Remaining species Common lambsquarters

Fig. 6.7. Effect of four types of tillage on (a) seed distribution, and (b) the density of common lambsquarters (Chenopodium album) in the seed bank (Clements et al., 1996; with permission of the Weed Science Society of America).

Mouldboard Chisel plough Ridge-till

No-till

Fig. 6.7. Effect of four types of tillage on (a) seed distribution, and (b) the density of common lambsquarters (Chenopodium album) in the seed bank (Clements et al., 1996; with permission of the Weed Science Society of America).

A species' seed bank can be classified based on seasonal variation in germinable seeds (Thompson and Grime, 1979). The two main types are transient and persistent seed banks. These two types are further subdivided. Transient seed banks contain seeds that do not last for more than 1 year; they may contain either autumn-germinating seeds (Type I - e.g. perennial rye grass, Lolium perenne) or spring-germinating seeds (Type II - e.g. Himalayan balsam, Impatiens glandulifera) (Fig. 6.8).

Persistent seed banks contain seeds that remain viable for more than 1 year. They either contain many seeds that germinate in autumn but maintain a small seed bank throughout the year (Type III - e.g. Rhode Island bentgrass, Agrostis tenuis), or they may have a large persistent seed bank year round (Type IV - e.g. common chickweed, Stellaria media). Most of the world's worst weeds have large persistent seed banks (Holm et al., 1977). Species that are increasing in abundance are more likely to have a persistent seed bank than those that are decreasing in abundance (s.f. Hodgson and Grime, 1990).

Monocarpic species tend to have more persistent seed banks than polycarpic species. This allows a seed carry-over effect for monocarpic species and prevents them from becoming locally extinct. If all individuals of a monocarpic species die before they can reproduce, there will still be viable seeds that can germinate. In polycarpic perennial species, seed carry-over is not as important because seeds are produced repeatedly during the life span. Species with persistent seed banks tend to have small, rounded seeds (Thompson and Grime, 1979; Thompson et al., 1993). This is likely to occur because small seeds are more likely to become buried either by falling down cracks in the soil or through burial by soil organisms (Thompson et al., 1993). Once buried, seeds are more likely to persist because they have lower predation rates and are less like-

Fig. 6.8. Types of transient (Type 1 and 2) and persistent (Type 3 and 4) seed banks. Shaded areas show seed capable of immediate germination, while unshaded areas show viable seed that are not capable of immediate germination. Examples of each seed bank type are: Type 1: annual and perennial grasses of dry and disturbed habitats. Type 2: annual and perennial herbs colonizing gaps of early spring. Type 3: species mainly germinating in the autumn but maintaining a small persistent seed bank. Type 4: annual and perennial herbs and shrubs with large persistent seed banks. (Thompson and Grime, 1979; with permission of Blackwell Science.)

Type 1 - Transient

Type 3 - Persistent

Type 4 - Persistent

Type 4 - Persistent

Fig. 6.8. Types of transient (Type 1 and 2) and persistent (Type 3 and 4) seed banks. Shaded areas show seed capable of immediate germination, while unshaded areas show viable seed that are not capable of immediate germination. Examples of each seed bank type are: Type 1: annual and perennial grasses of dry and disturbed habitats. Type 2: annual and perennial herbs colonizing gaps of early spring. Type 3: species mainly germinating in the autumn but maintaining a small persistent seed bank. Type 4: annual and perennial herbs and shrubs with large persistent seed banks. (Thompson and Grime, 1979; with permission of Blackwell Science.)

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ly to face germination-inducing environmental conditions, such as light, compared to seeds on the soil surface.

Seed dormancy

The main reason that seeds remain viable in the seed bank is because they are usually dormant, i.e. they cannot germinate until a specific set of environmental and physiolog ical conditions are met. During dormancy the seed exhibits little growth or development and respiration is reduced (Rees, 1997; Benech-Arnold et al., 2000). This allows the seed to persist but expend little resources on maintenance. Dormancy prevents germination while the seed is still on the parent plant and ensures temporal dispersal into environments favourable to seedling survival (Murdoch and Ellis, 1992). Long-lived perennials tend to have short-lived seed

Table 6.2. Definitions of terms associated with seed dormancy. Term Definition

After-ripening A process whereby seeds are gradually able to germinate over a broader range of conditions

Dormant Seeds unable to germinate even though they have imbibed and are under favourable environmental conditions Non-dormant Being able to germinate under favourable environmental conditions

Primary dormancy Seeds that are unable to germinate when they mature and are either dispersed or still attached to the maternal parent plant Quiescent Being unable to germinate due to unfavourable environmental conditions

Secondary dormancy Dormancy that is imposed on the seed after being dispersed banks, whereas short-lived species are more dependent on dormancy. Table 6.2 is a list of definitions associated with dormancy.

