Communities of annual plants are generally a response to either regular cycles of disturbance, as in agricultural situations, or highly seasonal rainfall patterns. In regions with temperate climates and reliable rainfall, annuals are mostly associated with agriculture (Figure 6.5). In Britain, cornfields with poppies, corn marigolds and other species are the best known and most charismatic annual plant community. Such communities are now rare due to
Annuals in fallowed field in Austria,
Papaver rhoeas and Consolida ambigua dominate
improved seed cleaning and germination-inhibiting herbicides, and are best developed where lowintensity farming has been practiced. Before agriculture, annuals were far less significant plants in these regions, probably restricted to animal migration routes and other heavily disturbed sites. Most of the major annual plant communities of the world are, however, associated with winter rainfall, Mediterranean climates, for example, California, Arizona, southern Europe, northern Chile and Peru, southern Africa, western and central Australia, plus summer rainfall climates, such as eastern Mexico and Texas. Here severe seasonal drought restricts competition from perennial herbaceous plants other than bulbs, allowing the development of rich annual plant communities.
Most of the winter rainfall species in Table 6.6 are referred to in the horticultural literature as ‘hardy annuals’, for example, Mansfield (1949) and Lloyd and Rice (1997), in that they can be established by sowing outside where they are to grow. As a designed vegetation, annuals are valuable because they often have very attractive flowers that are produced within three to four months of sowing.
Spontaneously occurring synanthropic vegetation, with lupins intermixed with native species, such as cow parsley, outside a garden in Hampshire
Table 6.6. Commonly cultivated annual forbs found in plant communities in various parts of the world (derived from Phillips and Rix 1999; plus the observations of the author)
* Species found in climates experiencing summer rainfall. All other species are primarily from winter rainfall climates. Most species associated with European cornfields are essentially winter rainfall species, whose flowering is often delayed into summer by low winter and spring temperatures.
With such a short timescale, providing the soil is moist, if a sowing fails, it can be repeated and still get flowers in the same season. Many species demonstrate high – establishment rates, and are fast growing and initially able to compete with weedy species on-site. A significant disadvantage of annuals is that they are transient and, even with management to encourage regeneration from self-sown seed, often require over-sowing on a yearly basis. The key design challenges in developing annual plant communities is to select a range of species that will extend flowering from summer through to autumn, rather than the four weeks typically associated with standard cornfield annual mixes. In Britain, this sometimes involves the addition of annuals from summer rainfall climates, for example Coreopsis tinctoria and Rudbeckia hirta, which have higher temperature requirements for growth and, consequently, flower in Britain from late summer into autumn (Dunnett 1999) (Table 6.6).
Annuals also have a role to play in providing seasonal colour in the first year of sowings of perennial species, providing species are chosen that do not compete too aggressively with seedlings of the latter.
Human sponsored ‘synanthropic’ urban vegetation
Many of the plant communities described so far are associated with agricultural practice, and thus are intimately associated with human beings. More often, however, human – sponsored vegetation is seen as associated with the following.