Facilitation and interference of Quercus douglasii on understory productivity in central California

In some studies, understory grassland productivity was <25% of open grassland productivity, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. Grassland productivity is likely to be facilitated by nutrient inputs via litterfall and throughfall under all trees, but trees with low understory grassland productivities had substantially higher amounts of fine roots in the upper 50cm of soil and much lower predawn xylem pressure potentials than trees with high understory productivities and presumably deeper root systems. Root exclosures reduced the negative effects of these trees on the dominant understory grass species. The negative effects of the tree roots may partially result from allelopathic oak root exudates. Thus, shallow fine tree roots may inhibit understory productivity, and variations in Q. douglasii root morphology may explain the intertree variations in facilitating/interfering effects on understory species. -from Authors