression progressively declined. Finally, when the moisture content was about 30% (i.e. at the FSP) there was no effective compression. A more general discussion of these important results follows.

The concept that water is chemically ‘bound’ onto the cell wall matrix implies that the forces involved in the cell wall–water linkages are stronger than the forces involved in water–water linkages. Thus, the ‘bound’ water molecules should be, on average, slightly closer together than the free water molecules – they are in effect ‘more bound’. Hence, the partial specific volume (and partial molar volume) should be slightly less for the ‘bound’ water than for the free water, as has been observed (Fig. 5). In Fig. 5, the partial specific volume equals the specific volume of pure liquid water at a moisture content of about 30% (the FSP). This can be used to define the phase boundary between the ‘solid solution’ and adjacent phases. Provided that the temperature was uniform throughout, the chemical potential of water would be equal in the adjacent phases on either side of this boundary (Roderick, 2001) because the escaping tendency of water from both phases would be equal (Gibbs, 1875; Lewis & Randall, 1961). This location of the boundary is also consistent with the conception that the water below the FSP ‘belongs’ to the cell wall–water mixture, but water above the FSP ‘belongs’ to the adjacent pure water.

The magnitude of the chemical forces involved in this effective compression can be estimated by noting that to compress liquid water isothermally by 1% (i.e. decrease the specific volume) a pressure of about 210 bar (21 MPa) is required. Hence, the effective compression of roughly 5% (note that the effective compression is difficult to determine precisely because of the asymptotic nature of the relation) that occurs near zero moisture content (Fig. 5) equates to about 1000 bar (100 MPa) (Stamm & Hansen, 1937). This emphasises the strength of the chemical forces, and thus why it is so difficult to dry timber completely.

IV. Inside the FSP

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

In the previous sections, the FSP was examined first as a phenomenon, and then from the point of view of equilibrium thermodynamics. The timber (or wood) was treated as a bulk quantity. This level of understanding is sufficient for many bulk engineering and industrial applications. However, this is not by itself sufficient for understanding the dynamics (e.g. rate of drying, etc.). We pursue that in this section by examining the FSP at the molecular and subcellular scale. Initially, we build on the previous section by summarising the bulk energetics from a chemical thermodynamic viewpoint. After that, we review the composition and arrangement of molecules within the cell walls because this is important for a molecular level understanding of the FSP. Finally, we summarise what the engineering–materials science disciplines have found about the movement of water through wood, and especially in cell walls.

1. The chemistry of bound water

When previously dried wood tissues are exposed to water (either liquid or vapour), the water molecules form strong hydrogen bonds (H-bonds) with the exposed hydroxyl groups of the cell wall molecules (Browning, 1963). In Fig. 6, this initial wetting is represented schematically by the single H2O molecule bound to the cell wall molecule at low moisture content. This bonding occurs because the attractive forces between the surface hydroxyl groups and water molecules exceed the attractive forces between water molecules for themselves. The binding force is stronger for the initial layer of water molecules and then progressively declines over successive layers. Because of the bipolar nature of water molecules, other water molecules align above those that are bonded to the hydroxyl groups and form H-bonds to produce hydration layers. This is denoted in Fig. 6 by the gradual increase in the number of ‘bound water’ molecules with the moisture content. Concurrently, there is a gradual increase in specific enthalpy of water in the ‘solid solution’ (hq,s) until it equals the specific enthalpy of pure liquid water (hw) at the same temperature and pressure. (Note that this is yet another way of defining the phase boundary previously discussed in Section III.3.) To move into an adjacent liquid (or vapour) phase, a molecule of water in the ‘solid solution’ must have sufficient activation energy (ha) to overcome the H-bonding forces between it and the solid. The vapour phase has the highest specific enthalpy (hv).

image

Figure 6. Schematic model of the relationships among cell wall moisture content (expressed as mass of water per unit dry mass), number of hydration layers and specific enthalpy of water molecules. Note that the spacing between hydration layers increases with distance from the cell wall as per the changes in partial specific volume (Fig. 5). The specific enthalpy levels are: hv, water vapour; ha, activated water molecules, in other words the activation energy level required to break the hydrogen bonds and thereby allowing water molecules to move from one binding site to the next, or to move into the vapour phase; hw, pure liquid water; hq,s, water in the ‘solid solution’. All enthalpy levels refer to the same temperature. Modified from Skaar (1988, Fig. 5.2).

