In terms of the linkage between weed seed rain and in-crop weed populations, Gallandt et al. (2010) found that by preventing weed seed rain they could reduce subsequent years' weed seedbanks compared with other autumn treatments between 45% and 93% and weed seedling densities by 23% to 90%. In Western Australia preventing the seed rain of annual ryegrass ( Lolium rigidum) reduced in-crop ryegrass emergence by 90% in 4 years ( Walsh et al., 2013 ). Another perspective is illustrated by a study by Rahman et al. (1998) whereby they tilled soil monthly for 4 years, achieving an exponential decline in the weed seed bank, represented by four weed species, both monocotyledons and dicotyledons ( Fig. 5.10 ).
Paolo Bàrberi , in Advances in Agronomy , 2019
Integrated Weed Management in Organic Farming
While this level of tillage in real-world farming is clearly excessive and would be highly damaging to soil, it, along with the other examples, clearly illustrates the importance of minimizing weed seed rain and the ability to reduce an existing weed bank. Depleting the seedbank also underlies the false and stale seedbed techniques, as these deplete the emergable weed seedbank (see Section 10.8.2.1 ).
Norris (1992) proposed that with proper use of herbicide and weed management technology, we can eliminate weeds from an area by preventing weeds from producing seed. He further stated that the economic threshold, defined as the pest population at which control action should be initiated in order to prevent the population from increasing to or exceeding the economic injury level, should not be adopted in weed management as it has been in entomology for insect management. Weed management must recognize long-term weed population dynamics, including the nature of the seedbank. He recommended that weed management, especially for serious problem weeds, should adopt a ‘no-seed’ threshold. This threshold implies that weeds should not be permitted to set seed. He cited several cases where this has worked in California on high-value crops where the same growers are in control of the land for many years. Norris (1999 , 2000 ) further stated that a ‘no seed’ threshold can only be successful when weed management technologies are integrated, including the use of hand labor for controlling low-weed populations that have not succumbed to other management tools.
To illustrate the direct relationship between the weed seedbank and in-crop weeds, a study by Rahman et al. (1996) studying the number of emerged weeds vs. the number of viable weed seeds in the soil found a clear, almost one-to-one relationship ( Fig. 5.9 ). Clearly, the larger the weed seedbank, the larger the population of in-crop weeds.
soil seed bank, natural storage of seeds in the leaf litter, on the soil surface, or in the soil of many ecosystems, which serves as a repository for the production of subsequent generations of plants to enable their survival. The term soil seed bank can be used to describe the storage of seeds from a single species or from all the species in a particular area. Given the variety of stresses that ecosystems experience—such as cold, wildfire, drought, and disturbance—seed banks are often a crucial survival mechanism for many plants and maintain the long-term stability of ecosystems.
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The role of seed dormancy
Variation in the characteristics of seed dormancy determine whether a species’s soil seed bank is transient (temporary) or persistent. Transient seed banks are composed of species that produce seeds with a brief or no period of dormancy. Such seeds generally germinate prior to the next round of seed production, and the seed bank is thus continually depleted and reestablished. Transient seed banks are typical for many plants, especially long-lived perennials such as trees and shrubs. Often, such species rely on other strategies or life-history stages for persistence. For example, species may depend on long-lived adults, “banks” of seedlings in a forest understory, or extensive seed dispersal. In contrast, species with persistent seed banks have seeds that can remain dormant for more than a year, meaning that there is always some viable seed in the soil as a reserve. Persistent seed banks are common in annual plants and some woody plants, in which the failure of seed to establish the next generation would mean the collapse of the population. Scientists sometimes further classify persistent seed banks based on the extent or pattern of dormancy.
Seed dormancy and environmental constraints on germination influence various characteristics of soil seed banks. For example, seed dormancy determines how long a seed can remain viable in the soil. Factors such as embryo immaturity, chemical inhibitors, and physical constraints influence seed dormancy. Light filtered through plant canopies, for example, can inhibit germination in some species, while a long winter chilling may break dormancy in other species. The result is a considerable variety in the patterns of germination of the seed banks by seasons, disturbances, or other environmental shifts.
In addition to dormancy, considerable variation occurs in seed bank germination because of seasonal or other environmental shifts. Disturbances such as fire, flooding, windstorms, plowing, or forest clearing are frequently strong selective forces and may increase the overall germination response of seeds. Ecosystems characterized by wildfire often have extreme cases of persistent seed banks, as is common for many areas with Mediterranean climates, such as Australia, California, and South Africa. In those ecosystems the germination of many species requires signals provided by fire, such as a heat pulse into the soil or chemicals from smoke or charred wood. Germination may not occur until after a wildfire, which then results in mass germination from the seed bank the following spring. Similarly, the seed banks of agricultural weeds are often well adapted to the almost continuous human-made disturbances of their environment. Such weeds frequently have complex dormancy patterns that reflect the agricultural practices under which they evolved.
Organic growers aim to manage their weed seed banks in the opposite fashion from a long term savings account: minimize “deposits,” and maximize “withdrawals” (Forcella, 2003). Weed seed bank deposits include:
The weed seed bank is the reserve of viable weed seeds present on the soil surface and scattered throughout the soil profile. It consists of both new weed seeds recently shed, and older seeds that have persisted in the soil from previous years. In practice, the soil’s weed seed bank also includes the tubers, bulbs, rhizomes, and other vegetative structures through which some of our most serious perennial weeds propagate themselves. In the following discussion, the term weed seed bank is defined as the sum of viable weed seeds and vegetative propagules that are present in the soil and thus contribute to weed pressure in future crops. Agricultural soils can contain thousands of weed seeds and a dozen or more vegetative weed propagules per square foot.
What is the Weed Seed Bank, and Why is it Important to Organic Farmers?
Keep in mind that this method is not likely to reveal all the species present in a field. However, in combination with field observations on seasonal patterns of weed emergence, greenhouse weed emergence tests can help anticipate when control tactics are likely to be needed in the coming season, and to begin developing a seed bank management strategy.
While it is sometimes advantageous to cause weed seeds to germinate, it is important at other times to keep them quiescent long enough for the crop to get well established. Several practices can help reduce the number of weeds emerging in the crop.
Maintaining excellent weed control for several consecutive seasons can eliminate a large majority of the weed seed bank, but a small percentage of viable, highly dormant seeds persist, which can be difficult to eliminate (Egley, 1986). Researchers are seeking more effective means to flush out these dormant seeds through multiple stimuli (Egley, 1986).