John Yarie

Modeling soil thermal and carbon dynamics of a fire chronosequence in interior Alaska

[1] In this study, the dynamics of soil thermal, hydrologic, and ecosystem processes were coupled to project how the carbon budgets of boreal forests will respond to changes in atmospheric CO2, climate, and fire disturbance. The ability of the model to simulate gross primary production and ecosystem respiration was verified for a mature black spruce ecosystem in Canada, the age-dependent pattern of the simulated vegetation carbon was verified with inventory data on aboveground growth of Alaskan black spruce forests, and the model was applied to a postfire chronosequence in interior Alaska. The comparison between the simulated soil temperature and field-based estimates during the growing season (May to September) of 1997 revealed that the model was able to accurately simulate monthly temperatures at 10 cm (R > 0.93) for control and burned stands of the fire chronosequence. Similarly, the simulated and field-based estimates of soil respiration for control and burned stands were correlated (R = 0.84 and 0.74 for control and burned stands, respectively). The simulated and observed decadal to centuryscale dynamics of soil temperature and carbon dynamics, which are represented by mean monthly values of these variables during the growing season, were correlated among stands (R = 0.93 and 0.71 for soil temperature at 20- and 10-cm depths, R = 0.95 and 0.91 for soil respiration and soil carbon, respectively). Sensitivity analyses indicate that along with differences in fire and climate history a number of other factors influence the response of carbon dynamics to fire disturbance. These factors include nitrogen fixation, the growth of moss, changes in the depth of the organic layer, soil drainage, and fire severity. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0330 Atmospheric Composition and Structure: Geochemical cycles; KEYWORDS: carbon, fire, nitrogen, hydrology, permafrost

Interaction of Temperature, Moisture, and Soil Chemistry in Controlling Nutrient Cycling and Ecosystem Development in the Taiga of Alaska

Dominating all aspects of forest ecosystem structure and function in the Alaskan taiga is the cold environment. Low mean annual temperature (-3.5°C) and a short growing season (90–100 days) result in a restricted period during which biological activity may occur in these forests. Low soil temperature restricts chemical weathering, organic matter mineralization, and soil profile development. Permafrost occurs in those soils with average annual temperatures of -1°C or less. Thus, inceptisols, entisols, and histosols occupy 78%, 12%, and 7%, respectively, of the land area or 97% of approximately 33,000,000 ha of interior Alaska. More intensively developed soils, including mollisols and spodosols, encompass only 3% (approximately 840,000 ha). The semiarid nature of the environment (29.4 cm mean annual precipitation, potential evapotranspiration [PET] approximately 45 cm) (Patric and Black 1968) further dictates that water will be limited to chemical and biological processes, and movement of solution through the soil will be markedly restricted compared with these conditions in many temperate latitude forest environments.

Resilience of Alaska's Boreal Forest to Climatic Change

This paper assesses the resilience of Alaska’s boreal forest system to rapid climatic change. Recent warming is associated with reduced growth of dominant tree species, plant disease and insect outbreaks, warming and thawing of permafrost, drying of lakes, increased wildfire extent, increased postfire recruitment of deciduous trees, and reduced safety of hunters traveling on river ice. These changes have modified key structural features, feedbacks, and interactions in the boreal forest, including reduced effects of upland permafrost on regional hydrology, expansion of boreal forest into tundra, and amplification of climate warming because of reduced albedo (shorter winter season) and carbon release from wildfires. Other temperature-sensitive processes for which no trends have been detected include composition of plant and microbial communities, long-term landscape-scale change in carbon stocks, stream discharge, mammalian population dynamics, and river access and subsistence opportunities for rural indige...

