Temperatures warming up, the snow melting, migratory birds making their way back – all these are harbingers of spring. In deciduous trees, the opening of buds, or budburst, is a discrete, but essential phenomenon. This stage means the end of dormancy and the start of the growing season.
Good winter protection
Trees have developed strategies to survive harsh winter conditions. The young leaves and flowers that emerge early in the spring were formed at the end of the previous summer. To survive the cold season, these structures must spend the winter in a state of dormancy, well protected by the bud scales.
Dormancy begins in the fall with the coming of shorter days and cooler temperatures. Progressively, cell division stops. Then, through different metabolic processes, woody plants manage to lower the freezing point of the water inside their cells. This allows the buds to avoid frost damage.
Dormancy, one stage at a time
So that buds don’t open prematurely in the spring and risk being harmed by a recurrence of frost, they’re subject to two types of dormancy: endodormancy and ecodormancy.
Endodormancy, which inhibits all cellular growth, is regulated by physiological factors inside the buds. For it to be lifted, the tree must first accumulate chilling units, or hours of coldness. This is why trees of temperate regions, if lacking sufficient exposure to cold, will be unable to break bud dormancy in the spring.
Once these needs for cold are met, buds enter the ecodormancy phase, during which growth is limited by environmental conditions. At this point, it’s the accumulation of warmth that initiates the process leading to budburst.
Budburst, sooner or later…
With the return of warmer weather and the accumulation of heat, dormancy comes to an end. Growth begins again slowly and imperceptibly inside the buds. These then swell up and spread their protective scales, allowing the young leaves and flowers to slowly unfurl.
Each woody plant has its own period of budburst, which depends primarily on the accumulated heat, and therefore on the climate of the region where the tree is found. It also varies based on the species, because other factors such as photoperiod or the organization of the conducting vessels in the wood can also influence the period of bud break. In Québec, willows, maples, poplars and birches are early-season species, whereas ashes, oaks, hickories and walnut trees bud later.
Flowers or leaves, which gets to go first?
For certain species, like silver maples and willows, it’s the flowers that will appear first, when budburst takes place. For others, it’s the new leaves that open out. In wind-pollinated trees, it’s generally the floral buds that open first, allowing pollen to circulate freely before the emergence of leaves.
Note: At the Jardin botanique, starting in the month of March, witch-hazel (H. x intermedia) flowers, willow catkins and silver-maple flowers can be seen. And very soon forsythias and then magnolias will follow.