The soils and food webs associated with mid to high elevation, forested, headwater streams in northeastern North America are potential hotspots for mercury (Hg) methylation and bioaccumulation, but are not well studied. Our goals were to quantify total Hg (THg) and methyl Hg (MeHg) concentrations in soils and terrestrial food webs associated with headwater streams of northern hardwood forests to identify predictors of small-scale spatial variation in Hg bioaccumulation. We sampled soil characteristics that promote Hg methylation including pH, sulfur and calcium content, and organic matter. To assess spatial variation, we sampled at high (~700 m asl) and mid elevations (~500 m asl), both adjacent to (<1 m) and away from (>75 m) three replicate headwater streams in each of two watersheds of the White Mountains region, New Hampshire, USA. Soils of these forested watersheds differed significantly in pH and the content of calcium, sulfur, organic matter and THg. Conditions for methylation were more favorable in the upland forest sites compared to streamside sites. Significant bioaccumulation of THg occurred in all measured components of the food web, including insects, spiders, salamanders, and birds. Trophic position, as determined by δ15N, was the best predictor of both THg and MeHg bioaccumulation across the sampled taxa and was also a better predictor than spatial location. However, the degree of bioaccumulation at which MeHg significantly affects animal behavior, reproduction or survival is unknown for most taxa in terrestrial habitats, particularly for invertebrates. These findings show that Hg methylation and bioaccumulation is not limited to areas traditionally classified as wetlands or to areas with exceptionally high THg inputs, but that it is a widespread and important phenomenon in the moist deciduous forests of eastern North America.

Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.

The seasonal change in soil oxygen availability was determined in several habitats along a topographic moisture gradient in an arctic watershed. The effect of changes in soil aeration on soil chemical and plant properties was examined by comparison of the driest (tussocks) and wettest (wet sedge tundra) sites along this gradient. Spatial variability and seasonal change in soil oxygen availability was closely linked to the hydrologic regime and the thickness of the organic soil horizon. The greatest extension of the aerobic soil layer was found beneath well-drained tussocks, while less than 10% of the unfrozen soil layer is aerated in flooded wet sedge tundra. Intertussock areas and watertracks (channels of water drainage) have intermediate levels of aeration. In tussock tundra, soil oxygen diffusion is restricted in the mineral soil layer below the organic horizon due to reduced pore space. Organic matter constituents and their change with depth were similar beneath tussocks and in wet sedge tundra, indicating that factors other than soil aeration (e.g. low soil temperatures, short growing season) are the primary controls on decomposition in these two arctic tundra systems. NH4+, the dominant form of inorganic N, was more available in wet sedge tundra than in tussock tundra. At both sites, extractable and soil solution NO3- concentrations increased 4 to 8 fold in the second part of the growing season, indicating increased nitrifier activity with improved soil oxygen availability. Although soils thawed as deep as 60 cm, approx. 90% of the root biomass was concentrated within 20 cm of the surface. Despite the anaerobic soil environment in wet sedge tundra, the dominant species there, Eriophorum angustifolium, reached slightly greater rooting depths than E. vaginatum, whose roots grow in the elevated, aerobic portion of tussocks. E. angustifolium had a root porosity of 31%, within the range found for wetland species, while roots of E. vaginatum had a porosity close to 12%. Rhizome porosity were low in both species (11%).

Eriophorum vaginatum and E. angustifolium are dominant arctic sedges of the well-drained tussock tundra and the permanently flooded wet-sedge tundra, respectively. We determined diurnal courses of gas exchange and water relations of the two species in their natural habitat and compared their responses to changes in light, air temperature, and humidity. Mean photosynthetic response to light was similar between E. angustifolium and E. vaginatum and carbon gain in both species was light limited during most of the growing season. On sunny and dry days, both species closed stomata in response to high leaf-to-air vapor pressure deficits. Even though E. angustifolium was growing in standing water, it exhibited a tighter control of transpirational water loss and had lower hydraulic conductivity in the soil-root-shoot pathway than E. vaginatum. The different response pattern between the two species is discussed in the context of differences in habitat conditions.

