RETRANSLOCATION OF SHOOT NITROGEN TO RHIZOMES AND ROOTS IN PRAIRIE GRASSES MAY LIMIT LOSS OF N TO GRAZING AND FIRE DURING DROUGHT

1. It has previously been shown that perennial C-4 grasses of tallgras s prairie retranslocate up to 30% of shoot nitrogen (N) to rhizomes an d roots in response to water stress and that retranslocation contribut es to drought-related decreases in shoot N concentration and photosynt hetic capacity, resulting in decreased post-drought carbon gain for 1- 2 weeks. 2. In this paper the following hypothesis is tested: under N- limited conditions, the benefits of retranslocation may include limiti ng loss of shoot N to grazing (or fire) during drought, resulting in i ncreased end-of-season whole-plant biomass, N content, and reproductio n. All shoot tissue was removed from young N-limited plants either bef ore or after drought, thereby preventing or allowing the opportunity f or retranslocation, and the effects of each clipping treatment on biom ass and N content after flowering and senescence were determined.3. In Spartina pectinata, a mesic species that remobilizes 20-30% of shoot N during drought, plants clipped before drought (no retranslocation) h ad decreased biomass, N content, and tiller (but not seed) production relative to plants clipped after drought. In contrast, Schizachyrium s coparium, a xeric species that retranslocates little shoot N, exhibite d decreased biomass, N content, and tiller and seed production in plan ts clipped after drought: the result of growth-related increases in to tal shoot N during drought, and thus greater N loss in plants clipped after drought. Time of clipping had no effect on Andropogon gerardii, a species of intermediate drought tolerance that retranslocates ca. 10 % of shoot N during drought. 4. These results support the hypothesis t hat drought-induced shoot N retranslocation to below-ground tissues re presents a trade-off between N protection and post-drought carbon assi milation in prairie grasses.