AN EVALUATION OF SOIL AERATION STATUS AROUND HEALTHY AND DECLINING OAKS IN AN URBAN ENVIRONMENT IN CALIFORNIA

Examination of soil conditions around healthy and declining oak trees in an urban area in California revealed that soil aeration in the upper soil strata was critical to tree vigor. It also was found that measurements of oxygen diffusion rate gave a better indication of oxygen availability in soil than measurements of oxygen concentration. Oxygen diffusion rate was lowest in soils with high bulk density and high soil moisture content. In a poorly-aerated soil, oxygen diffusion rate did not increase significantly following air injections de- signed to loosen soil structure. Poor soil aeration is frequently cited by tree care professionals as the cause of tree decline. Trees are diagnosed as suffering from "wet feet", "suffocation", "anoxia", "oxygen stress", etc., all of which are terms descriptive of poor soil aeration. Often there is a reasonable basis for the diag- nosis of poor aeration. It is understood that roots require oxygen for proper function, and that oxy- gen moves from the atmosphere to roots primarily by diffusion through the soil. When soil has nu- merous, large-diameter pores, oxygen diffusion is rapid. Conversely, if soil contains few large-di- ameter pores, diffusion is inhibited. Healthy root function, therefore, requires a soil structure that enhances oxygen diffusion. It is apparent, however, that conditions exist in the root zone of many trees that interfere with oxygen diffusion into and through soils. For ex- ample, fine-textured soils (such as clays) or compacted soils have fewer macropores to sup- port rapid diffusion; excessive soil moisture causes pores to be filled with water (through which oxy- gen diffuses very slowly); surface barriers such as asphalt or concrete may slow the rate of oxygen entry into soil; and grade changes (fill soils) in- crease the distance over which oxygen must diffuse to reach an established root system. These and other conditions exist in many treescapes and frequently lead to the conclusion that tree decline is the result of poor aeration. The frequent occurrence of factors that may cause (or lead to) restricted oxygen movement in urban treescapes has prompted arborists and planners to employ a number of practices and design strategies to enhance soil aeration. These include venting systems to alleviate the effects of grade changes, porous pavers as substitutes for asphalt or concrete, core venting and vertical mulching to open large air channels, and high pressure water or air injections (e.g., Grow Gun®) to fracture compacted zones. There is, however, very little experimental evidence indicating whether these strategies successfully protect or enhance soil aeration within tree root systems. In fact, some fundamental questions remain uncertain, including: