Bjartmar Sveinbjörnsson

COUPLING FINE ROOT DYNAMICS WITH ECOSYSTEM CARBON CYCLING IN BLACK SPRUCE FORESTS OF INTERIOR ALASKA

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. M...

Global change and arctic terrestrial ecosystems

Global warming is likely to have the greatest impact at high latitudes, making the Arctic an important region both for detecting global climate change and for studying its effects on terrestrial ecosystems. The chapters in this volume address current and anticipated impacts of global climate change on Arctic organisms, populations, ecosystem structure and function, biological diversity, and the atmosphere.

Evidence of soil nutrient availability as the proximate constraint on growth of treeline trees in northwest Alaska

The position of the Arctic treeline, which is a key regulator of surface energy exchange and carbon cycling, is widely thought to be controlled by temperature. Here, we present evidence that soil nutrient availability, rather than temperature, may be the proximate control on growth of treeline trees at our study site in northwest Alaska. We examined constraints on growth and allocation of white spruce in three contrasting habitats. The habitats had similar aboveground climates, but soil temperature declined from the riverside terrace to the forest to the treeline. We identified six lines of evidence that conflict with the hypothesis of direct temperature control and/or point to the importance of soil nutrient availability. First, the magnitude of aboveground growth declined from the terrace to the forest to the treeline, along gradients of diminishing soil nitrogen (N) availability and needle N concentration. Second, peak rates of branch extension, main stem radial and fine-root growth were generally not coincident with seasonal air and soil temperature maxima. At the treeline, in particular, rates of aboveground and fine-root growth declined well before air and soil temperatures reached their seasonal peaks. Third, in contrast with the hypothesis of temperature-limited growth, growing season average net photosynthesis was positively related to the sum of normalized branch extension, main stem radial and fine-root growth across trees and sites. Fourth, needle nonstructural carbohydrate concentration was significantly higher on the terrace, where growth was greatest. Fifth, annual branch extension growth was positively related to snow depth, consistent with the hypothesis that deeper snow promotes microbial activity and greater soil nutrient availability. Finally, the tree ring record revealed a large growth increase during late 20th-century climate warming on the terrace, where soil N availability is relatively high. Meanwhile, trees in the forest and at the treeline showed progressively smaller growth increases. Our results suggest temperature effects on tree growth at our study sites may be mediated by soil nutrient availability, making responses to climate change more complex and our ability to interpret the tree ring record more challenging than previously thought.

Nitrogen cycling at treeline: Latitudinal and elevational patterns across a boreal landscape

ABSTRACT We studied spatial and temporal patterns of nitrogen pools and fluxes in soils at treeline and forested sites within three mountain ranges across a 785-km transect in Alaska during 2001–2002. We measured pools of soil mineral (ammonium and nitrate) and organic (amino acid and microbial biomass) nitrogen, in situ rates of net mineralization, net nitrification, net amino acid production, and decomposition, as well as soil carbon turnover in a laboratory incubation experiment. Soils at treeline were mostly colder than forested soils, particularly during fall and over winter, and had reduced rates of nitrogen cycling and litter decomposition relative to forested stands. Treeline soils also had lower rates of potential respiration per unit carbon, suggesting reduced soil organic matter quality relative to forest soils. Therefore, effects of both colder temperatures and poorer substrate quality appeared to suppress rates of nitrogen turnover at treeline. Seasonal patterns of nitrogen turnover were similar across latitudes (i.e., mountain ranges). On average, 70% of total annual net nitrogen mineralization occurred from August through May, suggesting that fall and winter are critical periods for soil nitrogen transformations in both forested and treeline ecosystems. Among mountain ranges, pool sizes and fluxes of nitrogen were similar despite significant variation in growing season length and mean annual temperatures. Soil moisture and soil organic matter quality may have stronger effects on variation in nitrogen cycling than temperature at our sites.

Microtopographic Control of Treeline Advance in Noatak National Preserve, Northwest Alaska

Arctic treeline positions are of fundamental importance to the function of high latitude landscapes, as regulators of surface energy exchange and carbon cycling. Most studies aimed at explaining current and predicting future treeline positions have examined growth trends in mature treeline trees, but treeline advance requires seed production, germination, seedling establishment, and recruitment of new trees beyond the treeline and these processes may not be well correlated with growth of mature trees. Tussock tundra is a widespread, microtopographically complex vegetation type, covering vast areas of northern Alaska and Siberia. We examined a site where the white spruce (Picea glauca) treeline has recently advanced into tussock tundra and asked if white spruce seedlings (~36-cm tall) occurred in tussocks, inter-tussocks, and frost boils disproportionate to the abundance of these habitats in the treeline ecotone. We found that seedlings disproportionately occurred in tussocks and frost boils. Seedlings found in tussocks and frost boils also showed greater branch growth than those in inter-tussocks. Tussocks and frost boils had higher soil temperatures than inter-tussocks. Tussocks had higher N, P, and K availability, whereas frost boils had greater secondary and micronutrient availability. The disproportionate occurrence of seedlings in tussocks and frost boils can likely be explained by the combination of warmer soils and greater nutrient availability. It is also possible that low competition for primary nutrients contributes to the success of seedlings in frost boils. Results of the study highlight the importance of biotic and abiotic facilitation as mediators of treeline advance in a changing Arctic.

