2.3.1 Deciduous versus evergreen habit
Of special note in phenology is the seasonal complete shedding of
foliage that characterises the deciduous habit, versus the evergreen
habit. This was reviewed by Holttum (1953) for tropical forests and
Axelrod (1966) for temperate forests. These important papers exemplify
most subsequent enquiry on the evolutionary drivers of deciduousness, in
pointing to factors of the abiotic environment. Both authors postulated
that seasonal drought was the original driver. Specifically, Axelrod
postulated that this habit, having originated during the Cretaceous, was
a preadaptation to cold winters that subsequently characterised
high-latitude environments along with their widely fluctuating
daylengths. Yet, in many harsh, high-latitude climates, with very severe
winters, both deciduous and evergreen tree species co-occur. Moreover,
even within some genera, notably Nothofagus in South America,
both deciduous and evergreen species can grow adjacently. In adaptation
to climates with severe winters there are some obvious trade-offs. The
deciduous habit entails heavy seasonal turnover of biomass, but the
leaves do not require investment in anatomical features needed for
overwintering. This habit also allows the leaves to be
photosynthetically very efficient relative to their dry matter (Reich et
al. 1992). Apart from co-occurring species, sometimes close relatives,
including both evergreens and deciduous ones, there are other obstacles
to facile climatic interpretation. For instance, deciduousness exists
among many species without severe winters (e.g. Suc 1984; Li et al.
2013). Also, there are deciduous tropical tree species that produce new
foliage well before dry seasons end, which we will revisit.
For the evergreen habit, the anatomical requirements for leaves
surviving winters or other seasonal stresses will also tend to make them
less attractive or rewarding to herbivores in general. Such protection
against herbivory is very often complemented by toxin production. All
these defences, along with defences against abiotic factors, require
additional investments (Loehle & Namkoong 1987; Strauss et al. 2002;
Kursar & Coley 2003; Villar et al. 2006), or “higher construction
costs” (Smith et al. 2019). However, such investments can obviate the
cost of the complete seasonal turnover of foliage in deciduous plants.
Also, early flushing is not crucial for evergreens to resume
photosynthesis in the spring (cf Panchen et al. 2014). Osada (2020),
comparing sympatric evergreen and deciduous species, found the former to
have later and longer periods of leaf expansion. One might expect
foliage toxicity to be more prevalent and more severe in evergreen
species than in deciduous ones, but we have found no published survey on
this question. Anecdotally, however, yews (Taxus spp) which are
evergreen are both notoriously toxic and often associates of deciduous
species. Within a deciduous example (Populus deltoides × P. nigra
‘Robusta’ ) a subtle, seasonal effect of seasonal decline in phenolic
compounds being associated with increasing susceptibility to the leaf
rust Melampsora larici-populina Kleb. was observed by
Maupetit et al. (2018). This suggests that, if phenolic compounds
represent a defence mechanism, that decline reflects an energetic cost
of their production and decreasing value of the protection that they
confer.
While much attention given to the energetic costs and associated
trade-offs in alternative strategies of deciduous or evergreen habit,
little seems to have been given to the possible roles of pathogens and
herbivores in generating or maintaining the deciduous habit. In relation
to the evolutionary pressures imposed by both biotic and abiotic
factors, we will consider also the evolutionary hurdles to be overcome
in shifting from one habit to the other. Regarding deciduousness, the
presence and life cycles of pathogens may play a role, and we now
consider a probable example.