The Day of the Bracken

In a previous post, I described the aftermath of a fire that scorched 2,000 acres in the southern Shawangunk Mountains.  Just two weeks after the blaze, some friends and I ventured into the charred wasteland and discovered young ferns emerging from the blackened soil.  I was fascinated by nature’s response to the fire and eager to return and observe further changes in the environment  — and I was especially curious to see what the ferns would make of their early start.

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Five weeks after the first visit, I ascended the ridgeline on the Shawangunk Ridge Trail where it is co-aligned with the Long Path, just north of Ferguson Road.  Volunteers from the New York-New Jersey Trail Conference had been quick to put up new blazes, and the trail was as easy to follow as it had been before the fire.

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The trail along this section of the Shawangunk Ridge is marked with Long Path, Shawangunk Ridge Trail, and DEC blazes

Midway up the slope, I had not encountered a single fern, although other plants were now growing in the black soil.  Where were the ferns?

While I pondered this question, I paused to admire the views.  Spread out in the valley below was a small airstrip, and behind it lay the Bashakill, southern New York’s largest wetlands.  Twenty-five miles distant, an almost microscopic needle could be seen rising atop the ridge:  this is the memorial tower in New Jersey’s High Point State Park, where the Shawangunk Ridge Trail meets the Appalachian Trail.

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View south from the Shawangunk Ridge across Wurtsboro outskirts and Bashakill

In the foreground, the burnt stems of thousands of scrub oak bushes stood out in black, but the rest of the landscape was green.  It was a warm, sunny day.  A glider spiraled in the distance, and here on the ridge a pleasant breeze was blowing.

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View southwest from the Shawangunk ridgeline

I walked another quarter mile and ascended another hundred feet up the ridge, and suddenly there they were: a sea of ferns flanking the path and stretching off in every direction.  With the scrub oak burned to a crisp, sunlight now reached the ground, and the young ferns had unfurled their fronds to absorb the nourishing light.  This was quite a change from before the fire, when the trail tunneled through dense thickets of scrub oak and blueberry, with ferns and other plants limited to the trail’s edge and other breaks in the brush.

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A sea of ferns beneath the blackened scrub oak

But now the ferns were running rampant, a thicket of wild-looking plants, with broad triangular fronds thrusting aggressively in every direction.  Upon close inspection, each frond was composed of multiple leafs, and each leaf had rows of small, weirdly-lobed leaflets.

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Ferns once ruled the forests.  During the Carbinoferous period (300-350 million years ago), the forests were dominated by giant fern trees and club moss, a fern relative, which today grows 3 or 4 inches on the forest floor, but back then soared 100 feet in the air.

Carboniferous era forest.  Credit:  Mary Parrish, Smithsonian National Museum of Natural History
Carboniferous era forest. Credit: Mary Parrish, Smithsonian National Museum of Natural History

Like other plants, ferns have vascular systems to transport water and nutrients from roots to leaves.  But unlike flowering plants, ferns reproduce through spores, which lack the protective shells of seeds as well as the nourishment contained in the seed which helps the young plant become established.

Thanks to seeds, flowering plants have significantly outgrown the more primitive ferns, displacing them to marginal environments, like swamps or cliff faces or the dim light underneath the forest canopy.  Firms adapted and survived, but they were no longer dominant.

After intensive research, I identified the fern spreading across the burned mountain as the common bracken (Pteridium aquilinum).  It is an aggressive plant, reproducing not only through spores, but also by spreading vegetatively from underground roots (rhizomes) that can tunnel 10 feet deep.  Given the opportunity, it forms dense thickets, and in some places there are enormous colonies which are thousands of years old.  Bracken is also a wily competitor:  it releases chemicals into the soil that impede the growth of other plants, and its dense foliage prevents smaller plants and seedlings from taking root.  In some parts of the world, it has become a problem.  For example, in Mexico, bracken has expanded considerably over the last fifteen years, exploiting human disturbances such as farming and ranching as well as hurricane blow-down.  In Great Britain, bracken is thought to be a greater menace to biodiversity than foreign invasive species.

Scientists who have studied bracken describe it as a “postfire colonizer” which sprouts profusely from surviving rhizomes.  Following fire, its new growth is more vigorous, and the fern becomes more fertile.  In one study of Arizona pine communities affected by logging and fire, bracken fern grew to cover up to 30 percent of the area.  A study in northeastern hardwood stands found that repeated fires could lead to bracken “domination.”  After two successive wintertime prescribed burns in a Florida forest, the bracken was found to have doubled its biomass.

In the Shawangunks, a variety of ferns lurk in the forest shadows (hay-scented, Christmas, interrupted, and New York ferns are common), but I had never seen a colony of bracken expanding across the side of a mountain.  I wondered whether the bracken would keep growing until it eventually engulfed the entire mountain range.  It would be as if the Shawangunks had returned to the Carboniferous era.

But the bracken’s window to dominate is probably limited.  The scrub oak (Quercus ilicifolia) is also known for aggressively responding to fire.  Even if a bush is burned to a crisp, new shoots rapidly sprout from the stout taproot, as I could see happening all around me on the mountain.  Indeed, not only is scrub oak an early colonizer of post-fire environments, it depends on frequent burning to flourish, because it cannot tolerate the shade of taller trees.  This section of the Shawangunk Mountains is unique for the dense scrub oak cover, which indicates that it was likely clear cut or burned repeatedly prior to its acquisition by the Open Space Institute and designation as a state forest preserve in the 1980s.

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Scrub oak resprouting vigorously after fire destroyed the shrub

For now, the ferns are running rampant, exploiting their day in the sun.  No doubt the rhizomes are tunneling deeper into the soil and expanding the geographic reach of the colony.  The bracken will flourish until the light is once again blocked by scrub oak and perhaps taller trees over time.  Back in the shadows, the colony of ferns will bide its time, infinitely patient, waiting for the next fire, the next chance to expand, another shot at domination.

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The bracken is running rampant, for now
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The Day of the Bracken

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