Leaf-out in the Biosphere
In the Mi’kmaw calendar, June is Nipniku’s which translates to ‘Trees Fully Leafed Time’. If you travel around the Bras d’or Lake Biosphere in early June, you will notice that there is a substantial difference in the degree of maturity of leaves among different trees, and in the same tree species in different places. So, what causes the leaves to mature and what effect does the environment have on the timing? Even though the leaves are unfolding in the spring, the whole process actually started last autumn. As the days get shorter and the temperature drops in the autumn, trees start to prepare themselves for winter by becoming dormant. Scientists have determined that the dominant cues are decreasing temperature and day length. After a certain period of shorter days and cooler temperatures, hard buds form over the developing leaves to protect them from the winter weather. But what controls the time that that buds open in the spring, exposing the delicate new leaf tissue? If they open too early, the young leaves will freeze solid. Most species of trees are adapted to the levels of winter cold followed by spring warmth common in the environment where they are native. In the Bras d’or Lake Biosphere, certain trees tend to leaf out early (birches, alders, many poplars) and others late (oaks and ashes). So why is there a difference? It would make sense to push the new leaves out as early as possible because they carry out photosynthesis and provide sugars for the tree. A tree that leafs out in mid-may has four additional weeks to photosynthesize than a tree that leafs out in mid-june. However, the early-leafing tree faces the danger of a late frost that will kill its leaves and damage its vessels, the main tissues that conduct water from the roots to the rest of the tree. So, the tree faces a trade-off between early food production and potential damage. The type of vessel structure is a good clue to explain why some trees leaf out early and some later. Early leafing species tend to have smaller vessels that are less prone to frost damage than the larger vessels of other species that leaf out later in the spring. Also important is the evolutionary history of a plant group. Just because a tree is native to the Biosphere now does not mean that it evolved here. If it originated in a warmer climate (for example, red oak) , it may not have fully adapted mechanisms for dealing with extreme cold and therefore may have different factors regulating leaf-out than a plant group originating in a colder climate (for example, white birch).
So, as I was explaining the influence of temperature on leaf-out times, I’ll bet that you were wondering about the impacts of global warming on our spring forest-scape. The rule of thumb has been that spring leaves come out five days earlier in the year for every degree of warming. Monitored native trees in New Hampshire (beech, sugar maple, and yellow birch) are leafing out 5 to 10 days earlier than they were 50 years ago. That may provide longer periods of photosyntesis for the trees but it has many repercussions for the rest of the ecosystem. Many birds and insects are cued into the timing of leaf-out in particular habitats. Newly emerged leaves provide critical habitat for some insects that have gone through a long winter with little available food, and these insects may be a critical food source for bird species completing an energy-demanding migration north. Timing is everything! Everything in the natural world is connected and in Mi’kmaq this principle is beautifully put as ‘MSIT No’kmaq’, or ‘all my relations’.
Dr. Annamarie Hatcher is an adjunct professor at Unama’ki College, Cape Breton University and a board member of the Bras d’or Lake Biosphere Reserve Association. For more information about the Bras d’or Lake Biosphere Reserve, check out blbra.ca.