Of interest is the entire reproductive dispersule (= dispersal structure or propagule) as it enters the soil. The dispersule may correspond with the seed; however, in many species, it constitutes the seed plus surrounding structures, e.g. the fruit. Dispersule size is its oven-dry mass. Dispersule shape is the variance of its three dimensions, i.e. the length, the width and the thickness (breadth) of the dispersule, after each of these values has been divided by the largest of the three values. Variances lie between 0 and 1 and are unitless. Small dispersules with low shape values (relatively spherical) tend to be buried deeper into the soil and live longer in the seed bank. Seed size and shape are then fundamental for seed persistence in the soil (seed-bank persistence).
What and how to collect?
The same type of individuals as for leaf traits and plant height should be sampled. Of interest is the unit that is likely to enter the soil. Therefore, only parts that fall off easily (e.g. pappus) are removed, whereas parts such as e.g. wings and awns remain attached. The flesh of fleshy fruits is removed too, because the seeds are usually the units to get buried in this case (certainly if they have been through an animal gut system first). The seeds (or dispersules) should be mature and alive. The dispersules can either be picked off the plant or be collected from the soil surface. In some parts of the world, e.g. in some tropical rain forest areas, it may be efficient to pay local people specialised in tree climbing (and identification) to help with the collecting.
Storing and processing
Store the dispersules in sealed plastic bags and keep in a cool box or fridge until measurement. Process and measure as soon as possible. For naturally dry dispersules, air-dry storage is also okay.
Remove any fruit flesh, pappus or other loose parts (see above within the present Protocol). For the remaining dispersule, take the highest standardised value for each dimension (length, width and thickness) using callipers or a binocular microscope and calculate the variance. Then dry at 60°C for at least 72 h (or else at 80°C for 48 h) and weigh (= dispersule size).
Special cases or extras
We recommend complementing this trait with other direct or indirect assessment of banks of seeds or seedlings for future regeneration of a species. For seed-bank assessment, there are good methods to follow (see More on methods below in the present Section); however, (above-ground!) canopy seeds banks of serotinous species of fire-prone ecosystems (e.g. Pinus and Proteaceae such as Banksia, Hakea and Protea) and long-lived seedling banks of woody species in the shaded understorey of woodlands and forests may also make important contributions. Vivipary as in some mangroves could also be part included in such assessments.
References on theory, significance and large datasets: Hendry and Grime (1993); Thompson et al. (1993, 1997); Leishman and Westoby (1998); Funes et al. (1999); Weiher et al. (1999); Peco et al. (2003).
More on methods: Hendry and Grime (1993); Thompson et al. (1993, 1997); Weiher et al. (1999); Pons and Pausas (2007).