We covered the three stages of dormancy here, wrapping up with a description of the internal changes during the last stage which prepare a tree for spring. These changes are invisible externally, but set in motion the changes that do become visible externally.

Depending on your location, late August/early September through to late September/early October is the time of year jujube trees begin to show those visible changes that mark them ready for spring. Let’s explore what is happening.

At the end of the final stage of dormancy, ecodormancy, growth-suppressing hormones are switched off, growth-inducing hormone production increases, and cell-to-cell communications within the tree are restored.

The restored communications means that phloem (sap) can flow again, bringing the food reserves that were laid down in the roots and stems prior to dormancy to the buds. It is vital that the tree has good reserves of nutrients stored away to sustain bud growth, as it cannot yet photosynthesise to make its own food.

Phloem also delivers the growth hormones to the buds, which signal their time to grow and develop.

The tree is fully heterotrophic at this stage — it is as wholly reliant on its stored organic compounds (sugars and starches) to get it into spring as an animal is reliant on its stored organic compounds (fats, mostly) to get it through a winter.

The root on this little Ta-Jan might not look like much, but it is packing a lot of energy.

This particular tree has been in a box, in the dark, covered with moist wood shavings, since early August and whilst still in ecodormancy. It’s still in that box, and it is only the stored energy in the Ta-Jan’s root and trunk now fuelling this new green growth at the top in preparation for spring:

Back in early August, the trunk was the typical grey colour all jujube wood is during dormancy. The photo below is of a similarly-aged, but still sleepy, Redlands trunk showing this colour. Sleepy, but awakening — if you look very closely you may just make out the red flush returning to the trunk (distinct from the shadow along right hand edge), and the active buds:

It is only once buds grow and produce leaves that a tree can become autotrophic and make its own organic compounds again. This it does via inorganic carbon dioxide and photosynthesis.

Photosynthesis also requires water, which is transported by the xylem from the roots to the leaves. This is a passive process, driven by capillary action and transpiration, whereby water evaporating from the leaves creates a water potential difference which draws water up from further down the plant.

Xylem becomes blocked during endodormancy by air bubbles formed from a freeze-thaw cycle as temperatures drop over winter. These air bubbles are removed during ecodormancy, when the xylem is repaired. While there are no leaves as yet to drive photosynthesis and transpiration, the xylem is ready to move water immediately it is needed.

Water can still reach the buds, where it is needed for growth, via the phloem.

The carbohydrate reserves in the roots also go into new root growth, seeking out the water and nutrients needed when phtosynthetic and growth processes return and replace those reserves.

In time enough foliage will develop that the tree is once again able to produce enough carbohydrates via photosynthesis (called photosynthates) that it can feed itself, grow, develop and reproduce, and subsequently build up new carbohydrate reserves in preparation for the next period of dormancy.

And the little Ta-Jan? I’ve become quite attached to this little fellow that packs quite a punch for his (lack of) height, and will make him the feature blogging tree for the 2020-21 year!