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Trees Playing Footsie: How Root Grafts Work

By Brandon Gallagher Watson

 

Have you ever come across one of the fabled “ghost trees” while walking in the woods? Don’t feel bad if you haven’t, as there are fewer than 250 of these trees known worldwide. Ghost trees are trees that completely lack chlorophyll, thus instead of being green, their foliage is stark white. This condition is often compared to albinism that causes pale skin and pink eyes in animals. Albino animals are unable to produce the skin pigment melanin, and appear whiter than normal for the species. While achlorophyllous plants also appear white, they have a much more serious condition than do albinos. These trees have a genetic mutation preventing the formation of chlorophyll, the molecule that produces their food. So if these trees cannot photosynthesize, how is it possible they are alive? There are legends from native cultures regarding these trees as forest spirits, but researchers have a less mystical idea. The answer comes not from these trees, but from their neighbors. Turns out these trees and the trees nearby have root systems that are connected, allowing the sugars produced by a “normal” tree to be shared with their white-leaved companion. Just as it takes a village to raise a child, it takes a forest to raise a ghost tree.

Shared root systems occur when the roots of two or more individual trees grow into each other. As the roots increase in girth they crush each other’s cortex tissue, eventually becoming grafted together. These root grafts are capable of sharing pretty much anything that can be transferred through a tree’s vascular system. Photosynthates, water, even inorganic nutrients obtained through a mycorrhizal relationship can all be moved from one tree to the next though root grafts. How common is this phenomenon in the wild? Much like mycorrhizae, the more we look for them, the more we find them. About 300 species of trees have been shown to form root grafts thus far. The total number is likely much higher, but the difficulty of excavating intact root systems is a definite roadblock to wider discovery. Nonetheless, the trees with this ability are thought to have some competitive advantages over purely solo trees.

For many years, researchers thought the evolutionary purpose of root grafts was just in the sharing of resources such as sugars, water and nutrients. This makes sense where a gradient of resources exists, such as along a slope. Trees near the moister bottom of the hill can essentially pass water up to trees on the drier top of the hill. The same works for nutrients, where trees in richer soils can assist nearby trees growing in poorer soils. Increasingly, there is evidence that there may be additional benefits as well. Canopy space is limited in any forest setting, so there are also advantages to a taller, more vigorous tree sharing carbohydrates to a shorter, less vigorous tree in the understory. Trees found growing in watery sites, such as the black tupelo, were found to have significantly greater ability to form root grafts than trees of the same species growing in upland sites. It is believed this evolved as a mechanism for helping anchor the trees in the muddy ground. Not only would conjoined root systems increase stability in waterlogged soils, but there is evidence they help protect against wind throws as well. A study of hurricane-affected trees in 1993 showed that trees with grafted root systems were better able to resist root plate lifting damage than stand-alone trees, showing there are mechanical, as well as metabolic, benefits to grafted roots. All of these interactions are showing that tree populations are more communal than originally thought.

How likely two trees are to form grafted roots depends on a few factors. First, they need to be close enough together for their roots to touch. Next, they need to be of a certain size as it takes time for two touching roots to grow with enough pressure that they fuse and begin sharing resources. The vast majority of root grafts that have been observed are between trees of the same species, (such as a red oak to another red oak) but there have been rare instances where two species (such as a Douglas fir and a black spruce) have been found with adjoining roots. Whether these trees are actually transmitting fluids to one another is unclear, and the unlikelihood of even finding them makes studying them difficult to say the least. Studies have shown grafts are more likely to occur in natural stands over plantation settings, and soil type can have an influence on the likelihood of graft formation. Trees planted close together in urban settings are known to graft as well, as is commonly found with boulevard American elm trees.

There are, of course, situations where sharing with the neighbors can have some negative side effects. Just as water and nutrients can pass from one tree to the next, so could pathogens. Fungi that affect the vascular system are known to pass from an infected tree to a healthy tree through these underground hallways. In some cases, such as oak wilt, transmission through root grafts is responsible for up to 90 percent of the new infections. That means a beetle brought the fungus to one tree, and nine of its neighbors were infected through these connected roots. Dutch elm disease is also caused by a fungus, but the infection ratio is almost exactly opposite with nine out of 10 trees contracting the fungus through overland spread by bark beetles and one out of 10 dying through root graft spread. These stats are likely related to the sites in which these two species are normally found, rather than the nature of the pathogen themselves. Oak wilt can be an urban tree issue in some parts of the country, but the vast majority of oaks killed by this are in forest stand or savanna settings. One tree contracts the fungus from a beetle and it quickly spreads to its neighbors. Dutch elm disease, on the other hand, is more common in urban areas and, thanks to prompt sanitation, infected elms are often removed before the fungus can grow into adjacent elm trees.

Proper management of both oak wilt and Dutch elm disease often requires physically breaking the root grafts between an infected tree and a healthy tree. Like many things in arboriculture, this is often easier said than done. In the loamy soils of the Great Plains, root grafts can be severed using a vibratory plow. Trenches are usually cut four- to six-feet deep between the trees. In urban areas, this can be challenging with site factors such as buried utilities, sprinkler systems, driveways, and sidewalks. In the hard limestone soils of central Texas, the blade of a vibratory plow just won’t do. Where root graft severing needs to take place, the power of a rock saw is often employed. This is a difficult and noisy operation but necessary to prevent the spread of the oak wilt fungus to healthy stands of trees.

Studying the formation, functions, and implication of shared root systems in urban trees could open up a new understanding of how these living communities interact with each other. New sensing techniques could certainly improve how we study underground interactions, and perhaps lead to new ways to care for trees. At the very least, shared root systems serve as a reminder that there are many aspects of arboriculture we are just beginning to discover. And, who knows, it might lead to an albino tree planting craze.

 

Brandon Gallagher Watson is creative director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).

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