Effects of flooding and nutrient enrichment on biomass allocation in Acer rubrum seedlings

A greenhouse experiment was conducted on Acer rubrum seedlings to evaluate the effects of flood frequency on production and allocation of biomass and to test the effects of N and P fertilization on production and allocation. Seedlings from the Dismal Swamp were subjected to three flood treatments (no flooding, intermittent flooding, and continuous flooding) and four enrichment treatments (no enrichment, N additions, P additions, and N + P additions). More continuous flooding resulted in less biomass production. Biomass increased during the study in all treatments except for root mass in the continuously flooded treatment. However, production of abundant adventitious roots compensated for the lack of normal root growth. Root/ shoot ratios exhibited the greatest decreases in the continuously flooded plants. Plants with N + P added had significantly more leaf, stem, and total mass than the nonenriched plants four months into the study. The N + P additions had apparently compensated for the effects of flood stress in the continuously flooded plants by the end of the study. The fertilized seedlings accumulated higher concentrations of N and P, but their nutrient use efficiency (biomass production per unit nutrient absorbed) was lower than in the nonenriched plants. Acer rubrum seedlings survive flooded conditions through several adaptations; however, their growth is slowed by continuous flooding. FLOODING AFFECTS rooted plants in a variety of ways. The immediate environmental effects are depletion of soil oxygen and accumulation of toxic elements (Keeley, 1979). These conditions result in altered plant metabolism, inhibited growth, stomatal closure, reduced photosynthesis, reduced nutrient and water absorption, and altered hormone balance in the plants (Kozlowski, 1984). Many believe that anoxia is the most likely cause of flooding injury (Whitlow and Harris, 1979; Kozlowski, 1984). Flooding during the growing season and stagnant water are more harmful than dormant season flooding or flowing water, even to tolerant species (Kozlowski, 1984). The periodic nature of flooding in some swamps raises interesting questions about the influence of hydroperiod on ecosystem processes. Periodically flooded systems offer intermediate habitats (Keeley, 1979) that are aquatic part of the year and terrestrial the rest. The deleterious effects of flooding on trees can be ameliorated when they are only periodic, and growth can even be promoted with the pulsing of a wet-dry cycle (Wharton et al., 1982). ' Received for publication 14 August 1986; revision accepted 3 February 1987. This material is based upon work supported by the National Science Foundation under Grant No. BSR-8405222. I thank Jeff Owens for technical assistance throughout the study and Susan West for help in obtaining seedlings. Revision of the paper was facilitated by the helpful comments of David Peterson and Richard Newsome. This study is part of a major research effort to evaluate the effects of hydroperiod on production and nutrient cycling. A greenhouse study of Acer rubrum (red maple) seedlings from the Dismal Swamp was conducted under controlled conditions. The specific objectives of this study were 1) to evaluate the effects of flood frequency on production and allocation of biomass in the seedlings and 2) to test the effects of nutrient (N and P) additions on biomass production and allocation. I hypothesized that nutrient enrichment would promote seedling growth because nonriverine swamps are commonly N or P limited. Particular attention was paid to root dynamics because of the great importance of belowground processes in ecosystem functions coupled with a scarcity of quantitative data on regulators of root growth (Harris, Santantonio, and McGinty, 1980). Acer rubrum was chosen as the test species because it is the most abundant species in many swamps throughout the southeastern United States and is considered tolerant to flooding (Whitlow and Harris, 1979). METHODS-Experimental design-Approximately 400 Acer rubrum seedlings were carefully dug up in August 1984 at a single site in the Great Dismal Swamp, Virginia, and transported to the greenhouse. The seedlings were all about the same height and biomass and were the same age. Only newly germinated seedlings