ALBERT

Description: Nitrogen (N) mineralization and nitrification were compared among ecosystems representing a primary successional sequence on the Tanana River floodplain of interior Alaska. These processes displayed marked seasonality, were closely related to substrate chemistry, and reflected the impact of vegetation clearing. The highest rates of N mineralization were encountered in the June to July incubation periods, and rates generally declined during the remainder of the summer. The early season period (June to July) was the interval of most favorable litter and mineral soil temperature and most available energy supply for microbial mineralization of detrital materials. Minimal rates were encountered during the winter. Litter layer N mineralization rates were highest in the early-successional poplar–alder (Populusbalsamifera–Alnustenuifolia (Nutt.) stage and declined with advancing succession in poplar (Populusbalsamifera) and mature white spruce (Piceaglauca) (Moench) Voss) stands. The poplar–alder stage displayed the highest rate of nitrification. Nitrate constituted 98% of the mineralized N in early-successional poplar–alder forest floors but fell to 4 and 0% in poplar and white spruce forest floors, respectively. Nitrogen mineralization was closely related to significant increases in the lignin/N ratio across the sequence of vegetation types. The rate of surface mineral soil net N mineralization increased with succession in response to higher soil organic matter content. The range of average total seasonal net N mineralization (260–1600 mg N•m−2) for litter layer plus mineral soil among successional stages in this study was generally lower than the 1200–8400 mg N•m−2 reported by investigators for other studies in temperate latitudes. Vegetation clearing increased the magnitude of temporal fluxes as well as total annual mineral N production. The most consistent increases were encountered in the poplar–alder vegetation type. The average seasonal total net N mineralization for forest floor plus mineral soil in this vegetation type increased from 1500 to 3264 mg N•m−2 as a result of clearing. Soil temperature declined with advancing succession and generally increased as a consequence of clearing. However, these changes were not as closely correlated with N mineralization as were the changes in substrate chemistry encountered across this successional sequence.

Description: The comparative analysis of a large set of long-term fertilization and thinning studies in the major forest types of interior Alaska is summarized. Results indicate that nutrient limitations may only occur during the early spring growth period, after which moisture availability is the primary control of tree growth on warm sites. The temperature dynamics of both air and soil set seasonal bounds on the nutrient and moisture dynamics for all forest types. Air and soil temperature limitations are the primary control of intraseasonal growth in the colder topographic locations in interior Alaska. These locations are usually dominated by black spruce (Picea mariana (Mill.) Britton, Sterns, Poggenb.) vegetation types. The seasonal progression of factors controlling growth is strongly tied to the state factor structure of the landscape.

Description: Fine root production and turnover were studied in hardwood and coniferous taiga forests using three methods. (1) Using soil cores, fine root production ranged from 1574 ± 76 kg•ha−1•year−1 in the upland white spruce (Piceaglauca (Moench) Voss) stand to 4386 ± 322 kg•ha−1•year−1 in the floodplain balsam poplar (Populusbalsamifera L.) stand, accounting for 49% of total production for coniferous stands and 32% of total production for deciduous stands. Fine root turnover rates were higher in floodplain (0.90 ± 0.06 year−1) stands than in upland (0.42 ± 0.10 year−1) stands. Across all sites, the ratio of fine root turnover to litter fall averaged 2.2 for biomass and 2.8 for N. Both values were higher in floodplain stands than in upland stands, and in coniferous stands than in deciduous stands. (2) The C budget method showed that C allocation to fine roots varied from 150 to 425 g C•m−2•year−1 and suggested that soil respiration was more dependent on C derived from roots than from aboveground inputs. The C allocation ratio (C to roots: C to litter fall) was inversely correlated with litter-fall C and varied from 0.3 to 69.5; there was a tendency for higher proportional belowground allocation in coniferous stands than in deciduous stands and the highest levels were at the earliest successional sites. (3) Estimates of apparent N uptake (Nu), N allocation to fine roots, and fine root production based on N budget calculations showed that annual aboveground N increments exceeded Nu estimates at half the sites, indicating that the method failed to account for large amounts of N acquired by plants. This suggests that plant and (or) mycorrhizal uptake of soil organic N may be more significant to ecosystem N cycling than mineral N turnover by the soil microbial biomass.

Description: 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 indigenous communities. Projections of continued warming suggest that Alaska’s boreal forest will undergo significant functional and structural changes within the next few decades that are unprecedented in the last 6000 years. The impact of these social–ecological changes will depend in part on the extent of landscape reorganization between uplands and lowlands and on policies regulating subsistence opportunities for rural communities.

