Posts Tagged ‘Rachel’

Searching for Squirrels, Finding the Night

Written by Rachel Garwin

A week ago, I joined my friend Teage (a Field Naturalist alum) and a group of his UVM students on an “owl prowl,” Teage’s own euphonic term for a night hike.  We gathered at the edge of Centennial Woods, where gauzy tufts of white pines and bare hardwood twigs strained the clear moonlight.  A wall of darkness met our eye-level gaze, while the raspy sounds of drying beech leaves in the understory added to a sense of disquiet.  The primary goal for the evening was to listen for flying squirrels and call them in.  Before we entered the darkness, however, we observed a requisite pre-night-hike ritual.

Since leaving a well-lit environment too soon for the darkened woods might lead to undesirable confrontations with tree trunks and branches, Teage informed us about night vision while our eyes adjusted.  Human eyes require 20-30 minutes to become accustomed to low light conditions.  Coincidentally, it finally gets dark about 30 minutes after the sun first sets.  The students oohed appreciatively at the revealed secret of the universe.

I considered Teage’s implication, reflecting on whether twilight length was consistent enough to provide a uniform selective pressure.  The period between sunset and full dark (termed “Civil Twilight”) varies with latitude and season, as it reflects the time the sun takes to drop 6° below the horizon.  On the same night, civil twilight in Burlington, VT, lasted 9 minutes longer than in Bogota, Colombia, a city near the equator.  Atmospheric conditions and local weather can also affect our perception of available light from the setting sun, which increases variability.  Pole-to-pole variation aside, the length of civil twilight appears consistent enough at low and middle latitudes to suggest the plausibility of the relationship (though it does not prove it).  What mechanisms would select for correctly timed physiological processes?  Perhaps some hairy, fanged predator was involved?

Suddenly, I heard Teage ask, “Rachel, do you have anything to add before we head into the woods?”

Bits and pieces of the night hikes I used to lead rushed to mind.  Out of the torrent, what would be most relevant to this group of students?  Our pupils dilate to accept more light, just like a camera aperture changes size.  Our eyes comprise not only lenses (e.g., the cornea) that focus light, but also a receptor structure (the retina), which translates received light into neural signals our brains can understand.  The retina, in turn, is made of two types of cells: rods and cones.  Rods are far more abundant (about 17 for every cone cell); however, the cones are concentrated in the center of the retina.  Responsible for receiving color and fine resolution, cone cells are better suited for working in high-light environments.  Rod cells cannot understand color, but they register exceptionally more light than cone cells.  The operational ranges of the two cells overlap to some degree; in true twilight, both cells help parse the dim picture before us.

Instead of a long-winded physiology lesson, I settled on a practical and safety-oriented piece of advice.  “If you’re having trouble finding the trail in the dark, try using your peripheral vision.  Your rod cells—the receptor cells that are really good at picking up light—are arranged on the periphery of your retina, so that part of your eye sees better in low light conditions.”  With that, we were off.

As the trail contoured the side of a hill, I followed it with my feet as much as with my eyes.  Hard packed dirt spotted with only a few fallen leaves firmly resisted my feet, whereas my wandering steps sunk into noisy leaf litter.  After a few quiet minutes, we angled from the trail and picked our way down the gentle slope.  Looking out of the bottom of my eyes—as if I wore tiny slivers of half-moon spectacles—proved the best technique for avoiding the tangle of hardy ferns and woody shrubs.  Teage motioned for us to stop; this would be our first attempt to call the flying squirrels.

I listened intently.  Wind gusted through the upper boughs of white pines and red maples.  Still-hanging, papery beech leaves rubbed together, sending bursts of unwarranted excitement running through my mind.  Sirens howled close-by.  One of Teage’s students fired up the iPod, and high-pitched “chip chip chip” alarm calls radiated into the night to serve as bait.  I cupped my hands around my ears to exclude the droning car engines circling the outskirts of the woods.  Still nothing.  Perhaps the squirrels’ huge eyes, dominated by rod cells, picked us out as we muddled through their habitat.

I relaxed my eyes and marveled at the increased input from my peripheral vision.  Intermediate wood ferns stood distinct from the ground; before, they had dissolved into the dimness.  It seemed we had been away from bright light long enough for rhodopsin, a photopigment in the rod cells, to build up.  Rhodopsin and other photopigments in the cone cells help increase light sensitivity within the receptor cells, though at different rates and degrees. Cone cells, which never develop the light sensitivity attained by rod cells, take only 5-7 minutes.  Rod cells, however, may need over 30-45 minutes to achieve full light sensitivity.  In the presence of too much light, these photosensitive compounds break down; enough time in dark conditions is thus needed for photopigments to build up to a functional level.

The flying squirrels proved reticent, so we walked through the underbrush back to the trail.  The undergrads hesitated less between steps, and they seemed to run into fewer obstacles.  After walking a circuit across Centennial Brook and back along the lower slope on the other side, we paused to make their eyes’ night adaption more explicit.  Crouched beneath a closed hemlock canopy, we covered our left eyes and stared at the flame of Teage’s lighter with our right ones.  I smiled, startled by the pervasiveness of the “Pirate Patch” myth.  As lore would have it, pirates did not wear eye patches to cover gaping eye sockets.  Instead, they kept one eye in darkness to allow them to see below and above deck without needing time for rhodopsin to develop.  While no historical evidence supports this story, the folks at MythBusters put their weight behind its plausibility and likely had a role in its propagation.

