Gambling in the forest canopy

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Spring in England means every day is longer than the one that precedes it by more than a few minutes. With the lengthening of the days comes the re-emergence of the forest. Earlier this week I went out with our research group to the canopy walkway at Wytham Woods.

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It is a short climb up several ladders to reach the top of the forest.

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The world changes on the journey up to the top. Leaves that look like small green dots from the ground begin to take their proper form, and branches that seemed impossibly far away come within an arm’s reach. The perspective changes from upward to sideways, and one gets a sense of how trees might see themselves and each other. As neighbors in a crowded environment, fighting amidst the sway of the wind and the bright light of the sun.

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Looking out at all these trees, it becomes clear that not every tree is responding to the longer days in the same way. Each individual is following its own particular strategy, taking its own counsel on how best to respond to the environment.

fagus sylvatica

This beech tree (Fagus sylvatica (Fagaceae)) has elongated its buds, but has no leaves deployed yet.

acer pseudoplatanus

in contrast, this sycamore (an unfortunate British common name for Acer psuedoplatanus (Sapindaceae) – the same name elsewhere refers to Platanus spp. (Platanaceae)) has leafed out several days earlier, and has flowers deployed too.

acer campestre

The congeneric species field maple (Acer campestre) is still in bud, and has no leaves or flowers deployed at all. These buds have a beautiful waxy-red color that may assist in protected buds from ultraviolet light damage.

quercus robur

This English oak (Quercus robur (Fagaceae)) tree has begun to leaf out, but there seems to be wide variation among individual trees.

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For example, this other English oak tree still has only buds.

Hyacinthoides non-scripta and Primula vulgaris

Back on the ground, the understorey is also still beginning to develop. Some species look to be near their flowering peak, as for this cream-colored primrose (Primula vulgaris (Primulaceae). Others, like this purple-colored bluebell (Hyacinthoides non-scripta (Asparagaceae)) are just beginning to flower.

Why is there such divergence in the spring strategies of these different plants? Proximately, each species is likely responding to or receiving different cues. Plants have marvelous sensory abilities. Spring leaf-out and flowering is thought to be triggered by a combination of day length (via light color detection) and temperature, with the thresholds for each varying among species. Forests also support a range of different temperature and light microenvironments. The oak tree with leaves may be experiencing very different conditions than the oak tree with buds. Ultimately, however, each species has evolved to follow a different set of rules for beginning spring growth. Grow too early and delicate leaves may freeze; grow too late and those leaves may not gain as much carbon as others deployed sooner. The physiology of each species means there are different tradeoffs each species must consider. These rules work well enough in the evolutionary and statistical average for each species to successfully grow and reproduce. What happens in any given year is a different question.

Looking in the canopy, I saw these trees making gambles. Each of these individuals has chosen to either begin or delay its investment in new leaves and flowers. Some days it is sunny; some days it snows. It is too early to know whether each developmental program will fare well or badly this year. But the air is full of hope for growth and new life.

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2 Comments Add yours

  1. Georgia Ehlers says:

    I never understood the T.S. Eliot quote until I lived in Ohio. With the tour and commentary of your English forest (a real forest!), I instantly returned to:

    “April is the cruelest month, breeding
    lilacs out of the dead land, mixing
    memory and desire, stirring
    dull roots with spring rain.”

    Here in Tucson our kitchen window looks out on the Catalinas. There are two mesquites near the edge of our property that more and more obscure my view of the mountains. The one on the left always bravely leafs out so early that I start to think the other one must have died over the winter. Then one day in late April, the reticent one emerges pale yellowish green, ready to again obscure my view of the mountains. Now, I know it isn’t personal.

    1. bblonder says:

      Yes, there is something magical about the new life that a temperate forest spring season will bring. And I am glad your unruly mesquites seem a bit more sensible now!

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