We examined the extent to which carbon investment into secondary compounds in loblolly pine (Pinus taeda L.) is changed by the interactive effect of elevated CO2 and N availability and whether differences among treatments are the result of size-dependent changes. Seedlings were grown for 138 days at two CO2 partial pressures (35 and 70 Pa CO2) and four N solution concentrations (0.5, 1.5, 3.5, and 6.5 mmol l−1 NO3NH4) and concentrations of total phenolics and condensed tannins were determined four times during plant development in primary and fascicular needles, stems and lateral and tap roots. Concentrations of total phenolics in lateral roots and condensed tannins in tap roots were relatively high regardless of treatment. In the smallest seedlings secondary compound concentrations were relatively high and decreased in the initial growth phase. Thereafter condensed tannins accumulated strongly during plant maturation in all plant parts except in lateral roots, where concentrations did not change. Concentrations of total phenolics continued to decrease in lateral roots while they remained constant in all other plant parts. At the final harvest plants grown at elevated CO2 or low N availability showed increased concentrations of condensed tannins in aboveground parts. The CO2 effect, however, disappeared when size differences were adjusted for, indicating that CO2 only indirectly affected concentrations of condensed tannins through accelerating growth. Concentrations of total phenolics increased directly in response to low N availability and elevated CO2 in primary and fascicular needles and in lateral roots, which is consistent with predictions of the carbon-nutrient balance (CNB) hypothesis. The CNB hypothesis is also supported by the strong positive correlations between soluble sugar and total phenolics and between starch and condensed tannins. The results suggest that predictions of the CNB hypothesis could be improved if developmentally induced changes of secondary compounds were included.

The North Slope of the Brooks Range in Alaska spans three physiographic provinces: the Coastal Plain, the Brooks Range Foothills, and the Brooks Range proper (Hultén 1968). The vegetation of the foothills province, which includes the Imnavait Creek watershed, is composed of a complex mosaic of communities that is tightly coupled to environmental gradients. This mosaic encompasses exposed upland ridges and mountain slopes, which have communities similar to those found in large areas of the High Arctic (Bliss and Matveya 1992), and, at the other extreme, the communities in valley basins resemble the sedge meadows of the Coastal Plain (Webber 1978). The distribution of these tundra communities is influenced by glacial history and topography, soil development, cryoturbation effects, and local erosion (Walker and Everett 1991; Chap. 4 this Vol.), as well as by summer thaw depth and soil water mobility (Giblin et al. 1992).

Ecosystem carbon dioxide (CO2) exchange is important because it is an indicator of energy captured by the system, it is related via decomposition to nutrient turnover, and long-term carbon storage and release affect atmospheric CO2 concentrations and the global carbon budget. The large amount of carbon stored in northern soils — and the potential for its release to the atmosphere with climate warming (Miller 1981) — has stimulated much research on ecosystem gas exchange in the Arctic. This work has focused primarily on the potential response of these systems to shifts in climate factors (Oechel and Billings 1992). Because of its stature, tundra is one of the few natural ecosystems for which whole-system CO2 fluxes have been determined (e.g., Grulke et al. 1990; Oechel et al. 1992, 1993; Tenhunen et al. 1995), because relatively small chambers can be placed over “representative patches” of tundra vegetation (Grulke et al. 1990; Vourlitis et al. 1993).

Carbon dioxide efflux and soil microenvironment were measured in three upland tundra communities in the foothills of the Brooks Range in arctic Alaska to determine the magnitude of CO2 efflux rates and the relative importance of the belowground factors that influence them. Gas exchange and soil microenvironment measurements were made weekly between 14 June and 31 July 1990. The study communities included lichen-heath, a sparse community vegetated by lichens and dwarf ericaceous shrubs on rocky soils, moist Cassiope dwarf-shrub heath tundra, dominated by Carex and evergreen and deciduous shrubs on relatively deep organic soils, and dry Cassiope dwarf-shrub heath of stone-stripe areas, which was of intermediate character.Rates of CO2 efflux were similar for the three communities until mid-season when they peaked at rates between 4.9 and 5.9 g m(-2) d(-1). Following the midseason peak, the rates in all three communities declined, particularly in the lichen-heath. Seasonal patterns of CO2 efflux, soil temperature, and soil moisture suggest changing limitations to CO2 efflux over the course of the season. Rates of carbon dioxide efflux followed changes in soil temperature early in the season when soil moisture was highest. Mid-season efflux appeared to be limited by soil, moss, and lichen hydration until the end of July, when temperature again limited efflux. Differences between the communities were related to microenvironmental differences and probable differences in carbon quality. The presence of peat-forming mosses is suggested to play an important role in differences in efflux and microenvironment among the communities.

