One of the joys of living in the temperate world is to experience the days lengthening in spring, the temperatures increasing, and our woody vegetation waking up in response. After a long hiatus, we suddenly see trees flushing flowers and leaves and we realize that their development has silently snuck up on us. These noiseless changes in trees are more intriguing to me every year, even though, as a graduate student studying developmental biology, I diligently try to find out everything there is to know about winter-to-spring transitions.
These transitions that we get to observe for a couple of weeks every spring are part of a larger story that spans the entire year. From a developmental point of view, plants are interesting because they are continually developing, constantly producing new organs—leaves, flowers—as they age. (Many of us animals, in contrast, have to go through life with just one set of organs.) Being a continually developing organism becomes extra interesting if you also happen to be long-lived and growing in a climate with a harsh winter. On an annual basis, you would need to slow your growth to prevent frost from killing your delicate new leaves, and then start back up again at the appropriate time.
Expansion of leaves in the spring is one frost avoidance strategy, a mechanism that has evolved to minimize costs associated with cold damage and to optimize the photosynthetic gains of the growing season. This alone is not enough for a tree to function in a temperate climate. Additional strategies for frost tolerance are required to keep the overwintering parts of the tree—buds, wood, roots—healthy during spells of extreme cold.
Through an evolutionary lens, the Arnold Arboretum may be seens as a collection of long-lived organisms that have risen to the challenge of developing in the highly seasonal Bostonian climate. Many of the lineages represented in the living collection have arrived at being woody in a temperate climate through different evolutionary trajectories. This makes the Arboretum an exceptional place for thinking about the manifold adaptations that exist in trees to avoid and tolerate long periods of cold temperatures.
If you would like to hear more about the challenges of winter and traits that allow trees to live in cold climates, I will lead a more in-depth discussion at a Botany Blast on April 15 of what is happening right now as trees prepare for spring, and why this is best understood in the context of an entire year. Come and take a tour of the seasons, and explore what we know and what we still don’t know about being woody in the temperate world.