Robert Stottlemyer

Nitrogen excess in North American ecosystems: predisposing factors, ecosystem responses, and management strategies

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen-fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greate...

Expansion of forest stands into tundra in the Noatak National Preserve, northwest Alaska

AbstractTemperatures across the northern regions of North America have been increasing for 150 years, and forests have responded to this increase. In the Noatak National Preserve in Alaska, white spruce (Picea glauca [Moench] Voss) forests reach their northern limit, occurring primarily on well-drained sites and as gallery forests along streams. Rolling plateaus of tundra separate the white spruce forests into disjunct stands. We examined patterns of tree age, tree growth, and tree encroachment into tundra ecosystems in six stands along the Agashashok River. Warming over the past 150 years appears to have increased tree growth and resulted in forest expansion into adjacent tundra ecosystems. The forest/tundra ecotone shifted by about 80 to 100 m into the tundra in the past 200 years, as evidenced by declining maximum tree age with distance towards the tundra. The decadal-scale pattern of tree establishment at the farthest extent of trees into the tundra (the tundra-forest ecotone) correlated with the detr...

Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado

Abstract Streamwater samples were collected during 1987–1988 from two adjacent gauged watersheds, the subalpine-alpine East St. Louis and the Fool Creek Alpine, in the Fraser Experimental Forest, Colorado. The study objective was to compare the relationships between streamwater discharge and ion concentration in alpine and alpine-subalpine watersheds at a site receiving low inputs of atmospheric contaminants. Streamwater discharge accounts for much of the variation in ion concentration. Trajectories of time, discharge, and ion concentration suggest that patterns of nutrient flux are controlled primarily by the magnitude of streamwater discharge, and seasonal differences in the relative contributions of snowmelt and soil water. In the subalpine catchment, increased streamwater discharge accounted for most of the decline in concentration of ions, with high concentrations in soil water relative to precipitation. This relationship was not seen in the alpine catchment, probably because of the influence of large diurnal variation in the ratio of snowmelt to soil water. In both catchments, ions with comparatively high concentrations in precipitation and the snowpack relative to soil water showed less concentration decline with increased streamwater discharge. The recurring nature of the trajectories, especially in the subalpine catchment, suggests that the time, discharge, and ion concentration patterns may represent a general characteristic in moderate-sized, undisturbed Rocky Mountain catchments which do not receive high inputs of airborne contaminants.

Soil nitrogen availability in some arctic ecosystems in Northwest Alaska: responses to temperature and moisture

AbstractWe explored patterns in soil nitrogen (N) availability in five ecosystems in the Noatak National Preserve in northwestern Alaska, and the potential response of N availability to changes in soil moisture and temperature. Nitrogen availability appeared to be highest in an ecosystem dominated by Alnus crispa (Ait.) Pursh, and lowest in ecosystems dominated by willows or by cotton grass tussocks. All soils responded strongly to changes in moisture; repeated watering during 30-day incubations raised net N mineralization rates by 1.5 to 3.0 fold. Net N mineralization at 12°C was 5 to 10 fold greater than at 5°C. The responses of net mineralization rates derived from strong effects of treatments on both microbial mineralization (release) of N, and microbial immobilization (uptake) of N. For example, incubation of soils from the alder site at 12°C slightly increased gross mineralization of N, more than doubled microbial immobilization of N, reduced gross nitrate production by more than 80%, and more than ...

Stream chemistry and hydrologic pathways during snowmelt in a small watershed adjacent Lake Superior

