Root Barriers and Windthrow Potential

Materials and Methods

Thirty-six 1.5-in. (4-cm) caliper green ash (Fraxinus pennsylvanica) were dug with a 32-in. (81-cm) diameter tree spade set to cut an 18-in. (46-cm) diameter root ball and planted on November 11 and 12, 1996. Half of the trees were installed centered in 22-in. (55-cm) top diameter by 18-in. (46-cm) deep round preformed tree root barriers (Deep Root Partners, L.P., San Francisco, CA, Product #RP22-30-18) planted according to manufacturers recommendations (Figure 1). The other half were planted in backhoe-dug holes, twice the width of the root ball. No wire baskets or burlap were used. All trees were irrigated during drought periods and fertilized equally in the fall of 1997 and 1998.

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Figure 1.

Planting specifications for Deep Root surround planter (RP 22-30-18).

On July 20, 1999, three trees growing in the barrier and three control trees were attached to a 0.25-in. (6.3-mm) steel cable using a nylon sling attached 24 in. (61 cm) above soil level (Figure 2). The opposite end of the cable was attached to a Dillion 4,000 lb (1,800-kg) peak recording mechanical dynamometer (Weight-Tronix Inc., Fairmont, MN) then to a tractor. Trees were pulled until they either broke or were pulled out of the ground. Tree height, spread, caliper at 6 in. (15 cm) above soil level, and force required to pull it over were recorded. Mean breaking strength, diameter, height, and branch spread were statistically compared using a T-test.

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Figure 2.

Ash trees planted 2.5 years before in surrounding barriers or open soil were attached to a dynamometer using a nylon sling and steel cable and pulled until they broke or came out of the ground.

Soil at the site was a Cecil sandy clay loam (CeB2). Soil samples were collected from around the tree roots to determine soil moisture content. Samples were dried to a constant weight and moisture was determined gravimetrically. After six trees were pulled, the field was flood irrigated for a total of 5 hours before the next set of trees were pulled. A second set of six trees was then pulled after the soil was saturated.

After each set of two trees was pulled, the trees were severed near the soil line, mounted in the back of a pickup truck (Figure 3) and driven at set speeds. The trunk of the tree was tied to a Dillion HR 2000 electronic dynamometer at 24-in. (61 cm) above the original ground level, to record the force of the wind on the tree. The base of the tree was contained loosely within a wooden frame, and ropes were loosely connected from the trunk to the sides of the truck to prevent excess lateral movement. Vehicle speed was monitored with a Garmin GPS 45 global positioning system. Wind resistance was measured on the first six trees. Results were statistically analyzed using a regression analysis.

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Figure 3.

Tree mounting system used to measure the wind resistance of ash trees. Note that there are only two contact points for the system—the dynamometer connecting line and the base of the platform that restrains moveme nt of the bottom of the trunk. The guy lines going to the corners of the truck were not under tension unless the tree moved laterally.

The remaining trees will be pulled over in 3 to 5 years to determine if stability changes as size increases.

Results

Under dry soil conditions, the trees within the root barriers were pulled out of the ground at an average force of 2,341 lb (1,060 kg, Table 1). These trees failed after the roots in the 1-to-2-in. (2-to-5-cm) diameter range broke. The control trees broke with an average force of 1,961 lb (888 kg) when the lower stem/root collar broke. Average soil moisture was 14.5 % water (w/w).

Under saturated soil conditions, the trees within the root barriers pulled out of the ground with an average force of 2,860 lb (1,296 kg, Table 1). These trees failed when the root system broke. The control trees failed with an average force of 2,063 lb (934 kg). The control average may be high because one tree was pulled 2 hours after the other trees—and the force to pull it out was over twice the average of the other two. The control trees all failed when their root systems pulled out of the soil. Average soil moisture content when the last tree was pulled was 33% water.

The measure of wind resistance showed a linear increase with vehicle speed (Figure 4). Using the calculated regression line to extrapolate beyond the data to a 100 mph (160 kph) wind speed, the force would be 365 lb (165 kg). This value is one third less than the lowest value (1,290 lb [584 kg]) required to pull a treated or control tree over under saturated soil conditions

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Figure 4.

Wind resistance of green ash (Fraxinus pennsylvanica) measured at various speeds 24 in. (61 cm) above original grade. Odd-numbered trees were grown within root barriers. Bold line is the regression line for all trees.

Visual observations indicated that root barrier grown trees appeared healthier. Average caliper, height, and branch spread of root-barrier-grown trees, however, were not significantly greater than controls (Table 2).

Discussion

Young ash trees were very wind stable with or without surrounding root barriers. The wind speed equivalent force required to break or throw these trees was far in excess of 100 mph (160 kph).

