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.
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 (Picea glauca (Moench) Voss) stand to 4386 +/- 322 kg.ha-1.year-1 in the floodplain balsam poplar (Populus balsamifera 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.
An understanding of taiga ecosystem controls is important for predicting global responses to climate change. An examination of taiga ecosystem controls is presented.
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 (Populus balsamifera - Alnus tenuifolia (Nutt.) stage and declined with advancing succession in poplar (Populus balsamifera) and mature white spruce (Picea glauca) (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.
The concept of nutrient limitation, as developed in agriculture, applies well to wild plants grown under controlled conditions, although plants adapted to infertile soils are less responsive to ...nutrient addition than are most crop species. There are serious difficulties in transferring the concept of nutrient limitation directly to plant communities, however, because (1) in comparing communities with different dominant species, the species characteristic of nutrient-rich sites are inherently more responsive to nutrient supply and may be more strongly nutrient-limited than species in low-nutrient sites, and (2) ecosystem-level feedback complicates the analysis of experiments involving fertilization. We suggest that nutrient limitation in communities can be best measured by assessing the plants' response to large nutrient additions that are sufficient to saturate chemical and microbial immobilization processes and still meet plant nutrient requirements. The magnitude of community-level nutrient limitation is highly sensitive to the potential growth rate of component species; it may be greatest in sites of intermediate fertility.
Alluvial soils on the Tanana River floodplain near Fairbanks, Alaska, were examined for development of physical and chemical properties in relation to soil depth and across a 200-year vegetation ...development sequence. Development was mediated by ecosystem controls including successional time, vegetation, terrace height, soil physical and chemical properties, and microclimate. These controls interact and are conditioned by the state factors time, flora, topography, parent material, and climate, respectively. On early-successional (< 5 years) lower alluvial surfaces, terrace height above groundwater, soil particle size, and microclimate (through soil surface evaporation) interacted through capillary rise to produce salt-affected surface soil. Calcium salts of carbonate and sulfate were the principal chemicals encountered in these soils. Establishment of a vegetation cover between 5 and 10 years introduced evapotranspiration as a new mechanism, along with capillarity, to control moisture suction gradients. In addition, newly formed surface litter layers further helped eliminate evaporation and formation of high salt content surface soil. Continued sedimentation raised terrace elevation, so on older terraces only infrequent flood events influenced soil development. Moreover, in these successional stages, only the highest river stages raised groundwater levels, so transpiration and capillarity influenced water movement to tree root systems. During the first 25-30 years of succession, plant deposition of organic matter and nitrogen, associated with the growth of alder, markedly changed soil properties. Nearly 60% (or 240 g.m-2) of the 400 g.m-2 nitrogen encountered at 100 years was accumulated during this early period. After 100 years of vegetation development, soil carbonate content dropped to about half the peak values of about 1600 g.m-2 encountered between 4 and 25 years. By the time white spruce was the dominant forest type at 180 years, carbonate carbon declined to about 500 g.m-2, one-third that of the 1600 g.m-2 high. By this time surface soil pH declined from high values of 7.5 to between 5.5 and 6.0. Organic carbon continued to accumulate to about 6300 g.m-2 in the white spruce stage, twice that encountered in the alder-poplar stage at 25 years. Indices of moisture retention were most strongly related to either soil particle size (low moisture tension and available moisture range) or vegetation-mediated soil organic matter content (high moisture tension). Cation exchange capacity was most strongly related to a vegetation-mediated index of organic matter (OM) content (%N, %C, or %OM).
