Geophysical Applications of Vegetation Modeling

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 (

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

Climate change and Arctic ecosystems: 2. Modeling, paleodata-model comparisons, and future projections

Jed Oliver Kaplan

Large variations in the composition, structure, and function of Arctic ecosystems are determined by climatic gradients, especially of growing-season warmth, soil moisture, and snow cover. A unified circumpolar classification recognizing five types of tundra was developed. The geographic distributions of vegetation types north of 55degreesN, including the position of the forest limit and the distributions of the tundra types, could be predicted from climatology using a small set of plant functional types embedded in the biogeochemistry-biogeography model BIOME4. Several palaeoclimate simulations for the last glacial maximum (LGM) and mid-Holocene were used to explore the possibility of simulating past vegetation patterns, which are independently known based on pollen data. The broad outlines of observed changes in vegetation were captured. LGM simulations showed the major reduction of forest, the great extension of graminoid and forb tundra, and the restriction of low- and high-shrub tundra (although not all models produced sufficiently dry conditions to mimic the full observed change). Mid-Holocene simulations reproduced the contrast between northward forest extension in western and central Siberia and stability of the forest limit in Beringia. Projection of the effect of a continued exponential increase in atmospheric CO2 concentration, based on a transient ocean-atmosphere simulation including sulfate aerosol effects, suggests a potential for larger changes in Arctic ecosystems during the 21st century than have occurred between mid-Holocene and present. Simulated physiological effects of the CO2 increase (to >700 ppm) at high latitudes were slight compared with the effects of the change in climate.

2003

The role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations

Jed Oliver Kaplan

Recent analyses of ice core methane concentrations suggested that methane emissions from wetlands were the primary driver for prehistoric changes in atmospheric methane. However, these interpretations conflict as to the location of wetlands, magnitude of emissions, and the environmental controls on methane oxidation. The flux of other reactive trace gases to the atmosphere also controls apparent atmospheric methane concentrations because these compounds compete for the hydroxyl radical (OH), which is the primary atmospheric sink for methane. In a series of linked biosphere-atmosphere chemistry-climate modeling experiments, we simulate the methane and biogenic volatile organic compound emissions from the terrestrial biosphere from the Last Glacial Maximum (LGM) to the present. Using a state-of-the-art chemistry-climate model, we simulate the atmospheric concentrations of methane, OH, and other reactive trace gas species. Over the past 21,000 years, methane emissions from wetlands increased slightly to the end of the Pleistocene but then decreased again, reaching levels at the preindustrial Holocene that were similar to the LGM. Global wetland area decreased by 14% from LGM to the preindustrial time. Emissions of biogenic volatile organic compounds (BVOCs), however, nearly doubled over the same period of time. Atmospheric OH burdens and methane concentrations were affected by this major change in BVOC emissions, with methane lifetimes increasing by more than 2 years from LGM to the present. We simulate a change in methane concentration of ∼385 ppb, accounting for 88% of the ∼440 ppb increase in methane concentrations observed in ice cores. Thus glacial-interglacial changes in atmospheric methane concentrations would have been modulated by BVOC emissions. In addition, the increase in atmospheric methane concentrations since the mid-Holocene is partly caused in our results by the increases in anthropogenic methane emissions over this period. While the interplay between BVOC and wetland methane emissions since the LGM cannot explain the entire record of ice core methane concentrations, consideration of BVOC source dynamics is central to understanding ice core methane. Rapid changes in atmospheric methane concentrations, also observed in ice cores, require further study.

2006

The role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations

Jed Oliver Kaplan

2006

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