Decline of Old‐Growth Redwood Forests in Relation to Some Soil Microbiological Processes

Observations on a number of old—growth coast redwood Sequoia sempervirens (D. Don) Endl. forests indicated unusual features of stand decline. To test the hypothesis that this decline resulted from some deterioration in the forest—site relationship, a series of studies was made on forest soils under old—growth redwood and at various stages of the hardwood succession following logging. Proliferation of redwood seedling roots was restricted in old—growth redwood soil and some evidence of seedling susceptibility to a root pathogen was found. Soil respiration patterns indicated that the availability of carbonaceous material from the organic matter in a form suitable for microbial digestion may be the primary factor limiting biological activity. The end results may be failure of, or abnormalities in, the process of nitrogen mineralization, and the establishment of an edaphic condition in which pathogens can attack seedling roots. Although nitrogen was found to be strongly immobilized in soils from an age sequence of tan oak developed on former redwood sites, it seems possible that tan oak litter could stimulate a higher level of microbial activity and so accelerate the decomposition of redwood humus. Deterioration in both the soil microbiological processes and the plant—soil relationship in a redwood forest are discussed in terms of the long—term incorporation of redwood litter into the soil.