THE ROLE OF UNDERSTORY VEGETATION IN THE NUTRIENT CYCLE OF FORESTED ECOSYSTEMS IN THE MOUNTAIN HEMLOCK BIOGEOCLIMATIC ZONE

The biogeochemical role of understory vegetation was investigated in three sites rep- resenting a typical topographic sequence of xeric, mesic, and hygric site types characteristic of the Mountain Hemlock Biogeoclimatic Zone. Overstory biomass on the three sites was estimated to be 60.9, 55.7, and 34.5 kg/M2 for the hygric, mesic, and xeric site types, respectively. Average values for understory biomass were: 44.1, 66.1, and 399 g/m2 for the hygric, mesic, and xeric site types. Understory aboveground production (UAP) values of 26.0, 14.2, and 63.1 g.m-2-yr- for the hygric, mesic, and xeric site types were equivalent to 11.3, 6.1, and 48.6% of the estimated overstory aboveground production. The understory was found to cycle a much greater proportion of its total nutrient (N, P, Ca, Mg, K, Zn, Cu) standing crop annually compared to overstory. Approximately 80%o of the macronutrients present in the understory standing crop are found in the understory annual production on the hygric site type. Estimates of 17.6, 8.3, and 20.6 g-m2.yr-t of total understory aboveground litterfall (ex- clusive of the moss layer) were obtained for the hygric, mesic, and xeric sites, respectively. Under- story was shown to return a significant proportion of the litterfall nutrients on a yearly basis, the bulk of which was returned as a single pulse during the first autumn snowfall. Understory vegetation above the moss layer was shown to have a significant effect on the quantity of nutrients present in throughfall precipitation. The effect was seasonal with P04-P, NO:-N, and NH4-N being removed in the spring and Ca, Mg, and K being added to overstory throughfall in the autumn. Modifications of water chemistry previously attributed to the forest floor may in some cases reflect unmeasured influences of understory vegetation.

Evidence and implications of recent and projected climate change in Alaska's forest ecosystems

The structure and function of Alaskas forests have changed significantly in response to a changing climate, including alterations in species composition and climate feedbacks (e.g., carbon, radiation budgets) that have important regional societal consequences and human feedbacks to forest ecosystems. In this paper we present the first comprehensive synthesis of climate-change impacts on all forested ecosystems of Alaska, highlighting changes in the most critical biophysical factors of each region. We developed a conceptual framework describing climate drivers, biophysical factors and types of change to illustrate how the biophysical and social subsystems of Alaskan forests interact and respond directly and indirectly to a changing climate. We then identify the regional and global implications to the climate system and associated socio-economic impacts, as presented in the current literature. Projections of temperature and precipitation suggest wildfire will continue to be the dominant biophysical factor in the Interior-boreal forest, leading to shifts from conifer- to deciduous-dominated forests. Based on existing research, projected increases in temperature in the Southcentral- and Kenai-boreal forests will likely increase the frequency and severity of insect outbreaks and associated wildfires, and increase the probability of establishment by invasive plant species. In the Coastal-temperate forest region snow and ice is regarded as the dominant biophysical factor. With continued warming, hydrologic changes related to more rapidly melting glaciers and rising elevation of the winter snowline will alter discharge in many rivers, which will have important consequences for terrestrial and marine ecosystem productivity. These climate-related changes will affect plant species distribution and wildlife habitat, which have regional societal consequences, and trace-gas emissions and radiation budgets, which are globally important. Our conceptual framework facilitates assessment of current and future consequences of a changing climate, emphasizes regional differences in biophysical factors, and points to linkages that may exist but that currently lack supporting research. The framework also serves as a visual tool for resource managers and policy makers to develop regional and global management strategies and to inform policies related to climate mitigation and adaptation.

Sensitivity of soil methane fluxes to reduced precipitation in boreal forest soils

