Simulating Impacts of Ice Storms on Forest Ecosystems

Summary

Ice storms are important weather events that are challenging to study because of difficulties in predicting their occurrence. Here, we describe a novel method for simulating ice storms that involves spraying water over a forest canopy during sub-freezing conditions.

Abstract

Ice storms can have profound and lasting effects on the structure and function of forest ecosystems in regions that experience freezing conditions. Current models suggest that the frequency and intensity of ice storms could increase over the coming decades in response to changes in climate, heightening interest in understanding their impacts. Because of the stochastic nature of ice storms and difficulties in predicting when and where they will occur, most past investigations of the ecological effects of ice storms have been based on case studies following major storms. Since intense ice storms are exceedingly rare events it is impractical to study them by waiting for their natural occurrence. Here we present a novel alternative experimental approach, involving the simulation of glaze ice events on forest plots under field conditions. With this method, water is pumped from a stream or lake and sprayed above the forest canopy when air temperatures are below freezing. The water rains down and freezes upon contact with cold surfaces. As the ice accumulates on trees, the boles and branches bend and break; damage that can be quantified through comparisons with untreated reference stands. The experimental approach described is advantageous because it enables control over the timing and amount of ice applied. Creating ice storms of different frequency and intensity makes it possible to identify critical ecological thresholds necessary for predicting and preparing for ice storm impacts.

Introduction

Ice storms are an important natural disturbance that can have both short- and long-term impacts on the environment and society. Intense ice storms are problematic because they damage trees and crops, disrupt utilities, and impair roads and other infrastructure^1,2. The hazardous conditions that ice storms create can cause accidents resulting in injuries and fatalities². Ice storms are costly; financial losses average $313 million per year in the United States (US)³, with some individual storms exceeding $1 billion⁴. In forest ecosystems, ice storms can have negative consequences including reduced growth and tree mortality^5,6,7, increased risk of fire, and proliferation of pests and pathogens^8,9,10. They can also have positive effects on forests, such as enhanced growth of surviving trees⁵ and increased biodiversity¹¹. Improving our ability to predict impacts from ice storms will enable us to better prepare for and respond to these events.

Ice storms occur when a layer of moist air, that is above freezing, overrides a layer of subfreezing air closer to the ground. Rain falling from the warmer layer of air supercools as it passes through the cold layer, forming glaze ice when deposited on sub-freezing surfaces. In the US, this thermal stratification can result from synoptic weather patterns that are characteristic of specific regions^12,13. Freezing rain is most commonly caused by Arctic fronts that move southeastward across the US ahead of strong anticyclones¹³. In some regions, topography contributes to the atmospheric conditions necessary for ice storms through cold air damming, a meteorological phenomenon that occurs when warm air from an incoming storm overrides cold air that becomes entrenched alongside a mountain range^14,15.

In the US, ice storms are most common in the “ice belt” that extends from Maine to western Texas^16,17. Ice storms also occur in a relatively small region of the Pacific Northwest, especially around the Columbia River Basin of Washington and Oregon. Much of the US experiences at least some freezing rain, with the greatest amounts in the Northeast where the most ice prone areas have a median of seven or more freezing rain days (days during which at least one hourly observation of freezing rain occurred) annually¹⁶. Many of these storms are relatively minor, although more intense ice storms do occur, albeit with much longer recurrence intervals. For example, in New England, the range in radial ice thickness is 19 to 32 mm for storms with a 50-year recurrence interval¹⁸. Empirical evidence indicates that ice storms are becoming more frequent at northern latitudes and less frequent to the south^19,20,21. This trend is expected to continue based on computer simulations using future climate change projections^22,23. However, the lack of data and physical understanding make it more difficult to detect and project trends in ice storms than other types of extreme events²⁴.

Since major ice storms are relatively rare, they are challenging to study. It is difficult to predict when and where they will occur, and it is generally impractical to “chase” storms for research purposes. Consequently, most ice storm studies have been unplanned post hoc assessments occurring in the wake of major storms. This research approach is not ideal because of the inability to collect baseline data before a storm. Additionally, it can be difficult to find unaffected areas for comparison with damaged areas when ice storms cover a large geographic extent. Rather than waiting for natural storms to occur, experimental approaches may offer advantages because they enable close control over the timing and intensity of icing events and allow for appropriate reference conditions to clearly evaluate effects.

