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 pointed to
factors of the abiotic environment as the evolutionary drivers of
deciduousness. This focus on abiotic factors appears to have dominated
subsequent enquiry into the topic. 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.