In order to better predict soil sinks of methane, we need to examine soil methane flux patterns and responses to altered soil moisture regimes. Estimates of the global atmospheric CH4 budget must also account for fluxes in the vast boreal region. We measured methane fluxes into the soil surface, methane concentrations, water content, and temperature in the soil profile in two interior Alaskan forests, over two growing seasons. At each site, a 0.10 ha rain-shelter limited summer precipitation from entering the soil. Limiting summer precipitation at the upland site generally increased that site’s soil uptake of methane. Average rates of soil methane uptake among upland plots ranged from 0.10 to 0.95 mg m ˇ2 day ˇ1 . At the floodplain site, limiting precipitation decreased the soil methane uptake of that site, and the rates here ranged fromˇ0.02 to 0.57 mg m ˇ2 day ˇ1 . Using soil profile methane concentrations, we calculated CH4 fluxes using Fick’s Law. Our inability to precisely measure the concentration gradient across the soil surface resulted in calculated flux estimates that more likely represent fluxes within the soil profile. Methane sources and sinks in the soil profile also confounded the comparison of measured and calculated fluxes. 7 2000 Elsevier Science Ltd. All rights reserved.

Vulnerability of white spruce tree growth in interior Alaska in response to climate variability: dendrochronological, demographic, and experimental perspectives

This paper integrates dendrochronological, demographic, and experimental perspectives to improve understanding of the response of white spruce (Picea glauca (Moench) Voss) tree growth to climatic v...

Flooding and Ecosystem Dynamics along the Tanana River Applying the state-factor approach to studies of ecosystem structure and function on the Tanana River floodplain

Forests found on interior Alaskan floodplains are some of the most productive in the taiga (Neiland and Viereck 1978), although they cover only a small portion of the total 45,900,000 ha of boreal forest in interior Alaska. The high primary productivity of these forests is partially a function of the frequent flooding of the rivers, which results in a dynamic equilibrium between active erosion and alluvial bar formation. Bar formation shapes the terraces on which primary succession proceeds. Silt deposited by flooding buries organic layers, provides the mineral soil seedbed required for woody plant establishment, and adds nutrients to the soil (Gill 1973, Brady et al. 1979). The high rates of sediment deposition associated with frequent flooding inhibit the establishment of some floodplain species (Walker et al. 1986). The Tanana River is a glacier-fed river sustained by north-draining tributaries from the Alaska Range. The Tanana River valley is bordered on the south by a large alluvial slope originating from the Alaska Range. This group of tributaries supplies

Spatial prediction of climatic state factor regions in Alaska

Temperature and precipitation data from weather stations in Alaska and western Canada were analyzed via universal kriging to estimate mean annual and mean growing season temperature and mean annual and mean growing season precipitation values on a 10 km grid in Alaska. These values were used to produce 35 annual eco-climatic regions and 40 growing season eco-climatic regions for the state, as well as to estimate average temperature and precipitation values for each region. The regions were compared with maximum normalized difference vegetation index (NDVI) values produced from Advanced Very High Resolution Radiometer (AVHRR) imagery. The regions alone can not be used to predict vegetation distribution, but are a valuable new source of information for those interested in parameterizing ecological models with temperature and precipitation as state factor values.

Effects of Carbon, Fertilizer, and Drought on Foliar Chemistry of Tree Species in Interior Alaska

Changes in foliar chemistry resulting from changes in forest-floor and min- eral-soil moisture availability, forest-floor microbial energy supply, and nitrogen availability were investigated across the successional sequences in both upland and floodplain landscape positions. Three amendments, sugar, sawdust, and nitrogen fertilizer (NH4NO3), were ap- plied to a series of three upland and four floodplain successional sites. The sugar and sawdust treatments were designed to increase the carbon: nitrogen ratio (C/N) of the forest floor to values typical of black spruce sites (C/N = 50). The nitrogen fertilizer treatment was designed to equal estimated yearly N mineralization in an attempt to double available nitrogen in the forest floor. A moisture exclusion treatment was designed to remove all summer rainfall from the treatment plots. Foliar phosphorus concentrations were higher in the upland sites than on the floodplain. No consistent differences were reported among successional stages within a landscape unit. The effect of either sugar or sawdust treatment was to decrease foliar phosphorus concen- trations. Sugar produced more significant differences than did sawdust. Sugar treatments decreased foliar nitrogen in all tree species except for white spruce, while fertilizer tended to increase foliar nitrogen. In the second year following treatment there was not an increase in foliar nitrogen concentration resulting from fertilizer treatment.