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The differences in enthalpy are commonly observed as the heat of wetting. That is, there is an evolution of heat when water is added to timber (Stamm, 1964) that is entirely analogous to the familiar heat of solution. Changes in enthalpy can, and have been, used to determine the FSP experimentally. For example, Simpson & Barton (1991) used differential scanning calorimetry to determine the FSP at 0°C of wood of Eucalyptus marginata (Jarrah), E. diversicolor (Karri) and Pinus radiata. Using this method, samples of wood of known moisture content are first cooled to −22°C using liquid nitrogen, and the enthalpies of melting are measured as the samples are subsequently warmed. Only the free water can be easily frozen, and this water is located in the larger voids of the cell lumens. As a sample warms, the frozen water in the larger voids melts, and a peak in temperature is observed along with an associated heat flux.

The average number of hydration layers at the FSP is estimated to be about five or six, but the water molecules are not necessarily distributed evenly – some sites may hold 10 layers whilst others hold two (Stamm, 1967a). When timber at the FSP is heated, some of the water molecules obtain sufficient energy to overcome the binding forces to the solid and enter the liquid or vapour phase. Thus the average number of hydration layers and the FSP decrease as the temperature increases (also see Section III.2).

2. The major chemical components of cell walls and their capacity to bind water

The dry matter in cell walls is mostly comprised of cellulose (42% ± 2% in both hardwoods and softwoods), hemicelluloses (28% ± 4% in softwoods, 33% ± 3% in hardwoods) and lignin (30% ± 4% in softwoods, 25% ± 3% in hardwoods) (Siau, 1984). Cellulose provides the skeleton, hemicelluloses provide the matrix and lignin encrusts the wall and binds it together, making it rigid (Siau, 1984). All of these molecules have exposed hydroxyl groups with which water molecules form strong H-bonds (Browning, 1963). The heartwood contains, in addition, low molecular weight organic compounds (extractives) that are generally located in the lumens, but sometimes also occur in the cell wall (Siau, 1984).

As water binds with exposed hydroxyl groups, the spatial arrangement of the tissues (and the hydroxyl groups) is important for understanding bulk properties like the FSP. A detailed description is provided by Siau (1984), and summarised below. Cellulose is a polymer of glucose residues and it always occurs in a fibrillar form. In nature, about 50–60% of the cellulose occurs in a crystalline form in which the fibrillar cellulose molecules lay parallel and on top of each other, bound together by strong hydrogen bonds. The crystalline regions of the cellulose are rigid and inaccessible to water molecules, but they are separated by amorphous regions to which water molecules may bind. The hemicelluloses are various polysaccharides that have much lower molecular weights than cellulose. The arrangement of the lignin and hemicelluloses is amorphous. Lignin has fewer hydroxyl groups to bind water than do cellulose or the hemicelluloses. Hence, wood with higher lignin content will have a lower FSP, all else being equal.

Of the three major materials (lignin, hemicelluloses, cellulose) in cell walls, lignin has the lowest affinity for water and hemi-cellulose the highest. The affinity of cellulose for water is between these two. A useful way to understand this is to express the sorption capacities of the three major materials relative to the sorption capacity for intact wood. For example, data for Eucalyptus regnans show that, relative to the intact wood, the sorption capacity of lignin was 0.60; for hemicelluloses it was 1.56, whereas for cellulose it was 0.94 (Kelsey & Clarke, 1956; Christensen & Kelsey, 1958; Browning, 1963). This means that the FSP would increase as the relative proportion of hemicelluloses increased, and would decrease as the relative proportion of lignin increased. Hence, the observed fact that wood often has a FSP similar to cellulose (Stamm, 1964) arises because the two other major wood components (lignin and hemicelluloses) have sorption capacities either side of the value for cellulose.

3. The physical structure of cell walls with regard to water sorption

The cell wall is a laminated structure, and the number of layers and their thickness vary with the cell type. Detailed descriptions of the formation, arrangement and composition of the layers are presented in Panshin & de Zeeuw (1980) and Higuchi (1997). The outermost layer, the middle lamella, occurs between adjacent cells and is largely comprised of pectins (Higuchi, 1997; Jarvis et al., 2003; Wi et al., 2005). Pectins have a high affinity for water, but the amount of water in this layer is small because it is very thin. Moving inward, there is the primary (P) layer, then up to three secondary layers, and finally, in vessels and tracheids, the warty layer, which is comprised of the residue of cell contents following death of the protoplasm (Panshin & de Zeeuw, 1980).

