Search

We manipulated air temperature in tussock tundra near Toolik Lake, Alaska, and determined the consequences for total plant biomass, aboveground net primary production (ANPP), ecosystem nitrogen (N) pools and N uptake, and ecosystem C[O.sub.2] flux. After 3.5 growing seasons, in situ plastic greenhouses that raised air temperature during the growing season had little effect on total biomass, N content, or growing-season N uptake of the major plant and soil pools. Similarly, vascular ANPP and net ecosystem C[O.sub.2] exchange did not change with warming, although net primary production of mosses decreased with warming. Such general lack of response supports the hypothesis that productivity in tundra is constrained by the indirect effects of cold temperatures (e.g., low nutrient availability or short growing-season length) rather than by cold growing-season temperatures per se. Despite no effect on net ecosystem C[O.sub.2] flux, air warming stimulated early-season gross photosynthesis (GP) and ecosystem respiration (ER) throughout the growing season. This increased carbon turnover was probably associated with species-level responses to increased air temperature. Warming increased the aboveground biomass of the overstory shrub, dwarf birch (Betula nana), and caused a significant net redistribution of N from the understory evergreen shrub, Vaccinium vitis-idaea, to B. nana, despite no effects on soil temperature, total plant N, or N availability. Thus, although air warming had no effect on total ecosystem N pools, it did mediate N distribution within the plant community. The early-season stimulation of GP is consistent with warming effects on leaf expansion of dominant shrubs (including B. nana) observed in other studies in tussock tundra. The stimulation of ER probably resulted from maintenance and growth respiration associated with higher aboveground B. nana biomass production and higher root N uptake and/or turnover in the greenhouses. The species responses to warming are consistent with changes in plant species assemblages associated with past warming and suggest that future warming may increase the abundance of B. nana in tussock tundra. We conclude that a 4 ° C rise in air temperature alone will significantly stimulate carbon turnover in tundra but will have little effect on net carbon balance. Key words: arctic; Betula nana; carbon; C[O.sub.2]: global warming: microbial biomass; nitrogen; photosynthesis; plant biomass: primary production; respiration; tussock tundra; Vaccinium vitis-idaea., INTRODUCTION Studies of the temperature response of arctic plant physiological processes such as photosynthesis and nutrient uptake suggest that arctic plants (1) are well adapted to low temperature, having low [...]

The effect of elevated C[O.sub.2] on plant-microbial interactions and nitrogen (N) cycling is critical to predicting plant growth responses to elevated C[O.sub.2], because plant growth is often N-limited. We investigated whether the effects of elevated C[O.sub.2] on plant-microbial N dynamics differed among six annual plant species: three European grasses that have invaded California grasslands, and one grass and two forbs native to California serpentine grassland. Elevated C[O.sub.2] altered plant N pools and 15N[[H.sub.4].sup.+] uptake, but the direction and magnitude of the changes were species dependent. The introduced grasses showed increased plant N pools and 15N[[H.sub.4].sup.+] uptake, whereas the native species showed smaller increases or even decreases in plant N pools and 15N[[H.sub.4].sup.+] uptake. Under nutrient enrichment, soil microbial N and 15N[[H.sub.4].sup.+] uptake differed among soils with different plant species, but they were not affected by elevated C[O.sub.2]. At low nutrients, elevated C[O.sub.2] altered soil microbial N and 15N[[H.sub.4].sup.+] uptake, but the direction and magnitude of the changes were species dependent. The changes in soil microbial N were positively correlated with changes in the plant N pool, suggesting that there was no trade-off in N uptake between plants and microbes. These results also suggest that plant species composition will partly determine the direction of changes in soil N cycling in response to elevated C[O.sub.2]. Key words: annual grassland; elevated C[O.sub.2]; functional groups; introduced vs. native species; Jasper Ridge, California; 15N; nitrogen cycle; nitrogen immobilization, uptake, and partitioning; plant-microbe N interactions., INTRODUCTION Nitrogen (N) strongly limits plant growth in many terrestrial ecosystems (Vitousek and Howarth 1991), potentially constraining the response of terrestrial ecosystems to elevated C[O.sub.2] (Bazzaz and Fajer 1992, Field [...]

Traditional models of nutrient cycling assume that soil microorganisms must decompose organic matter, releasing inorganic N, to make N available to plants. Several lines of evidence have raised doubts about this assumption in arctic tundra, but no firm evidence existed. Here we demonstrate that Eriophorum vaginatum and Carex aquatilis, two tundra sedges, compete well for glycine and aspartate N relative to N[[H.sub.4].sup.+] in situ. At relatively high concentrations (25 [µgram] N/g soil), during the peak of the season, E. vaginatum took up amino acid N more rapidly than N[[H.sub.4].sup.+], while later in the season and at lower concentrations (2-4 [µgram] N/g soil) both E. vaginatum and C. aquatilis took up glycine N and N[[H.sub.4].sup.+] at similar rates. These results are incompatible with a simple mechanism of amino acid mineralization followed by plant uptake of the released N. These results indicate that these tundra plants have active mechanisms for enhancing their access to amino acid N in situ. Key words: amino acids; arctic; N cycling; N uptake; tundra; tussock tundra; wet meadow., INTRODUCTION A central dogma of plant and soil science has long been that soil microorganisms must decompose organic matter, releasing inorganic N, before that N becomes available for plant uptake. [...]

