K. Van Cleve

Element Cycling in Taiga Forests: State-Factor Control

ronment characterized by drastic seasonal fluctuations in day length and temperature, a short growing season, low soil temperatures, and permafrost (Van Cleve and Alexander 1981). The taiga is part of the circumpolar forest region near the latitudinal limit of tree growth. The taiga occupies large areas of Alaska, Canada, Scandinavia, and the Soviet Union (Van Cleve and Alexander 1981). With changing climate, ecological relationships within the taiga could assume global importance, because this region contains 20% of the worlds stored carbon and is a large but unexplored source of methane and carbon dioxide, two gases implicated in causing climate change (Billings 1987, McBeath 1984, Reeburgh 1990). Flux rates of these gases are expected to change An understanding of taiga ecosystem controls is important for predicting global responses to climate change

Taiga Ecosystems in Interior Alaska

For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the black spruce type. Black spruce forests are widespread in interior Alaska and are the most fire-prone forest type. They are also the most nutrient-limited and least productive forest type, especially in the late stages of succession. Ecosystem differences in productivity and degree of nutrient limitation are controlled mainly by soil and forest-floor temperatures. (Accepted for publication 3 August 1982)

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.

Relationship of ion absorption to growth rate in taiga trees.

SummaryRates of nutrient absorption were measured on excised roots of taiga tree seedlings grown in the laboratory. Phosphate and to a lesser extent ammonium (relatively immobile ions in the soil) were absorbed most rapidly by poplar and aspen, two species with rapid growth rates and most slowly by alder and/or black spruce, species with slow growth rates. In contrast, potassium (which is more mobile in soil) was absorbed most rapidly by slowly growing species. All species had low rates of nitrate and chloride absorption. Absorption rate of each ion was most temperature sensitive in those species that typically occupy the warmest soils (i.e. poplar and aspen). We suggest that in infertile soils a high capacity for uptake is an important component of root competition only in the case of mobile ions (e.g. potassium, nitrate), because only for these ions do diffusion shells of adjacent roots overlap; in contrast plants compete for immobile ions (e.g. phosphate) only by increasing absorptive surface via root growth or mycorrhizal association.

Fire in Taiga Communities of Interior Alaska

Most forest communities in interior Alaska have been extensively influenced by recurring fire. To a large extent, the distribution of the dominant tree species has been shaped by fire. First-time visitors are often struck by the small-scale mosaic of forest types (white spruce, aspen, and paper birch) they observe on some sites in interior Alaska. Fire, working in the context of the influence of soil and topography, is most influential in the distribution of these forest types.

The role of mosses in the phosphorus cycling of an Alaskan black spruce forest

SummaryMosses account for 75% of the annual phosphorus accumulation in aboveground parts of an Alaskan black spruce forest, although they comprise only 17% of the phosphorus pool in aboveground vegetation. Sphagnum subsecundum and feathermosses (Hylocomium splendens and Pleurozium schreberi) have a higher capacity to absorb phosphate than do the fine roots of black spruce (Picea mariana) that are situated beneath the moss layer. In three of the four moss species studied, phosphate absorption capacity increases with increasing age of green tissue and decreases with increasing age of brown tissue. In the two feathermosses, which acquire moisture primarily from the air, and in Sphagnum, phosphate absorption is more rapid in green than in brown tissue. In contrast, the endohydric moss Polytrichum commune, which transports water through stem tissue from soil, absorbs phosphate most rapidly from stems in mineral soil. Two treatments designed to reduce activity of mycorrhizae (cutting of roots extending beneath the moss carpet or application to the moss surface of a fungicide that kills mycorrhizal hyphae) tended to increase phosphate retention by mosses and reduce phosphate transfer out of the experimental plots. This suggests that mycorrhizae are an important avenue of phosphorus movement out of the moss carpet and a means by which the black spruce competes with the overlying mosses for nutrients.

The stable isotope geochemistry of CaCO3 on the Tanana River floodplain of interior Alaska, U.S.A.: Composition and mechanisms of formation

Abstract On the river floodplains of interior Alaska, forests exist on calcareous, alluvial soils. The objectives of this study were to determine the stable 13 C and 18 O isotopic composition of CaCO 3 along a plant primary successional sequence (250 yr.) and to examine possible mechanisms controlling the formation of CaCO 3 in these floodplain soils. Soil samples were analyzed from duplicate plots of three successional stages: open shrub (Stage III, 4 yr. old), young balsam poplar-alder (Stage V, 30 yr. old), and mature white spruce (Stage VIII, 170–250 yr. old). The early stages of plant succession showed little variation in the mean soil δ 13 C PDB (−4.3 to −4.0%), while the Stage-VIII sites showed the greatest carbon depletion δ 13 C PDB = −7.9 to −6.2%). The mean soil δ 18 O PDB -values ranged from −16.3 to −14.6%. These low δ 18 O-values reflect, in part, the very depleted meteoric precipitation ( δ 18 O PDB = −50.3%) for this cold, continental site. A few surface “salt crust” samples showed significant enrichment in both C and O isotopes. Six calcite-bearing rock samples from the Alaska Range, the source of the alluvial parent material, fell into two classes with means for δ 13 C PDB of −0.2 and −5.2% and means for δ 18 O PDB of −14.6 and −18.7%, respectively. The early Stage-III profiles showed little variation in isotopic composition with soil depth, suggesting that the CaCO 3 was primarily inherited with the alluvial material and was not formed in situ. Surface evaporation of water played a minor role and transpirational loss of water played a major role in altering the isotopic composition of soil CaCO 3 along the successional sequence. There was no evidence supporting freezing as a mechanism controlling soil CaCO 3 precipitation. Over the 170–250-yr.-old plant successional sequence, the biotic factor significantly altered the isotopic composition of soil CaCO 3 .

Interaction between moisture, nutrients and growth of white spruce in interior Alaska

Abstract Two thinning and fertilization studies, the first in 1969 and the second in 1971, were established to evaluate the question of nutrient limitation to tree growth and the consequences of stand manipulation of soil moisture supply. Fertilizer was applied yearly for the first 5 years in both studies; growth response has been measured through 1987. Results indicate that thinning is necessary to obtain a growth response to fertilizer applied at the rate of 111 kg nitrogen ha−1. The response to fertilization after fertilization ended lasted for 4 years in plots thinned to 800 stems ha−1, while a significant response continued for only 2 years in plots thinned to 1600 stems ha−1. A soil water-balance model was calibrated for the control and treatment plots of these two studies. Soil water-deficits were estimated and correlated with yearly average basal-area growth per tree. Results indicated that there is a correlation between seasonal soil-moisture deficit and growth during the years when soil moisture was measured for the unthinned control plots (r2 = −0.787, P = 0.002) but not for the thinned and fertilized plots (r2 = −0.652, P = 0.057).