3.2. Area of a leaf

The area of a leaf (also called leaf area, LA) is the most common metric for leaf size and is defined as the one-sided or projected area of an individual leaf, expressed in mm2 (see Section 3.1). Interspecific variation in LA has been variously related to climatic variation, geology, altitude and latitude, where heat stress, cold stress, drought stress, nutrient stress and high-radiation stress all tend to select for relatively small leaves. Within climatic zones, variation in the LA may also be linked to allometric factors (plant size, twig size, anatomy and architecture, leaf number, number of lateral buds produced) and ecological strategy with respect to environmental nutrient stress and disturbances, and phylogenetic factors can also play an important role.

What and how to collect?

For the leaf-collecting protocol, see under Section 3.1. LA is rather variable within plants and we recommend collecting a large number of replicates (i.e. close to the higher end of the number of replicates recommended in Appendix 1). For storing leaves, see Section 3.1.

Measuring

Measure the individual leaf lamina for species with simple leaves. For compound-leaved species, either the leaflet area or the whole LA can be measured, and the appropriate decision depends on the research question at hand. For the heat balance, the leaflet area is important, which is functionally analogous to a simple leaf. When analysing total light capture, the whole leaf should be measured. Ideally, determine for compound-leaved species both the leaflet area and whole LA, because this allows one to address more questions and to compare the results with other studies. Measure the laminae with or without petiole and rachis, according to the objectives of your study (see Section 3.1), and always report this in your publication. Note that this whole LA may be different from the area used to determine SLA.

Special cases or extras

(1) Leafless plants. Because leaflessness is an important functional trait, record LA as zero for leafless species (not as a missing value). However, be aware that these zeros may need to be excluded from certain data analyses. Alternatively, sample leaf analogues (see Succulent and leafless plants in Section 3.1).
 
(2) Heterophyllous plants. See Section 3.1.

(3) Ferns. See Section 3.1.

(4) Leaf width. This is measured as the maximum diameter of an imaginary circle that can be fitted anywhere within a leaf, and is an additional trait of ecological interest related to leaf size. Narrow leaves, or divided leaves with narrow lobes, tend to have a smaller boundary layer and a more effective heat loss than do broad leaves with the same area. This is considered adaptive in warm, sun-exposed environments. There is also emerging evidence that leaf width contributes more positively than does the area of the whole leaf to the expression of canopy dominance.

(5) Leaf number per node. Leaf size is a compromise between functional and resource-use efficiency. Plants are modular in construction and, as a result, these functions can be partially uncoupled. Species with alternate, opposite and whorled leaves frequently co-exist and leaf dry mass or area multiplied by the number of leaves per node provides additionally a crude estimate of the size of each growth module. This may in extreme cases be 10 times the value of a single leaf.
 
References on theory, significance and large datasets: Raunkiaer (1934); Parkhurst and Loucks (1972); Givnish (1987); Cornelissen (1999); Ackerly et al. (2002); Westoby et al. (2002); Milla and Reich (2007); Niinemets et al. (2007); Niklas et al. (2007); Poorter and Rozendaal (2008); Royer et al. (2008).

More on methods: see references in More on methods of Section 3.1.