2.3.1.4 Discussion
We are not postulating that pathogens have necessarily created a biotic
lock-in of deciduousness, but rather that the evolutionary inertia may
have been decisive in some cases but not others. The evolutionary
outcome could likely be influenced by various factors, including details
of the pathogen life cycle. Possibly relevant details include: longevity
of fungal fruiting bodies, level of host specificity, and absence or
presence of alternate hosts. If alternate hosts exist, whether they are
obligate, and what phases of the pathogen life occur in which host, are
also likely to be relevant. A useful framework to examine these complex
co-evolutionary dynamics that includes the context of abiotic factors is
the ‘disease triangle’ (Stevens 1960), which illustrates the interplay
between host, pathogen and environment, and their respective interaction
and influence on disease and impact upon the host (Figure 3).
The story for tropical tree species does appear to be different. The
production of foliage during the dry season in some species suggests
strongly that, if pathogens are a selective force, they would have
sometimes driven a switch to deciduousness.
The picture concerning the general incidence of deciduousness among
woody perennial taxa is mixed. Conifers, which represent the ancient
taxon, are very predominantly evergreen, suggesting profound
evolutionary inertia against a shift to deciduousness. One exception is
the entire genus Larix , within the Pinaceae, being deciduous. The
remaining exceptions are all Taxodiaceae members of the
Cupressaeae/Taxodiaceae complex, namely the genera Taxodium ,Glyptostrobus and Metasequoia , although T.
mucronatum Ten. is semi-deciduous. They also tend to have few
pathogens, along with some other taxonomically isolated species. Among
angiosperms, which are evolutionarily more recent, deciduousness is far
more common, in numbers of species if not in percent of total species.
Even with genera, sympatric species can include both deciduous and
evergreen members. This is so with Nothofagus in South America
(as already mentioned), and Quercus in North America (where some
species are semi-evergreen/deciduous) and the Mediterranean basin.
Overall, the general incidence of deciduousness among taxa argues
against any completely overriding influence of abiotic environment or
taxonomic lineage per se .
2.3.2 Swiss needle cast
and Douglas-fir
With native populations of Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco in western north America, dates of spring bud burst show
a coast-to-inland gradient from the Pacific coast. At a given elevation
bud burst comes earlier the further from the coast (Campbell and Sugano
1979). This has been observed not only in situ but also in
common-garden experiments, ruling out a simple effect of cooler coastal
temperatures caused by the cold ocean current. One possible reason is
that coastal populations have a longer humid season in which to complete
vegetative growth and cone ripening, especially compared with more
easterly populations where summer drought starts earlier (cf Campbell
and Sorensen 1978). Another possible factor may be less insolation early
in spring. Both factors might reduce the advantages of early bud burst
in coastal populations. Yet another possibility, not mutually exclusive,
is that later bud burst there escapes the worst of the seasonal hazard,
created by humidity, of infection by foliage pathogens. Of such
pathogens the most prominent is Phaeocryptopus gaeumannii(T.Rohde) Petrak, cause of Swiss needle cast (Boyce 1940; Mulvey et al.
2013), which is most aggressive in the humid coastal climates. Given the
postulated selection pressure imposed by the pathogen, one might expect
fog-belt Douglas-fir grown in very mild climates without the pathogen to
show a genetic shift to earlier bud burst. However, any such study would
both be long-term and face the difficulty of finding anywhere that will
remain free of the pathogen. Chemical control of the pathogen would need
to be long-term and is expensive, and could be complicated by the
pathogen evolving resistance.
2.3.3 Two Pinusspecies
Two closely related coastal California pine species appear to be another
case in point. They are Pinus radiat a D.Don (either Monterey pine
or radiata pine) and P. muricata D.Don (bishop pine or muricata
pine). They have overlapping (but almost entirely allopatric) geographic
distributions. Pinus radiata ranges discontinuously from 28−37°N,
and P. muricata discontinuously from
311/2−41°N. Pinus radiata has a
very long, opportunistic growing season, being able to make growth all
year if temperatures permit, which evidently accounts for a very fast
growth potential (e.g. Burdon 2001; Burdon et al. 2017). By comparison.P. muricata makes minimal shoot elongation during winter, with
apical buds remaining sealed, and is slower growing. Correspondingly, it
has a later pollination season. These species are naturally challenged
by strongly overlapping sets of pathogens, notably ones causing needle
casts and shoot galls.