The P layer of cell walls in living trees is comprised of ≈70% water and ≈9% cellulose in an amorphous matrix of hemicelluloses, pectic materials and lignin. In living trees, the P wall layer is ≈0.1 µm thick, but shrinks to ≈0.03 µm when the wood is dry (Panshin & de Zeeuw, 1980).

The secondary wall layers contain high proportions of cellulose and are responsible for much of the strength of woody cells (Panshin & de Zeeuw, 1980). In tracheid and fibre cells, the secondary wall generally consists of three layers: S1, S2 and S3. The thin S1 layer is a transition layer between the thin P layer and the thick central S2 layer which constitutes the bulk (70–80% by volume) of the cell wall. The innermost S3 cell wall layer is thin like the S1 layer (Panshin & de Zeeuw, 1980). Much of the water in woody tissues is held in the S2 layer of fibre cells and tracheids. However, as the FSP is expressed on a dry mass basis, tissues that have a thin S2 layer (thus thin cell walls and a larger ratio of P : S2 layers) have a higher FSP and lower [D].

The cell wall is partly porous on a molecular scale and water can enter these spaces. However, this is not considered a capillary phenomenon. On the contrary, because of the large chemical forces involved (Stamm & Hansen, 1937), water can force its way into these spaces and effectively makes its own space (Stamm, 1964; Panshin & de Zeeuw, 1980).

4. Within- and between-tree variation in [D] and FSP

Single values of [D] and the FSP are commonly reported in the literature. With reference to Eqn 9, we see that this obviously only has meaning for bulk samples, i.e. those whose volume is large relative to the constituents. Consequently, the FSP (and [D]) will obviously vary in different parts of a single stem. There are also bulk differences between trees, especially between the softwoods (conifers) and hardwoods (angiosperms).

In softwoods, fluid conduction is via the longitudinal and ray tracheids, and food reserves are stored in longitudinal and ray parenchyma, which typically comprise between 3 and 12% of the softwood tissue volume (Panshin & de Zeeuw, 1980). These living cells have relatively thin cell walls. Earlywood tracheids have a larger radial diameter and a thinner cell wall S2 layer than latewood (Siau, 1984). Hence, as is well known, [D] and FSP both show interannual variation in a single growth ring of conifer wood.

In hardwoods, a single measure of [D] or FSP for a given volume represents a much more complex mixture and spatial arrangement than in the softwoods. The relatively thin-walled vessels of hardwood are surrounded by tracheids and thick-walled fibre cells and libriform fibres (very thin and short fibres), which, depending on the species, may occupy between 20 and 70% of the wood volume (Siau, 1984). The longitudinal and ray parenchyma of hardwood commonly comprise 1–18% and 5–33%, respectively, of the wood volume (Siau, 1984). The wood may be ring-porous (having vessels which vary greatly in diameter within the rings of earlywood and latewood) or diffuse porous (having vessels which have a relatively uniform diameter throughout the wood). The earlywood, produced during the period of terminal growth of the tree (Shigo & Hillis, 1973), has a much lower value of [D] (and a higher FSP) than the latewood. For example, in plantation-grown Eucalyptus nitens in southern Australia, [D] values varied within an annual ring from ≈0.4–1 g cm−3 (Wimmer et al., 2002).

In addition to variation within annual growth rings, the FSP may vary with position within a tree, such as along the radial and vertical axes owing to varying proportions of juvenile wood, sapwood and heartwood. Juvenile wood is produced by cambial cells located near the pith or centre of the elongating stem. It has a much lower value of [D] and a much higher moisture content than the mature wood into which it intergrades (usually over several years) with distance from the centre of the tree and increasing age of the cambium (Zobel & van Buijtenen, 1989).

