Seed and fruit dispersal traits

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Contributors : Adrienne Nicotra 

 

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This article is modified from Pérez 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 trait 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

Dispersal syndrome

The mode of dispersal of the ‘dispersule’ (or propagule = unit of seed, fruit or spore as it is dispersed) has obvious consequences for the distances it can cover, the routes it can travel and its final destination.

How to classify?

This is a categorical trait. Record all categories that are assumed to give significant potential dispersal (see Box 4), in order of decreasing importance. In the case of similar potential contributions, prioritise the one with the presumed longer-distance dispersal; e.g. wind dispersal takes priority over ant dispersal.

It is important to realise that dispersules may (occasionally) get transported by one of the above modes even though they have no obvious adaptation for it. This is particularly true for endo-zoochory and exo-zoochory. Note that there is ample literature (e.g. in Floras) for dispersal mode of many plant taxa.

Table 1. Dispersal syndromes

(1) Unassisted dispersal; the seed or fruit has no obvious aids for longer-distance transport and merely falls passively from the plant

(2) Wind dispersal (anemochory) includes: minute dust-like seeds (e.g. Pyrola, Orchidaceae), seeds with pappus or other long hairs (e.g. willows (Salix), poplars (Populus), many Asteraceae), ‘balloons’ or comas (trichomes at the end of a seed), flattened fruits or seeds with large ‘wings’, as seen in many shrubs and trees (e.g. Acer, birch (Betula), ash (Fraxinus), lime (Tilia), elm (Ulmus), pine (Pinus)); spores of ferns and related vascular cryptogams (Pteridophyta) and ‘tumbleweeds’, where the whole plant or infructescence with ripe seeds is rolled over the ground by wind force, thereby distributing the seeds. The latter strategy is known from arid regions, e.g. Baptisia lanceolata in the south-eastern USA and Anastatica hierochuntica (rose-of-Jericho) in northern Africa and the Middle East

(3) Internal animal transport (endo-zoochory), e.g. by birds, mammals, bats; many fleshy, often brightly coloured berries, arillate seeds, drupes and big fruits (often brightly coloured), that are evidently eaten by vertebrates and pass through the gut before the seeds enter the soil elsewhere (e.g. holly (Ilex), apple (Malus))

(4) External animal transport (exo-zoochory); fruits or seeds that become attached e.g. to animal hairs, feathers, legs, bills, aided by appendages such as hooks, barbs, awns, burs or sticky substances (e.g. burdock (Arctium), many grasses)

(5) Dispersal by hoarding; brown or green seeds or nuts that are hoarded and buried by mammals or birds. Tough, thick-walled, indehiscent nuts tend to be hoarded by mammals (e.g. hazelnuts (Corylus) by squirrels) and rounded, wingless seeds or nuts by birds (e.g. acorns (Quercus spp.) by jays)

(6) Ant dispersal (myrmecochory); dispersules with elaiosomes (specialised nutritious appendages) that make them attractive for capture, transport and use by ants or related insects.

(7) Dispersal by water (hydrochory); dispersules are adapted to prolonged floating on the water surface, aided for instance by corky tissues and low specific gravity (e.g. coconut)

(8) Dispersal by launching (ballistichory); restrained seeds that are launched away from the plant by ‘explosion’ as soon as the seed capsule opens (e.g. Impatiens)

(9) Bristle contraction; hygroscopic bristles on the dispersule that promote movement with varying humidity

Dispersule size and shape

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.

Measuring

Remove any fruit flesh, pappus or other loose parts (see above). 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 (=dispersule shape). Then dry at 60°C for at least 72 h (or else at 80°C for 48 h) and weigh (= dispersule size).

Dispersal potential

Dispersal potential is defined as the proportion of dispersules produced by one individual that travels a certain distance, which can be chosen arbitrarily depending on the question. The dispersules may be seeds or fruits or vegetative propagules. In contrast to dispersal syndrome, dispersal potential allows the assessment of dispersability of a seed in relation to distance. It varies not only among species, but also strongly among species with the same dispersal syndrome. Therefore, it is a crucial variable when asking if dispersal is limiting the occurrence of a species in suitable habitats or species richness of plant communities, or if fragmentation is a threat to the survival of species or populations. The capacity to survive in disturbed habitats or in fragmented landscapes is often correlated with a high dispersal potential. Both seed production and also seed characters may be correlated with dispersal potential. The more seeds are produced, the higher the probability that one seed spans larger distances. The seed characters such as e.g. mass, form and structure of seed surface responsible for a high dispersal potential depend on the dispersal vector. There may be a trade off between dispersal potential (in space) and maximum plant lifespan as well as seed-bank persistence (dispersal in time). Long-lived species often exhibit a low dispersal potential, as do species with a long-term persistent seed bank.

How to record?

Dispersal potential is a continuous variable and may be recorded either by direct measurements in the field or can be identified by measurements of traits related to the dispersal potential, or by modelling approaches. Wind-dispersal potential is correlated with dispersule-releasing height and terminal velocity, dispersal potential by water to buoyancy of the dispersules and animal-dispersal potential to either attachment potential or survival after digestion. Dispersal by humans, machines or vehicles is very complex. Measuring dispersal potential, therefore, requires studies adapted to the specific question.

Measurements should be carried out on the intact dispersule, i.e. seed or fruit with all the structures, such as e.g. pappus and awns, that are still attached when it is released. Releasing height should be measured during dispersule release and is the difference between the highest elevation of the seed or fruit and the base of the plant. Terminal velocity is measured on freshly collected air-dry dispersules and, most simply, by the actual rate of fall in still air. Floating capacity (proportion of dispersules floating after a defined time) is measured by putting dispersules in glass beakers that are placed on a flask shaker moving with a frequency of 100 min^–1 Attachment capacity (proportion of dispersules still attached after a defined time) is measured by putting seeds on the respective animal fur, which is then shaken by a shaking machine. Survival after digestion is measured either by digestion experiments with the respective animals or by simulating ingestion by a standardised mechanical treatment and digestion by a standardised chemical treatment, which have to be calibrated by digestion experiments.

To assess animal-dispersal potential, field studies should be added where possible, because the behaviour of animals (e.g. selection of species by grazing animals) strongly influences dispersal potential. Predicting animal-dispersal potential requires process-based models with the ability to predict over a range of scenarios.

Notes and troubleshooting tips

Dispersule size and shape 

We recommend complementing Dispersule size and shape 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  Dispersule size and shape - More on methods below); 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.

Dispersal potential

(1) For water plants, seed releasing height is the distance between the highest point of seeds or fruits and water surface.

(2) Secondary process, e.g. dispersal by wind on the ground, may strongly affect dispersal potential. Such processes are often obvious only from field studies and may require the establishment of additional new methods.

Literature references

Dispersal syndrome

References on theory,significance and large datasets:

More on methods:

Dispersule size and shape

References on theory,significance and large datasets:

More on methods:

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

Dispersal potential

References on theory and significance:

More on methods:

 

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