In regions with airborne contaminants and large snowpacks, there is concern over the impact that snowmelt chemical “pulses” — periods of sharp increase in meltwater solute concentration — could have on aquatic resources during spring runoff. A major variable in determining such an effect is the flow path of snowmelt solutes to the stream or lake. From December 1988, to late April 1989, the quality and quantity of precipitation, snowmelt, soil solution and streamwater were measured in a 176-ha gauged watershed on the south shore of Lake Superior. The main objectives were to (1) examine the change in flow path meltwaters take to the stream during distinct winter and spring hydrologic periods, (2) quantify ecosystem-level ion budgets prior to, during, and following snowmelt, and (3) examine if streamwater chemistry might be a sensitive indicator of change in ecosystem flow paths. Prior to peak snowmelt, groundwater made up 80% of stream discharge. During peak snowmelt, the groundwater level rose to the soil surface resulting in lateral water movement through near-surface macropores and as overland flow. Near the end of snowmelt, melt-waters exerted a piston action on deeper soil solution again increasing its relative contribution to streamwater discharge. Net groundwater drawdown during the study resulted in streamwater discharge about equal to precipitation inputs. Unfrozen soils and brief mid-winter thaws resulted in steady snowmelt throughout early and mid-winter. The snowpack lost > 50% of most ions prior to the period of major snowmelt and high stream discharge in late March and early April. Snowmelt and streamwater NO3− and NH4 pulses occurred before the period of overland flow and peak streamwater discharge (April 4–24). During overland flow, stream discharge of total N, P, DOC, and AI peaked. Nutrient budgets computed for distinct hydrologic periods were much more helpful in explaining ecosystem pathways and processes than were changes in solute concentration. For the study period, watershed base cation (CB) discharge was 23 times input and SO42− discharge exceeded input by 42%. H+ was the most strongly conserved ion with output < 0.2% of input. Also conserved were NH4+ with only 1.4% of input leaving the ecosystem and NO3− with output equal to 9.4% of input.

Seasonal change in precipitation, snowpack, snowmelt, soil water and streamwater chemistry, northern Michigan

We have studied weekly precipitation, snowpack, snowmelt, soil water and streamwater chemistry throughout winter for over a decade in a small (176 ha) northern Michigan watershed with high snowfall and vegetated by 60 to 80 year-old northern hardwoods. In this paper, we examine physical, chemical, and biological processes responsible for observed seasonal change in streamwater chemistry based upon intensive study during winter 1996-1997. The objective was to define the contributions made to winter and spring streamwater chemical concentration and flux by processes as snowmelt, over-winter forest floor and surface soil mineralization. immobilization, and exchange, and subsurface flowpath. The forest floor and soils were unfrozen beneath the snowpack which permitted most snowmelt to enter. Over-winter soil mineralization and other biological processes maintain shallow subsurface ion and dissolved organic carbon (DOC) reservoirs. Small, but steady, snowmelt throughout winter removed readily mobilized soil NO 3 - which resulted in high over-winter streamwater concentrations but little flux. Winter soil water levels and flowpaths were generally deep which increased soil water and streamwater base cation (C B ), HCO 3 - , and Si concentrations. Spring snowmelt increased soil water levels and removal of ions and DOC from the biologically active forest floor and shallow soils. The snowpack solute content was a minor component in determining streamwater ion concentration or flux during and following peak snowmelt. Exchangeable ions, weakly adsorbed anions, and DOC in the forest floor and surface soils dominated the chemical concentration and flux in soil water and streamwater. Following peak snowmelt, soil microbial immobilization and rapidly increased plant uptake of limiting nutrients removed nearly all available nitrogen from soil water and streamwater. During the growing season high evapotranspiration increased subsurface flowpath depth which in turn removed weathering products, especially C B , HCO 3 - , and Si, from deeper soils. Soil water was a major component in the hydrologic and chemical budgets.

Effect of subalpine canopy removal on snowpack, soil solution, and nutrient export, Fraser Experimental Forest, CO

