The following summary concerns the ratio of leaf area to sapwood area in woody plants, taken as a measure of the balance between transpiration and stem water supply.
The leaf to sapwood area ratio is normally defined as the total projected area of leaves divided by the cross-sectional area of sapwood supplying water to those leaves. The inverse of this ratio, i.e. the sapwood to leaf ratio, is also used in the literature, and often referred to as the -Huber value’ (Huber 1928).
Terminology, units and equations
The leaf to sapwood area ratio (AL:AS) describes the relationship between projected leaf area and the area of sapwood supplying water to the leaves. The AL:AS ratio is considered a key component of the biomass allocation and the plant hydraulic architecture. Being a quotient between two areas, the AL:AS ratio can be considered dimensionless, but for convenience, it is usually expressed in units of m2 – mm-2 or m2 – cm-2. The AL:AS ratio can be measured for whole plants (Mencuccini & Bonosi 2001) or for individual branches (e.g. Martínez-Vilalta et al. 2009). Alternatively, the balance between transpiration and sapwood water supply has been quantified by the reverse ratio (AS:AL), also referred to as the -Huber value’ (Tyree & Zimmermann 2002), and originally as a sapwood cross-sectional area to leaf mass ratio (Tyree & Zimmermann 2002). Another related hydraulic trait ratio is the leaf specific conductivity or LSC. This LSC takes the hydraulic conductivity of sapwood KS into account, and is calculated as KS:(AL:AS) (Tyree & Zimmermann 2002). This leaf specific conductivity is considered a better proxy for the balance between transpiration and water supply by the sapwood (e.g. Sterck et al. 2008). This ratio is described in the Water Relations section under Hydraulic conductance and conductivity.
Significance and sources of variation
The rationale behind using the AL:AS ratio to characterize plant form stems from the early observations of Leonardo da Vinci on branching patterns and related water flow patterns in trees (Tyree and Zimmermann 2002). These early ideas were formalized mathematically in the pipe model theory (Shinozaki et al. 1964), according to which the amount of leaves sustained by each tree is linearly related to the stem sapwood cross-sectional area. We now know that the relationship between AL and AS across trees of a given species is not necessarily linear and frequently scales allometrically, not isometrically. But the fact remains that these two variables are highly correlated in most cases and that the slope of their linear relationship (which is exactly the AL:AS ratio, provided that the regression is forced through the origin) is a key component of plant hydraulic architecture. A simple way of gauging the importance of the AL:AS ratio and its relationship with other components of the plant hydraulic system is provided by the more complete steady state model of Whitehead et al. (1984), according to which AL:AS = KS – D / (gs – D – L), where KS is sapwood specific hydraulic conductivity, D is the water potential gradient driving water flow, gS is the conductance for water vapour between leaves and bulk air, D is the vapour pressure deficit of the atmosphere, and L is path length (e.g. tree height). This framework was expanded by Magnani et al. (2000) by including roots in the equation. The AL:AS ratio is known to show strong acclimation across environmental gradients (Mencuccini & Bonosi 2001) and, at least for some species, it has been shown to be the most plastic hydraulic property when comparing locations with contrasted water availability (DeLucia et al. 2000; Martínez-Vilalta et al. 2009). Additionally, the AL:AS ratio has also been shown to vary during ontogeny, decreasing with tree size in the majority of species and potentially providing a homeostatic mechanism to compensate for increased hydraulic resistance as trees grow in height (Mencuccini & Grace 1996; McDowell et al., 2002).
Quantifying the AL:AS ratio normally implies the independent measurement of leaf area (AL) and cross-sectional sapwood area (AS). In each case, the choice of method will depend upon the scale at which the AL:AS ratio needs to be estimated (branch or whole plant) and on the specific questions being asked in the study. The AL:AS ratio can be established from a single AL and AS measurement, and this may be enough if one wants to characterize this value for a given branch or tree. However, if representative population level estimates are required, it is advisable to estimate the AL:AS ratio based on multiple measurements taken from samples covering a wide size range, as the slope of the relationship between AL and AS (forced through the origin). Another important consideration is that leaf area (and sapwood area, to a lower extent) can show large temporal fluctuations. It is thus advisable to use maximum seasonal AL values and to measure AL and AS simultaneously.
Branch level estimates
Measuring the leaf area of an excised branch normally involves removing all leaves distal to the stem segment of interest. Once the leaves have been removed, their area can be estimated either directly (e.g., with a ((PROTOCOL: Leaf Area Meter, LI-3100C|LI-COR LI-3100C Leaf Area Meter)) or similar equipment) or indirectly by weighting the leaves and multiplying the resulting biomass by the corresponding specific leaf area (SLA, defined as the ratio between leaf area and leaf dry mass). On the other hand, measuring cross-sectional sapwood area in (reasonably small) branches is normally straightforward, as they normally have no heartwood and, therefore, sapwood area can be approximated by the total cross-sectional area. In very small branches, it is advisable to subtract the pith area from total stem cross-sectional area.
Whole-plant level estimates
Estimating the AL:AS ratio at the whole-plant level is trickier than for branches. Firstly, getting accurate estimates of tree-level leaf area from leaf area index (LAI) measurements at the stand level is very difficult due to the inherent inaccuracies of available techniques and the difficulty in separating the contribution of different individual trees to overall leaf area. Secondly, directly measuring the total leaf area of a whole tree is destructive, impractical and can be hugely time consuming. For this reason, some sort of sub-sampling is usually used. This sub-sampling normally consists in the following steps:
- Sample a number of branches (typically around 20 or more) from the target trees and characterize their size (e.g. base diameter, cross-sectional area) and position in the canopy (e.g. height above the ground, whorl number, distance to the top or bottom of the living crown). Try to sample branches over the whole range of positions within a tree canopy.
- Measure the leaf area of the sampled branches as explained in the previous section.
- Use the previous data to fit a statistical model (for example, GLM or multiple regression) predicting branch leaf area from branch size and/or position.
- Record the value of the significant explanatory variables in the previous model for all the branches in target trees. If necessary, canopy branches may be accessed by climbing the trees or measured from the ground using laser pointers (e.g. Haglöf Gator Eyes).
- Apply the model to estimate the leaf area of all branches.
- Add up the estimated leaf area of all branches in a given tree to get its total leaf area.
Tree-level cross-sectional sapwood area is normally measured at breast height (ca. 1.3 m above the ground), although some studies show that the relationship with tree leaf area is better if sapwood area is measured at the base of the living crown (Mencuccini & Bonosi 2001). In either case, its quantification requires distinguishing the sapwood from the heartwood, which depending on the species can be accomplished visually on the basis of color or translucency, or using specific laboratory protocols involving water content measurements or staining (e.g. Shain 1967). When full wood discs are not available, the cross-sectional sap-wood area is normally estimated from measurements on wood cores extracted at different sides of the stem.
Ranges of values
AL:AS: m2 – cm-2
Angiosperm, branch: 0.01 – 10
Gymnosperm, branch: 0.01 – 2
Angiosperm, whole-plant: 0.04 – 1.4
Gymnosperm, whole-plant: 0.07 – 0.8
Related topics and techniques
Biomass allocation e.g. Dry Weight biomass
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