Effects of sedimentation on soil nutrient dynamics in riparian forests.

The influence of sedimentation rates on biogeochemistry of riparian forests was studied near ephemeral streams at Fort Benning, GA. Upper reaches of seven ephemeral streams had received varying rates of sedimentation stemming from erosion along unpaved roadways at the military installation. Two reference catchments were also included in the study. Decomposition of foliar litter, microbial C and N, N mineralization, and arthropod populations were compared within and among catchments. Rates of sedimentation over the past 25 yr ranged from 0 in references to 4.0 cm yr(-1). Decomposition rates declined exponentially with sedimentation rates as low as 0.20 to 0.32 cm yr(-1) and appeared to reach an equilibrium at a sedimentation rate of 0.5 cm yr(-1). Nitrogen mineralization and microbial C and N followed the same trend. Sedimentation had no discernible effect on arthropod populations. These data suggest that biogeochemical cycles may be altered by sedimentation rates that commonly occur in some floodplain forests.

Ecological Responses of an Oligotrophic Floodplain Forest to Harvesting.

Aerial and ground-based harvest systems were compared in terms of effects on gaseous and hydrologic transfers of N and P. The South Alabama study sites were narrow floodplains of low order, blackwater streams and were dominated by P-deficient histolsols and a mixed deciduous, evergreen forest. There was no statistically significant harvesting effect on either denitrification or surface and groundwater chemistry. Denitrification exhibited a strong seasonal trend and considerable within-site variation with annual rates ranging from 7 to 20 kg/ha per yr. Nitrate and phosphate concentrations in waters were quite low and did not vary significantly between the harvested zone and unharvested forests immediately upstream and downstream from the harvest. An unexpected result of the harvests was a statistically significant lowering of groundwater tables, an apparent evaporation response to elevated temperatures in the dark-colored organic soils.

Response of total tannins and phenolics in loblolly pine foliage exposed to ozone and acid rain.

Tannin and total phenolic levels in the foliage of loblolly pine (Pinus taeda L.) were examined in order to evaluate the effect of atmospheric pollution on secondary plant metabolism. The trees were exposed to four ozone concentrations and three levels of simulated acid rain. Tannin concentration (quantity per gram) and content (quantity per fascicle) were increased in foliage exposed to high concentrations of ozone in both ozone-sensitive and ozone-tolerant families. No effect of acid rain on tannins was observed. Neither total phenolic concentration nor content was significantly affected by any treatment, indicating that the ozone-related increase in foliar tannins was due to changes in allocation within the phenolic group rather than to increases in total phenolics. The change in allocation of resources in the production of secondary metabolites may have implications in herbivore defense, as well as for the overall energy balance of the plant.