The Effect of Temperature Preconditioning on the Temperature Sensitivity of Net CO2 Flux in Geographically Diverse Populations of the Moss Polytrichum Commune

Gametophytes of five geographically diverse populations of the moss Polytrichum commune ranging from Barrow, Alaska (71VN), to Gainesville, Florida (29?N), were grown under constant temperature of 5? and 20?C with 16 h of light each day. The net CO2 exchange of the shoot tissue produced under these conditions was subsequently studied in relation to temperature. The following patterns were noted. (1) The maximum net photosynthetic rate decreased in P. commune populations from sites along a gradient of increasing latitude, except that highest rates were found in the temperate St. Hilaire population; the difference in photosynthetic rates between the extreme values was about sevenfold. (2) Cool growth conditions resulted in low maximum net CO2 uptake rates. (3) The growth temperature affected the maximum net photosynthetic rate of low-latitude populations more than high-latitude populations. (4) The optimum temperatures for photosynthesis were equal to or higher than the growth temperatures. (5) The temperature at the center of the estimated optimal range for net CO2 uptake was correlated with both preconditioning temperature and the latitude of origin. (6) The upper temperature compensation points, which were unusually high for bryophytes, were correlated positively with the preconditioning temperature and negatively with the latitude of origin. (7) Northern populations had dark-respiration rates which were equal to or lower than those of southern populations. (8) After low-temperature acclimation, the dark respiration increased only in the southernmost population.

Drought‐induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska, USA

Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought-induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west-to-east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (Δ13 C), intrinsic water-use efficiency (iWUE), and needle intercellular CO2 concentration (Ci ) from δ13 C in tree-ring alpha-cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in Δ13 C, a decline in Ci , and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature-growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining Δ13 C. Meanwhile, estimates of Ci showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and Δ13 C along moisture gradients within each watershed, suggest that drought-induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.

Environmental Controls on Needle Gas Exchange and Growth of White Spruce (Picea glauca) on a Riverside Terrace near the Arctic Treeline

Abstract Recent studies have revealed positive and negative trends in the radial growth of treeline white spruce (Picea glauca) as temperatures have warmed in recent decades. Investigators have speculated that negative growth trends reflect the increasing importance of temperature-induced drought stress, yet direct observations of drought-induced stomatal closure have not been made in white spruce near the Arctic treeline. In this study, we measured needle gas exchange, a variety of needle traits, and branch growth in contrasting growing seasons on a riverside terrace near the Arctic treeline in Noatak National Preserve, northwest Alaska. Needle gas exchange was limited by cold soils (<7 °C), nighttime frosts, large vapor pressure deficits (VPD) and/or low soil water contents during the majority of our midday measurements. Near optimal conditions for needle gas exchange were consistently found in late August, when soils were relatively warm, air temperatures were moderate, and the VPD was relatively small. Defoliation during a two-year bud moth infestation (Zeiraphera spp.) substantially reduced branch growth, obscured potential relationships between needle gas exchange and growth, and revealed the importance of whole canopy gas exchange measurements. Results of our study show there is a very narrow window of environmental conditions for near optimal needle gas exchange in white spruce near the Arctic treeline. Although we identified many abiotic constraints on needle gas exchange, a single biological factor likely had the greatest effect on annual branch growth.

Controls on CO 2 Exchange in Two Polytrichum Moss Species. 1. Field Studies on the Tundra near Barrow, Alaska

Microclimatic habitat conditions and diel field CO2 flux were studied in Polytrichum alpinum and P. commune during the 1973 growing season and again in the late autumn of 1974, near Barrow, Alaska. Incident photon flux density differed among the three different habitats a mesic meadow, polygon center and polygon rim while differences in moss surface temperature and water content were small. Diel CO2 flux remained positive during June and much of July in unshaded samples of both species while shaded samples of P. alpinum showed night respiration and reduced daytime CO2 uptake during much of the season. The proportion of the diel assimilate pool respired at night increased sharply from first sunset on 2 August. No regular seasonal changes in maximum net photosynthetic rate could be detected in either species. Samples of P. alpinum from the shaded mesic meadow reached light compensation at lower photon flux densities than did the samples from the more open low polygon centers, especially at higher temperatures. P. commune showed a greater temperature sensitivity in net photosynthesis than P. alpinum. A seasonal total CO2 uptake of 700-800 mg g-l is estimated for nonshaded aboveground shoots.

Reindeer lichen productivity: Problems and possibilities

Reindeer lichens are important in the structure and function of tundra and taiga ecosystems, as exemplified by cover values, biomass, mineral content, and effect on other ecosystem components. They are particularly important for winter ecology of reindeer and caribou which largely relay on them. Growth measurement is difficult due to the very slow rate and the methods that have been used are not sufficiently documented, precise, or appropriate. Use of carbon dioxide exchange models, coupled with models of lichen microclimate and water relations, based on microclimatic data are suggested as alternatives for land managers. The assumptions of such models are discussed and the performance of mixed species lichen mats and of the lichen CO2 environment and its effect on lichen CO2 exchange.