Paleoclimate and Amerindians: evidence from stable isotopes and atmospheric circulation.

Two Amerindian demographic shifts are attributed to climate change in the northwest plains of North America: at approximately 11,000 calendar years before present (yr BP), Amerindian culture apparently split into foothills-mountains vs. plains biomes; and from 8,000-5,000 yr BP, scarce archaeological sites on the open plains suggest emigration during xeric "Altithermal" conditions. We reconstructed paleoclimates from stable isotopes in prehistoric bison bone and relations between weather and fractions of C(4) plants in forage. Further, we developed a climate-change model that synthesized stable isotope, existing qualitative evidence (e.g., palynological, erosional), and global climate mechanisms affecting this midlatitude region. Our isotope data indicate significant warming from approximately 12,400 to 11,900 yr BP, supporting climate-driven cultural separation. However, isotope evidence of apparently wet, warm conditions at 7,300 yr BP refutes emigration to avoid xeric conditions. Scarcity of archaeological sites is best explained by rapid climate fluctuations after catastrophic draining of the Laurentide Lakes, which disrupted North Atlantic Deep Water production and subsequently altered monsoonal inputs to the open plains.

Stable isotope analysis of termite food habits in East African grasslands.

Stable carbon isotope techniques were employed to study the food habits of the termite Macrotermes michaelseni (Isoptera: Termitidae) in a semiarid savanna habitat in Kenya. At Kajiado this species utilized approximately 70% herbaceous vegetation (mostly grass) and 30% woody vegetation, while at Ruiru approximately 64% of the vegetation utilized was woody and 36% herbaceous. Stabel carbon isotope ratios varied between castes within sites, but were consistent with the manner in which carbon flows through termite colonies. δ(13)C values increased in the sequence: diet→fungus comb→nonreproductive castes→reproductive castes. These results are in agreement with the idea that organic carbon becomes enriched in (13)C as it passes through a food chain.

Fractionation and turnover of stable carbon isotopes in animal tissues: Implications for δ13C analysis of diet.

The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleoecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood processes. This study shows that tissues of the gerbil (Meriones unguienlatus) have different δ13C values when equilibrated on corn (C4) or wheat (C3) diets with constant 13C/12C contents. Lipids were depleted 3.0‰ and hair was enriched 1.0‰ relative to the C4 diet. Tissue δ13C values were ranked hair>brain>muscle>liver>fat. After changing the gerbils to a wheat (C3) diet, isotope ratios of the tissues shifted in the direction of the δ13C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was adequately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair.The results of this study have important implications for the use of δ13C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C3 vs. C4, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.

Effects of simulated grazing on foliage and root production and biomass allocation in an arctic tundra sedge (Eriophorum vaginatum).

Eriophorum vaginatum is a tussock-forming member of the Cyperaceae, widespread in the arctic tundra, and a common food item for grazing herbivores on the Alaskan North Slope. Populations of this sedge at Atkasook, Alaska, were subjected to a variety of simulated grazing experiments to determine tiller responses to frequent and intensive defoliation.The intial, short-term response of Eriphorum to defoliation was an increase in leaf production at the expense of belowground structures. Multiple defoliations, however, resulted in proportionately greater reductions in leaf weight than leaf area. Leaf blades of defoliated plants weighed 0.71 mg/cm of length, compared to an average weight/length ratio of 0.91 mg/cm in control plants. Declines in leaf production were accompanied by weight losses in stem base and sheath components and a curtailment of root growth. Root initiation was reduced by 28 and 63%, respectively, after one and two seasons of multiple defoliations, and the depth of penetration of these annual roots was reduced substantially. Total root biomass was reduced 24% in the least severe defoliation treatment and 85% in the most severe treatment. The allocation of dry matter into new tillers on defoliated plants, relative to new tiller production of control plants, was reduced by 75% after two seasons of defoliation, although equivalent numbers of tillers were initiated. The reduced biomass of daughter tillers was restored to levels observed in control plants during the season of rest following the season of multiple defoliation. Sexual reproduction was significantly depressed in the most severe defoliation treatment and stimulated by the least severe treatment. Defoliation treatments of intermediate severity had no significant impact on flower initiation.Over 80% of the tillers subjected to complete defoliation at 10-day intervals for one entire growing season survived, overwintered and initiated growth the following season. Recovery from multiple defoliations was partially achieved by the stimulated growth and extended longevity of older leaves on the tiller, although one season of rest was not enough to fully replenish weight losses of storage organs. Tillers were capable of withstanding more defoliation events when clipping was initiated early in the growing season. Biomass of storage organs of tillers subjected to multiple defoliations imposed at 10-day intervals for two entire growing seasons was 34% above the estimated minimum biomass necessary for tiller survival. The data suggest that Eriophorum tillers can survive 100% leaf removal at 10-day intervals for 50 to 75% of their estimated three to four year lifespan.

The effects of simazine and temperature on photosynthesis in rye.

Temperature during growth had a strong influence on light-saturated rates of photosynthesis and respiration in rye (Secale cereale). Carbon dioxide uptake was depressed at the low temperatures and reached maximal values higher than 30 milligrams of carbon dioxide per square decimeter per hour at the intermediate temperatures. Respiration rates increased substantially while growth temperature decreased. Simazine treatment (0.00, 0.03, 0.08, 0.12 milligrams per liter) resulted in no significant stimulatory or inhibitory effects at low temperatures, but progressively inhibited growth and photosynthesis at the higher temperatures. Respiration rates were not significantly influenced. Thus, the effect of simazine is strongly temperature-dependent and is more pronounced on photosynthesis than respiration.