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.