Simulating Impacts of Ice Storms on Forest Ecosystems

Ice storms are challenging to study because it's difficult to predict when and where they will occur. This protocol outlines a novel method for simulating natural ice storms. The experimental approach described offers the advantage of control over the timing and amount of ice applied making it possible to create ice storms of different frequency and intensity.

We tested this method in a forest ecosystem, but it can also be applied in different ways, such as to evaluate the impacts of ice loads on utility lines and other infrastructure. Select a location in an area where there is access to a water source during winter, making sure that the water supply is adequate for ice application based on the pump rate and other factors such as the diameter of the hose, length of the hose, nozzle used and water pressure. Set up a supply pump at the water source and connect a suction hose.

Then connect a strainer to the end of the suction hose to keep debris out of the lines. Break through any surface ice and fully submerge the strainer. The minimum depth of the water supply should be about 20 centimeters.

Place a booster pump in the bed of the utility task vehicle to improve water pressure. In some cases a booster pump may not be necessary, especially for low stature vegetation. Run a firefighting hose from the supply pump to the booster pump.

Use a fire fighting monitor mounted on the back of a utility task vehicle to enable safe manual control over the high pressure hose. The monitor can also be freestanding. Take care to avoid interruptions to the flow of the water, such as kinks in the hose, water draw down at the supply source or running out of gasoline for the pumps.

Create ice by spraying water vertically through gaps in the canopy. Make sure the water extends above the height of the canopy so that it deposits vertically and freezes on contact with sub-freezing surfaces. Avoid stripping branches and bark from trees while spraying.

Evenly distribute spray over the forest canopy by slowly driving the utility task vehicle back and forth along the edge of the application area. If freestanding monitors are used move these manually to ensure that the coverage is even. Make ground based caliper measurements of radial ice thickness on lower level branches or twigs near the edge of the application area to monitor ice accretion and determine when the target thickness has been attained.

Obtain more accurate estimates of ice accretion with passive ice collectors after the application. The passive ice collectors are constructed from 30 by 2.54 centimeter dowels joined with a six-way steel connector. To measure ice accretion use an arborist throw weight to string a parachute cord over sturdy branches that can withstand the ice load.

Attach the passive ice collectors to the cord and raise them up into the canopy. Once the application is completed lower the collectors to the ground, being careful not to lose any ice from the collector. Make vertical and horizontal measurements of ice thickness with calipers at multiple locations on the collector before and immediately after ice application.

To determine ice thickness with the water volume method use a reciprocating saw to cut each dowel. After bringing the dowels to a heated building and melting off the ice measure the volume of the melt water with a graduated cylinder. An ice storm simulation was performed in a 70 to 100 year old forest stand at the Hubbard Brook Experimental Forest in Central New Hampshire.

Ice accretion was measured on passive ice collectors using both a caliper and water volume methods. Average ice accretion on individual collectors indicated a strong positive relationship between caliper and water volume measurement methods. Measurements using the water volume method exceeded measurements with the caliper method when there was more than about eight millimeters of ice.

There was a significant positive relationship between spray time and ice accretion measured with the water volume method and the caliper method. The average rate of ice accretion ranged from 1.4 to 4.2 millimeters per hour across plots. There was a marginally significant inverse relationship between air temperature and ice accretion measured with the water volume method and no significant relationship with the caliper method.

Canopy cover data showed no significant differences in pre-treatment surveys. Whereas, post-treatment surveys indicate significant decreases in canopy cover in the mid-ice treatment, the mid-ice treatment that got sprayed twice and the high-ice treatment relative to the control. The effects of the simulated ice storms on surface soil temperatures was evaluated during sampling.

Soils in the treated plots where significantly warmer than the control plots at both depths where all three levels evaluated. With this method it's important that the spray reaches above the tallest trees and that the water is distributed evenly over the forest canopy. The ice storm simulation technique has made it possible to identify critical thresholds in forest ecosystems which is important for predicting and preparing for ice storm impacts.