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Title: MICROCLIMATE AND PEDOGENIC IMPLICATIONS IN A 50-YEAR-OLD CHAPARRAL AND PINE BIOSEQUENCE

Author
item JOHNSON-MAYNARD, J - UNIVERSITY OF IDAHO
item Shouse, Peter
item GRAHAM, R - UC RIVERSIDE, CA
item CASTIGLIONE, P - UC RIVERSIDE, CA
item QUIDEAU, S - UNIV OF ALBERTA, CANADA

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2004
Publication Date: 5/20/2004
Citation: Johnson-Maynard, J.L., Shouse, P.J., Graham, R.C., Castiglione, P., Quideau, S.A. 2004. Microclimate and pedogenic implications in a 50-year-old chaparral and pine biosequence. Soil Science Society of America Journal. 68:876-884.

Interpretive Summary: The ultimate characteristics of a soil can conceptually be viewed as a result of the balance of soil-forming processes through time. Soil forming processes result from interactions between five soil-forming factors, of which vegetation is one factor. Vegetation can alter the balance of soil-forming processes through direct and indirect mechanisms. Directly, plant roots create pores and alter the chemical, physical, and hydraulic properties of soil. Indirectly, vegetation may select for soil fauna through litter palatability, and alter the energy balance creating unique microclimates. The objective of this study, therefore, is to determine if vegetation has directly caused divergent pathways of soil formation, or if indirect effects of microclimate and selection of soil organisms are more important. The biosequence of lysimeter soils at the San Dimas Experimental Forest provided a unique opportunity to study the effects of vegetation on soil formation. In the San Dimas biosequence, the formation of distinctly different surface soil horizons under oak and pine was probably caused more by the action of earthworms, or their absence, than microclimate. Since the palatability of litter determines the distribution of earthworms, vegetation type indirectly controls incorporation and decomposition rates through the influence of litter composition rather than through its effect on microclimate. Although it is impossible, given the current data, to separate the effects of the absence of earthworms and production of organic acids under pine, either or both of these factors are more important influences on the transport of silicate clay than is microclimate. Soil earthworm communities appear to have a greater effect than microclimate in causing the divergent soil morphologies under oak and pine. Microclimate created by chamise may play an important role in the decomposition rate of litter, but appears to have little influence on soil processes occurring within the mineral soil. The influence of microclimate on soil processes under chamise should be further investigated, considering its widespread distribution in the southwestern USA.

Technical Abstract: Vegetation, which is generally considered a co-variable controlled by climate in studies of pedogenesis, can itself alter the balance of soil-forming processes by modifying the microclimate. The chaparral and pine biosequence at the San Dimas Experimental Forest (SDEF) in Southern California offers an opportunity to determine the effect of individual species on soil microclimate and related soil properties. Soil temperature and moisture were monitored under pure stands of Coulter pine (Pinus coulteri B. Don), chamise (Adenostoma fasciculatum Hook. and Arn.), and scrub oak (Quercus dumosa Nutt.). Despite the appreciable differences in soil morphology that have evolved under pine compared with oak, the microclimates created by the dense canopies and thick litter layers (7-10 cm) of these two vegetation types were similar. Average monthly soil temperatures within the top 65 cm ranged from about 8 to 20 degrees C under pine and 7 to 18 degrees C under oak. Average monthly water contents ranged from 8 to 32 percent (by volume) under pine and 6 to 32 percent under oak. In contrast, average monthly soil temperatures within the top 65 cm under chamise ranged from 6 to 23 degrees C and average monthly water contents were 6 to 36 percent. Diurnal variation in soil surface temperature under chamise was much more pronounced than under oak and pine because of the relatively open canopy and thin litter layer. Microclimate created by chamise may play an important role in the decomposition rate of litter, but appears to have little influence on soil processes occurring within the mineral soil. Soil macrofaunal communities appear to have a greater effect than microclimate in causing the divergent soil morphologies under oak and pine.