Movement of alpine timberline has been an observed consequence of recent and historic climate warming, and is expected to continue in the future. Evidence from the southern Rocky Mountains during the early and mid Holocene also indicates an upward expansion of timberline, and has been interpreted as a response to increased temperatures related to enhanced warm-season insolation.  However, little independent evidence of temperature change exists to put such ecological changes in context. Such data are needed to help evaluate the sensitivity of timberline and other aspects of regional vegetation patterns to long-term climate change. We use sediment archives from American Lake, Colorado in the hydrologically important region around the headwaters of the Colorado River to investigate how forest expansion corresponds to long-term temperature trends.  Fossil pollen and stomata are used to reconstruct past elevation limits of timberline.  We reconstruct temperature with two methods.  First, we derive temperature from fossil pollen abundances and, then, we use compound specific hydrogen isotopes that record the temperature-dependent isotopic values of meteoric precipitation. Comparison between the two approaches helps to evaluate the potential for long-term vegetation-climate equilibrium at timberline.
Results/Conclusions

Calibration of compound-specific hydrogen isotopes around the headwaters of the Colorado River (n=10, elevation range of 2400-3400m) demonstrates that behenic acid hydrogen isotopes record lake-water δD values (r = 0.88), and are heavily influenced by temperature variation. The behenic acid hydrogen-isotopic record from American Lake shows a cold excursion at ca. 8200 years before present, which is consistent with Front Range evidence of glacier advances.  During the mid-Holocene, temperatures rose by ~2°C and high temperatures persisted until 4 ka. The magnitude of isotope-inferred temperature changes in the Holocene, therefore, is the same as those observed in the twentieth century. Pollen and macrofossil analysis show an upward expansion of timberline concurrent with high accumulation of organic material from loss-on-ignition data that persist until about 4ka. Our results suggest that future warming will exceed mid-Holocene warming and may change the extent of mountain forests more dramatically than during the Holocene.  The impact of expanded forests on carbon budgets and water resources may be critical in a warmer world.