Experimental approaches also pose challenges, especially in forested ecosystems. The height and width of trees and the canopy makes them difficult to experimentally manipulate, as compared to lower-stature grasslands or shrublands. Additionally, disturbance from ice storms is diffuse, both vertically through the forest canopy and across the landscape, which is difficult to simulate. We know of only one other study that attempted to simulate ice storm impacts in a forest ecosystem²⁵. In this case, a rifle was used to remove up to 52% of the crown in a loblolly pine stand in Oklahoma. Although this method produced results that are characteristic of ice storms, it is not effective at removing larger branches and does not cause the trees to bend over, which is common with natural ice storms. While no other experimental methods have been used to study ice storms specifically, there are some parallels between our approach and other types of forest disturbance manipulations. For example, gap dynamics have been studied by felling individual trees²⁶, forest pest invasions by girdling trees²⁷, and hurricanes by pruning²⁸ or pulling down whole trees with a winch and cable²⁹. Of these approaches, pruning most closely imitates ice storm impacts but is labor intensive and costly. The other approaches cause mortality of whole trees, rather than the partial breakage of limbs and branches that is typical of natural ice storms.

The protocol described in this paper is useful for closely mimicking natural ice storms and involves spraying water over the forest canopy during sub-freezing conditions to simulate glaze ice events. The method offers advantages over other means because the damage can be distributed relatively evenly throughout forests over a large area with less effort than pruning or downing whole trees. Additionally, the amount of ice accretion can be regulated through the volume of water applied and by selecting a time to spray when weather conditions are conducive for optimal ice formation. This novel and relatively inexpensive experimental approach enables control over the intensity and frequency of icing, which is essential for identifying critical ecological thresholds in forest ecosystems.

Protocol

1. Develop the experimental design

1. Determine the intensity and frequency of icing based on realistic values.
2. Determine the size and shape of the plots.
   1. If the goal is to evaluate tree responses, select a plot size that is large enough to include multiple trees and most of their root systems, which varies depending on factors such as tree species and age.
   2. For safety purposes, design the plots so that the entire plot area can be sprayed from outside the boundary.
   3. Space plots far enough apart (e.g., 10 m) so that a treatment in one plot does not affect another.
   4. Establish a buffer zone (e.g., 5 m) around plots to reduce edge effects and ensure a more even distribution of the ice coverage.
   5. Establish subplots within the larger plots for specific sampling needs.
3. Decide on the number of replicate plots.

2. Select and establish a study location

1. Select a homogeneous forest stand with similar features, such as tree species composition, soils, lithology, and hydrology.
2. Select a location for the application in an area where there is access to a water source during winter.
3. Ensure that the supply of water is adequate for the ice application based on the pump rate and other factors such as the diameter of the hose, length of hose, nozzle used, and water pressure.
4. Mark the boundary of the plots, buffer zone, and subplots.
5. Conduct a complete forest inventory with descriptions of tree health conditions including assessments of dead, dying and damaged trees. Additionally, record any potential stressors (e.g., evidence of insect damage or disease) to help interpret the response to the ice treatment.
6. If using UTVs to spray water, create passable trails along the sides of the plots while being careful to minimize disturbance.
7. Once the plots are established, randomly assign a treatment to each plot and type of sampling that will be conducted in each subplot (e.g., coarse woody debris, fine litter, soil samples).

3. Timing of the application

1. Select an appropriate window of time to perform the spraying.
2. Perform the experiment when the weather conditions are conducive (e.g., when air temperature is less than -4 °C and wind speed is less than 5 m/s).
3. If spraying at night, deploy high powered lights around the edge of plots and run them on generators if electricity is not available.

4. Set up the water supply

1. Set up a supply pump at the water source and connect a suction hose.
2. Connect a strainer to the end of the suction hose to keep debris out of the lines.
3. Break through any surface ice and fully submerge the strainer. The minimum depth of the water supply should be about 20 cm.
4. Place a booster pump in the bed of a UTV to improve water pressure. In some cases, a booster pump may not be necessary, especially for low-stature vegetation.
5. Run a firefighting hose from the supply pump to the booster pump.
6. Use a fire-fighting monitor to enable safe, manual control over the high-pressure hose. The monitor can be free standing or mounted on the back of a UTV.
7. Avoid situations that may interrupt the flow of water such as kinks in the hose, water drawdown at the supply source, and running out of gasoline for the pumps.