Following the death of the living cells, juvenile and mature wood is referred to as heartwood. The heartwood, which forms an approximately conical-shaped core within the stem, is generally darker and usually (but see below for exceptions relating to ‘wetwood’) has a lower moisture content than the sapwood (Shigo & Hillis, 1973; Skaar, 1988). Heartwood is often coloured because of the presence of extractives that have been deposited by the surrounding ray and longitudinal parenchyma cells into the cell lumens, and often also in the cell walls. The presence of these extractives (e.g. polyphenols and tannins) results in increased [D] and reduced FSP. Because the higher [D] in heartwood is mostly a result of the presence of extractives, this is not expected to have any effect on the mechanics of the live tree (Gartner, 1995). Although the moisture content of heartwood is usually less than that of sapwood, in some trees a condition commonly known as ‘wetwood’ affects the heartwood, such that the heartwood has a similar or even higher moisture content than the sapwood. In some cases (Schink et al., 1981; Xu et al., 2001), but not all (e.g. Yamamoto et al., 2003), ‘wetwood’ is associated with microbial infection.

5. The movement of water in cell walls

Wood scientists and engineers are interested in how long it takes to dry wood because this is a common industrial process. Of special interest is the commercial imperative to avoid the distortion that occurs when timber is dried too quickly. As wood is dried from the FSP to lower moisture contents, the motion of water is (mostly) by diffusion. Stamm (1967b) provides a mathematical treatment of the possible types of diffusion in wood, including diffusion of water vapour within the void structure, and diffusion of water in the ‘solid solution’. Both are involved in drying. The water vapour pathway is relatively straightforward and will not be discussed further (see Stamm, 1967b for details). The diffusion of bound water in the ‘solid solution’ is more interesting.

If the water is chemically ‘bound’ to materials in the cell walls, then a ‘diffusion’ process would presumably involve a series of discrete jumps from one binding site to the next. That conception is consistent with the experimental evidence (Stamm, 1967b). The final equilibrium would equate to a thermodynamic state of maximum entropy. Interestingly, experiments show that the rate of diffusion of the ‘bound’ water in the longitudinal (fibre) direction is about twice that in the radial direction, and three times that in the tangential direction (Stamm, 1967b). The faster rate along the longitudinal direction presumably means that there are a greater number of available binding sites in that direction. Alternatively, it might also result because the binding energy per binding site in that direction could be less than the other directions. Interestingly, the diffusion of water through cell walls would also provide a means for water to move into, or out of, heartwood in which the lumens have been occluded by tyloses.

V. Living trees and the FSP

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

The focus of the wood science literature is on the properties of timber and wood products and the experiments are mostly carried out on timber samples. These are fragments of dead trees. However, it is the processes in living trees that are of interest to biologists. In this section, we consider the FSP and hydration of cell walls as they relate to the water relations of living trees. First ,we examine the ways in which stems and other woody tissues in living trees differ from timber. Following that, we present an engineering perspective on the shrinkage and swelling of timber and living trees. Finally, we summarise observations on the shrinkage and swelling of living stems and attempt to discuss those dynamics in terms of the FSP.

1. Differences in the behaviour of timber and wood in living trees

A piece of timber is a dead fragment of a tree stem that has been dried and prepared for use as a construction material. Disconnected from the rest of the plant, including the surrounding stem tissues, bark, leaves and roots, it responds passively to the surrounding environment. In contrast, the wood in living trees is part of the body of a functioning organism. It serves to support the canopy, acts as a store (e.g. for starch) and as a conduit for material transport to and from roots and leaves.

Perhaps the most marked difference between a piece of timber and an intact stem in a living tree is that the latter has metabolically active cells that occupy some of the voids that exist in timber. Hence, the ‘solid solution’ that we have previously described may be adjacent to living cells in a tree stem instead of ‘bulk water’ or a gas phase in a drying piece of timber. This makes the description of living stems more challenging, and more interesting, because it is possible that metabolically active cells may play a role in the movement of water to and from cell walls – for example, by starch–sugar interconversion (Canny, 1997; Bucci et al., 2003; Salleo et al., 2004).

