2.4.1 Woody perennials
It has been noted that the timing synchronisation of seasonal flushing can represent a balance between the advantages of flushing as soon as conditions favour growth and the safety of delaying it until after almost all threat from climatic damage. In temperate zones, where seasons are defined primarily by temperatures, the classic danger is late (spring) frost. However, it is unclear whether close synchronisation of flushing is necessarily an adaptive consequence of meeting that balance. An alternative interpretation is that close synchronisation of bud burst and shoot and leaf extension means a brief period of ‘predator satiation’ or ‘predator swamping’ (cf Emlen 1966; Molles 2002) This would represent the period of maximum food value for browsers or other herbivores, after which the food value declines sharply. That decline, if it helps to limit the herbivore carrying capacity of an ecosystem, also represents a feedback mechanism to reduce browsing pressure. That, in turn may reduce the ecological importance of other defences against herbivory. A question arising with deciduous woody perennials, is whether, or how much, any such predator-satiation defence may be incidental to the advantage of quick and complete seasonal restoration of photosynthetic capacity The Quercus petraea /powdery mildew case, with tree-to-tree variation in flushing date, suggests that it is not entirely so.
With evergreens, in which complete seasonal restoration of photosynthetic capacity is not an issue, the expected premium for earliness of flushing is less. In this connection, while apparently not documented, the very brief flushing periods of some conifers, notably some firs (Abies spp} in very mesic habitats, strongly suggests a predator satiation strategy. Proving the postulate, however, is far more difficult than proposing. But there would be scope for documenting the phenology in relation to climate and moisture status, which could provide evidence against a simple climatic explanation.
Synchronisation of flowering has its own advantages, for efficiency of pollination, but it can also create a brief superabundance for herbivores. This could be another ‘predator satiation’ situation (cf Emlen 1966; Molles 2002), but Ims (1990) concluded that for this to operate the reproductive synchrony needs to spread across subpopulation units. Similarly, synchrony may militate against a build-up of pathogens that specialise on reproductive structures.
For the tropics, where seasonal effects are often dominated by wet or dry seasons rather than temperatures, the flushing behaviour can be more complicated than in temperate regions. There, the likely role of herbivory as a phenotypic driver has attracted more attention. Lamarre et al. (2014) report a study, and cite a number of others, of “synchrony of leaf production” in relation to herbivory. The situation represents widespread operation of predator satiation within the tropics. The story, however, can be more complicated. Aide (1992) documents a case of a woody shrub flushing outside the wet season, at a time when herbivore density is low, in ‘herbivore escape’. But that species also has brief and intensive flushing and flowering, which is consistent with an element of predator satiation. One may suggest that in herbivore escape the species benefits from an outlier status. Such a status can occur among the typically numerous woody species of tropical forests. In this connection, Bagchi et al. (2014) have documented how “negative density dependent” selection pressures from pathogens can tend to favour high species diversity.
Table 2 summarises, very broadly, inferred levels of alignment between predator satiation and optimisation of growth/competitive ability in temperate and tropical zones.
2.4.2 Non-woody plants
Outside woody perennials trees, three cases are adduced for herbaceous plants, two for pasture plants, and one for a collection of species in a Mediterranean climate. In one pasture-plant case, the seasonal phenology, seemingly driven by climate, may actually be driven largely by biotic factors in conjunction with climate. Key aspects of pasture-plant phenology are seasonality of forage growth and flowering. An extended growing season, meaning extended forage production, is widely sought by plant breeders. However, there is classically a trade-off between a long growing season and the persistence that avoids a need for frequent sowings to ensure sward renewal.