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

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 secondary influence. Sensitivity simulations in which climate was allowed to vary while CO2 was constant agreed better with reconstructed LAIs and regional vegetation histories than did the simulations in which CO2 varied and climate was constant. Our work emphasizes the need for models to incorporate the effects of both CO2 and climate on late-Quaternary vegetation dynamics and structure. We show that LAI is a viable paleo-vegetation proxy that can be used as a benchmark for rigorous, quantitative data-model comparisons focusing primarily on vegetation history, particularly in regions with high densities of fossil pollen data.