Evaluation of vegetation and fire simulations using palaeodata

Abstract

This project uses the Land Processes and eXchanges (LPX) model to analyse fire-vegetation-climate interactions under different climate regimes since the Last Glacial Maximum (LGM, 20 kyr. ago). The work includes constraints based on real and hypothetical scenarios, and comparisons with observational records to test the model’s performance. The first model experiment aims to separate effects on biomass burning due to CO2 changes from those of climatic changes alone. Two different climates: Last Glacial Maximum (LGM) and Pre-Industrial (PI), and two different atmospheric CO2 concentrations: 185 ppm and 280 ppm, are used for this purpose. The experiment shows that CO2 influence on biomass burning is substantial – but it has been generally overlooked. This research therefore highlights the importance of including the CO2 effect in future fire simulations in a high-CO2 world. In the second experiment, a factorial design is used to evaluate the influences of fire, climate and CO2 on net primary production (NPP) and biome distribution, combining different scenarios in a series of simulations (with vs. without fire, 185 ppm vs. 280 ppm CO2, and LGM vs. PI climate). Several synergies were observed among the studied variables, the most dramatic being the reduction in forest cover under warm climate (PI), low CO2 (185 ppm) and fire. Fire generally reduces the extent of woody biomes, and allows greater production per unit area of each biome. However, as forest cover is reduced, total global NPP stays lower than it would be without fire. The final chapter explores fire patterns under last millennium (pre-industrial) climate, by modelling carbon and CO fire emissions, and comparing them against sedimentary charcoal and ice core CO concentration records. Simulated CO emissions are passed through the MOGUNTIA atmospheric chemistry-transport model (with prescribed OH concentration) in order to simulate past CO concentrations. The simulations reproduce the broadest features of the charcoal record in the northern and southern extratropics, notably the decline in biomass burning towards a minimum in the Little Ice Age, and the subsequent rapid increase. There is little agreement between simulations and data in the tropics, however. The simulated CO concentrations have the right magnitude but the observed values show changes of much greater amplitude than is indicated by the isotopically derived valued of “biomass burning CO” in Antarctica in particular. These model simulations have provided insight into the consequences of fire-vegetation interactions, and have shown the ability to reproduce some key features of the palaeorecord of biomass burning as shown in charcoal records. The strong effect of CO2 concentration on fire regimes, and the non-linear ways in which CO2 concentration interacts with fire to influence vegetation distributions and fuel loads, indicate that future projections of fire risk require continued research in process-based fire modelling that takes account of how plants respond to their total environment, including climate, CO2 and fire.