Is your tree leafing out?
By Jesse Batty
Ahhh, the sun is out, the birds are chirping, spring has officially sprung!
Or has it? Why is it that some trees have leafed out when others are still dormant? At FOT, we often get the question in the spring time, “Is my tree dead?”
Patience, my friends. Some trees leaf out earlier than others because of genetics and environmental factors. In fact, there are many people who dedicate their lives to studying the timing of leafing out and other annual natural occurrences, a study that is called phenology.
For the most part, the timing of leaf out for the majority of trees is related to temperature and/or daylight length or both. Some trees need a certain number of days below a certain temperature threshold (chilling days, most often 32-50 degrees), followed by a certain number of days above a certain temperature threshold (warming days). Other trees need a certain number of daylight hours in order to experience bud break.
Trees need to balance their desire to leaf out and begin photosynthesis and produce sugars with the potential of leafing out too early and experiencing frost damage. The evolutionary history must be taken into account, as some species originated in cold climates and others in warm climates. These hereditary genetic traits have become learned and have aided trees in their survival over many generations.
Even the same species can leaf out at different times. This can usually be explained by environmental factors like the previous autumn’s conditions, altitude, proximity to buildings, streets or other location factors, direct sunlight or shade, soil type and soil moisture, and other variable cultural affects. Scientists now are studying leaf out times in relation to climate change as well. Early findings show earlier bud break times for some species and the potential for plant ranges to shift. Remote sensing promises to be a useful tool to analyze this phenomena moving forward. (Polgar & Primack)
In the meantime, we hope that you enjoy your spring and the snowbells, birches,and stewartias and all their glory. Soon the oaks, black tupelos, and ashes will be joining in the springtime fun!
For a great article about leaf out times and climate change, read Polgar & Primack’s entry in Arnoldia, the magazine of the Arnold Arboretum, the greatest arboretum in the world.
–Batty is a Neighborhood Trees Specialist for Friends of Trees.