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When Water Meets Wilderness

Newts from the Field is a seasonal installment written by Wildlands Ecology Director Shelby Perry, bringing you the wonders of nature from out in the wilderness.

Atlantic hurricane season lasts from June 1st through November 30th every year, a fact that never crossed my mind growing up in Vermont, but that seems to be a growing part of my awareness now.  Already this year a conga line of named storms has sashayed through the Northeast, leaving varying degrees of chaos in their wakes.  Elsa, Fred, Henri, and Ida each became household names for a brief window this season, and hurricane Sam is stewing in the mid-Atlantic as I write.  Pretty much the whole region is on a first name basis with Irene and Sandy, who each left an indelible mark on our area and in our collective memory in 2011 and 2012 respectively.

A stream in a wild forest connected to its natural floodplain.

It is common knowledge that warmer seas and more humid air leads to stronger storms, and reports by the Intergovernmental Panel on Climate Change and others indicate that we can expect things to get worse on that front, even if we meet all of the emissions cutting goals of the Paris Climate Accord.  So what will a future of super storms mean for our forests?  What does a climate-resilient forest look like?  The answer, it turns out, has a lot to do with how well a forest handles water.

Large storms dump a tremendous amount of water on the landscapes they pass through, often coupled with high winds.  Anything in excess of normal rainfall amounts is usually categorized by humans as “stormwater.”  The biggest problem with stormwater is that it moves.  It falls to the ground and collects in low spots and valleys, seeks the path of least resistance, and rushes away, usually to the nearest body of water where it changes names to “floodwater.”  Human stormwater controls are usually aimed at collecting that flowing water and sending it somewhere it can’t do damage, like a sewer system or a roadside ditch. Sometimes they work, and sometimes the road washes out, or worse.

At the Vickie Bunnell Preserve in New Hampshire, a debris dam large enough to walk across has created a plunge pool beneath where fish congregate. This land is protected with a Northeast Wilderness Trust forever-wild easement.

Forests, and especially old forests, deal with water differently. Rather than concentrating the water, they spread it out. The lumpy-bumpy floor of a forest, lined with spongy layers of leaves and debris and punctuated with large downed logs sends the water in all kinds of directions, and every obstacle the water hits slows it down. Slower moving water has less energy, and water with less energy does less damage.

The energy of water is what allows floodwaters to carry away big rocks and logs, and sometimes even cars and houses. A natural river system is connected to a floodplain, and when stormwater enters the river it overflows its banks into a wide flat area filled with obstacles (trees, rocks, the debris dropped in previous floods). Almost immediately after leaving its banks the water will use up its energy to spread out, and when that energy is gone it can’t carry big objects anymore, so they get dropped right where they are. This is true for smaller, forested systems too, when a stream is connected to its floodplain (and not cut into a deep channel from erosion or human alteration) it tends to drop what it is carrying quickly, just outside its banks, even in a pretty significant flood.

The mature trees in this New Hampshire forest have caught the debris carried by floodwaters as well as slowing their flow. The woody debris adds to the complexity of the forest floor.

Once it has been dropped in the floodplain, as shown above, that debris from floodwaters serves many roles in natural stormwater control. It further spreads and slows water moving on the landscape. It also catches small organic matter like leaves and wood, which not only catch and hold water moving through like a sponge, but also decay in place, building thick, spongy soils around the stream rich with nutrients. Many trees and plant species are floodplain specialists, perfectly adapted to that particular niche of wet roots, occasional floods, and rich soil.

Trees in the floodplain catch more debris, sometimes catching large logs that span the stream banks, which then in turn catch more smaller debris, plugging up the channel until even water has a hard time getting through. This is called a “debris dam” (pictured above) and adds important complexity to the stream channel. It slows the stream flow above the dam (and causes sand and sediment to drop out) and creates a deep plunge pool below it (the water spilling over the dam adds oxygen and carves out deeper pools loved by fish). Trees grown in the moist and flood-enriched soil beside a stream also help to hold that soil in place, their roots adding structure to the banks, and less erodes away when faster water does show up after a storm.

A frog sits in clear, clean water free of sediment thanks to the healthy, mature forest around it.

On a recent site visit to a property in southern New Hampshire shortly after Ida swept through the region I was reminded of the brilliance of natural stormwater systems. The dirt road leading up to the property had been undermined and washed out in places by swiftly flowing stormwater in its ditches.  The water in those ditches was the color of chocolate milk, it was so laden with sediment. The broad expanse of impervious roadway that drained into had no lumps or bumps, no piles of debris to slow, spread, or absorb the water.

But step off that hard road surface, and into the wild forest beside and the stream ran clear. The banks felt soft and spongy underfoot, so much so that water squeezed out of them as I walked around. Piles of sticks and debris had collected around the trunks of the trees beside the stream, and birds and chipmunks picked through it for treasures washed down by the floodwaters. Frogs plopped into the clear stream as I passed by, their breathing unimpeded by dirt suspended in the water. Having trained as both an engineer and an ecologist, I had to admit that the natural system for dealing with stormwater had far outperformed the human one.

Resilience is built into natural systems, and the longer natural processes have been allowed to play out in a forest, the more resilient it will be. Protecting more wilderness now helps build a more resilient future for everyone.

Photography by Shelby Perry