The amount of leaf area a species produces per unit cross-section of sapwood (the inverse of Huber value, expressed in mm2 mm–2) is crucial for both water transport (with related effects on photosynthetic rate) and mechanical strength.
What and how to collect
The ratio leaf area : sapwood area (LA : SA) depends strongly on leaf phenology. Furthermore, there is variation between wet and dry seasons, variation among populations of a given species along moisture gradients, ontogenetic trajectories for given individuals, and within trees along a branch from trunk to tip. Declining function of sapwood with age is one reason why LA : SA generally increases when one moves from larger (older) towards smaller branches. Unfortunately, the age-related decline in sapwood function is not always well understood, can be difficult to measure, and may vary among species. All this should be considered when designing a sampling methodology and interpreting this trait (see Point 3 of Special cases or extras in the present Section).
To make meaningful comparisons among species, we recommend sampling terminal, sun-exposed shoots from the outer canopy. This means sampling terminal shoots either of a certain standard length, or of a certain age (1–3 years) (for shoots in which terminal bud scars allow their age to be determined). This approach maximises the likelihood that all the sapwood in the branch is still functional. We recommend sampling at the peak of the growing season when leaf area is highest. At this time, LA : SA should be at a maximum for the year; this is similar to the efforts to measure maximum photosynthetic rate as a way of making meaningful comparisons across species. Care should be taken to select shoots that have not lost leaves or parts of leaves to mechanical damage, herbivory or early senescence and abscission.
Leaf area : sapwood area ratio can be measured at different scales, namely, from whole plant to just terminal branches (and this should be taken into consideration when scaling up measurements). Total leaf area of leaves distal to the collection point is measured by the same method as the area of individual leaves (see Section 3.2). Sapwood area at the collection point is most precisely measured with digital micrographs and image-analysis software (see Section 3.1 for free software); however, a calliper should work for most species in most situations. In measuring sapwood area, care should be taken to exclude bark, phloem, heartwood and pith from the area measured.
Special cases or extras
(1) For herbaceous species, similar methods can be applied; however, care must be taken to identify the parts of the stem that can conduct water; this distinction may not be as clearly distinguished as it is within most woody species. It can be quantified with a dye-transport experiment (see Point 3 below in the present Section).
(2) Seasonal changes. Because cambial growth in many trees continues well after spring flush of bud growth is completed and the final leaf area for the season is attained, the LA : SA ratio is best measured as late in the growing season as possible, when all the season’s newly produced leaves remain attached, but (for evergreens) before the seasonal abscission of older leaves has occurred.
(3) In ring-porous trees, the effective conductivity of xylem drops precipitously in older sapwood, sometimes within a very few annual rings. For these species, the conductivity of the sapwood (and its decline with sapwood age) can be quantified by placing the cut end of the shoot into a fairly strong solution of a dye, such as eosin, and allowing the foliage to transpire in air, and after 10–20 min, cutting a cross-section of the stem a few centimetres above its cut end and measuring the dye-stained area.
References on theory, significance and large datasets: Chiba (1991); Eamus and Prior (2001); Maherali and DeLucia (2001); Mäkelä and Vanninen (2001); McDowell et al. (2002); Preston and Ackerly (2003); Addington et al. (2006); Buckley and Roberts (2006); Maseda and Fernández (2006); Wright et al. (2006); Cornwell et al. (2007); Litton et al. (2007).
References on meta–analysis: Mencuccini (2003).