Research on the effects of vegetation manipulation on snowpack, soil water, and streamwater chemistry and flux has been underway at the Fraser Experimental Forest (FEF), CO, since 1982. Greater than 95% of FEF snowmelt passes through watersheds as subsurface flow where soil processes significantly alter meltwater chemistry. To better understand the mechanisms accounting for annual variation in watershed streamwater ion concentration and flux with snowmelt, we studied subsurface water flow, its ion concentration, and flux in conterminous forested and clear cut plots. Repetitive patterns in subsurface flow and chemistry were apparent. Control plot subsurface flow chemistry had the highest ion concentrations in late winter and fall. When shallow subsurface flow occurred, its Ca 2+ , SO 4 and HCO 3 - concentrations were lower and K + higher than deep flow. The percentage of Ca 2+ , NO SO 4 2- , and HCO 3 - flux in shallow depths was less and K + slightly greater than the percentage of total flow. Canopy removal increased precipitation reaching the forest floor by about 40%, increased peak snowpack water equivalent (SWE) > 35%, increased the average snowpack Ca 2+ , NO 3 - , and NH 4 + content, reduced the snowpack K + content, and increased the runoff four-fold. Clear cutting doubled the percentage of subsurface flow at shallow depths, and increased K + concentration in shallow subsurface flow and NO 3 - concentrations in both shallow and deep flow. The percentage change in total Ca 2+ , SO 4 2- , and HCO 3 - flux in shallow depths was less than the change in water flux, while that of K + and NO 3 - flux was greater. Relative to the control, in the clear cut the percentage of total Ca 2+ flux at shallow depths increased from 5 to 12%, SO 4 2- 5.4 to 12%, HCO 3 - from 5.6 to 8.7%, K + from 6 to 35%, and NO 3 - from 2.7 to 17%. The increases in Ca 2+ and SO 4 2- flux were proportional to the increase in water flux, the flux of HCO 3 - increased proportionally less than water flux, and NO 3 - and K + were proportionally greater than water flux. Increased subsurface flow accounted for most of the increase in non-limiting nutrient loss. For limiting nutrients, loss of plant uptake and increased shallow subsurface flow accounted for the greater loss. Seasonal ion concentration patterns in streamwater and subsurface flow were similar.

Bottom-up factors influencing riparian willow recovery in Yellowstone National Park

ABSTRACT. After the elimination of wolves (Canis lupis L.) in the 1920s, woody riparian plant communities on the northern range of Yellowstone National Park (YNP) declined an estimated 50%. After the reintroduction of wolves in 1995–1996, riparian willows (Salix spp.) on YNPs northern range showed significant growth for the first time since the 1920s. However, the pace of willow recovery has not been uniform. Some communities have exceeded 400 cm, while others are still at pre-1995 levels of <80 cm mean height. We took intensive, repeated measurements of abiotic factors, including soil and water-table characteristics, to determine whether these factors might be contributing to the varying pace of willow recovery. Willows at all of our study sites were “short” (<250 cm max. height) prior to 1995 and have recovered to varying degrees since. We contrasted “tall” (>250 cm max. height) willow sites where willows had escaped elk (Cervus elaphus L.) browsing with “short” willow sites that could still be browsed. Unlike studies that manipulated willow height with fences and artificial dams, we examined sites that had natural growth differences in height since the reintroduction of wolves. Tall willow sites had greater water availability, more-rapid net soil nitrogen mineralization, greater snow depth, lower soil respiration rates, and cooler summer soil temperatures than nearby short willow sites. Most of these differences were measured both in herbaceous areas adjacent to the willow patches and in the willow patches themselves, suggesting that they were not effects of varying willow height recovery but were instead preexisting site differences that may have contributed to increased plant productivity. Our results agree with earlier studies in experimental plots which suggest that the varying pace of willow recovery has been influenced by abiotic limiting factors that interact with top-down reductions in willow browsing by elk.

Parent material depth controls ecosystem composition and function on a riverside terrace in northwestern Alaska

Abstract:Many studies have focused on factors that influence ecosystem composition and function, but little is known about the influence of varying quantities of a single parent material without confounding effects of age or location. On a riverside terrace of the Agashashok River, the depth of the cap of silt and sand over the gravel floodplain strongly influenced species composition, production, and response to additions of nitrogen (N) and water. Thin siltcaps (< 0.25 m) had vegetation dominated by herbaceous species, whereas thicker siltcaps had a strong component of shrubs. The depth of the siltcap accounted for about 50% of the variation in the first principle-component of the variation in species composition and cover. In situ net N mineralization increased with increasing siltcap depth, but net nitrification declined. Production by herbs increased by about 20% with water additions but not with N additions, and the responses were strongest at the two intermediate siltcap depths. Shrub production in...

Ecosystem development on terraces along the Kugururok River, northwest Alaska

Riverside terraces along the Kugururok River in the Noatak National Preserve provided an opportunity to study primary succession, considering general trends that apply across all terraces, and uniq...