Description: Results of the study of salt-affected soils on the Tanana River floodplain in interior Alaska raised a number of key issues that are important to resolve with future research in this fluvial environment. The results emphasized the prominent role of plant succession in ecosystem structure and function in northern boreal forests. For example, alder plays a crucial role in organic matter and nitrogen dynamics in floodplain ecosystems. This plant species has an impact on ecosystem processes in successional stages beyond its period of dominance on the floodplain. The potential facilitative nature of nitrogen accumulation, mediated by alder, to plant community development during and subsequent to the alder phase of succession should be examined in future studies. Organic matter chemistry and soil temperature appeared to be more important controls of soil processes than high soil salt content. Moreover, secondary plant chemicals may play a role in determining rates of soil nitrification in these floodplain ecosystems. The importance of ion balance to plant nutrition and primary production in salt-affected soils is an important topic for future research. Groundwater may be a unique source of water and nutrients to floodplain plant communities. The magnitude of water flux to the rooting zone in relation to terrace elevation and river level and its importance to plant growth is a significant consideration in the semiarid environment of interior Alaska. The established plant community indicates which species are successful in this environment. Optimum species-terrace combinations may exist that maximize productivity through use of unique moisture and nutrient supplies associated with a shallow water table. Results of this work suggested that new research initiatives are crucial to advance the fundamental understanding of controls of ecosystem processes and as a base of information to support forest resource management.

Description: Because a large fraction of the world's carbon exists in the soil of boreal forests, understanding how soil temperature and moisture affect soil respiration is vital for predicting soil response to climate change. We measured soil respiration and CO2 concentrations within soils of floodplain and upland forests in interior Alaska from 1996 to 1997. At each site, a 0.10-ha-area shelter was constructed that prevents summer precipitation from infiltrating into the soil. Measurements of soil profile CO2, soil respiration, soil temperature, and soil moisture were made inside (treatment) and outside (control) the sheltered areas through two growing seasons and the winter of 1996-1997. Sheltered soils had decreased profile concentrations and surface flux of CO2. At the upland control site, individual flux rates ranged from 0.10 to 0.95 g·m-2·h-1 in the summer and at sites under the shelter from 0.10 to 0.53 g·m-2·h-1. Rates at the floodplain control site ranged from 0.11 to 1.45 g·m-2·h-1 and under the shelter from 0.11 to 0.55 g·m-2·h-1. Fick's Law could predict surface CO2 flux when the CO2 concentration gradient within the profile accurately represented the soil surface gradient and biological sources and sinks of the gas did not overwhelm flux calculations.

Description: The structure and function of taiga ecosystems over a 3 600 000 ha area of northeastern interior Alaska was shown to be consistent with a hypothesis relating vegetative structure and dynamics to site nutrient status and soil temperature. Ordination of modal community descriptions and correlation of the ordination values with environmental parameters indicated that controls of vegetative structure and function found for the Fairbanks area, where the hypothesis was developed, can be applied to the interior Alaska taiga. High productivity sites were associated with warmer soil temperatures, smaller accumulation of soil organic layers, and lower C:N ratios. Lower productivity sites were associated with the opposite trends. Black spruce (Piceamariana (Mill.) B.S.P.) generally dominated the less productive sites, while white spruce (Piceaglauca (Moench) Voss) and hardwoods occupied the more productive sites. The successional trends described for other areas of interior Alaska appear to be valid for this remote study area.

Description: 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 variability in interior Alaska. The dendrochronological analyses indicate that climate warming has led to widespread declines in white spruce growth throughout interior Alaska that have become more prevalent during the 20th century. Similarly, demographic studies show that white spruce tree growth is substantially limited by soil moisture availability in both mid- and late-successional stands. Interannual variability in tree growth among stands within a landscape exhibits greater synchrony than does growth of trees that occupy different landscapes, which agrees with dendrochronological findings that the responses depend on landscape position and prevailing climate. In contrast, the results from 18 years of a summer moisture limitation experiment showed that growth in midsuccessional upland stands was unaffected by moisture limitation and that moisture limitation decreased white spruce growth in floodplain stands where it was expected that growth would be less vulnerable because of tree access to river water. Taken together, the evidence from the different perspectives analyzed in this study clearly indicates that white spruce tree growth in interior Alaska is vulnerable to the effects of warming on plant water balance.

Description: A total of 58 trees was sampled from eight stands across a large area of interior Alaska. Regression equations were developed to estimate standing aboveground biomass for 22 white spruce (Piceaglauca (Moench) Voss) stands. Aboveground standing biomass of white spruce in mature stands ranged from 0.92 to 23.28 kg/m2 and current annual production ranged from 89 to 2853 g•m−2•year−1.