WHAT makes deciduous trees and other perennial plants start to grow in the spring?
I used to think that I knew the answer but now I’m beginning to have doubts.
It’s fairly well known that the main factor that triggers grass into growth is soil temperature. In the spring rising soil temperature lags behind rising air temperature, often by a matter of weeks. But when the top few centimetres of soil reach 6 degrees the grass starts to grow. This can even happen in January if we get a prolonged spell of mild weather. But if the weather gets cold again the grass will stop growing.
In the autumn the soil stays warmer than the air, which is why you may still be mowing the lawn in November. The other, less important, factor controlling the behaviour of grass is soil moisture. It will stop growing in a drought or if the soil gets completely water-logged.
But I always believed that deciduous trees were different. The textbook wisdom is that day-length is the factor that controls spring growth and that temperature or soil moisture have little or no effect on when they break dormancy.
But early this year I was involved in setting up an experiment under the Discover Primary Science scheme at the Lullymore Heritage Park. We inserted tubes into several different species of tree so that we could, among other things, record the date on which the sap started to rise in the spring. We set everything up in January so as to be in good time and the experiment was designed to run for several years so that we could find out if there was any variation from year to year. We installed instruments to measure as many factors as we could think of, including day-length, light intensity, soil temperature and soil moisture.
We were expecting the birch trees to be the first ones to yield results and the suggestion was that this would happen in late February or early March. Obviously the sap starts to rise some time before the first leaves appear and its movement from the roots, through the cambium layer under the bark and up into the branches is the first stage in the cycle.
Then we waited and waited. The whole of March passed and nothing happened. We began to think we’d made some mistake in the way we’d set up the experiment and that we’d missed the event we were trying to record.
So I phoned a Welshman I know who has a farm in the hills above Cardiff. He’s been tapping the sap from his birch trees for many years now in order to make wine out of it.
“The same thing is happening over here,” he said. “I got a very small amount of sap in early March, a fraction of what I usually get, and then the flow stopped. We’ve had a very cold, dry month over here and I reckon that’s the reason. I’m expecting it to start again soon.”
This was reassuring but also surprising. The weather in Kildare had been similar to the weather in south Wales. But the conventional wisdom that day-length is the controlling factor seemed to be wrong. It appeared that either temperature or soil moisture, or both, were delaying the date on which the birches broke dormancy.
Of course that’s a very preliminary assumption and only applies to one of the four species of tree we are monitoring. We’re starting to get some birch sap now and we think it’s at least four weeks late. In some ways this is quite exciting. An experiment that yields unexpected results is a valuable experiment. The next month or so is going to be very interesting.
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