There were different failure patterns between barriers and control trees. Under dry conditions, the barrier treatments allowed the roots to move more, increasing the breaking force required. Because the root system did move, there were no trunk failures with the barrier. The control trees failed at the root collar or when the stem broke.

Under saturated conditions, the pattern of failure was the same for all trees: The roots pulled out of the ground with breakage occurring in roots 0.25 to 0.5 in. (0.6 to 1.3 cm) in diameter. The force required to pull the trees out differed depending on treatment. Trees with root barriers were able to withstand higher forces than the control trees. It appeared that the reason for this increased resistance was deeper rooting of the barrier-surrounded trees. Roots grew beneath the barrier to a depth typically 12 to 16 in. (30 to 40 cm) deeper than the control trees. After growing under the barrier or through the slots near the bottom of the barrier, root growth varied. Most roots turned upward in to the soil outside of the gravel that surrounds the barrier, then became horizontal at a depth of 4 to 10 in. (10 to 25 cm). No girdling roots were observed; many roots inside the barrier were deflected downward by the ribs in the surface of the barrier.

The root system configurations of trees surrounded by root barriers were very different from the control trees. After three growing seasons, this difference resulted in ash trees being more resistant to windthrow within root barriers than nontreated trees. The long-term effects of circling root barriers needs to be studied to determine if these trends continue.

Résumé

Cette étude a été créée pour determiner si les barrières à rainures qui sont disponibles sur le marché peuvent réduire ou accroître la stabilité des arbres soumis à des stress latéraux sévères. Des frênes de Pennsylvanie (Fraxinus pennsylvanica) ont été plantés en novembre 1996: 6 entourés de barrières racinaires et 6 sans barrière. La moitié des arbres de chacun des groupes ont été renversés en juillet 1999 sous des conditions de sol sec (14% d’eau) et l’autre moitié Font été sous des conditions de sol saturé (33% d’eau). La force requise pour renverser les arbres a été mesurée comme si c’était celle de la resistance face au vent des arbres. Une force légèrement plus importante a été requise pour renverser les arbres qui s’étaient développés à ?intérieur d’une barrière racinaire que ceux sans cette barrière. Quoiqu’il en soit, la force requise pour renverser le plus faible des arbres a été beaucoup plus élevée que celle exercée par un vent de 160 km/h. La raison de l’accroissement de la resistance des arbres à l’intérieur d’une barrière racinaire s’expliquerait par un enracinement plus profond.

Zusammenfassung

Diese Studie wurde entwickelt, um zu bestimmen, ob im Handel erhãltliche gerippte Barrieren die Stabilität von Bäumen mit großem lateralen Stress reduzieren oder unterstũtzten. Fraxinus pennsylvanica Wurde im November 1996 gepflanzt: 6 mit einer umgebenden Wurzelbarriere und 6 ohne Schutz. Die Hãlfte jeder Gruppe wurde im Juli 1999 unter trockenen Bodenverhältnissen (14 % Wasser) und die andere Hälfte bei wassergesättigen Verhältnissen (33 % Wasser) umgezogen. Die aufgewendete Kraft, sowie der Windwiderstand der Bäume wurde gemessen. Fūr die Bāume mit der Wurzelbarriere war bei beiden Bodenverhältnissen etwas mehr Kraft erforderlich. Die erforderliche Kraft für den schwächsten Baum lag dennoch viel h?her als die Windkraft mit 100 mph (160-kph). Der Grund fūr die ansteigend aufzubringende Kraft fůr die Bäume mit Wurzelbarriere schien in dem tieferen Wurzelsystem zu liegen.

Resumen

Este estudio fue desarrollado para determinar si las barreras reforzadas, disponibles comercialmente, reducen o incrementan la estabilidad de los árboles bajo condiciones severas de estrés lateral. Árboles de fresno (Fraxinus pennsylvanica), fueron plantados en noviembre de 1996: 6 con barreras para raíces del tipo común en la zona y 6 sin barreras. La mitad de cada grupo fueron extraídos en julio de 1999 bajo condiciones de suelo seco (14% de humedad) y la otra mitad fueron extraídos bajo condiciones de suelo saturado (33% de humedad). La fuerza requerida para sacar a los árboles fue medida como la resistencia al viento por parte de los mismos. Se requirió levemente más fuerza para sacar a los árboles que crecieron con barreras que los de control. La fuerza requerida para extraer al árbol más débil, sin embargo, fue mucho mayor que la ejercida por un viento de 160 km/h (160 mill/h). La razón de la mayor resistencia de los árboles, con raíces en barreras, parece ser su sistema más profundo de raíces.