Nurse Log

The process of temperate rainforest succession from

the point of view of a Sitka Spruce

BY SEAN GREALISH ART BY BISMIKADO

Over 180 years ago, a typical winter storm battered the Olympic Peninsula, shaking the temperate rainforest ecosystem established there. It is certainly strange to have a rainforest this far north, but as clouds come off the Pacific Ocean they hit the Olympic Mountain range and drop 14 feet of rain on the coast there every year. As a McCormick scholar in Professor Woods’ Ecology lab last summer, I had the pleasure of getting up close and personal with this fascinating ecosystem and its succession processes. The following is the approximate story of a single nurse log from the night it fell around 180 years ago until I sampled it over last summer.

To experience this story, please enjoy briefly “stepping into the bark” of a Sitka Spruce tree just as it begins to fall:

In the heart of the gale I am thrown around by gusts of wind and with a splintering noise my trunk finally gives out. After growing for more than 200 years and standing at many hundreds of feet tall, all of my majesty tumbles to the ground with a massive crash, destroying boughs of neighboring trees and crushing the understory beneath me. Then, everything is quiet. A bird occasionally flutters past me and elk cross the raging Hoh River nearby, but no heed is paid to me, the giant on the ground. For years I lie silently on the forest floor while the moss communities that grew on my trunk while I was upright continue to thrive in their new horizontal habitat. Only a few centimeters tall and sparsely distributed, they stand in stark contrast to the dense and thick forest floor mosses that surround me on all sides and suffocate any new tree seedlings attempting to germinate upon them.

It has now been 40 years since I fell, and

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Sitka Spruce and Western Hemlock seedlings are starting to find the safe haven of my trunk high above the inhospitable ground moss. To start, only a few manage to grow upon my trunk, but after 90 years on the ground there are up to 40 germinates (<5cm tall) and seedlings (>5cm tall) growing on a single meter of my trunk! This is where the name “nurse log” originates, but to say that I am consciously caring for these little trees like children does seem a bit anthropomorphic. At this point I begin to decay; most of my bark has fallen off or rotted away, and if you were to kick me (please don’t!) debris would fall off me. Nevertheless I stand strong and would support your weight if you walked on me. However, the ground mosses are turning the tide against me, and 120 years after I fell in that cold winter storm they now cover 50% of my surface. After 140 years the seedlings are less numerous, only those that had previously found a foothold remaining on my trunk. As the ground moss continues to crowd out my original tree mosses, I slowly become just as inhospitable to new seedlings as the forest floor that surrounds me.

Finally, I reach the final phase of my journey on the

ground. It has taken 180 years for me to completely decay, but I now exist as just a hump in the ground, completely covered in ground mosses like a paper mache sculpture over a disintegrating mold. In reality there is hardly any of me left; if you stepped on me you would find a 20-centimeter-deep moss layer with an empty hollow underneath where I used to be. The only things growing successfully on me are the trees that established themselves many decades ago, who no longer rely on my support for their growth. They too will grow up tall and strong for over 200 years, and they too will eventually fall to the ground. Just as I have, they will spend the next 200 years supporting a new generation of towering giants in this unique temperate rainforest ecosystem as the cycle continues slowly and silently in the falling rain.

The story above is rooted in the field research that I (the author) conducted on the Olympic Peninsula last summer. By taking cores of nurse logs and aligning their tree rings with those of standing trees, I was able to establish when the nurse log had fallen. Combining these measurements with surveys of the nurse log surface community allowed me to trace how the community on the surface of a nurse log changes as the log decays into the forest floor. While the story above has been slightly embellished, all numbers or percentages are taken directly from the data collected to yield a fascinating story of temperate rainforest succession.

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