Soil aggregate stability under chaparral species in southern California

- Chaparral soils have relatively low water stable aggregation.
- Aggregate stability does not vary significantly by chaparral plant species.
- Organic matter, roots, fungal hyphae, earthworms, and clay are aggregating agents.
- Soil erosion and minimal earthworm activity inhibit development of stable aggregates.

Aggregation is a key soil property that increases infiltration and aeration, and reduces erosion and runoff. This study sought to determine the amount and sources of aggregation in soils under four common chaparral species in southern California: bigberry manzanita (Arctostaphylos glauca Lindl.), scrub oak (Quercus dumosa Nutt.), hoaryleaf ceanothus (Ceanothus crassifolius Torr.), and chamise (Adenostoma fasiculatum H. & A.). Research was conducted at the San Dimas Experimental Forest in the San Gabriel Mountains. Total water stable aggregation, determined by wet sieving, averaged 45% in A and 40% in B horizons and differences were not significant by soil horizon or under the four vegetation species. Organic matter, roots, fungal hyphae, macrofauna, and clay all contribute to soil aggregation, but none of these is a clear, dominant, overriding factor in these soils. Development of water stable aggregation is inhibited by low densities of earthworms, relatively frequent wildfires, and soil erosion on the steep slopes in this chaparral ecosystem.