Blow Downs are Natural Disturbances--Part I

Disturbance is a natural force, and blow downs are part of nature. This is my view and I stand by it.

Disturbances have value and importance in the Adirondack forest. They may be as innocuous as a bear footprint on a decaying log or a winter frost heave on exposed soil. Disturbances may also be as grand as a hundred year flood on a river plain, a fire that burns of days, or the blow down of July 15, 1995. Nature is full of disturbance. In the short term, the disturbance may radically change the ecosystem. For the frost heave, the soil becomes freshly aerated changing the composition of soil bacteria, thin hyphal filaments of fungi shredded, winter hibernacula of insects and amphibians destroyed. The Adirondack blow down opens the forest soil to more sunlight, air flow, and dryer air that affect the birds, amphibians, ferns, insects and microbes. In the long term, nature recovers and the disturbance no matter how grand or how small disappears to the forest.

There are three elements of disturbance that are important. First, the type of disturbance sets in place a series of immediate imprints and changes. For example, a fire removes ground vegetation, recycles nutrients, and reduces competition for survivors. A flood on the other hand, inundates the soil and kills seeds, animals, and microbes than can’t survive anoxic conditions. The flood may also deliver a different slate of organisms adapted to periodic soil saturation.

The frequency and magnitude of the disturbance are important features that drive the evolution of adaptive traits in organisms. Seasonal floods are frequent and predictable disturbances along some rivers. Thus, if you were to visit a river basin in the lower Mississippi valley or southeastern Atlantic coast in December, you might find a flat dry bed of crusty soil in a bald cypress water tupelo forest. Return to the exact same spot the following April and you might be ten feet underwater. Seasonal flooding selects for tree species whose seed bank remains alive, germinate, growth and reproduction under the influence of disturbance. In this case it is bald cypress and water tupelo that survive and selection works against sugar maple and American beech.

The magnitude or size of the disturbance is a final feature of the disturbance that determines the community response. Fires can be so intense, hot and devastating that nothing survives. Fires may also appear as minor, albeit important, occurrences that burn small areas of ground cover. It is important to note that magnitude is not determined by the physical size of the affected area, but more the scale of the impact within an area. In a similar way, meteorologists use the Fujita Scale to measure the magnitude of a tornado in a give area and not the distance traveled by the storm.

Why are these features important? The biological response and recovery time to pre-disturbance conditions is determined the type, frequency, and magnitude of the disturbance. Biological organisms and communities adapt to disturbances of predictable nature, rate of occurrence, and size of impact. Many pine forests in the southeastern and western United States are adapted to periodic (on the level of decades), low magnitude fires. The ponderosa pine retains its seeds in closed cones for many years until heat from a fire break the seal on the hard cones and allows the seeds to fall on fresh mineral rich soil days after the area cools. Long leaf pine saplings grow slowly at first. The young growing tissues are insulated from low magnitude fires by a dense covering of green needles. Pitch pine and California Redwoods grow a thick protective bark and self-prune lower branches to protect the canopy from fires that frequently race through their communities. Just like bottomland forests of the lower Mississippi valley have adapted to seasonal floods, many pine lands are adapted for periodic, low intensity, fires. The community does not radically change follow these forms of disturbance.

To humans the blow down of July must seem like a massive, unpredictable, once-in-a-lifetime disturbance. Certainly we don’t find organisms adapted to the intense winds experience from 4 to 6 A.M. that day. Trees fell and roots were turned upright. Animal nest and dens were destroyed. The interior of the forest was opened to strongest sunlight, wind and dryness that the soil has experienced in several hundred years. Our concern, sadness, and pain over such a “disaster” elicit emotional and perhaps irrational responses. Do we promptly harvest the wood and plant seedlings to start the process of recovery? Do we harvest the wood to reduce the threat of fire on human settlements? We want to help, but should we? And, would our actions help or hinder the natural process of recovery?

My next installments will discuss the pathway to recovery of the Adirondack forests following the 1995 blow down. I will also discuss if this disturbance was truly one of disastrous magnitude and as unpredictable as a short-lived human might believe.