Primary and secondary dormancy

Seeds that are unable to germinate when they first mature have primary dormancy. When dormancy is imposed after seeds have dispersed, this is called secondary dormancy. Dormancy is usually imposed when environmental conditions are unfavourable for prolonged periods of time. Dormancy is adaptive because it prevents seeds from germination during seasons when environmental conditions are unsuitable for the growth of that species.

Seeds may cycle in and out of dormancy, changing from dormant to conditionally dormant (where they germinate under a smaller range of conditions) to non-dormant; this cycle repeats and can result in annual dormancy cycles observed in many weeds such as barnyardgrass (Honek et al., 1999) (Fig. 6.9). Dormancy of summer annuals such as common ragweed (Ambrosia artemisiifolia) and lady's-thumb (Poly-gonum persicaria) is released in the spring by low winter temperatures and re-induced by early summer high temperatures. Winter annuals such as ivy-leaved speedwell

(Veronica hederifolia) and henbit (Lamium amplexicaule) require higher summer temperatures to release dormancy for fall germination. Dormancy cycles ensure that seeds remain viable over seasons (by not expending resources) but are able to germinate when conditions are appropriate for them.

Secondary dormancy is maintained by several mechanisms. Physiological mechanisms in the seed embryo may prevent it from germinating. Physical mechanisms can also enforce dormancy (Foley, 2001). A hard seed coat is one type of secondary dormancy as it prevents water and/or gasses from entering the seed. Seeds with a hard seed coat usually require some sort of physical or chemical abrasion to break dormancy. Alternatively, chemical inhibitors within the seed coat can maintain dormancy. These chemicals must be removed, for example by leaching, before the seed can germinate. Velvetleaf (Abutilon theophrasti) and field bindweed (Convolvulus arvensis) are examples of weeds with seed coat-imposed dormancy, while wild oat (Avena fatua) and annual sunflower (Helianthus annua) have embryo dormancy.

Seeds exhibiting dormancy usually have to experience periods of favourable environmental conditions during a period called 'after-ripening' (Fig. 6.6). After-ripen

Fig. 6.9. Patterns of cyclic dormancy in barnyardgrass (Echinochoa crus-galli). Seeds buried in 1 993 (open squares) and 1994 (black squares) were periodically retrieved and germinated in light at 25°C (Honek et al., 1999; with permission of Blackwell Science).

ing is a process whereby seeds are gradually able to germinate over a broader range of conditions (Baskin and Baskin, 1989). The environmental conditions required for after-ripening to occur are specific to individuals (and often broadly to populations and species). For example, common and giant ragweed (Ambrosia artemisiifolia and A. trifida) require cool, moist conditions (Bazzaz, 1970; Ballard et al., 1996) whereas common cocklebur (Xanthium strumarium) requires warm dry conditions (Esashi et al., 1993).

Breaking dormancy

The conditions required to break dormancy often vary among species, but also vary within species or populations. This is especially true if their geographical range or habitats have a high degree of environmental variation (Allen and Meyer, 1998). In species that normally experience wide environmental variation, e.g. beard-tongues (Penstemon spp.) and blue flax (Linum perenne), only some seeds respond to dormancy-breaking conditions and they maintain a long-term seed bank as a hedge against sudden environmental change within a growing season. Within a given species, populations that experience colder winters have more dormant seeds and require longer periods of cold to break dormancy than populations with milder winters. Intermediate populations often have variable dormancy, with differences occurring either within or among individual plants (Allen and Meyer, 1998).

Even seeds produced from one individual may have different dormancy-breaking requirements. This often occurs in species with polymorphic seeds where seeds with different morphs have different types of dormancy. For example, seeds from the peripheral flowers on tansy ragwort (Senecio jacobaea) are larger, heavier and require longer periods of time to break dormancy; whereas seeds from central flowers are small, lighter, disperse further and are less dormant (McEvoy, 1984). Similarly, common lambsquarters produces mainly dormant black seeds but a few (< 1% of total) are non-dormant brown seeds (Roman et al., 2000).

How the cues for breaking dormancy and germination differ

When dormancy is 'released' or 'broken', this does not necessarily lead to seed germination as conditions required to break dormancy are not necessarily the same as those required for germination (Benech-Arnold et al., 2000). A seed that loses dormancy may either germinate, become dormant again, or it may die. Release from dormancy and seed germination generally are considered as two sequential processes.

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