10 Seed Release and Dispersal Mechanisms
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10 Seed Release and Dispersal Mechanisms For seedling recruitment to occur seeds need to be dispersed into an environment that promotes germination and seedling survival. Dispersal consists of two phases. Primary dispersal is defined as the initial transport of seeds or seed-bearing fruits (collectively seeds and fruits are called diaspores) to the ground or water body, or for aerial parasites, a host branch. Secondary dispersal relates to any subsequent movement to the seed’s final resting place. Primary dispersal may be active (e.g. seeds released explosively from the fruit, e.g. dehiscence (opening) of Hardenbergia pods), passive (e.g. seeds fall out when the capsules of Eucalyptus open), or require a vector to aid in seed removal (e.g. wind uplift of winged seeds of Hakea or winged fruits of Nuytsia; Amyema berries consumed by mistletoe birds). Secondary dispersal involves either a biotic (e.g. ants) or environmental (e.g. wind, water) vector, and it is usually a different mechanism than that involved in primary dispersal. While primary dispersal is usually only for a few metres, secondary dispersal may cover several kilometres, and sometimes thousands for tiny seeds. This chapter covers some of the dispersal mechanisms exhibited by the SouthWest flora following their release. Terminology used to describe seed dispersal mechanisms is provided in Table 10.1. Table 10.1: Seed dispersal terminology. Term Definition Anemochory Wind dispersed Chamaechory Dispersal by rolling along the ground (wind assisted) Zoochory Animal dispersed (general) Myrmecochory Ant dispersed Ornithochory Bird dispersed Mammalochory Mammal dispersed Hydrochory Water dispersed Barochory Unassisted (gravity causes seeds to drop to the ground) Autochory Dispersal assisted by the actions of the parent plant (e.g. swaying in the wind) Bolochory Dispersal by propulsive mechanisms (ballistic) Endozoochory Animal dispersed, seed (or diaspore) eaten Epizoochory Animal dispersed; seed carried in fur, feathers, feet Synzoochory Animal dispersed, seeds carried intentionally (for consumption) Dyszoochory Animal dispersed, seed eaten intentionally Nautohydrochory Water dispersed, seeds carried in flowing water Xerochasy Seed release induced by fruit desiccation Hydrochasy Seed release induced by moisture © 2015 Philip K. Groom, Byron B. Lamont This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Brought to you by | Graduate School, CAS Authenticated Download Date | 10/10/16 8:15 PM Pyriscence of Serotinous Fruiting Structures 173 10.1 Pyriscence of Serotinous Fruiting Structures Prior to their dispersal by wind, seeds must be released from the fruits or cones of serotinous species. This is usually through the action of fire, called pyriscence (Lamont, 1991). For other groups of plants, seed and fruit release is simply a matter of spontaneous abscission following maturation, often hastened by strong winds that dislodge the diaspores, as in Nuytsia floribunda (Chapter 5). Omnivores like emus pluck fruits from the plant. Seeds of bird-dispersed acacias, with bright-red arils, remain displayed in the open pods. Succulent diaspores, such as Persoonia fruits and Macrozamia seeds, drop to the ground when ripe where they are found by frugivores. Fruits of Adenanthos cygnorum are released into the surrounding leafy cup where they are located by granivorous ants (Fig. 10.6). We illustrate the requirements for pyriscent seed release using the genus Banksia as it is by far the most interesting and best known. That fire is the key is supported by observations that the level of serotiny among three widespread banksias increases over a 500-km gradient north of Perth (Cowling & Lamont, 1985b). As the climate becomes drier, hotter and more seasonal so the height of the vegetation declines (scrub-heath) and fire is more likely to reach the position of the cones (crown rather than ground fires) so that serotiny can be relied on as a device for ensuring follicle opening and seed release at the time when seedling recruitment is most likely. In the open woodland further south, crown-reaching fires are less guaranteed and serotiny is weaker—interfire recruitment is more likely as well since the climate is milder. Most Banksia species retain their seeds in closed follicles for 5−10+ years accompanied by persistent dead florets—both traits are ancestral among banksias and can be traced back to the origin of the genus, 61 Ma (Fig. 10.1). Many species, such as B. brownii, never open all their follicles in the absence of fire, even when the plant dies from drought or disease (Lamont, 1996). Later (26 Ma), the persistence trait evolved further to conceal the fruits, possibly to hide them from efficient granivores like cockatoos whose ancestors were evolving at this time (White et al., 2011) (Fig. 10.1). Also at this time, species began to retain their dead foliage, a trait restricted