We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska to determine how global changes in these parameters might affect community and ecosystem processes. Some of these manipulations altered nutrient availability, growth-form composition, net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climatic change. In general, short-term (3-yr) responses were poor predictors of longer term (9-yr) changes in community composition. The longer term responses showed closer correspondence to patterns of vegetation distribution along environmental gradients. Nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen shrubs and nonvascular plants. Light attenuation reduced biomass of all growth forms. Elevated temperature enhanced shrub production but reduced production of nonvascular plants. These contrasting responses to temperature increase and to nutrient addition by different growth forms 'canceled out' at the ecosystem level, buffering changes in ecosystem characteristics such as biomass, production, and nutrient uptake. The major effect of elevated temperature was to speed plant response to changes in soil resources and, in the long term (9 yr), to increase nutrient availability through changes in N mineralization. Species within a growth form were similar to one another in their responses to changes in resources (light or nutrients) but showed no consistent response to elevated temperature. Species richness was reduced 30-50% by temperature and nutrient treatments, due to Joss of less abundant species. Declines in diversity occurred disproportionately in forbs, which are important for animal nutrition, and in mosses, which maintain soil thermal regime. There was no increased abundance of initially rare species in response to any treatment. During our 9-yr study (the warmest decade on record in the region), biomass of one dominant tundra species unexpectedly changed in control plots in the direction predicted by our experiments and by Holocene pollen records. This suggests that regional climatic warming may already be altering the species composition of Alaskan arctic tundra. Key words: arctic; climate change; diversity; light; nitrogen; phosphorus; productivity; temperature; tundra., INTRODUCTION Global warming is expected to be most pronounced at high latitudes (Hansen and Lebedeff 1987, Lashof and Ahuja 1990). In the Arctic, anticipated changes include warmer air and soil [...]

TIMOTHY C. CATER [1] Abstract. We used a combination of surveys of natural vegetation and seed-sowing and seedling transplant experiments to determine the relative importance of competition and microenvironmental modification [...]

Water vapor and CO[SUP]2[SUP] exchanges were measured by the eddy covariance method in 24 ecosystems along a transect from the Arctic coast to the latitudinal tree line in northern Alaska during three growing seasons. Variations in net ecosystem exchange across the region were controlled by differences in the net uptake of CO[SUP]2[SUP] due to photosynthesis, rather than by differences in ecosystem respiration. Daytime CO[SUP]2[SUP] uptake was related mainly to site differences in leaf area index, whereas nighttime CO[SUP]2[SUP] efflux was related to leaf area index and soil moisture. Temperature had no effect on regional patterns of ecosystem respiration during the growing season. Regional variations in surface conductance were largely explained by the water content of the moss-soil layer. Water vapor and CO[SUP]2[SUP] fluxes were poorly coupled because water vapor exchange was determined largely by evaporation from mosses, whereas CO[SUP]2[SUP] exchange was controlled by vascular plant activity. This is in contrast to the close correlation between maximum rates of surface conductance and CO[SUP]2[SUP] assimilation that is observed at the global scale. The results also suggest that the relationship between the maximum rates of surface conductance and stomatal conductance in arctic tundra departs markedly from the global pattern, because the moss layer dried more readily than a moist soil. These spatially distributed eddy covariance measurements revealed new functional relationships among water vapor and CO[SUP]2[SUP] exchanges and their environmental controls in arctic tundra; however, there remains a need for winter flux observations and long-term measurements to understand the net effect of these processes on the annual carbon balance of the region. [ABSTRACT FROM AUTHOR]

Many attempts have been made to link invasions of exotic plants to specific plant traits and key attributes of invaded ecosystems. While these factors play a role in determining the potential for invasion, they are often inadequate in predicting the success of a specific invasion. We show that interactions of an invasive grass with other members of the community determine the local pattern of invasion. A fungus, Ulocladium atrum, aids the establishment of barbed goatgrass (Aegilops triuncialis) by weakening the grass's tough seed head, thereby accelerating germination and seedling establishment. In contrast, gophers, Thoraomys bottae, decrease establishment of this invader by selectively burying patches of goatgrass seedlings under mounds. Plants that survive these gopher disturbances produce seeds that are uninfected by Ulocladium atrum, which may further decrease the establishment of the next generation of goatgrass. A field survey indicated that goatgrass achieves dominance in areas with minimal gopher disturbance, but has limited establishment in pastures with high gopher activity, indicating that the landscape pattern of gopher activity influences patterns of goatgrass invasion by manipulating gopher-plant-fungal interactions. [ABSTRACT FROM AUTHOR]

Examines the effects of the interaction between fertilizer and species composition on the long term change in tundra productivity in Alaska. Insights into the interactions between changes in resource availability; Research duration; Increase in wood production.

Looks at an experiment to determine Physiological and growth responses of artic plants. Description of methods used in specific categories; Information on results; Discussions on findings in selected categories.

Presents a study that examines the relative importance of phenotypic and genotypic factors in controlling nutrient resorption by sampling several mediterranean trees and shrubs growing in serpentine and nonserpentine soils in northern California. Tissue analysis; Decline in concentrations of nitrogen in mature leaves in spring; No significant differences among species adapted to poor versus rich soils.