Mast seeding is the synchronous intermittent production of large flower or seed crops by a population of perennial plants (Kelly 1994). The species that perform mast seeding have to be long-lived species; otherwise the cost of lost opportunities for reproduction would be too great. The variability of the seed output among years by these species is what makes this so noticeable, and also raises important ecological questions about why plants should skip opportunities to reproduce. Another puzzling aspect is how individual plants become synchronised between one another to produce high seed output in the same year. Since plants cannot communicate, this requires them all to be responding to a common cue, usually a climate signal like a warm summer.
In the absence of selection for higher or lower variability, plants will vary in tandem with the environment (resource matching). Two selective factors often favour the evolution of masting: increased pollination efficiency in wind-pollinated species, and satiation of seed predators. Other factors select against masting, including animal pollination and frugivore dispersal. For a review of these, see Kelly (1994) and Kelly and Sork (2002).
Illustration of the variability in seed out put over time and the comparison of synchrony, within populations of three varying species over different time frames. Note the extreme synchrony over time in (c) L. latifolia
Why study mast seeding?
There are two main reasons, scientific and practical. The scientific reasons are to do with wanting to understand the evolutionary forces that make it beneficial to plants to not reproduce in some years, when selection usually favours early and repeated reproduction. We ask whether variable seeding is simply a plant's reproductive response to variable weather conditions, or is a reproductive trait that has evolved through natural selection despite the costs of lost opportunities for reproduction (Walter 1979) and probable higher density-dependent seedling mortality in mast years (e.g., Hett 1971). Also, there is interest in unravelling the mechanisms used to ensure synchrony among plants, and the impacts of global warming on these mechanisms.
Practically, a reproductive episode that results in a superabundance of seeds can be a remarkable phenomenon. Anecdotes about entire forests being swamped with seeds, such as Malaysian forests dominated by Dipterocarpaceae (Janzen 1974, Ashton et al. 1988, Curran & Leighton 2000), or an entire area of a bamboo species suddenly coming into flower (Janzen 1976), draw attention. The pulse of resources through masting can have effects throughout the ecosystem. For example, in the eastern United States masting by oaks and other tree species triggers interacting density fluctuations in rodents, deer, Lyme disease and gypsy moths (Ostfeld et al. 1996).
Food web interactions in an Oak (Quercus) forest in response to a mast seeding event by Quercus species.