Research on Saving California Oaks
Lee K. Klinger
P.O. Box 644, Big Sur, California 93920
Pathologists investigating the recent death of many oak trees in northern California have concluded that the problem is due to a new plant disease, dubbed sudden oak death (SOD), caused by the fungal pathogen Phytophthora ramorum. While not disputing that P. ramorum is involved in the final demise of many oaks, there are a growing numberof experts who do not agree that this pathogen is the fundamental cause of the decline. They point out that most of the dying oaks in SOD-affected forests show no expression of P. ramorum. Instead, they suggest that acidic conditions create mineral imbalances and deficiencies in soils that weaken the trees, raising their susceptibility to secondary pests and pathogens. Here I present evidence that, due to fire suppression, there has been progressive acidification of oak forests, leading to greater incidence of disease.This helps us understand both why the SOD phenomena is occuring now andwhat can be done to solve the problem.
The etiology of SOD in California coincides closely with the symptoms of decline seen in aging forest ecosystems elsewhere. Dieback starts with the upper and outer branches in the crown, showing a pattern of wilting and browning of leaves along with dying small branches and progressively spreading to the lower parts of the crown over several years. The decline affects many kinds of oaks, as well as bays, buckeyes, and pines, hitting mainly the larger trees in mixed-oak savannas and forests, most of which have been under strict fire control for more than 50 years. Areas near the coast and those experiencing frequent seasonalfog are especially hard hit by SOD. Affected trees tend to occur in mature forests (>100 years old) and are usually found in association with a heavy cover of mosses and lichens. Moss mats have been shown in both laboratory and field studies to be associated with the mortality of underlying fine roots and mycorrhizae, which leads to water and nutrient stress and reduced radial growth in nearby trees. Mosses and lichens are also observed to degrade the tree’s protective bark layer, allowing for pests/pathogens to more easily infest/infect the tree.
Data on pH from 28,577 soil samples taken from a wide range of landscaped and agricultural soils in California indicate that only 10.2% of the soils are acidic (pH < 6.0). Data on samples taken from disease site soils (mostly with SOD) indicates that 79.2% of these soils are acidic (median pH = 5.7; n = 136). Soils from the disease sites were also found to be consistently low in Ca and very high in soluble Al and Fe. Spatial analysis reveals a strong coastal gradient in soil pH with the lowest pH values found near the coast. Strong coastal gradients are also apparent in soil Ca, which is lowest near the coast, and in soil Al, which is highest near the coast.
These results lend further support to the theory that systemic acidification is adversely affecting the health of the trees and soils, predisposing trees to infection by the SOD pathogen. It is recommended, thus, that the scope of SOD research be expanded to include studies of acidification by mosses and lichens in the context of forest and soil ecology.