Thomas W. D. Edwards

Rapid response of treeline vegetation and lakes to past climate warming

FUTURE greenhouse warming is expected to be particularly pronounced in boreal regions1, and consequent changes in vegetation in these regions may in turn affect global climate2–4. It is therefore important to establish how boreal ecosystems might respond to rapid changes in climate. Here we present palaeoecological evidence for changes in terrestrial vegetation and lake characteristics during an episode of climate warming that occurred between 5,000 and 4,000 years ago at the boreal treeline in central Canada. The initial transformation — from tundra to forest-tundra on land, which coincided with increases in lake productivity, pH and ratio of inflow to evaporation — took only 150 years, which is roughly equivalent to the time period often used in modelling the response of boreal forests to climate warming5,6. The timing of the treeline advance did not coincide with the maximum in high-latitude summer insolation predicted by Milankovitch theory7, suggesting that northern Canada experienced regionally asynchronous middle-to-late Holocene shifts in the summer position of the Arctic front. Such Holocene climate events may provide a better analogue for the impact of future global change on northern ecosystems than the transition from glacial to nonglacial conditions.

Holocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden

The oxygen-isotope composition of local precipitation (d18OP) is reconstructed from carbonate lake-sediment components in a sediment core covering the last 10000 calendar years from Lake Tibetanus, a small, hydrologically open, groundwater-fed lake in the Abisko area, northern Sweden. Comparison of the d18OP history with a pollen-based palaeotemperature record from the same core clearly reveals pronounced deviations from the normally expected temporal d18OP-temperature relation (so-called ‘Dansgaard relation’) that may be a function of changing oceanicity. The transition from relatively moist, maritime conditions in the early Holo cene to a much drier climate after 6500 cal. BP is re‘ ected by major changes in forest extent and composition as recorded by pollen and plant macrofossil data. At the time of maximum in‘ uence of westerly air-mass circulation (high zonal index) c. 9500 cal. BP, brought about by high summer insolation and enhanced meridi onal pressure gradients, d18OP at Lake Tibetanus was about 2‰ higher than would be predicted by the modern isotope-temperature relation. The occurrence of long-term changes in d18OP-temperature relations, which are more sensitive measures of palaeoclimate than either d18OP or temperature alone, needs to be taken into account when extracting palaeoclimatic information from continental oxygen-isotope records.

δ13C response surface resolves humidity and temperature signals in trees

Stem cellulose of bean plants (Vicia faba) grown under controlled conditions exhibits inverse linear carbon–isotope reactions to changes in both relative humidity (RH) and temperature (T), readily mappable as a planar δ13C response surface in RH–T space. The analogous response surface for annual late-wood cellulose δ13C from a field calibration using fir trees (Abies alba) in the Black Forest, southern Germany, also supports resolution of independent δ-RH and δ-T effects. The response of cellulose δ13C to RH and T derived from this new calibration differs markedly from estimates based on univariate linear regression analysis: The sensitivity of δ13C to RH is stronger than that inferred previously (c. −0.17‰/% vs. −0.12‰/%, respectively), whereas the δ-T coefficient is weaker and reversed in sign (c. −0.15‰/K vs. +0.36‰/K). This new perspective on the coupled influence of moisture and temperature changes on tree-ring cellulose δ13C helps to unify divergent observations about carbon–isotope signals in trees, especially the broad range of apparent δ-T relations obtained in calibration studies, which are often used as paleoclimate transfer functions. Although this highlights the large potential uncertainties surrounding paleoclimate reconstruction based solely on δ13C data, coupling of the carbon–isotope response–surface approach with equivalent response surfaces for hydrogen or oxygen isotopes may afford new opportunities for investigating the nature of past climate variability and change from tree-ring sequences.

Carbon and Oxygen Isotope Analysis of Lake Sediment Cellulose: Methods and Applications

Carbon and oxygen isotope analysis of lake sediment cellulose is a recently developed paleolimnological approach that is gaining increasing usage, especially in carbonate-free sedimentary systems. As with carbonate-based paleolimnological investigations (see Ito, this volume), lake sediment cellulose can provide a record of lake paleohydrology. As a result, studies incorporating this technique typically aim to address the following research questions:

Palaeolimnological and sedimentary responses to Holocene forest retreat in the Scandes Mountains, west-central Sweden

A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake Spaime, a small, hydrologically open water body in the modem alpine tundra zone of the Scandes Mountains, west-central Sweden. The study aimed to evaluate (1) the nature of climate changes that forced the late-Holocene lowering of altitudinal tree limit in the region, the timing of which is known from prior studies based on radiocarbon dating of subfossil wood, and (2) the impact of these vegetational changes on an aquatic ecosystem. Arboreal pollen and plant macrofossil data confirm the persistence of trees in the lake catchment at least from c. 9700 cal. BP until c. 3700 cal. BP. Although growing-season temperature is commonly believed to be the dominant factor driving boreal forest tree-limit variations in the region, a chironomid-based reconstruction of mean July air temperature suggests that local deforestation during the late Holocene was not accompanied by a significant cooling. The tree-limit retreat was more likely caused by increasing effective moisture and declining length of the growing season. The ecohydrological response of Lake Spaime to this combination of climate and vegetational changes included a decline in primary productivity, as indicated by an abrupt decrease in sediment organic matter content, while associated increases in organic 613C, 615N and C/N point to diminished fluxes and altered balance of catchment derived nutrients following deforestation. The decline in aquatic productivity is also marked by a distinct change in the mineral magnetic properties, from a high magnetic concentration assemblage dominated by fine-grained magnetite of biogenic origin to one dominated by background levels of coarse-grained detrital magnetite.

