Vol. 32, No. 1, February 2000
Extreme Tree Rings in Spruce (Picea abies [L.] Karst.) and Fir (Abies alba Mill.) Stands in Relation to Climate, Site, and Space in the Southern French and Italian Alps (pp 1-13)
The similarity over long distances of dendroecological pointer years (with extreme ring-widths) were studied at both regional and country scales, in order to investigate the geographical extension of climate influences on tree-rings. Two common species, Norway spruce (Picea abies Karst.) and white fir (Abies alba Mill.) were compared. The regional study was carried out on 33 populations located in four alpine valleys along a climatic gradient of summer aridity (Tarentaise, Maurienne, and Brianšonnais, in France, and Susa valley in Italy). For most of species and regions, several negative ring-width pointer years with abrupt growth reductions such as 1976, 1922, 1986, and 1944 were common (listed in order of decreasing importance). However, spruce growth was more reduced in 1948 than that of fir. At the country scale, some of the strongest positive (e.g., 1932, 1964, 1969) and negative (e.g., 1956, 1962, 1976, 1986) pointer years extended over the whole of France, whereas the geographic variability was explainable by the autoecology of species. At both studied scales, evident climatic interpretations such as severe winter frosts, unusual summer droughts, or excessive wet and cold springs can explain most of the negative pointer years. Conversely, most positive growth responses are caused by a local combination of favorable climatic factors rather than simple extreme events, and therefore are less efficient for wood dating.
Tree Growth near Treeline: Abrupt or Gradual Reduction with Altitude? (pp 14-20)
Natural climatic treelines are relatively discrete boundaries in the landscape established at a certain elevation within an otherwise continuous gradient of environmental change. By studying tree rings along elevational transects at and below the upper treeline in the European Alps, we (1) determine whether radial stem growth declines abruptly or gradually, and (2) test climatic influences on trees near treeline by investigating transects for climatically different historical periods. While tree height decreases gradually toward the treeline, there is no such general trend for radial tree growth. We found rather abrupt changes which imply threshold effects of temperature which moved upslope in a wave-like manner as temperatures increased over the past 150 yr. Currently radial tree growth at treeline in the Alps is the same magnitude as at several hundred meters below current treeline. Over short intervals, tree-ring width is more dependent on interannual climatic variability than on altitudinal distance to treeline. We conclude that (1) the elevational response of tree-rings includes a threshold component (a minimal seasonal temperature) and that (2) radial growth is more strongly correlated with year to year variation in climate than with treeline elevation as such. Our data indicate that the current treeline position reflects influences of past climates and not the current climate.
Humus Forms in the Forest-Alpine Tundra Ecotone at Stillberg (Dischmatal/Switzerland): Spatial Heterogeneity and Classification (pp 21-29)
In the forest-alpine ecotone at Stillberg (Dischmatal/Switzerland) the morphology of humus forms and the spatial variability of organic layer properties were investigated. At northeast-exposed gully sites mulls with high acidity in the A-horizon occur. They were classified after the Canadian classification of humus forms as Rhizomulls. Mors occur on ridges and on their east- and north-exposed aspects. They can be differentiated by the ratio between the thickness of the F-horizon and the combined thickness of the F- and H-horizon. The relative thickness of the F-horizon increases significantly in the order: east aspects ridges north aspect. The humus forms of the east aspects and the ridges were classified as Humimors and those of the north aspects as Hemimors. The Canadian classification was suitable to describe the properties of the horizons and to classify the humus forms. The results of a grid sampling at the study sites and the computation of nonergodic correlograms show that the spatial variability of organic-layer thickness, bulk density, and moisture is high (CV around 50%), with a pronounced small-scale heterogeneity (range usually below 2 m and more than 50% variability occurs within 0.3 m). Only 33% of the variance of organic-layer thickness were explained by site and vegetation structure, but in spite of the low percentage both proved to be a significant factor. In the forest-alpine tundra ecotone about 30 to 35 soil samples per site are needed for a reliable estimation of the mean of the organic-layer thickness.
