Protocol

 

Author

This article is modified from Perez-Harguindeguy et al (2013). The “New handbook for standardised measurement of plant functional traits worldwide” is a product of and is hosted by Nucleo Diversus (with additional Spanish translation). For more on this and on its context as part of the entire trait handbook visit its primary site Nucleo DiverSus at http://www.nucleodiversus.org/?lang=en

Overview

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.

Procedure

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, 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 SLA), and process and measure as soon as possible. Otherwise air-dry storage is also appropriate.

Measuring

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.

Notes and troubleshooting tips

(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 x L1 x L2 x 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.

Literature references

References on theory, significance and large datasets:

Cornelissen JHC (1999) A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy. Oecologia 118, 248-255. doi:10.1007/s004420050725

Leishman M,Wright I, Moles A, Westoby M (2000) The evolutionary ecology of seed size. In The ecology of regeneration in plant communities. 2nd edn. Ed. M Fenner, pp. 31-57. CAB International: London

Mazer J (1989) Ecological, taxonomic, and life history correlates of seed mass among Indiana dune angiosperms. Ecological Monographs 59, 153-175. doi:10.2307/2937284

Moles A, Westoby M (2006) Seed size and plant strategy across the whole life cycle. Oikos 113, 91-105. doi:10.1111/j.0030-1299.2006.14194.x

Moles A, Ackerly D, Webb C, Tweddle J, Dickie J, Pitman A, Westoby M (2005) Factors that shape seed mass evolution. Proceedings of the National Academy of Sciences, USA 102, 10540-10544. doi:10.1073/pnas.0501473102

Reich PB, Tjoelker MG, Walters MB, Vanderklein DW, Bushena C (1998) Close association of RGR, leaf and root morphology, seed mass and shade tolerance in seedlings of nine boreal tree species grown in high and low light. Functional Ecology 12, 327-338. doi:10.1046/j.1365-2435.1998.00208.x

Seiwa K, Kikuzawa K (1996) Importance of seed size for the establishment of seedlings of five deciduous broad-leaved tree species. Vegetatio 123, 51-64. doi:10.1007/BF00044887

Westoby M, Falster D, Moles A, Vesk P, Wright I (2002) Plant ecological strategies: some leading dimensions of variation between species. Annual Review of Ecology and Systematics 33, 125-159. doi:10.1146/annurev.ecolsys.33.010802.150452

Wright IJ, Ackerly D, Bongers F, Harms KE, Ibarra-Manríquez G, Martínez-Ramos M, Mazer SJ, Muller-Landau HC, Paz H, Pitman NCA, Poorter L, Silman MR, Vriesendorp CF, Webb CO, Westoby M, Wright SJ (2007) Relationships among ecologically important dimensions of plant trait variation in seven neotropical forests. Annals of Botany 99, 1003-1015. doi:10.1093/aob/mcl066

More on methods:

Hendry GAF, Grime JP (1993) Methods in comparative plant ecology. A laboratory manual. Chapman and Hall: London

Thompson K, Bakker JP, Bekker RM (1997) The soil seed bank of North West Europe: methodology, density and longevity. Cambridge University Press: Cambridge, UK.

Weiher E, Clarke GDP, Keddy PA (1998) Community assembly rules, morphological dispersion, and the coexistence of plant species. Oikos 81, 309-322. doi:10.2307/3547051

Westoby M, Cunningham SA, Fonseca C, Overton J, Wright IJ (1998) Phylogeny and variation in light capture area deployed per unit investment in leaves: designs for selecting study species with a view to generalizing. In Variation in growth rate and productivity of higher plants. Eds H Lambers, H Poorter H, MMI Van Vuuren, pp. 539-566. Backhuys Publishers: Leiden, The Netherlands.

Wright IJ, Ackerly D, Bongers F, Harms KE, Ibarra-Manríquez G, Martínez-Ramos M, Mazer SJ, Muller-Landau HC, Paz H, Pitman NCA, Poorter L, Silman MR, Vriesendorp CF, Webb CO, Westoby M, Wright SJ (2007) Relationships among ecologically important dimensions of plant trait variation in seven neotropical forests. Annals of Botany 99, 1003-1015. doi:10.1093/aob/mcl066