At a practical level, the presence of active cells also means that the calculations (Eqns 1–8; Fig. 1) used previously to estimate the bulk phases need to be modified slightly to recognise that a small amount of the dry matter is located in liquid phase (i.e. in the cells). Hence, in addition to ‘allocating’ water to the ‘solid solution’ (Fig. 1), some of the dry matter needs to be ‘allocated’ to the liquid phase. One approach to this partitioning that might have some practical benefits is based on observations showing that in leaves the mass of nitrogen per unit liquid mass is relatively conserved (Leigh & Johnston, 1985; Roderick et al., 1999b). [An even more conservative relationship exists between the mass of potassium in leaf tissue and the water (Leigh & Johnston, 1983; Leigh & Jones, 1984; Walker et al., 1996). However, it is not common practice to report the potassium concentration so this might not be initially as useful as the nitrogen-based relationships.] This means that one could at least approximately estimate the mass of dry matter in the liquid phase as a certain (small) fraction of the water in the solution, and this could be easily implemented into the framework (Fig. 1; Eqns 1–8). Note that although the main focus of the review is plant stems, the concept of the FSP and the partitioning of water into phases can also be applied to leaves and roots.

In addition to the above-noted functions, tree stems also store water. Water that enters from the roots resides in the stem tissues for some period of time before it exits to the leaves. In smaller plants, like herbaceous crop species, the total daily transpiration stream can be, and often is, many times larger than the total mass of water in the plant (Canny, 1990). However, in large trees, the storage can be equivalent to the total transpiration over several days or perhaps longer (Waring & Running, 1976; Waring et al., 1979). More generally, the regular occurrence in forest trees of diurnal and seasonal swelling and shrinkage of stems resulting from changes in stem water content has been the focus of much research in the forestry and wood science literature. It will be discussed in Section V.3, but before that we examine timber shrinkage and swelling from an engineering perspective.

2. Swelling and shrinkage in timber: an engineering perspective

As previously outlined (see Section II.1), the swelling of dry timber when exposed to water is mostly attributed to an increase in the mass of water in, and hence in the volume of, the ‘solid solution’. Shrinkage is the reverse. This latter change is perhaps easier to visualise and is outlined in a highly stylised schematic way in Fig. 7.

image

Figure 7. Schematic diagram showing possible ways that the volume of a piece of timber can reduce. The shaded area is the ‘solid solution’ of volume Vs and the remaining space has volume Vau (= Va+Vu; see Eqn 1) representing the combined liquid and gas phase. Note that the diagram does not refer to a single cell, but instead refers to the total volume of all the respective phases in a timber sample. (a) Vs has its maximum value before shrinking, and the sample moisture content is at the fibre saturation point (FSP) at a given temperature. (b) Vau is reduced but Vs remains constant. (c) Vs is reduced but Vau remains constant. (d) Vs and Vau are both reduced. Modified from Skaar (1972).

Download figure to PowerPoint

Imagine starting with a timber sample whose moisture content is equal to the FSP (Fig. 7a). Note that because the moisture content is at the FSP, the combined liquid and gas phase will only contain gas. Possible ways for timber shrinkage to occur from this condition are for a reduction in the volume of the gas phase (Fig. 7b), or a reduction in the volume of the ‘solid solution’ (Fig. 7c), or a combination of both (Fig. 7d). The option depicted in Fig. 7(b) is very unlikely to occur. For example, in this scenario the fluid phase is all gas, and the pressure in that phase will be somewhere between a vacuum and atmospheric pressure. Differences in pressure of this magnitude would have little impact on the volume of the ‘solid’ phase. Hence, in this scenario, the majority of the shrinkage can really only occur by a reduction in the volume of the ‘solid’ phase (Fig. 7c). That in turn can happen in one main way – the removal of water from the ‘solid’ phase into adjacent liquid and or gas phases, but there will also be a (small) density effect as well (Fig. 5). (Recall that the volume can change because of a change in mass or a change in density.) That is why engineers and wood scientists have long recognised that the volume of the fluid phases (Va, Vu) remains more or less constant while shrinkage occurs (Siau, 1984; Skaar, 1988).

As noted above, shrinkage in timber mostly occurs because the mass of water in the ‘solid solution’ declines. In the above scenario, we assumed that we began with a piece of timber with moisture content at the FSP. How does the shrinkage relate to the FSP? We need to distinguish two cases. The first is that the FSP remains constant while the mass of water in the ‘solid solution’ declines. A more subtle possibility is for the FSP itself to decline, which would also lead to a decline in mass, and a decline in the volume of the ‘solid’ phase, and hence would lead to shrinkage of the timber sample. For example, in Fig. 4 the FSP at 20°C is ≈0.31 and at 30°C it is ≈0.30. Thus as the temperature increases from 20 to 30°C, the associated decrease in mass of ‘bound’ water is ≈3%. Although this is small compared with the loss of water that would occur with a modest decline in relative humidity (see Fig. 3), it is nevertheless a possibility.