In arctic tundra soil, oxygen depletion associated with soil flooding may control plant growth either directly through anoxia or indirectly through effects on nutrient availability. This study was designed to evaluate whether plant growth and physiology of two arctic sedge species are more strongly controlled by the direct or indirect effects of decreased soil aeration. Eriophorum angustifolium and E. vaginatum, which originate from flooded and well-drained habitats, respectively, were grown in an in situ transplant garden at two levels of soil oxygen, nitrogen, and phosphorus availability over two growing seasons. In both species, N addition had a stronger effect on growth and biomass allocation than P addition or soil oxygen depletion. Net photosynthesis and carbohydrate concentrations were relatively insensitive to changes in these factors. Biomass reallocated from shoots to below-ground parts in response to limited N supply was equally divided between roots (nutrient acquisition) and perennating rhizomes (storage tissue formation) in E. angustifolium. E. Vaginatum only increased its allocation to rhizomes. In the flood-tolerant E. angustifolium, growth was improved by soil anoxia and biomass allocation among plant parts was not significantly affected. Contrary to our initial hypothesis, whole-plant growth in E. vaginatum improved in flooded soils; however, it only did so when N availability was high. Under low N availability growth in flooded soils was reduced by 20% compared to growth in the aerobic environment. Reduced biomass allocation to rhizomes and thus to storage potential under anaerobic conditions may reduce long-term survival of E. vaginatum in flooded habitats.

Changes in the whole plant carbon/nitrogen balance of loblolly pine grown at ambient and elevated CO{sub 2}-levels (35 and 70 Pa) and four N levels (0.5, 1.5, 3.5 and 6.5 mM NH{sub 4}NO{sub 3}) were determined by measuring concentrations of total proteins, free amino acids, carbohydrates, and phenolic compounds. Free amino acids and protein concentrations increased with N availability in needles and lateral roots, but were unchanged in stems and tap roots. Under elevated CO{sub 2} starch content was increased in needles but not in roots or stems. Changes in phenolic content in response to elevated CO{sub 2} or N availability generally followed the pattern of soluble sugar concentrations. Phenolic content was in primary needles>lateral roots>tap roots>fascicular needles>stems and decreased with increasing N availability (at>1.5 mM NH{sub 4}NO{sub 3}). Phenolic content was significantly increased at 70 Pa CO{sub 2} only in needles and stems, but not in roots.

Carbon dioxide efflux and soil microenvironmental factors were measured diurnally in Carex aquatilus-and Eriophorum angustifolium-dominated riparian tundra communities to determine the relative importance of soil environmental factors controlling ecosystem carbon dioxide exchange with the atmosphere. Measurements were made weekly between 18 June and 24 July 1990. Diurnal patterns in carbon dioxide efflux were best explained by changes in soil temperature, while seasonal changes in efflux were correlated with changes in depth to water table, depth to frozen soil and soil moisture. Carbon dioxide efflux rates were lowest early in the growing season when high water tables and low soil temperatures limited microbial and root activity. Individual rainfall events that raised the water table were found to strongly reduce carbon dioxide efflux. As the growing season progressed, rainfall was low and depth to water table and soil temperatures increased. In response, carbon dioxide efflux increased strongly, attaining rates late in the season of approximately 10 g CO2 m-2 day-1. These rates are as high as maxima recorded for other arctic sites. A mathematical model is developed which demonstrates that soil temperature and depth to water table may be used as efficient predictors of ecosystem CO2 efflux in this habitat. In parallel with the field measurements of CO2 efflux, microbial respiration was studied in the laboratory as a function of temperature and water content. Estimates of microbial respiration per square meter under field conditions were made by adjusting for potential respiring soil volume as water table changed and using measured soil temperatures. The results indicate that the effect of these factors on microbial respiration may explain a large part of the diurnal and seasonal variation observed in CO2 efflux. As in coastal tundra sites, environmental changes that alter water table depth in riparian tundra communities will have large effects on ecosystem CO2 efflux and carbon balance.