The leaves are once again green. It is spring.
I am always a little bit surprised when plants begin to get their new leaves each spring. I know it is going to happen, of course, but the change in color always astonishes me somehow. How do trees know when to bud out? Are they smarter than we think?
It turns out they are. The bursting buds are the result of a complex program worked out by trees over thousands of years. It takes only slight variability in the program to disrupt their timing.
Spring green-up is set up the previous fall. Our autumn days get shorter, the temperature begins to drop, thus triggering the trees to go dormant.
The nights get longer quickly in the fall, which in Alaska is more important than the temperature. Near the equator, the decline in daylight may be barely noticeable, so trees there are more sensitive to other cues. Decreasing temperatures and water availability are the main cues in more temperate climes.
Trees make a dormant bud in the fall. This is the little hard bud we see on our birches. On our willows, they are food for ptarmigan in the winter. The hard bud protects the tissue inside from ice storms, minus-40 temperatures and such.
They are set up to grow again in the spring — and smart enough to interpret their winter cues.
The program set up in the fall is all-important. Otherwise, a warm spell in March might cause the trees to leaf out. A cold snap after that would kill the early budding trees. To protect themselves from that disaster, trees have what is called a cold requirement. Once the required number of cold days has passed and there are enough warm days in a row to shake them awake — voila! Leaves.
A cold autumn usually means earlier leaves in the spring. A warm autumn normally means a later leaf-out because the required number of cold days has not been met. The combination of night length and temperature may not have been quite right to shut them down correctly.
A changing climate could mess up this sequences. Trees can adapt, but not quickly. They can’t pick up and move. They must adjust in place.
Climate change that takes place in a short period of time could make things difficult. A warm fall coupled with a very warm winter might leave an Alaska birch tree without enough cold days for them to leaf properly.
The first green leaves will indicate when pollen is released. Birches, the worst allergen producers of our trees, typically begin to release pollen a couple of days before leaves emerge. They continue to shed pollen for another 10 days or so.
Allergy sufferers are extremely sensitive to birch pollen. Pollen from birches is minuscule, too small to see. A single birch catkin can release millions of pollen grains, and there can be as many as 500 grains in a cubic foot of air. The pollen specks have a protective wall that reacts when it contacts moisture, much like the mucus membranes in your nose or linings of your eyelids. The protein molecules from the protective coating leach into your tissues, and your body reacts by triggering the release of histamines. Cold-like symptoms are the result.
Poplar and aspen release their pollen slightly ahead of the birches, though they leaf slightly later. These two closely related trees have pollen that is also mildly allergenic.
However, folks that park their cars under cottonwoods are more irritated by the damage to their paint job than a stuffy nose. When cottonwoods release their fluff, it is too late, they have already pollinated.
Willow and alder release pollen a little later. Both have allergenic pollen, but they are pollinated mostly by insects rather than the wind so have less of an effect on humans.
Pollen may not be great for the allergy sufferer, but it is manageable with the help of some of our more simple wonder-drugs. Conversely, the green leaves of spring are wonderful for all of us and are something we cannot do without.
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