That’s a sobering thought—a tree with a normal life span of 65 to 250 years may live less than 10 years when planted in any American city’s downtown landscape. Admit-tedly, that figure represents tree placement in the worst of our urban landscape sites: sidewalk cut-outs. These inhospitable planting sites are also known as tree coffins, tree burial mounds, or urban tree disposal units to frustrated urban foresters. When the mortality rate of downtown trees is compared to tree losses from Dutch elm disease, oak wilt, sudden oak death, and gypsy moth, it doesn’t take too long to realize that there’s an epidemic of urban tree loss going on and it’s largely under the radar (Figure 1).Another oft-quoted number is that the aver-age urban residential tree lives for 30 to 35 years (Moll 1989). That life span is three times as long as a sidewalk tree, yet only half as long as a paper birch in its natural environment. Grow-ing conditions in residential landscapes may not be quite as bad as sidewalk sites, but there are many natural and unnatural pressures on the trees that lead to briefer landscape lives. Residential landscape soils can be as stressful as downtown sites: poorly drained, outrageously alkaline, subjected to blends of every pesticide known to modern society, and compacted to such a degree that lawns may seem like nothing more than green concrete.With few exceptions (perhaps tornadoes and a few diseases), there are no “angels of death” that descend and quickly kill trees in landscapes. More commonly, a multitude of predisposing stresses that occur in our highly altered urban landscapes combine to weaken trees over the years. Often, inciting events such as floods or hailstorms and/or contributing agents such as target cankers or wood boring insects complete the job for the majority of tree losses. Mean-while, plant health care professionals attempt to determine the true causes of decline and death, and often the diagnoses are incomplete or incorrect because of the multiple offenders involved with the problem.Predisposing Factors and Tree DeclineWhen trees are chronically stressed (long-term drought, repeated defoliation, etc.), their nor-mal reserves of chemical energy—primarily as complex carbohydrates—are slowly depleted. Each year as stressed trees come out of dor-mancy, they emerge in a weakened state due to this energy depletion and find it increas-Dysfunctional Root Systems and Brief Landscape Lives: Stem Girdling Roots and the Browning of Our LandscapesGary JohnsonConsider this comparison of potential life spans for trees (Burns and Honkola 1990; USDA 1998)Quercus macrocarpa (bur oak), in upland site 250+ yearsAcer saccharinum (silver maple), in riparian site 125+ yearsAcer negundo (boxelder), in lowland site 100+ yearsPinus banksiana (Jack pine), in field site 80+ yearsBetula papyrifera (paper birch), in northern lowland forest 65+ yearsTree planted in urban core street site less than 10 yearsingly difficult to releaf, grow, and deal with the harsh realities of urban landscapes on a normal basis. It takes a tremendous amount of chemi-cal energy to push out new leaves and shoots, recover from accidental wounds on the stems, or produce flowers and fruit.As the tree’s energy reserves continue to decline—and thereby affect the tree’s ability to capture and store new energy through pho-tosynthesis—the entire system is affected and the decline spiral to premature death begins. So decline in a sense refers to the tree’s ability to deal with life’s normal stresses. A tree in decline may die suddenly because of an event such as a cold winter with no snow cover, a short-term summer drought, or a defoliation from insects or hail. The other trees in the land-scape tolerate the damage and survive, but the predisposed trees—those in decline—are unable to recover from the damage.Dysfunctional Root Systems as Predisposing AgentsDespite the fact that roots are seldom seen, dys-functional root systems are too often the predis-posing agents connected to tree health decline, and ultimately the reason why many urban landscape trees experience such brief lives. If the root system—approximately 50% of a tree’s bio-mass—is not operating normally, the entire sys-tem will be abnormal. Abnormal is not always harmful, as seen in bonsai plants and trees grow-ing on slopes. In bonsai plants, a restricted root system causes compacted growth in the rest of the plant system, but the system itself may be healthy and completely functional under most circumstances. In the case of a tree growing on a slope, the tree is anchored with a skewed and asymmetrical root system, but its overall health is not compromised even though the root system could certainly be considered abnormal.Figure 1. Trees in urban sidewalk sites are subjected to very unhealthy environments and live less than 10 years on average.PHOTO By GARy JOHNSONDysfunctional Root Systems 3But abnormal root systems that do affect the overall health or stability of the tree are considered dysfunctional. For example, when a container-grown tree with a severely pot-bound root system is planted, its rhizosphere does not occupy a large enough area to capture sufficient water and nutrients needed to support a normal sized tree without supplemental help. Dysfunc-tional root systems are also common on newly transplanted bare-root and balled-and-burlapped plants; these plants often lose 75% or more of their root systems during the harvest operation, resulting in transplant shock which may go on for several years until the root system regrows. And then there are stem girdling roots (SGRs), which create a root system so dys-functional that it can end up killing the entire tree.Stem Girdling Roots as Predisposing AgentsStem girdling roots are those roots that grow either partially or com-pletely against the tree’s stem and compress (girdle) the stem tissues (Figure 2). Xylem and phloem tissues in the stem become much narrower at the point of compression, imped-ing normal water movement and sap flow (Figure 3). This restriction affects energy reserves by directly and indirectly affecting photosyn-thesis. Trees become stressed and more vulnerable to secondary problems. For this reason, SGRs are considered to be primary predisposing agents in landscape tree decline and death.Some of the first symptoms of SGR-impacted tree health include leaf scorch or leaf wilting on a tree when no other plants in the area