Effects of Oak Tree Removal on Nutrient Cycling

To exploit the greater nutrient pools beneath oak canopies, tree removal has been advocated as a means of decreasing competition for water, nutrients, and light, presumably leading to increased forage production. The results of oak clearing trials on forage production are not consistent and appear to depend on canopy density and annual precipitation. To simulate the effects of oak tree removal, two blue oaks were cut and removed from plots in the month of August prior to the onset of the rainy season in December. Nutrient dynamics associated with oak tree removal were examined by collecting and analyzing nutrient concentrations in ecosystem waterflows (e.g., precipitation, canopy throughfall, and soil solutions). These waterflows provide a dynamic pathway for the redistribution of elements within different levels of the ecosystem. Because these solutions reveal current processes, their composition is sensitive to management practices and ecosystem disturbances. Soil solutions concentrations also reflect the plant-available nutrient status (i.e., soluble nutrients are readily available for uptake) and therefore provide a basis for comparing differences in nutrient availability resulting from various management practices. The soil solution data reported in the following section are from three collections during the January-February period following oak tree removal.

A solution charge balance diagram for the January-February time period indicates that large differences existed between nutrient concentrations in soil solutions collected beneath the oak canopy and in adjacent grasslands. In particular, concentrations of Ca, Mg, K, NO3, HCO3, and organic anions were observed to be considerably larger in soil solutions beneath the oak canopy. This reflects the greater solid-phase pools of organic matter and nutrients found beneath the canopy compared to the adjacent grasslands.