Nutrient and Thermal Regime, Microbial Biomass, and Vegetation of Antarctic Soils in the Windmill Islands Region of East Antarctica (Wilkes Land) (pp 30-39)
In the antarctic summer of 1996, permafrost-affected cold soils close to the Australian Casey Station in the Windmill Islands region (Wilkes Land) were investigated to determine in what way the thermal and nutrient regimes in the antarctic soils are related to microbial biomass and vegetation patterns. The soils are characterized by a high content of coarse mineral particles and total organic carbon (TOC) and a low C/N ratio (mean 11). Despite the low pH values (mean 4.0) the soils are rich in nutrients due to an input from seabirds (existing or abandoned nesting sites) and an eolian distribution of fine-grained soil material in the landscape. Vegetation influences TOC storage and the cation exchange capacity in the uppermost soil horizons, whereas total N and most nutrient levels are not affected by the vegetation, but by seabird droppings. The present nutrient level does not affect plant adaptation, because the K, Mg, and P contents are often extraordinarily high. This suggests that nutrient supply is not a limiting factor, whereas microclimate effects, such as moisture availability and ground-level wind speed, have a primary influence on plant growth. Soil-surface temperature measurements indicate a strong variability in microclimate due to small-scale variations in geomorphological surface features. Bacteria found in all soil horizons, but not algae and yeast. Soil microbial counts are weakly correlated to the C/N ratios and soil surface temperatures. High TOC and clay contents probably improve the soil water-holding capacity and TOC contributes to the microbial food supply. The investigated microbial parameters are weakly correlated to the present vegetation carpet, the lowest counts are found in the soils with scattered or no vegetation cover.
Constraints to Nitrogen Fixation by Cryptogamic Crusts in a Polar Desert Ecosystem, Devon Island, N.W.T., Canada (pp 40-45)
Polar desert ecosystems, which dominate the landscape throughout much of the High Arctic, are environmentally stressed and limited in their development. Scattered intermittently over these landscapes are areas of cryptogamic crust development that are associated with increased vascular plant abundance. Since nutrient limitation, especially nitrogen, is significant in these ecosystems, I wished to examine the role of these cryptogamic crusts in the supply of fixed nitrogen and the constraints to that fixation. Nitrogen fixation rates (as measured by acetylene reduction) were highest in sites with a well-developed cryptogamic crust, lowest in sites with only bare mineral soil, and intermediate in sites with a partially developed crust. Highest rates of acetylene reduction (i.e., nitrogen fixation) were seen within a few days of snowmelt (late June to early July) and declined as the season progressed, until near the end of the growing season (1-5 August) when rates were approximately 50% of early season rates. Late season precipitation events restored acetylene reduction rates to near original levels. In manipulative experiments, acetylene reduction rates dropped dramatically as crust moisture content declined and rates increased as soil surface temperature increased to 24░C. A significant finding was that acetylene reduction at 3░C was 40% of that found at 12 to 13░C. Thus, there is a potential for nitrogen accumulation even during the colder periods of the growing season. As calculations show, the quantity of nitrogen fixed by these cryptogamic crusts was adequate to support the nitrogen needs of the mosses and vascular plants of these developing ecosystems.
Pliocene Piedmont Glaciation in the RÝo Shehuen Valley, Southeast Patagonia, ArgentÝna (pp 46-54)
There are few sites in southern South America where late Tertiary glacial sediments have been radiometrically dated. Glaciations occurred near the Miocene-Pliocene transition, during the mid-Pliocene (3.5 Ma), and after 2.1 Ma ago. In the RÝo Shehuen valley at least four river terraces older than 2.25 Ma correlate with terminal moraines and outwash plains. The age of the youngest Pliocene advance is bracketed by two radiometric dates; glaciofluvial sediments lie on a 3.0 Ma old basalt lava and merge into a river terrace that is covered by 2.25 Ma old basalts. A remnant of a terminal moraine of the second oldest advance still exists. This glacier advance extended over 160 km from the Southern Patagonian Icefield. The oldest glaciation occurred well before 3.0 Ma. It may be possible to correlate these pre-Pleistocene glaciations to glacial deposits covering the Meseta Desocupada north of the glacial basin of Lago Viedma (250 m altitude). There, at an elevation of 1500 m, previous research found a till lying between two basalt flows with an age of 3.5 Ma. Till, separated by soil formations, covers the plateau surface.
