Oak Trees Create Islands of Fertility

The ability of plants to alter their edaphic environment occurs primarily through addition of organic matter and nutrient cycling. Blue oaks in the Mediterranean climate of northern California display a striking ability to enhance soil organic matter concentrations and nutrient pools beneath their canopies. These approximately 100 year old oak trees return an average 9100 kg/ha of organic matter to the soil surface as litterfall each year. The added organic matter contains nutrient within its structure (e.g., N, P, S) and also provides nutrient storage capacity in the form of cation exchange capacity. Additionally, canopy throughfall contributes appreciable fluxes of nutrients to the soil surface. Nutrient fluxes in canopy throughfall originate from root uptake and capture of atmospheric gases, aerosols, and particulate matter. Since oak roots are abundant at greater depths compared to the shallow rooted annual grasses, nutrient uptake by oak roots attenuates leaching losses from the soil profile. The extension of oak roots beyond the edge of the oak canopy may also contribute to nutrient differences between soils beneath the oak canopy and open grasslands. Selective uptake of nutrients by oak roots will deplete the open grasslands of nutrients while concentrating these nutrients beneath the oak canopy.

A further effect of the oak canopy on nutrient cycling occurs through canopy processes reducing the leaching and erosion potentials. Transpiration was approximately 7% greater in the oak/understory compared to the open grasslands and canopy interception reduces the amount of water reaching the soil surface by 23%. The combined effect is 30% less water available for leaching in the soils beneath the oak canopy. In addition to the positive effect of organic matter on the soil nutrient status, higher organic matter concentrations lead to lower soil bulk density and greater porosity. This in turn provides increased infiltration rates which reduce surface runoff, water erosion, and stream water suspended sediment with its associated nutrients. Thus, there are several biogeochemical processes by which oak trees concentrate nutrients and create islands of enhanced fertility beneath their canopy.