3.2 Two pines
In the main landmasses mammals are typically the main ground-dwelling
browsers. While extreme heteroblasty is seldom evident as it is in New
Zealand, there are cases where ontogenetic phenology can be interpreted
in terms of changes in both palatability and browse resilience.Pinus radiata is an example. In it, the juvenile to adolescent
habit is bushier and more intensively branched than the adult habit
(Burdon et al. 2017), and resilience to browsing is high. The adult
habit, however, is associated with greater palatability to
ground-dwelling browsers and less resilience. This is notoriously
evident in adult-phase scions grafted for seed orchards onto small
seedlings, the scions attracting far more deer browsing than seedlings.
There is evidently a trade-off of resistance or resilience to browsing
against pathogen tolerance. Juvenile material is more susceptible to
some pathogens, notably Endocronartium harknessii (Moore)
Hirats.) which causes western gall rust (Old et al. 1986), andDothistroma septosporum (Dorog.) M. Morelet (syn. D. piniHulbary) (Burdon et al. 2017). Losses of some individual seedlings to
pathogens, directly or through impaired competitive ability in the case
of western gall rust, as part of density-dependent selection, may not
impair population fitness in the thickets of regeneration that often
occur naturally. However, another advantage of a juvenile phase can lie
in how strongly a longer growing season (cf Norskov-Lauritsen 1963) can
contribute to growth potential while tree growth is still partly
exponential. At least one other pine, P. taeda L., shares the
feature of being susceptible to an important gall rust pathogen,Cronartium fusiforme Hedgcock et Hunt ex Cummins, in young trees
rather than later (Powers et al. 1981).
3.3 Eucalyptus nitens
While only incidentally reported as such, Eucalyptus nitens(H.Deane et
Maiden) Maiden is a
case involving a combination of strong ontogenetic phenology and both
herbivory and pathogen attack. This species has major geographic
(provenance) variation in the duration of juvenile foliage production
(Pederick 1979). The adult phase is subject to insect herbivory, while
the juvenile phase is very subject to a foliage disease (Johnson and
Wilcox 1989). Provenance variation evidently reflects the comparative
selective pressures imposed by insect pests and pathogens in respective
habitats. Thus, provenances with persistent juvenile characteristics,
while enjoying longer protection against insect herbivory, are
vulnerable to foliage disease with warm, humid summers. Conversely,
those provenances with a brief juvenile phase, while less vulnerable to
foliage disease are more so to insect herbivory.