FOR-40

Decline of Live Oak in Urban Florida Environments

by Roger Webb

A Valuable Ornamental Tree

Our live oak, Quercus virginiana is a highly desirable ornamental tree ranging in height at maturity from 40 to 70 feet and reaching an average 3 to 4 feet in diameter, although the short, buttressed trunk may reach a diameter of 6 to 8 feet. Typically, Q. virginiana develops two to four wide-spreading horizontal branches which support a close round-topped foliar canopy that may reach 125 to 150 feet across. Evolving in the southeastern coastal areas, Q. virginiana has developed a tolerance to salt spray and is frequently a dominant tree on sandy soils among coastal forests and nearby islands.

The massive, curved main branches of the canopy were highly prized by colonial shipwrights due to the great tensile strength of the wood and decay resistance. Since Q. virginiana trunks are usually short and buttressed, the species has never become a major resource commodity except in shipbuilding. However, minor uses of the wood were found, such as for cart hubs and axles, screws and cogs of mill wheels, and waterwheels, pilings and other needs involving submerged wood. The wood appears attractive for use as furniture or flooring, but at an average of 75 pounds per cubic foot (fresh weight), the wood is too hard for working and finishing. Live oak has little commercial value today except its occasional use as firewood or in construction of small boats.

Quercus virginiana is an excellent ornamental tree due to its fast growth, abundant shade, resistance to insects and disease-causing organisms, and its longevity, often in excess of 200 years. One specimen in South Carolina is estimated to be 1,500 years of age. The tolerance of live oak to salt spray permits its use in coastal areas, which usually are lacking in hardwood trees the size of Q. virginiana. Fire, the most important natural enemy of live oak, is rarely a problem in ornamental plantings in urban Florida. However, a decline of live oaks in urban settings has been observed dating back many years, and despite preliminary research efforts by state and university scientists, no biological cause for the premature loss of foliage and subsequent mortality has been determined.

Cultural Problems

Two fungal organisms have been associated with live oak decline in the southeastern United States. Two species of Endothia (E. parasitica and E. gyrosa) have been associated with occasional limb dieback of trees exposed to drought and/or unusually cold winter temperatures. A decline among Texas live oaks and other species has been attributed to the spread of the oak wilt fungus, Ceratocystis fagacearum In Florida, individual live oaks have declined and died in urban settings accompanied by attacks by saprophytic organisms, which have been encouraged by debilitating physical conditions in the environment.

The extensive use of Q. virginiana as an urban ornamental plant during the "boom years" of development in Florida near the turn of this century has made the decline problem acutely noticeable. In certain areas of central and south Florida towns and cities, such as Arcadia and Lakeland, live oaks have grown to such dimensions that plantings between streets and along sidewalks now exceed the capacity of those sites to support their root and canopy systems' enormous requirements for water, sunlight, and space (Figure 1).

In addition, freezes and droughts have undoubtedly caused additional stress on live oaks both above and below ground. This has opened the way for infection following invasion by many microorganisms feeding on dead or dying oak tissue. In concert with the- death of lower, shaded limbs in this crowded environment and improper pruning wounds which become infected, live oaks have experienced decline and death in the prime of their developmental years.

Also, a common practice in early planting was to "top" the sapling, forcing side branches to develop in an exaggerated vase-shaped form. As this growth habit developed, this pruning wound became the end of the "funnel" for rain water dripping from leaves and down the stems of side branches (Figure 2). This area would trap abnormally high levels of moisture, and decay processes would be initiated, leading much later in the tree's life span to branch or stem breakage. Once the tree becomes unsightly, it is frequently cut down, since its threat to life and property is increased should further breakage occur.

Reducing the Decline Potential

Decline of live oaks in the urban environment in Florida is a problem of older and larger trees competing for shrinking space in a progressively more hostile envirorunent. Preservation of live oaks requires more vigorous planning efforts in the future to allow for a century or more of unrestricted growth. Wider planting spaces in areas less likely to suffer road, sidewalk, driveway, water main or building construction damage should be considered. Pruning should be conducted when necessary by individuals competently trained in horticulture. During periods of drought, additional watering should be done routinely to reduce stress brought on by the demands of the enormous foliar component. Following simple rules of plant care will help preserve live oaks in the urban environment and extend the life and use of Q. virginiana as a highly prized ornamental tree.