3. Stem swelling and shrinking in living trees

Changes in stem diameter in living trees are well known (Haasis, 1934). It declines during the day, and increases at night. Note that for the stem diameter to increase over time, the night-time expansion must consistently exceed the day-time shrinkage. When the water availability remains roughly constant, such as during periods of abundance or great insufficiency of water, there is little diurnal variation in stem diameter.

The phenomenon of diurnal shrinkage and swelling is not confined to stems, but also occurs in branches, roots and reproductive structures (Kozlowski & Winget, 1964). Kozlowski & Winget (1964) found that the magnitude of the shrinkage may vary in different parts of the same stem. The amount of shrinkage is not trivial, and over seasonal periods in dry years the stem diameter can often decline (Buell et al., 1961; Fritts, 1976; Downes et al., 1999), even though cambial growth may be occurring (Kozlowski & Winget, 1964). More recently, Zweifel & Häsler (2001) measured the changes in stem diameter of Norway spruce trees at several different heights and described a slow ‘peristaltic’ wave of contraction from the canopy to the roots.

The shrinkage and swelling may occur in the bark and/or the sapwood. In some cases, it has been argued that the shrinking and swelling is confined to the bark. For example, in young (4–6 yr old, ≈1 m tall) Norway spruce (Picea abies) trees, the water loss responsible for the diurnal shrinking of stems appears to come from the living cells in the bark (i.e. tissues outside of the vascular cambium) (Zweifel & Häsler, 2000; Zweifel et al., 2000, 2001). Similar changes in the hydration of bark tissues were also reported by Haasis (1934). Further research on Norway spruce found that the swelling and shrinking of bark tissues of Norway spruce was not related to xylem sap flow, or to phloem capacitance, but corresponded to changes in relative humidity of the ambient air (Gall et al., 2002).

What is the role of the FSP in the above processes? The simple response is that we do not as yet know the answer because the FSP and the related research has not as yet been used to any significant degree in the biological literature. One aim of the review is to encourage that research.

VI. Conclusions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

An understanding of the FSP of timber has been central to both research and to practical applications in wood science and engineering for over a century. Research within these disciplines has resulted in a detailed understanding of the equilibrium behaviour of water in timber and that understanding is the basis for confidence in the FSP concept. This has been possible because water interacts with timber in a relatively predictable and repeatable way.

This research, firmly grounded in experiments, has potential practical applications for biologists. First, provided that the fresh volume of a sample of plant tissue (wood, leaves, etc.) is known, along with the dry mass and the moisture content, it is possible to partition the water into bound water (associated with cell walls and other solids) and free water (of cell lumens and vacuoles), and consequently to partition the plant tissue into the three bulk phase (gas, liquid, solid). This has applications for comparative studies.

Extending the FSP concept to help in the study of the water relations in living trees is in its infancy. That was the reason to write this review. The inclusion of the dynamic exchanges of water between cell wall–water mixtures and the adjacent liquid and/or gas phases, long observed in timber, may help advance our understanding of plant–water relations when put into an appropriate biological framework. To understand the overall significance, assume that the moisture content of a timber sample is 15%, which is a typical value (Skaar, 1988). Now turn back to Fig. 5 and note that the associated effective compression is about 0.5% and recall that the isothermal compression of liquid water of 1% requires a pressure of about 210 bar (21 MPa). Hence, an effective compression of about 0.5% implies an equivalent pressure of about 100 bar (10 MPa). The associated relative humidity would be in the range 70–80%. Hence, even at quite high values of relative humidity, the chemical forces involved in the cell wall–water mixture are still very strong.

Acknowledgements

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References

We thank Martin Canny, Paul Kriedemann, Stephen Roxburgh, Tony Winters, Chin Wong, Richard Norby and two anonymous reviewers for helpful comments on earlier drafts.

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  4. II. The FSP: an overview
  5. III. Equilibrium thermodynamics and the FSP
  6. IV. Inside the FSP
  7. V. Living trees and the FSP
  8. VI. Conclusions
  9. Acknowledgements
  10. References
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