Used by permission. Written by Faith Appelquist in December newsletter of Tree Quality
I find it fascinating at this time of year to try to pinpoint the day that I can say that the leaves are out and spring has finally arrived. Usually it's sometime in the second week of May, though it seems to have been inching forward over the past couple of decades. But even when I can declare that it's "spring," not every tree is clothed in green.
Certain groups of plants such as maples, birches, willows, and apples tend to leaf out early, while other groups such as oak, coffeetree, honeylocust, and catalpa tend to leaf out late. Why? The answer has to do with genetics and evolution, climate and weather.
A lot of a tree's leaf-out strategy has to do with how the tree's water-carrying vessels are arranged. Oaks, elms, and ashes are so-called "ring porous" species. Their water-conducting vessels are bigger and can carry more water, but are easily damaged by freezing temperatures. Ring-porous trees have to grow a new annual ring of wood before they can produce leaves, so they usually leaf out late.
Maple, birch and willows are "diffuse porous" species. Their water-carrying vessels are narrower and scattered throughout the growth wood and are not as susceptible to cold damage. It might seem logical that a tree that leafs out earlier would have an edge in the race for sunlight. But there is a safety versus efficiency tradeoff here. Those that leaf out early might get a head start, but they're also running a risk of a late spring frost that could kill their leaves and damage vessel elements, the chief water-conducting tissue.
This late leafing habit could also be because late-leafing species evolved in southerly or tropical areas, whereas the early-leafing species evolved in temperate or colder climates. In other words, if a plant originated in a warmer climate, it may not have fully adapted mechanisms for dealing with extreme cold and therefore may have different factors regulating leaf out than a plant originating in a colder climate.
As the climate warms, some plants are leafing out a full week earlier in the spring, perhaps extending the growing season and their competitive advantage. Just what this means for our future forests is uncertain. For example, in eastern North America, maple and birch trees, which leaf out early, may be replaced gradually by more heat-tolerant oak trees, which tend to leaf out later in the spring. Insects are involved too. If certain kinds of insects feed only on the young leaves of a particular plant, those insect species may decline if they emerge too early or late in the spring relative to their food supply.
In the end, a staggered leaf-out schedule might be a good thing. The variation may help in creating a healthy, resilient forest. Of course it might be a benefit to the forest as a whole that not everybody does everything at the same time. Climate change has brought a renewed emphasis on research into why trees do what they do when they do it. And there's still a lot to be learned.
Certain groups of plants such as maples, birches, willows, and apples tend to leaf out early, while other groups such as oak, coffeetree, honeylocust, and catalpa tend to leaf out late. Why? The answer has to do with genetics and evolution, climate and weather.
A lot of a tree's leaf-out strategy has to do with how the tree's water-carrying vessels are arranged. Oaks, elms, and ashes are so-called "ring porous" species. Their water-conducting vessels are bigger and can carry more water, but are easily damaged by freezing temperatures. Ring-porous trees have to grow a new annual ring of wood before they can produce leaves, so they usually leaf out late.
Maple, birch and willows are "diffuse porous" species. Their water-carrying vessels are narrower and scattered throughout the growth wood and are not as susceptible to cold damage. It might seem logical that a tree that leafs out earlier would have an edge in the race for sunlight. But there is a safety versus efficiency tradeoff here. Those that leaf out early might get a head start, but they're also running a risk of a late spring frost that could kill their leaves and damage vessel elements, the chief water-conducting tissue.
This late leafing habit could also be because late-leafing species evolved in southerly or tropical areas, whereas the early-leafing species evolved in temperate or colder climates. In other words, if a plant originated in a warmer climate, it may not have fully adapted mechanisms for dealing with extreme cold and therefore may have different factors regulating leaf out than a plant originating in a colder climate.
As the climate warms, some plants are leafing out a full week earlier in the spring, perhaps extending the growing season and their competitive advantage. Just what this means for our future forests is uncertain. For example, in eastern North America, maple and birch trees, which leaf out early, may be replaced gradually by more heat-tolerant oak trees, which tend to leaf out later in the spring. Insects are involved too. If certain kinds of insects feed only on the young leaves of a particular plant, those insect species may decline if they emerge too early or late in the spring relative to their food supply.
In the end, a staggered leaf-out schedule might be a good thing. The variation may help in creating a healthy, resilient forest. Of course it might be a benefit to the forest as a whole that not everybody does everything at the same time. Climate change has brought a renewed emphasis on research into why trees do what they do when they do it. And there's still a lot to be learned.
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