Seed mass, also called seed size, is the oven-dry mass of an average seed of a species, expressed in mg. Stored resources in large seeds tend to help the young seedling to survive and establish in the face of environmental hazards (e.g. deep shade, drought, herbivory). Smaller seeds can be produced in larger numbers with the same reproductive effort. Smaller seeds also tend to be buried deeper in the soil, particularly if their shape is close to spherical, which aids their longevity in seed banks. Interspecific variation in seed mass also has an important taxonomic component, more closely related taxa being more likely to be similar in seed mass.
What and how to collect?
The same type of individuals as for leaf traits and plant height should be sampled, i.e. healthy adult plants that have their foliage exposed to full sunlight (or otherwise plants with the strongest light exposure for that species). The seeds should be mature and alive. If the shape of the dispersal unit (e.g. seed, fruit) is measured too (see 6.2. above), do not remove any parts until dispersule measurement is finished . We recommend collecting at least 10 seeds from each of 10 plants of a species, although more plants per species is preferred. Depending on the accuracy of the balance available, 100 or even 1000 seeds per plant may be needed for species with tiny seeds (e.g. orchids).
In some parts of the world, e.g. in some tropical rain forest areas, it may be efficient to work in collaboration with local people specialised in tree climbing to help with collecting (and identification).
Storing and processing
If dispersule shape is also measured, then store cool in sealed plastic bags, whether or not wrapped in moist paper (see Section 3.1), and process and measure as soon as possible. Otherwise air-dry storage is also appropriate.
After measurements of dispersule shape (if applicable), remove any accessories (wings, comas, pappus, elaiosomes, fruit flesh), but make sure not to remove the testa in the process. In other words, first try to define clearly which parts belong to the fruit as a whole and which belong strictly to the seed. Only leave the fruit intact in cases where the testa and the surrounding fruit structure are virtually inseparable. Dry the seeds (or achenes, single-seeded fruits) at 80°C for at least 48 h (or until equilibrium mass in very large or hard-skinned seeds) and weigh. Be aware that, once taken from the oven, the samples will take up moisture from the air. If they cannot be weighed immediately after cooling down, put them in the desiccator until weighing, or else back in the oven to dry off again. Note that the average number of seeds from one plant (whether based on five or 1000 seeds) counts as one statistical observation for calculations of mean, standard deviation and standard error.
Special cases or extras
(1) Within individual variation. Be aware that seed size may vary more within an individual than among individuals of the same species. Make sure to collect ‘average-sized’ seeds from each individual, and not the exceptionally small or large ones.
(2) Available databases. Be aware that a considerable amount of published data are already available in the literature, and some of the large, unpublished databases may be accessible under certain conditions. Many of these data can probably be added to the database; however, make sure the methodology used is compatible.
(3) Seed volume. There are also many large datasets for seed volume, often measured as π/6 × L1 c L2 × L3 (i.e. assuming an ellipsoidal shape). Most of these databases actually include both seed mass and volume. Using the appropriate calibration equations, those data can be also successfully used.
(4) Additional measurements. For certain (e.g. allometric) questions, additional measurements of the mass of the dispersule unit or the entire infructescence (reproductive structure) may be of additional interest. Both dry and fresh mass may be useful in such cases.
References on theory, significance and large datasets: Mazer (1989); Seiwa and Kikuzawa (1996); Reich et al. (1998); Cornelissen (1999); Leishman et al. (2000); Westoby et al. (2002); Moles et al. (2005); Moles and Westoby (2006); Wright et al. (2007).
More on methods: Hendry and Grime (1993); Thompson et al. (1997); Westoby (1998); Weiher et al. (1999); Wright et al. (2007).