ALBERT

Description: Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding fire-climate-vegetation interactions is to quantify the effect vegetation has in mediating fire regime. Here, we explore the relative importance of Holocene land cover and dominant functional forest type, and climate dynamics on biomass burned in temperate and boreo-nemoral regions of Central and Eastern Europe over the past 12 ka BP years. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and novel statistical modelling. Biomass burned was highest during the early Holocene and lowest during the mid Holocene in all three ecoregions, but diverged more markedly over the past 3–4 ka BP. Although the climate was an important driver of fire hazard during the warm and dry early Holocene, tree cover was consistently the strongest predictor of past biomass burning. In temperate forests, biomass burned was high at ~ 45 % tree cover and decreased strongly towards 60 % tree cover. In needleleaf dominated forests, biomass burned was highest at ~ 60–65 % tree cover and abruptly declined at 〉 65 % tree cover. Biomass burned also increased when arable lands and grasslands reached ~ 15–20 %, although this relationship was highly dynamic depending on land use intensity throughout ignition and fuel type and availability. Our observations cover the full range of Holocene climate variability and land cover changes and illustrates that percentages of land cover is a key predictor of the probability of fire occurrence over timescales of centuries to millennia. We suggest that long-term fire risk may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene.

Description: Vegetation is crucial for modulating rates of denudation and landscape evolution as it stabilizes and protects hillslopes and intercepts rainfall. Climate conditions and atmospheric CO2 concentration ([CO2]) influence the establishment and performance of plants and thus have a direct influence on vegetation cover. In addition, vegetation dynamics (competition for space, light, nutrients and water) and stochastic events (mortality and fires) determine the state of vegetation, response times to environmental perturbations, and the successional development. In spite of this, state-of-art reconstructions of past transient vegetation changes have not been accounted for in landscape evolution models. Here, a widely used dynamic vegetation model (LPJ-GUESS) was used to simulate vegetation composition/ cover and surface runoff in Chile for the Last Glacial Maximum (LGM), Mid Holocene (MH) and present day (PD). In addition, we conducted transient vegetation simulations from LGM to PD for four sites of the Coastal Cordillera of Chile at a spatial and temporal resolution adequate for coupling with landscape evolution models. Using a regionally-adapted parametrization, LPJ-GUESS was capable of reproducing present day potential natural vegetation along the strong climatic gradients of Chile and simulated vegetation cover was also in line with satellite-based observations. Simulated vegetation during the LGM differed markedly from PD conditions. Coastal cold temperate rainforests where displaced northward by about 5° and the tree line and vegetation zones were at lower elevations than at PD. Transient vegetation simulations indicate a marked shift in vegetation composition starting with the past-glacial warming that coincides with a rise in [CO2]. Vegetation cover between the sites ranged from 13% (LGM: 8%) to 81% (LGM: 73%) for the northern Pan de Azúcar and southern Nahuelbuta sites, respectively, but did not vary by more than 10% over the 21,000yr simulation. A sensitivity study suggests that [CO2] is an important driver of vegetation changes and, thereby, potentially landscape evolution. Comparisons with other paleoclimate model driver highlight the importance of model input on simulated vegetation. In the near future, we will directly couple LPJ-GUESS to a landscape evolution model (see companion paper) to build a fully-coupled dynamic-vegetation/ landscape evolution model that is forced with paleoclimate data from atmospheric general circulation models.

Description: We present a numerical modelling investigation into the interactions between transient climate and vegetation cover with hillslope and fluvial processes. Model simulations were designed to investigate the effects of climate change and associated changes in surface vegetation cover on topographic basin metrics such as: slope, relief and channel steepness. The Landlab surface process model was used to evaluate the effects of temporal variations in vegetation cover on hillslope diffusion and detachment limited fluvial erosion. A suite of simulations were conducted to represent present-day climatic conditions and satellite-derived vegetation cover at the four EarthShape study areas as well hypothetical transient long term changes. Two different transient variations in climate and vegetation cover include a step change in climate or vegetation, as well as 100 kyr oscillations over 5 Myr. Results indicate that the coupled influence of surface vegetation cover and mean annual precipitation shifts basin landforms towards a new steady state, with the magnitude of change highly sensitive to the initial vegetation and climate conditions of the basin. Dry, non-vegetated basins show higher magnitudes of adjustment than basins that are situated in wetter conditions with higher vegetation cover. For coupled conditions when surface vegetation cover and mean annual precipitation change simultaneously, the landscape response tends to be weaker. When vegetation cover and mean annual precipitation change independently from each other, higher magnitude shifts in topographic metrics are simulated. Changes in vegetation cover show a higher impact on topography for low initial surface cover values whereas for areas with high initial surface cover, the effect of changes in precipitation dominate the formation of landscapes. This study demonstrates a sensitivity of catchment characteristics to different transient forcings in vegetation cover and mean annual precipitation, with a crucial role for initial vegetation and climate conditions. Ongoing research is developing fully-coupled landscape evolution and dynamic vegetation model (see companion paper) forced with predicted paleoclimate histories from an atmospheric general circulation model.