5. Creating the ice

1. 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 is deposited vertically and freezes on contact with sub-freezing surfaces. Avoid stripping branches and bark from trees as water is sprayed upwards.
2. Evenly distribute spray over the forest canopy by slowly driving the UTV back-and-forth along the edge of the application area. If free-standing monitors are used, move these manually to ensure that the coverage is even.
3. Keep track of the timing of the application to help determine factors such as the weather conditions during application and the volume of water sprayed.

6. Measure ice accretion

1. 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 during application and determine when the target thickness has been attained.
2. Obtain more accurate estimates of ice accretion with passive ice collectors after the application (Figure 1).
   1. Before the application, construct passive ice collectors with two dowels oriented on three cardinal axes³⁰ to create collectors with six component arms.
   2. Cut 2.54 cm dowels at a length of 30 cm.
   3. Join the dowels with a 6-way steel connector.
   4. Use an arborist throw weight to string parachute cord over sturdy branches that can withstand the ice load.
   5. Attach the passive ice collectors to the cord and raise them up into the canopy.
   6. Once the application is completed, lower the collectors to the ground, being careful not to lose any ice from the collector.
   7. Make vertical and horizontal measurements of ice thickness with calipers at multiple locations on the collector (e.g., three vertical and three horizontal measurements at three locations along each arm) before and immediately after ice application.
   8. Calculate ice thickness on each collector as the difference between the measurements before and after the application.
   9. To determine ice thickness with the water volume method, use a reciprocating saw to cut each dowel.
   10. Bring the dowels to a heated building, place them in buckets, and let the ice melt off at room temperature.
   11. Measure the volume of meltwater with a graduated cylinder.
   12. Calculate ice thickness based on the water volume and density of ice³¹.

7. Safety considerations

1. Stay well outside of the ice treatment area during spraying because ice loads can cause branches and limbs to break and fall.
2. Wear hard hats or helmets to provide protection while the ice is being applied and during any sampling that occurs in the treated area after the application.
3. Use a monitor to stabilize the hose during spraying.
4. Dress appropriately for hazardous conditions and sub-freezing weather. Wear bright, visible clothing. Be prepared to spend long periods in wet, cold conditions by wearing rain gear and layers of warm clothes. Bring multiple changes of clothes, especially for personnel who are designated to spray.
5. If working in a remote location, set up a temporary warming tent equipped with a portable heater.
6. Allow personnel to have adequate time for breaks, changing out of wet clothes, and addressing problems that arise with equipment, etc.
7. Use radios to communicate among personnel during the experiment. Maintain contact with personnel at a base station.
8. Develop a safety plan in case of medical emergencies. Have medical personnel (e.g., Emergency Medical Technicians) and emergency equipment and supplies on site during the experiment.

Results

An ice storm simulation was performed in a 70‒100 year-old northern hardwood forest at the Hubbard Brook Experimental Forest in central New Hampshire (43° 56′ N, 71° 45′ W). The stand height is approximately 20 m and the dominant tree species in the area of the ice application are American beech (Fagus grandifolia), sugar maple (Acer saccharum), red maple (Acer rubrum) and yellow birch (Betula alleghaniensis). Ten 20 m x 30 m plots were established and rando...

Discussion

It is critical to perform experimental simulations of ice storms under appropriate weather conditions to ensure their success. In a previous study³⁰, we found that the optimal conditions for spraying are when air temperatures are below -4 °C and wind speeds are less than 5 m/s. Natural ice storms most commonly occur when air temperatures are slightly less than freezing (-1 to 0 °C), and although the ideal temperatures for ice storm simulations are colder, they are still within the temper...

Materials

Name	Company	Catalog Number	Comments
Booster pump	Waterax	BB-4-23P	401 L min-1 maximum flow; 30.3 bar maximum pressure
Firefighting hose	ATI Forest Products	Forest-Lite G55H1F50N	3.8 cm diameter, polyester, single jacket
Monitor (ground placement)	Task Force Tips	Blitzfire XX111A	2000 L min-1 maximum flow; fits 3.8 cm hose
Monitor (UTV mount)	Potter Roemer	Fire Pro FP1S-125	1325 L min-1 maximum flow; fits 3.8 cm hose
Nozzle	Crestar	ST2675	Smooth bore; double stacked; 3.8 cm intake; 1.3 cm orifice
Strainer	Northern Tool	107902	7.6 cm hose fitting, 17.6 cm outside diameter
Suction hose	JGB Enterprises	A007-0489-1615	7.6 cm diameter; 4.6 m long
Water pump	NorthStar	106471E	665 L min-1; fits 7.6 cm hose