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.