Nineteenth- and Twentieth- Century Glacier Fluctuations and Climatic Implications in the Arco and Colonia Valleys, Hielo Patagónico Norte, Chile (pp 55-63)
Dendrochronology, lichenometry, and analysis of aerial photographs taken in 1944, 1979, and 1983 were used to date the 19th- and 20th-century fluctuations of the Arco, Colonia, and Arenales glaciers on the eastern side of the Hielo Patagˇnico Norte in southern Chile. This work has demonstrated that the glaciers retreated from their Little Ice Age maximum positions between 1850 and 1880, with retreat rates increasing during the 1940s and with surface thinning of at least 30 m since 1980. Comparison with the fluctuation behavior of other outlet glaciers of the icefield suggests a degree of synchrony in the timing of their variations and therefore argues for a common climatic control for these movements.
Variability in Winter Mass Balance of Northern Hemisphere Glaciers and Relations with Atmospheric Circulation (pp 64-72)
An analysis of variability in the winter mass balance (WMB) of 22 glaciers in the Northern Hemisphere indicates two primary modes of variability that explain 46% of the variability among all glaciers. The first mode of variability characterizes WMB variability in Northern and Central Europe and the second mode primarily represents WMB variability in northwestern North America, but also is related to variability in WMB of one glacier in Europe and one in Central Asia. These two modes of WMB variability are explained by variations in mesoscale atmospheric circulation which are driving forces of variations in surface temperature and precipitation. The first mode is highly correlated with the Arctic Oscillation Index, whereas the second mode is highly correlated with the Southern Oscillation Index. In addition, the second mode of WMB variability is highly correlated with variability in global winter temperatures. This result suggests some connection between global temperature trends and WMB for some glaciers.
Holocene Treeline and Climate Change in the Subalpine Zone Near Stoyoma Mountain, Cascade Mountains, Southwestern British Columbia, Canada (pp 73-83)
Multiproxy paleoecological investigation of a small lake in the high subalpine zone near Stoyoma Mountain, northern Cascade Mountains of British Columbia, reveals significant change in vegetation, limnic conditions, and inferred climate throughout the Holocene (last 10,000 radiocarbon years). Three zones of distinct pollen, plant macrofossil, and chironomid assemblages are apparent in the sediment core from 3M Pond (informal name). A dry, sparsely vegetated spruce parkland and a warm-adapted chironomid community existed in and around the study sites in the early Holocene (ca. 10,000 to 7000 14C yr BP). Between 7000 and 3500 14C yr BP, Engelmann spruce-subalpine fir forest conditions established and then declined around 3M Pond leading to modern subalpine parkland conditions from 3500 14C yr BP to present. Chironomid communities at 3M Pond between 7000 and 3500 14C yr BP are indicative of warmer waters than present, but show a transition to modern assemblages. Three climatic regimes are identified near Stoyoma Mountain: (1) the early Holocene xerothermic period (10,000 to 7000 14C yr BP, (2) a period of climatic transition in the mid-Holocene (7000 to 3500 14C yr BP), and (3) cool, modern neoglacial conditions (after 3500 14C yr BP). These findings confirm vegetation and inferred climate changes identified at Cabin Lake, British Columbia (a nearby lake in the subalpine forest). Changes in treeline position, plant communities, chironomid communities, and inferred climate are nearly synchronous and validate the multiproxy approach for paleoecological reconstruction. Chironomid-based paleotemperature reconstructions confirm earlier evidence that the early Holocene was significantly warmer than present, with estimated summer water surface temperatures up to 4║C higher than today.
