Planktonic heterotrophic bacterial production in Mirror 1,ake (New IIampshirc) was cstimated by several different methods. Production ranged between 3 and 8 g C * m 2* yr-' based on organic C fluxes and the sum of bacterial losses. The close agreement with the estimate of 6.5 * 1.8 (X ? SE) obtained by the ssSOa method (in which bacterial S uptake was multiplied by a C:S ratio of 50) supports the validity of using this method in aerobic freshwaters.

Within 2 km of a zinc (Zn) smelter in Palmerton, Pennsylvania, near the Lehigh Water Gap, up to 13.5% Zn by weight has been measured in the O2 horizon of the soil, and up to 8% Zn in the A1 horizon. The total numbers of bacteria, actinomycetes, and fungi (measured by dilution plate counts) were greatly reduced in the most severely Zn-contaminated soils compared with control soils. The reduction of microbial populations may be a partial cause of the decreased rate of litter decomposition at Lehigh Gap. Growth of most bacteria from control sites was reduced by 100 to 200 muM Zn, most actinomycetes by 100 muM Zn, and most fungi by 100 to 1000 muM Zn in thin-Pablum extract agar (TPab). All the tested actinomycetes and non-spore-forming bacteria isolated from Zn-contaminated Lehigh Gap soils were Zn-tolerant, growing normally in media containing 600-2000 muM Zn. Most fungi, regardless of source, were capable of at least 50% of normal growth at 700 muM Zn. Zinc-tolerant bacteria, actinomycetes, and fungi were readily isolated from low-Zn soils, suggesting that selection for Zn tolerance may proceed rapidly. Acidophilic Mortierella species have been selectively eliminated near the smelter, apparently because of elevated soil pH. Peryronellaea glomerata (Corda) Goidanich and Coniothyrium spp. were found only in the high-Zn soils.

Fire dependent pitch pine (Pinus rigida Mill.) and scrub oak (Quercus ilicifolia Wang.) barrens are found on coarse-textured, droughty soils in the northeastern United States. These communities are globablly rare and, in many areas, dependent on active management to sustain them. We used historic and recent aerial photographs of the Central Pine Barrens in New York to develop transition matrices and trajectories of vegetation change. A vegetation map for 1990 provided pre-burn cover type information for a 1,200 ha area burned by an extremely severe wildfire in August 1995. Geographic information system analyses, a firehistory study, post-wildfire research and existing literature provided additional information for development of three conceptual models that describe our understanding of vegetation change in response to fire and land clearing. A simulation model was then used to predict the amount of each cover type in 2046. The reasonableness of the prediction was assessed based on our understanding of pine barrens vegetation dynamics.In 1938, following a period of frequent, intense wildfires of the region, 90% of vegetation in the study area was open canopy barrens (including dwarf pine plains, pitch pine-scrub oak woodland, heath, pine-heath woodland, and scrub oak shrubland). Between 1938 and 1994, wildfires decreased in size, the average area burned per year generally declined, and there were no fires in 70% of the study area. As a result, barrens decreased to $45% of study area vegetation. In August 1995, an unusually severe summer wildfire converted 810 ha of woodland and forest into scrub oak shrubland, and increased barrens to 60% of study area vegetation. Most areas of heath and pine-heath, which comprised 24% of barrens vegetation in 1996, are successional types that developed on formerly cleared land. In the absence of extensive burning, we expect barrens vegetation to be converted to closedcanopy forest as coppice trees and new seedlings mature. Recent changes in fire regimes are likely to result in a loss of barrens vegetation that exceeds predictions of the simulation model. Careful use of prescribed fire and/or mechanical treatments to simulate the effects of fire are recommended for future restoration and maintenance of pitch pine barrens, and for protection of human life and property. #

