Thursday, June 13, 2013

Ancient Plants, Ancient Ecologies

It's the 20th anniversary of Jurassic Park. Pretty amazing how our imaginations got caught up in the images, real or not, from that movie. I have a survivor of the Jurassic growing in my garden, a lovely, lively Gingko biloba, the ancient maidenhair tree. Gingko is an antique gymnosperm, older than the other gymnosperms like pines, firs, yews, and cypresses. It has no extant relatives, a deciduous gymnosperm without cones, a real oddity.

The smelly fruits of Gingko biloba are edible

The gingko in my garden taught me something about ancient forests and their ecology. We think of the gingko as a large, upright, stately tree, a significant addition to the landscape. But the other day I saw it in a different light. Last fall I trimmed the main stem of my gingko, which was growing toward the second floor of my house and threatening to shade out the rest of the garden. As much as I love the form of this tree I love the light more, so I had to sacrifice height for light. I stuck some of the branches I trimmed into the cold damp soil of autumn, and this spring they were sprouting new leaves. I learned that you could clone a gingko, something I would never have expected. The main tree itself also thrived. In return for its severe trim, the gingko grew lushly this spring, several long branches competing for dominance, the whole thing like a large graceful shrub, long, cascading limbs heavy with leaves, nearly touching the ground.

Graceful low-slung branches

After a few days of heavy rain the tall new branches had joined the others in a downward sweep, threatening again to shade out the garden. Knowing how well this tree responds to a trim I went ahead and pruned. Four foot, five foot branches fell to the clipper. What amazed me was the texture of those new, fast growing branches. Instead of being super woody they were super soft and supple, something you might predict for new fast-growing tissue but still, in a gingko

It got me thinking about ancient forest ecology. Then suddenly this morning it occurred to me. Maybe gingkos weren't the dominant tree in their ancient forests. They were vying for space among giant ferns, selaginellas, and horsetails that were hundreds of feet tall. Early in its evolutionary history gingko was a newcomer on the block. A very shady block. A block with lots of established competitors. It had to think on its feet and act energetically, with stealth and a little bit of cunning.

Ancient horsetails were huge

My guess is that in its early history, gingko was an opportunistic plant that could take advantage of scarce forest light. And it was a fast grower that could use gaps in the forest, temporarily flooded with light, as a way to establish and grow. We have lots of contemporary forest plants that behave this way. And some ancients too. Consider the tree ferns of New Zealand, fast-growing "weeds" that grow when temporary gaps in the forest canopy let in light. But unlike the tree fern, gingko can sprout new branches all along the old branches. There is meristem tissue wherever the leaves emerge that can mobilize vertical growth and become a new branch. 

Imagine light through a temporary opening in the forest canopy

How does this connect to ancient forest ecology? So imagine the dark, dense, damp forests of old. Imagine a smallish gingko growing among towering giants. It survives but does not thrive in the shade. Now imagine one of the giants dying, being struck by lightning, or being felled by the wind. A little light comes through the canopy. Gingko responds by fast vertical growth. The faster it grows the weaker (softer actually) its branches, so with heavy rainfall or perhaps when something falls on them, the branches cascade toward the soil, eventually coming into contact with it. Still in touch with the mother plant, the branches get nutrients, vital perhaps as they find themselves shaded out by nearby giants. Eventually they establish themselves in the soil and by lateral growth and cloning the gingko claims an ever-larger part of the forest. More light becomes available and the gingko responds by renewed vertical growth. The process continues and gingko survives and thrives. 

I am always amazed by how much I can learn in the confines of my small garden. Gingko was discovered by western science only quite recently. Before that all we had was fossil evidence of this amazing tree. And it's no coincidence that I'm writing about a "living fossil" today, my 60th birthday!

Gingko in its glory

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