DEVELOPMENT AND VALIDATION OF AN ISOTOPIC METHOD FOR ESTIMATING LAKE EVAPORATION

A study designed to test the validity of an isotopic method for estimating evaporation was conducted within a small, tundra lake situated in the continental Arctic of Canada. Evaporation was determined using an isotope mass balance approach which accounted quantitatively for isotopic fractionation, progressive evaporative enrichment of δ 18 O and δ 2 H in lake water, and attenuation of enrichment by liquid inputs and atmospheric moisture. Concurrent determinations made using standard mass balance, energy balance, aerodynamic profile and class A pans permitted rigorous comparisons between methods. Results are presented for two summers which had distinct weather conditions and hydrological balances. Overall, the δ 18 O balance was found to be in good agreement with standard methods during both years over time intervals greater than about one week. Owing to a less systematic response of δ 2 H over short time periods, it use is not recommended for quantitative mass balance determinations over time intervals of less than about 50 days in this setting.

Climatic effects on the δ18O and δ13C of cellulose in the desert tree Tamarix jordanis

Abstract The 13 C 12 C and 18 O 16 O ratios of stem cellulose of Tamarix jordanis (a tree common in wadis of rid regions) increased with decreasing relative humidity (RH) in individual trees growing along a climatic gradient in Israel. The response to RH observed in the δ18O of the wood cellulose was strongly similar to that observed in leaf water over a diurnal cycle. Most of the data for δ13C and all of the data for δ18O could be fitted to two independent linear equations that, combined, allowed the reconstruction of RH and the δ18O of source water from the isotopic composition of ancient T. jordanis wood previously reported from the ancient fortress of Masada. Since the Roman period, RH at Masada decreased by about 17%, while the δ18O value of local groundwater remained similar to present-day values, suggesting that changing atmospheric circulation has played a role in climate change in the Middle East over the past two millennia.

Climate and environment during the Younger Dryas (GS-1) as reflected by composite stable isotope records of lacustrine carbonates at Torreberga, southern Sweden

Climatic and environmental changes during the Younger Dryas stadial (GS-1) and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient Lake Torreberga, southern Sweden. Event GS-1 is represented in the sediment sequence by 3.5 m of clay containing lacustrine carbonates of various origins. Comparison of isotopic records obtained on mollusc shells, ostracod valves, and Chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures. Variations in soil erosion rates can also be estimated from a simple isotope-mass-balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments. The well-known, rapid climatic shifts characterising the Last Termination in the North Atlantic region are clearly reflected in the isotopic data, as well as longer-term changes within GS-1. Following maximum cooling shortly after the Allerod-Younger Dryas (GI-1-GS-1) transition, a progressive warming and a slight increase in aquatic productivity is indicated. At the Younger Dryas- Preboreal (GS-1-PB) transition mean annual air temperature rapidly increased by more than 5°C and summer lake-water temperature increased by ca. 12°C. The subsequent Preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature. These results are in harmony with recent findings about large-scale climate dynamics during the Last Termination. Copyright

Changes in carbon and nitrogen cycling during tree-line retreat recorded in the isotopic content of lacustrine organic matter, western Taimyr Peninsula, Russia

Bulk organic and cellulose stable carbon isotope and bulk organic nitrogen isotope profiles from the sediments of a small tundra lake on the western Taimyr Peninsula, Russia, show changes that are correlated with climate cooling and treeline retreat at c. 4000 14C years BP. Increased soil organic matter decomposition, combined with a moist climate, probably provided a 13C-depleted source of CO2(aq) to lake phytoplankton thriving under favourable conditions during the forest interval. Increased concentration of CO2(aq) and re-utilization of respired CO2(aq) from organic matter settling through the water column may have also contributed to the relatively low δ13C values in the lower part of the lake sediment record. Bulk organic and cellulose carbon are more enriched in 13C after the boreal forest retreated probably because CO2 from the atmosphere became the dominant source of carbon to the lake as soil organic decomposition rates declined, the lake became more oligotrophic, and the climate became colder and drier. Reduced concentration of CO2(aq) may have also led to 13C-enrichment. Interpretation of the bulk organic δ15N record is somewhat more speculative but changes in drainage basin terrestrial vegetation, soil decomposition and hydrology also appear to have strongly influenced the lake water nitrogen cycling. Although considerably more effort is required to assess modern carbon and nitrogen isotope systematics, these promising results suggest that lake sediment organic matter δ13C, δ15N and cellulose δ13C combine to form useful tracers of past nutrient cycling in boreal tree-line watersheds.

Atmospheric circulation controls on precipitation isotope–climate relations in western Canada

δ18O and δ2H time-series of monthly composite precipitation (July 1975–June 1982) for three stations located in western Canada were examined to characterize the influence of atmospheric circulation on modern isotope–climate relations in the region. Spatially coherent trends in long-term isotope and temperature anomalies were evident among the three stations, with isotope and temperature anomalies showing the strongest correlations when weighted to reflect precipitation amount. Strong correlations were also found between unweighted isotope anomalies and the Pacific–North American (PNA) index, which is a key descriptor of air-mass circulation patterns across North America. Positive δ anomalies and variable temporal δ–temperature relations having relatively shallow slopes occur during periods of intensified meridional circulation (PNA+), especially during winter. Periods of stronger zonal circulation (PNA−), in contrast, are marked by negative δ anomalies and steeper δ–temperature relations, consistent with increased distillation and deepening of the isotope shadowin the lee of the Canadian Cordillera. Significant positive correlations with pressureheight and precipitable-water anomalies located over western Canada provide additional confirmation that seasonal and interannual variability in the strength of the PNA ridge-and-trough pattern profoundly influences the isotopic evolution of moisture reaching the interior. Analogous circulation-dependent shifts in the precipitation isotope–temperature relation in western Canada also occur over much longer timescales, as inferred from isotope palaeorecords in various natural archives.