Zinc ores have been smelted in Palmerton, Pennsylvania, since 1898. Within 2 km of the primary smelter, up to 8% Zn, 1500 ppm Cd, 1200 ppm Cu and 1100 ppm Pb by weight were found at the surface of the A1 soil horizon. Washed, oven—dried foliage of trees located near the smelters contained up to 4,500 ppm Zn and 70 ppm Cd by weight. The Lehigh Gap area of Blue Mountain near the smelters is sparsely vegetated or completely barren over an area of about 485 ha. Forests in burned and unburned areas were sampled for density and cover of trees, shrub, and herb species. Sassafras albidum and Nyssa sylvatica are the most common tree species in the burned, severely denuded areas; Arenaria patula is the most common herb. Very few tree seedlings are found near the smelters. Species which normally invade burned areas, including Populus tremuloides, Betula populifolia, Comptonia peregrina, and Pteridium aquilinum, are rare or absent at Lehigh Gap. Inhibition of seed germination and/or seedling growth by high levels of soil Zn is probably preventing establishment of these invader species. Quercus rubra and Pinus strobus seedlings planted in pots of high Zn soil collected near the smelters produced much less root and shoot growth than normal. Minimum toxic foliar levels for Q. rubra are approximately 250 ppm Zn and between 5 and 30 ppm Cd. Arenaria patula tolerated 100 ppm Zn in sand culture and a foliar content of 13,000 ppm Zn, and is probably a zinc—tolerant ecotype. Solution concentrations of up to 100 ppm Zn and 10 ppm Cd did not affect seed germination of Q. rubra, B. populifolia, and P. tremuloides, but at these metal concentrations the radicle fails to elongate. Radicle elongation occurs, but is significantly reduced, at > 1 ppm Zn or > 5 ppm Cd in solution culture. Saturation extracts of soils collected 1—5 km from the smelter contained 40—1.6 ppm Zn and 1.0—0.02 ppm Cd; thus only the Zn content of soils within 5 km of the smelter appears high enough to stunt root growth. The combined stresses of high soil—zinc levels plus fire appear to be the primary cause of the vegetation damage at Lehigh Gap. Either stress alone would have caused much less damage. Erosion and desiccation are important secondary factors preventing revegetation of the barren areas.

Nitrogen cycling was studied during the first 2 yr of spray irrigation of vegetation by treated wastewater in Falmouth (Cape Cod), Massachusetts. We attempted to take advantage of an unexpected natural 15 N enrichment during wastewater treatment to trace the fate of wastewater N following irrigation. Wastewater N was enriched by 6-20‰ relative to soil and by 13-25‰ relative to vegetation. We were able to estimate retention of wastewater N by soil using a mass balance of 15 N. However, under conditions of N saturation ␦ 15 N values for plants were lower than expected, possibly due to discrimination against 15 N during uptake of NH 4 ϩ and NO 3 Ϫ by plant roots. This is a potential weakness of trying to use low-level natural 15 N enrichment in a tracer study. This problem is not likely to occur in N-limited ecosystems, or at high levels of isotopic enrichment.Three different ecosystem types were irrigated: a successional pitch pine woodland 26 yr in age; a mixed oak-pitch pine forest Ͼ70 yr in age; and cleared areas revegetated with grasses and old-field weeds. Areas near the spray heads were overloaded with N at deposition rates of 370-480 kg·ha Ϫ1 ·yr Ϫ1 . Pine woods and grass areas rapidly became N saturated; soil pore water NO 3 Ϫ levels reached 800 mol/L in irrigation in year 1 and 1400 mol/L in year 2. In contrast, soil pore water NO 3 Ϫ levels in the oak forest did not consistently exceed 100 mol/L until late in year 2. Soil was a major sink for wastewater N in year 1, but in year 2 soil N retention fell to near zero, and N leaching losses greatly increased. At the onset of N saturation, irrigated forests were invaded by weedy species including Phytolacca americana, Polygonum convolvulus, and Solanum dulcamera, later followed by Celastrus orbiculatus and Lonicera tartarica.