Keith Van Cleve

Forest Succession in Relation to Nutrient Cycling in the Boreal Forest of Alaska

The successional sequences described in this chapter were located in the central part of Alaska, in the general vicinity of Fairbanks, but the general trends and conclusions derived from our studies should have broader application to most of the North American taiga.

Nitrogen availability and nitrification during succession: Primary, secondary and old-field seres

Suggestions that nutrient cycles become more strongly regulated and that nitrification is progressively inhibited in the course of ecological succession have stimulated numerous field measurements. Results of these are inconsistent; in some cases nitrogen turnover and nitrification decrease during succession, while in others both increase substantially.Consideration of the nature of disturbance which initiates each succession explains much of the difference in nitrogen dynamics. Primary succession (the development of ecosystems on wholly new substrates) invariably involves a low nitrogen availability and nitrification early in succession. In contrast, destructive disturbance followed by immediate regrowth (the ‘pure case’ of secondary succession) invariably increases nitrogen availability (and generally nitrate production) in recently disturbed sites; it is followed by a decline during later stages of succession. Succession following a period of chronic disturbance (i.e. prolonged agriculture) does not follow such clear patterns; the duration and intensity of disturbance may control whether nitrogen availability and potential nitrification increase or decrease early in such seres.

SEASONAL NUTRIENT DYNAMICS OF TUNDRA VEGETATION AT BARROW, ALASKA

Seasonal patterns of nutrient concentration and standing stock were documented in aboveground plant biomass of wet meadow tundra vegetation at Barrow, Alaska. Mobile elements, such as nitrogen, phosphorus, and potassium, were present in higher concentrations in tundra than in comparable temperate species. These elements reached peak concentration within 10 days of snowmelt and decreased to about half their maximum concentration in the course of the growing season. The total standing stock of these elements increased more rapidly aboveground than did biomass itself. Nearly half of the total nitrogen, phosphorus, and potassium complement of aboveground material was retranslocated belowground before the end of the growing season. That portion of these nutrients remaining in standing dead material at the end of the growing season was leached at rates reflecting their respective solubilities in plant tissue. Calcium and magnesium increased in concentration and in total content in the course of the growing season. Dicots had higher concentrations of phosphorus and of cations than did monocots. This is discussed with respect to their differences in mycorrhizal association and distribution. The implications for herbivores of seasonal and species differences in nutrient concentration are discussed. Fertilization with nitrogen, phosphorus,

Approaches to studying nutrient uptake, use and loss in plants

Although most basic principles and techniques for studying mineral nutrition were developed in agriculture, they can, with some modification, be used for studying the mineral nutrition of wild plants. In this chapter we describe procedures that have proven useful in ecology and forestry and discuss the advantages, disadvantages and potential sources of error involved with each method.

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

RESPIRATION RATES IN THE FOREST FLOOR OF BIRCH AND ASPEN STANDS IN INTERIOR ALASKA

This paper presents data on the influence of physical and chemical properties of the forest floor, and moisture and temperature on respiration rates in birch and aspen forest floors in central Alaskan Boreal forests. Physical and chemical properties of birch and aspen forest floors did not differ sufficiently to cause significant differences in forest floor respiration between the two vegetation types. Minimum forest floor moisture contents of 20 to 50% in aspen and 20 to 70% in birch produced marked decreases in forest floor respiration rates. This effect was greatest at 1 and 5?C where respiration dropped to zero. During periods of adequate moisture litter respiration rates were approximately 5 to 10 times greater at 20?C than at 1?C. Maximum average respiration rates during the experimental period, obtained at 30?C, were approximately 0.05, 0.20, and > 0.30 g CO2 m-2 hr-l for aspen L, F, and H layers, respectively, and 0.06, 0.21, and > 0.30 g CO2 m-2 hr-1 for birch L, F, and H layers, respectively. Multiple regression analysis showed that temperature is 2 to 5 times more important than moisture in explaining variability in respiration data. For both litter types the respiration rate on a per gram basis of the L layer > F layer > H layer. However, when the volume weight of each litter layer is considered, the respiration rate of the H layer > F layer > L layer.

A comparison of four methods for measuring respiration in organic material

Abstract Four methods for measurement of respiration (CO 2 evolution) were compared on uniformly prepared, birch forest-floor organic matter; the methods included infrared gas analysis (i.r.g.a.), gas chromatography (g.c.), KOH absorption of CO 2 (b.a.) and the Gilson respirometer. Respiration estimates were made at a single temperature and also at variable temperatures at constant moisture contents. The i.r.g.a. and b.a. methods gave maximum estimates for respiration; the g.c. gave lowest, with the Gilson intermediate. Lower estimates for the g.c. are attributed to the closed system employed, which included no means of increasing gas diffusion (such as a KOH-CO 2 sink or continuous gas flow) out of the sample into the unoccupied space of the respiration chamber. At 25°C, the minimum sensitivity of the g.c. and Gilson were 3.8 and 3.6 μg CO 2 , respectively. The minimum sensitivity of the b.a. method (44 μg CO 2 ) could have been improved with use of a micro-liter burette for titrations. The least minimum sensitivity, 0.31 μg CO 2 , was attained with the i.r.g.a. Conditions under which any of the techniques may be used to estimate respiration should be carefully worked out. The relationship between sample size, gas flow rate (open system), chamber atmosphere composition (closed system), and chamber design should be established with sufficient confidence so that stimulation of respiration by excessive gas flow rate, or exhaustion of O 2 or CO 2 inhibition through extended chamber closure does not adversely affect respiration estimates.

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.

Forest-floor nitrogen dynamics in a 60-year-old paper birch ecosystem in interior Alaska

SummaryAn analysis was conducted of nitrogen dynamics in the forest floor of a paper birch forest in subarctic Alaska. Low addition levels of high enrichment isotope (<1% of the total nitrogen pool with 95 to 99 atom percent excess15N) and isotope dilution analysis, were used to establish the kinetics of nitrogen flow. The pools examined were NH4, NO3+NO2, soluble organic-N and forest floor organic matter. This approach allows a more realistic assessment of rates of N movement at the levels of nitrogen concentration encountered in natural systems.