to many SouthWest species and genera (Fig. 10.1). Both dead floret and leaf retention increase flammability (Lamont & Cowling, 1984) and, together with their narrow, sclerophyllous leaves, the plants burn strongly and quickly, ensuring that the resin sealing the valves of the follicles melts (Enright & Lamont, 1989; Fig. 10.2) and they open as they dry out. If the fire is intense enough, seeds may be released a few hours after fire. Otherwise, the onset of autumn rains results in wet-dry cycles that cause the gap to widen and the separator, which splits down the centre and clasps the wing of each seed in the illustrated B. lanata, gradually levers the two seeds out of the follicle (Cowling & Lamont, 1985a; Lamont & Barker, 1988). Seeds drop to the ground or become airborne at the level of the cone and are usually redistributed by wind and water into post-fire, litter microsites (Lamont et al., 1993). Gusts of wind or wind vortices (‘willy-willies’), particularly prominent after fire, may pick up the seeds and carry them several km from the parent plants (He et al. 2004; 2009). Black cockatoos may also remove the cones, especially after fire (note that the separator conceals and protects the seeds and makes it difficult for birds to access them while they remain on the plant), but the significance of this is unknown. Brought to you by | Graduate School, CAS Authenticated Download Date | 10/10/16 8:15 PM 174 Seed Release and Dispersal Mechanisms Fig. 10.1: Sequence of steps involved in the release of seeds from serotinous cones of Banksia (data refer to 85 banksias and 5 dryandras from He et al., 2011). Pictured are: B. lemanniana (dead floret retention) - note that the cone hangs upside down in this species (it is wasp-pollinated) and that the first florets to open are the ones most likely to set fruits; B. hookeriana (dead florets conceal follicles); B. candolleana (dead leaf retention) a clonal shrub estimated to survive for up to 1,200 years (Merwin et al., 2012), that also retains its dead florets on cauliflorous heads, ideally located for pollination by honey possums; B. lanata (resin melts). Fig. 10.2: Removal of Banksia attenuata seeds following cone harvesting can be achieved by flaming the woody fruits. This melts the resin that holds the two valves together, causing the follicle to rupture immediately. Dunking the cone in water and allowing it to dry, and repeating many times, enables the follicle to open progressively and the enclosed separator gradually lifts the two seeds out of the fruit (Lamont & Barker, 1988). Cone is 15 cm long. Image provided by Haylee D’Agui. Brought to you by | Graduate School, CAS Authenticated Download Date | 10/10/16 8:15 PM Wind Dispersal 175 10.2 Wind Dispersal Seeds designed specifically for wind dispersal (anemochory) have structures that increase the diaspore’s wind resistance and make it buoyant. Anemochorous seeds can be classified as either gliders, parachutes, helicopters, flutterers or tumbleweeds. The three-winged, single-seeded fruit of Nuytsia floribunda (Loranthaceae) (Fig. 9.1) relies on strong gusts of wind in autumn to become airborne (Lamont, 1985b). Each fruit is held in place by three persistent bracteoles, preventing immediate dispersal. These winged diaspores can be dispersed at least 50 m from the parent plant, although the greatest density of fruits occurs within 10 m. Nuytsia fruits can be regarded as ‘flutterers’, with the wings enabling an increase in uplift by decreasing the fruit’s terminal velocity, defined as the speed at which a seed ceases to accelerate because the downward force of gravity equals the upward force of air drag. Seeds of Banksia, Hakea and Xylomelum (all Proteaceae) (Fig. 10.3) and Allocasuarina (Casuarinaceae) have diaspores subtended or encircled by a papery wing that accounts for at least half the diapore area. Casuarinaceae has woody bracts around what appears to be a winged seed but is actually a fruit. The fruits or woody cones open as a result of desiccation that is caused by death of the supporting stem from heat, drought or physical damage. Seeds can be released within a few hours of their protective woody-fruit opening in response to severe fire. With less intense fire and only partial opening of the fruit or cone, a wet-dry cycle is required (Cowling & Lamont, 1985a; Lamont, 1988). With each successive bout of autumn rain and subsequent drying the woody structure opens more until finally the seeds are released onto an ideal seedbed for germination. This also minimizes the time of exposure to summer heat and granivores before germination can commence. Taxa in the family Asteraceae (daisy family) characteristically produce a one- seeded fruit (achene) terminated by a feather-like crown of bristles (pappus). These act like a parachute—catching the wind so that the fruit floats in the air. Feathery parachutes also occur in other families (e.g. Clematis, Ranunculaceae). Hairs aid wind dispersal within certain Proteaceae (e.g.