Hydrology and Geochemistry of River-borne Material in a High Arctic Drainage System, Zackenberg, Northeast Greenland (pp 81-94)
The roles of chemical and mechanical weathering in permafrost regions were assessed, by measuring stream discharge and major, trace, and rare earth elements (REE) of suspended matter (SPM), river-bed sediments (RBS), and water in two lithologically different catchments in the High Arctic at Zackenberg, Northeast Greenland. The drainage basin contains sedimentary and crystalline rocks. In streams draining the sedimentary rock area, SPM and total dissolved solutes (TDS) are high with maximum values of 2500 mg L-1 and 105 ÁS cm-1, respectively. Variation of both relates to changes in vegetation and morphology. Mineral fractionation during transport and soil-forming processes in the sedimentary portion of the study area lead to characteristic chemical profiles for the SPM and RBS. Streams draining the crystalline rock area have low SPM (18 mg L-1) and TDS (14 ÁS cm-1) as a result of poor soil development and a lack of vegetation. Mechanical denudation exceeds chemical denudation by an order of magnitude for the entire catchment. Because the REE distributions of the crystalline differ from those in the sedimentary SPM differ, it is possible to quantify source rock contributions to the main outflow using a mixing calculation. A mass balance comparing the SPM in the main outflow with the tributaries, using the REEs as "fingerprints", indicates that about 90% of the sedimentary basin suspended matter is redeposited before reaching the outflow, at least over the period of observation. Taking this redeposition into account, the rate of chemical denudation (100 kg km-2 d-1) exceeds mechanical denudation (70 kg km-2 d-1) in the sedimentary drainage basin.
Seasonal Changes in Bed Elevation in a Step-Pool Channel, Rocky Mountains, Colorado, U.S.A. (pp 95-103)
Scour and fill patterns at East St. Louis Creek, Colorado, were investigated via repeat, detailed surveys of the channel bed at 11 cross sections during the 1995 snowmelt season. Spatial variability was remarkably high, with significant differences in cross section scour and fill patterns over distances as short as 0.5 m. Most sites had small net changes in bed elevation, both daily and over the entire runoff season. The data and observations indicate the presence of small pulses of fine material that are temporarily deposited on top of the channel pavement in wider areas of the channel and near woody debris complexes. Scour and fill are primarily limited to the finer material of such pulses. ANOVA analysis indicates that although discharge was important in predicting changes in bed elevation, the relationship between discharge and bed mobility is complicated by the effects of local channel morphology and a slight hysteresis. Regression analysis shows that variations in channel width determine where finer sediments are deposited, and therefore the locations of greater change in bed elevation. The proximity of morainal ridges and boulders to the channel edge locally influence the channel width and also the distribution of woody debris complexes. Results of this study suggest that the channel morphology and sediment transport along some reaches of small, high-gradient streams in watersheds with a glacial history may not respond as substantially to changes in discharge characteristic as do other types of alluvial channels.
A Note on Summer CO2 Flux, Soil Organic Matter, and Microbial Biomass from Different High Arctic Ecosystem Types in Northwestern Greenland (pp 104-106)
We measured CO2 flux, soil organic matter, and soil microbial biomass carbon in six high arctic tundra communities near Thule, Greenland, in July 1997, including polar desert, polar semidesert, and polar oasis ecosystems. Three of the four polar desert sites were in a contiguous toposequence originating at the receding margin of the Greenland ice cap and extending away from the ice approximately 400 m. The other sites ranged from 3 to 12 km from the ice margin. We measured net ecosystem CO2 uptake in the polar desert ecosystem most distance from the ice sheet (1.2 g CO2 m-2 d-1) and in the polar semidesert ecosystem (0.3 g CO2 m-2 d-1), but net CO2 loss in the polar oasis site and the three polar desert sites in the toposequence. Ecosystem respiration tended to be greatest in the ecosystems that have apparently been ice-free the longest, with efflux rates up to 3.7 g CO2 m-2 d-1. In the toposequence, soil organic matter was greatest adjacent to the icecap (3.10%) and decreased to 0.93% is the polar desert site 400 m from the ice. The polar semidesert and polar oases sites had 2.67 and 3.83% soil organic matter, respectively. Soil microbial biomass carbon ranged from about 1 mg C g-1 soil in the polar oasis ecosystem to about 0.2 mg C g-1 soil in one of the polar desert ecosystems but did not follow the patterns we found for soil organic matter. Our findings substantiate other recent studies showing significant CO2 flux between high arctic ecosystems and the atmosphere, and suggest that carbon exchange in these systems merit consideration in circumarctic estimates of carbon flux.