Tree Phenology (or Seasonal Cycles)

The term phenology is used to describe the life cycle of a biological organism like a tree. Phenological events for trees include bud development, bud break, flowering, fruiting and leaf & fruit drop, as well as other unseen changes such as sap rising, seed development, root growth, cambial activity or hardening off of tissues for winter.ref

Tree phenology is entwined with the environment in which the tree lives. As there are a very large number of different climates and micro-climates within them, there are accordingly many different nuances in tree phenology, according to the location and environment. Even the same species can show widely different phenology between two different places (at least from a timing point of view).

So to really understand how phenology would play out for your own trees, you need to understand the species phenology and how it varies based on location. You’ll often find bonsai articles are specific to the location of the author which won’t always be relevant to you.

The main phenological events relate to a tree’s growth and reproduction. For example, roots stop growing below 6°C, buds break when the tree detects a low chance of frost in the future (which might damage the tender buds and shoots), photosynthesis, energy production and growth is highest when there is the most sun, and reproduction happens in conditions which most favour seed survival.

  • In the boreal forests – “high-latitude environments where freezing temperatures occur for 6 to 8 month”ref phenology is mainly driven by temperature, affecting the timing of the start of the growing season and thereby its durationref
  • Temperate-zone forests are located between the tropics and the boreal forest zone – they have hot summers and cold winters with high temperature variationref, and their phenology is also mainly driven by temperatureref
  • Mediterranean coniferous forests are mainly driven by water availabilityref
  • Australian ecosystems are extremely diverse and also subject to irregular events such as fire, drought, cyclones and flooding, which can affect phenological events, but a key driver is water availability.ref Where evergreens dominate in this ecosystem, flowering is the main phenological event.
  • In tropical forests which have less variation in temperature and usually high water availability, leaf shedding and growth is continuous, but reproduction (flowering and fruiting) demonstrates ‘mast’ timing effects associated with drier than normal conditionsref (ie. all trees fruiting at the same time every seven years)

In boreal and temperate areas the phenology is described in this article and summarised in the images below. But if you’re keen to understand the specific phenology for your tree in your area, you could consult google scholar.

The chart below shows the proportion of Eucalyptus loxophleba flowering at any given time in a seed orchard in the southwest of Western Australia. The highest proportion of flowering happened in spring (Sept-Nov in Australia) but a significant portion also happened in winter (June-Aug). Flowering fell to zero in the hot, dry summer (Dec-Feb).

This all seems a bit confusing given how many different variables there are, but there are some basic principles you can use from a bonsai perspective:

  • Trees in their growth phase (usually when there is plenty of sun and water) will be able to recover more easily from significant damage (such as large trunk chops or carving wounds) and fight any pathogens which might seek to take advantage of these.
  • Similarly leaf pruning during active growth will result in more buds activating.
  • Trees which are in a strong vegetative growth phase (growing leaves and stems) deprioritise root growth. Root growth gets a turn after the leaves establish.
  • Trees which have set buds but haven’t flowered yet – if you prune indiscriminately – you will lose flowers! There is a way to identify flower buds on your tree but it involves a bit of effort. Flower buds differentiate from vegetative buds at a certain point prior to flowering/leafing out. You can identify different looking buds on your tree, then remove one example of each. Cut it open and look at it under a loupe or microscope and you will be able to see which one was the flower vs the leaf or shoot. Or if you’re both patient and organised, take a picture of some your tree with buds and then with flowers – and you should be able to see what the different bud shapes are.
  • Storage of carbohydrates to storage tissues will take place during growth phases, and these will be used in turn when less photosynthesis is happening, to drive respiration and other processes requiring energy. Read more about how storage varies in roots here: Root Food Storage (or, can I root prune before bud break?)
  • If you’re a fan of wiring, doing this before a stem hardens off will allow you more bendability (although watch out for growth around the wire)
  • Depriving a tree of resources (water, nutrients) will mimic ‘hard times’ and cause it to respond accordingly phenologically – drop its leaves earlier, produce less flowers/fruit or not flower at all, or push out emergency growth (like adventitious buds/suckers)
  • I think it’s important to say that although the term ‘dormant’ gets used in relation to trees, this is a little misleading. Trees are living organisms and still need to maintain their metabolism even during winter. This includes respiring (using oxygen and stored energy to maintain metabolism), photosynthesising (for any tree with green areas remaining including evergreen trees but also deciduous trees with green stems), transpiring (even deciduous trees still transpire during winter, although a lot less than when they have leaves and in particular they take up water to swell the buds prior to bud breakref), and taking up nutrients through the roots. As I’ve written elsewhere in this site, root growth can happen above 6 degrees C, so your tree may well be more ‘alive’ than you think during winter.

I know there will be people saying at this point – just tell me what happens when!! For those people here are some general guidelines for temperate zones.

You can expect conifers to cease xylem production in autumn and root growth in winter, and to pick these up again between 2-7 degrees C (cambium) and 6-9 degrees C (roots). Buds will burst from early spring onwards depending on the species and latitude and pollen cones will release their pollen. Seed cones will start maturing, which can take just one summer (Picea, Tsuga) or one or more years plus the summer (Pinus, Cedrus). Next year’s buds and future years’ seed cones will form in late summer, and old needles (2+ years depending on species) will drop in late autumn. Mature seed cones will drop or release seed from late autumn onwards. ref1 ref2 ref3 Hardening leaves for the winter also happens in late autumn.

The main differences for angiosperms in temperate zones revolve around xylem production, leaf growth and senescence within the season, and flowers & fruit. In spring xylem creation will commence – in diffuse porous trees buds can break earlier but ring porous trees need to create the new season’s xylem layer before budding. Some trees will burst bud based on temperature and others on photoperiod (or a combination of the two).ref Whether flowers or leaves come first depends on the species, and the timing of flowers is hugely variable (Frank P Matthews has a list of flowering times for ornamental trees in the UK). The leaves of deciduous trees start a structured senescence process in the autumn, when they remove cholophyll and other molecules from the leaves for storage and recycling (hence the colour changes). After this has been completed the tree creates a cork layer at the base of the leaf causing it to drop off. Fruit develops throughout the growing season and depending on the species will drop off from early summer through to winter.

There’s one more phenological domain which I haven’t covered in this article – the phenology of the microbiome. This is a whole other kettle of…microbes…and might be the subject for a future post.

Finally, the fabulous In ‘Defense of Plants’ podcast has covered phenology in this podcast episode.