EFFECTS OF PREVENTING NUTRIENT RESORPTION ON PLANT FITNESS AND FOLIAR NUTRIENT DYNAMICS

To determine whether autumnal nutrient resorption contributes significantly to plant fitness, resorption was prevented by defoliating scrub oak (Quercus ilicifolia) plants ("defoliated plants") just prior to the onset of foliar senescence in the years 1985 through 1987. At the same time, to account for the potential influence of the reduced photosynthetic season on defoliated plants, another group ("shaded plants") was subjected to darkness, resulting in senescence in the absence of concomitant photosynthesis. In 1988, senescence and abscission were allowed to proceed normally in all study plants. In the three years that followed treatment years, 1986-1988, radial stem growth and yearly increases in foliar biomass of defoliated plants averaged 41 and 54% less than in controls. Acorn production in defoliated plants in 1989 was only 10% of that in either controls or shaded plants. In contrast to such marked responses in growth and reproduction, content of nitrogen, phosphorus, copper, manganese, and zinc in presenescent (green) leaves exhibited little response to defoliation in the previous year. In 1988, when resorption efficiencies could be measured in plants previously subjected to defoliation, there were no significant differences between control and defoliated plants for nitrogen, copper, manganese, or zinc. The efficiency of phosphorus resorption was 10% lower in defoliated plants. In shaded plants, resorption efficiencies of nitrogen, phosphorus, and copper, but not of manganese and zinc, were substantially reduced relative to controls in treatment years, indicating both quantitative and qualitative differences between dark-induced and normal senescence. The significant negative effects on growth and reproduction that resulted from preventing resorption in Q. ilicifolia offer strong support for the hypothesis that resorption contributes significantly to plant fitness.