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.

Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between general circulation models using prescribed and computed sea surface temperatures

Jed Oliver Kaplan

The terrestrial biosphere model Carbon Assimilation in the Biosphere (CARAIB) was improved by introducing two vegetation storeys and implementing a new module which simulates the equilibrium distribution of the vegetation inferred from physiological processes and climatic constraints. In this fourth version of CARAIB, we differentiate ground-level grasses from tree canopies, which allows us to determine the light available to grasses as a direct function of the leaf area index (LAI) of the forest canopy. Both of these storeys are potentially composed of several plant functional types (PFT). The cover fraction of each PFT within each storey is estimated according to its respective net primary productivity (NPP). A biome is assigned to each grid cell on the basis of three physiological criteria: (1) the cover fraction, (2) the NPP, and (3) the LAI; and two climatic constraints: (1) the growing degree-days (GDD) and (2) the lowest temperature reached during the cold season (T-min), which are well-known indices of vegetation expansion boundaries. Total biospheric carbon stocks (vegetation + soil) are reconstructed by forcing the model with eight climatic scenarios of the Last Glacial Maximum (LGM, 21 ka BP), which were obtained from the Palco-Modelling Intercomparison Project (PMIP) from four general circulation models (MRI2, UGAMP, LMD4, and GEN2) using prescribed and computed sea surface temperatures (SSTs). The model was also forced with a current climate together with a preindustrial atmospheric CO2 level of 280 ppm as reference simulation, To validate the model, current biome distribution is reconstructed and compared, for the modem climate, with two distributions of potential vegetation and, for the LGM, with pollen data. The model simulations are in good agreement with broad-scale patterns of vegetation distribution, The results indicate an increase in the total biospheric carbon stock of 827.8-1106.1 Gt C since the LGM. Sensitivity analyses were performed to discriminate the relative effects of the atmospheric CO, level ("fertilization effect"), the climate (present or LGM), and the sea level. Our results suggest that the CO, fertilization effect is mostly responsible for the total increase in vegetation and soil carbon stocks. The four GCMs diverged in their predicted responses of continental climate to calculated SSTs. Only one of them, i.e., MRI2, predicted a marked decline of the continental temperatures in response to lower calculated SSTs. For this GCM, the effect of reduced SSTs on continental biospheric carbon stocks was a decrease of 544.1 Gt for the soil carbon stock and of 283.7.

2002

Leaf area index for northern and eastern North America ad the Last Glacial Maximum: A data-model comparison

Jed Oliver Kaplan

Aim To estimate the effects of full-glacial atmospheric CO2 concentrations and climate upon leaf area index (LAI), using both global vegetation models and palaeoecological data. Prior simulations indicate lowered LAIs at the Last Glacial Maximum (LGM), but this is the first attempt to corroborate predictions against observations. Location Eastern North America and eastern Beringia. Methods Using a dense surface pollen data set and remotely sensed LAIs from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, we evaluate the ability of analogue-based techniques to reconstruct modern LAIs from pollen data. We then apply analogue techniques to LGM pollen records, calculate the ratio of LGM to modern LAIs (RLAI) and compare reconstructed RLAIs to RLAIs simulated by BIOME4. Sensitivity experiments with BIOME4 distinguish the effects of CO2 and climate on glacial LAIs. Results Modern LAIs are skilfully predicted (r2 = 0.83). Data and BIOME4 indicate that LAIs at the LGM were up to 12% lower than modern values in eastern North America and 60–94% lower in Beringia. In eastern North America, LGM climates partially counteracted CO2-driven decreases in LAI, while in Beringia both contributed to lowered LAIs. Main conclusions In both regions climate is the primary driver of LGM LAIs. The decline in eastern North America LAIs is smaller than previously reported, so regional vegetation feedbacks to LGM climate may have been less significant than previously supposed. CO2 exerts both physiological and community effects upon LAI, by regulating resource availability for leaf production and by influencing the competitive balance among species and hence the composition and structure of plant communities. Pollen-based reconstructions using analogue methods do not incorporate the physiological effect and so are upper estimates of full-glacial LAIs. BIOME4 sensitivity experiments indicate that the community and physiological effects together caused 10% to 20% decrease in LAIs at the LGM, so simulated RLAIs that are 80–100% of reconstructed RLAIs are regarded as consistent with data.

2008

Geophysical Applications of Vegetation Modeling

Jed Oliver Kaplan

This thesis describes the development and selected applications of a global vegetation model, BIOME4. The model is applied to problems in high-latitude vegetation distribution and climate, trace gas production, and isotope biogeochemistry. It demonstrates how a modeling approach, based on principles of plant physiology and ecology, can be applied to interdisciplinary problems that cannot be adequately addressed by direct observations or experiments. The work is relevant to understanding the potential effects of climate change on the terrestrial biosphere and the feedbacks between the biosphere and climate. BIOME4 simulates the distribution of 15 high-latitude biomes, including five tundra vegetation types, for the present day using observed climate, and the LGM, mid-Holocene, and a “greenhouse” scenario for 2100 using the output of GCMs. In the LGM simulations, the high-latitudes show a marked increase in the area of graminoid and forb tundra, which is also the predominant feature in the paleodata. This vegetation has no widespread modern analog; it was favored by the cold, dry climate, and supported large mammoth populations. Mid-Holocene simulations indicate a modest, asymmetrical northward advance of the Arctic treeline compared to present, with greatest extension in central Siberia (up to 300 km), and little to no change in the Western Hemisphere. This result is in good agreement with pollen and megafossil data from the same period. Differential warming of the continents in response to increased high-latitude solar radiation is hypothesized to account for the asymmetry. Vegetation changes in the 2100 projection, which assumes a continued exponential increase in atmospheric GHG concentrations, are more radical than those simulated for the mid-Holocene. The year-round forcing due to GHGs increases both summertime and annual temperatures in the high latitudes by up to double the mid-Holocene anomaly. However the potential treeline advances and biome shifts in our simulation are unlikely to be realized within 100 years, because of the time required for migration and establishment of new vegetation types. Potential natural wetland area for the present day was simulated by BIOME4 as 11.0x106km2. This value is higher than other estimates but includes small (

Lund University2001

Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between general circulation models using prescribed and computed sea surface temperatures

Jed Oliver Kaplan

2002

Leaf area index for northern and eastern North America ad the Last Glacial Maximum: A data-model comparison

Jed Oliver Kaplan

2008

Geophysical Applications of Vegetation Modeling

Jed Oliver Kaplan

Lund University2001