Influence of tree canopies on grassland productivity and nitrogen dynamics in deciduous oak savanna

In oak and annual grassland savanna in the foothills of the Sierra Nevada, CA, soils under deciduous blue oak ( Quercus douglasii) canopies have higher nitrogen (N) turnover and inorganic N availability than surrounding open grassland soils, as measured by seasonal changes in inorganic N pools, mineralizable N (anaerobic incubation) and nitrification potential (chlorate inhibition assay). Higher N fertility is attributed to mineralization of oak leaf litter, which is several times more abundant under the oak canopy than in open grassland. Similar amounts of above-and below-ground litter of herbaceous plants were sampled in both areas. Soil water potential (0–30 cm) differed little between the understory and open grassland, even during the onset of summer drought in April and May. Over 3 years of study, annual above- and below-ground productivity and plant N accumulation in both communities were generally very similar. Although the two areas differ in species composition, rates of seedling establishment, seasonal phenology (more delayed in understory annuals), and dry weight and N allocation patterns (greater dependence of seed production on currently assimilated rather than stored resources in understory annuals), these differences are not great enough to affect productivity at this site. Even if forage production in the blue oak savanna does not benefit from the presence of oak canopies, soils under oaks harbor a reservoir of soil organic N that could be rapidly lost or redistributed if oaks are removed for management purposes.