Teage flicked off the lighter, and we switched our “eye patch” to the light-blinded eye.  Most students commented they could see more precisely with their night-adapted eye than with the light-blinded one.  Removing my hand, I winked back and forth at the hemlock boughs above.  Sure enough, my left eye discerned individual twigs against the dark sky; my right eye saw only fuzzy dimness.  While not proof that Blackbeard covered one eye so he could rush up to a darkened deck from a lamp-lit cabin, our experience supported the possibility.

We emerged from beneath the dense hemlock canopy onto a grassy hillside, where moon-cast shadows danced at our sides.  No longer relying on peripheral vision, the students carelessly walked down the trail towards home.  I smiled.  An hour ago, they had hung together timidly in similar light levels, still uncomfortable with moderate darkness.  Now they practically ran.  While the flying squirrels had remained elusive, the students found something more powerful: the ability to stride confidently through the night.

A Bypassed Giant

by Rachel Garwin

What’s the last amazing thing you overlooked?  I discovered mine last Wednesday in Centennial Woods, a 65-acre natural area near the University of Vermont campus.  A friend and expert naturalist was

white oak sketch

White oak sketch.

sharing his local knowledge with a group of undergrads, and I had tagged along.  The familiar path turned to the left in front of me, but I looked at the woods beyond it as if for the first time.  Gore-Tex dripping with unseasonably chilly rain, I stared unbelieving at the biggest white oak I’ve seen in Vermont.  A white oak I’d never noticed before, despite passing it scores of times in the last year.

Its whitish, ridged bark transfixed me, and I longed for the warmer conditions favored by this southern species.  I heard my friend estimate its age around 300 years, but how could that possibly be?  Too soon we walked away; commitments required our presence back on campus.

For the rest of the week, the white oak filled my thoughts.  My wonder pulled me back to the woods on the crisp, clear Saturday that followed.  I marveled at the oak itself, but also looked at the forest around me with new eyes.  Had I been asked to describe the patch a week before, I would have shared my general impressions of Centennial Woods: a weedy jumble of pioneer trees—red maples, paper birch, white pine—and invasive shrubs that grew up after field abandonment at least sixty years ago.  Now I wasn’t so sure.

White oak acorn caps, © 2011 Rachel Garwin

White oak acorn caps.

Instead of that theoretical assemblage, hardwoods covered the hillside above me. Red maples’ old, platy bark and smooth, young trunks textured the forest, spotted here and there with gargantuan, grooved-barked white pines.  The occasional pines shot straight into the canopy, towering above the broadleaves glinting yellow-green in the sun.  Instead of poison ivy—a scourge on the rest of Centennial Woods—intermediate wood fern and sensitive fern carpeted the duff-covered ground.  Sun flecks danced across fern and leaf alike; their shimmers added another lively layer atop the chatter of insects, blue jays, and red squirrels.

flaky oak bark, © 2011 Rachel Garwin

Up close, the rough, ridged bark is surprisingly flaky.

Cool air washed over me.  I thought of the jacket hanging on the newel post at home, but resisted the impulse to run after it.  Instead, I hopped off the fallen pine and wrapped my arms around the oak, fully two wingspans around.  Rough, flaky bark pressed into my cheek as I looked up along the trunk.

The spreading crown of layered branches dominates the sky.  The branching pattern differs between the lower and upper trunk: until the tree rises above the neighboring red maples and rotting white pines, branches grow only on the southeast side of the oak.  Once free of competing limbs and leaves from other trees, however, the branches radiate in 360 degrees.  Since branches in high light environments have more access to abundant energy, they grow more quickly—and are more likely to survive—than branches in shady environments.  Once the oak’s top emerges from beneath the canopy, however, its branches have equal energy opportunities and reach in any direction.  It follows, then, that this tree grew up on the edge of shady woodland to the west and north, much like the present patch of woods is situated today.

I walked around the massive trunk, careful of roots and uneven ground.  The barbed wire took me by surprise.  Extending from the very center of the tree, the rusted, twisted strands extended east-northeast.  I imagined the wire extending into a fence running further northeast and southwest, through the center of the oak and perpendicular to the growing direction of the lowermost limbs.

barbed wire in white oak, © 2011 Rachel Garwin

Rusty barbed wire protruding from the base of the white oak. The design looked similar to the common Glidden “Winner” variety patented in 1874 and widely available across America.

These waist-thick protrusions of wood would have reached towards the sun-rich pasture across the barbed fence, growing massive.  Today, however, the lowest branches are dead.  When they fall, the trunk will grow around the branch scar, resulting in a large burl.  Lower branch scars exist, but none as striking as what the future holds.  Does this lack of gigantic burls suggest that yesterday’s lower limbs were smaller?  Did the oak originally grow in less sunny conditions on that side, which reduced the amount of energy available to the tree to create huge branches?  Or had the tree simply been younger and smaller, yet unable to produce such girth?  Regardless, the branching pattern suggested that the patchy area to east and south—now growing with young, even-aged, forked white pines—was once open.

My friend claimed the oak was 300 years old, and I’d believe him based solely on the tree’s size.  The oak’s stately bearing is convincing as well.  The spreading canopy still bears a lush complement of waxy, dark green leaves; 300 can be considered middle-aged when oaks have been known to live as long as six centuries.  A relatively fresh acorn cap lay at the ground, however, suggesting the tree is still of reproductive age, or between fifty and 200 years old (though sometimes older).  Without coring the oak, we will never know for certain, and the mystery will continue.   How had I missed it so many times in the past?

fallen oak leaf, © 2011 Rachel Garwin

A freshly fallen white oak leaf lying among bits of old pine needles, maple leaves, and bark chunks on the forest floor.