Variations in leaf area index in northern and eastern North America over the past 21,000 years: a data-model comparison

• Published in: Quaternary Science Reviews
• Main Authors: Gonzales, L. M., Williams, J. W., Kaplan, J. O.
• Format: Text
• Language: unknown
• Published: 2009
• Subjects: Canada; Tundra; Alaska
• Online Access: http://infoscience.epfl.ch/record/136636; https://doi.org/10.1016/j.quascirev.2008.04.003

Over the past 10 years, there has been a growing debate about the relative importance of late-Quaternary variations in climate and CO2 as drivers of changes in vegetation structure. In this study, we compare new datasets of pollen-reconstructed leaf area index values (LAIs) for northern and eastern North America with BIOME4 simulations and regional vegetation histories in order to identify sources of data-model disagreements and assess the relative influence Of CO2 and climate on LAI. BIOME4, an equilibrium biogeochemistry-biogeography vegetation model, was forced by climate simulations from the Hadley Centre Unified GCM to simulate changes in LAI at 1000-year intervals for the past 21,000 years in six regions of North America. BIOME4 simulations included sensitivity experiments to investigate the effects Of CO2 and climate on LAI. Pollen-reconstructed LAIs were based on the modern analog technique and a cross-referencing of surface pollen samples against recent LAI observations from the MODIS sensor. Pollen-reconstructed LAIs were biased towards too high values due to lower-than-present atmospheric CO2; sensitivity experiments with BIOME4 suggested that this bias ranged from 13 +/- 7% during the Lateglacial period (21-12 ka) to 7 +/- 4% during the Holocene (11-0 ka).:Simulated and reconstructed LAI trends agreed well for western Canada, southeastern Canada/northern US, and eastern North America, but significant discrepancies were observed in the Midwestern LIS, Alaska, and southeastern US. Causes of data-model discrepancies varied by region and were due to (1) inaccurate simulation of the prairie-forest ecotone in the Midwestern US due to too-wet climate simulations; (2) a combination of too-high simulated LAIs for Lateglacial tundra and too-low reconstructed LAIs from Alaskan pollen samples that lacked modern analogs; and (3) too-high reconstructed LAIs for scrublands in the southeastern US. Sensitivity tests indicated that late-Quaternary changes in LAI were driven primarily by climate, with CO2 as a ...