Description: We present a numerical modeling investigation into the interactions between transient climate and vegetation cover with hillslope and detachment limited fluvial processes. Model simulations were designed to investigate topographic patterns and behavior resulting from changing climate and the associated changes in surface vegetation cover. The Landlab surface process model was modified to evaluate the effects of temporal variations in vegetation cover on hillslope diffusion and fluvial erosion. A suite of simulations were conducted to represent present-day climatic conditions and satellite derived vegetation cover at four different research areas in the Chilean Coastal Cordillera. These simulations included steady-state simulations as well as transient simulations with forcings in either climate or vegetation cover over millennial to million-year timescales. Two different transient variations in climate and vegetation cover including a step change in climate or vegetation were used, as well as 100 kyr oscillations over 5 Myr. We conducted eight different step-change simulations for positive and negative perturbations in either vegetation cover or climate and six simulations with oscillating transient forcings for either vegetation cover, climate, or oscillations in both vegetation cover and climate. Results indicate that the coupled influence of surface vegetation cover and mean annual precipitation shifts basin landforms towards a new steady state, with the magnitude of the change being highly sensitive to the initial vegetation and climate conditions of the basin. Dry, non-vegetated basins show higher magnitudes of adjustment than basins that are situated in wetter conditions with higher vegetation cover. For coupled conditions when surface vegetation cover and mean annual precipitation change simultaneously, the landscape response tends to be weaker. When vegetation cover and mean annual precipitation change independently from one another, higher magnitude shifts in topographic metrics are predicted. Changes in vegetation cover show a higher impact on topography for low initial surface cover values; however, for areas with high initial surface cover, the effect of changes in precipitation dominate the formation of landscapes. This study demonstrates the sensitivity of catchment characteristics to different transient forcings in vegetation cover and mean annual precipitation, with initial vegetation and climate conditions playing a crucial role.

Description: Vegetation is crucial for modulating rates of denudation and landscape evolution, as it stabilizes and protects hillslopes and intercepts rainfall. Climate conditions and the atmospheric CO2 concentration, hereafter [CO2], influence the establishment and performance of plants; thus, these factors have a direct influence on vegetation cover. In addition, vegetation dynamics (competition for space, light, nutrients, and water) and stochastic events (mortality and fires) determine the state of vegetation, response times to environmental perturbations and successional development. In spite of this, state-of-the-art reconstructions of past transient vegetation changes have not been accounted for in landscape evolution models. Here, a widely used dynamic vegetation model (LPJ-GUESS) was used to simulate vegetation composition/cover and surface runoff in Chile for the Last Glacial Maximum (LGM), the mid-Holocene (MH) and the present day (PD). In addition, transient vegetation simulations were carried out from the LGM to PD for four sites in the Coastal Cordillera of Chile at a spatial and temporal resolution adequate for coupling with landscape evolution models. A new landform mode was introduced to LPJ-GUESS to enable a better simulation of vegetation dynamics and state at a sub-pixel resolution and to allow for future coupling with landscape evolution models operating at different spatial scales. Using a regionally adapted parameterization, LPJ-GUESS was capable of reproducing PD potential natural vegetation along the strong climatic gradients of Chile, and simulated vegetation cover was also in line with satellite-based observations. Simulated vegetation during the LGM differed markedly from PD conditions. Coastal cold temperate rainforests were displaced northward by about 5∘ and the tree line and vegetation zones were at lower elevations than PD. Transient vegetation simulations indicate a marked shift in vegetation composition starting with the past glacial warming that coincides with a rise in [CO2]. Vegetation cover between the sites ranged from 13 % (LGM: 8 %) to 81 % (LGM: 73 %) for the northern Pan de Azúcar and southern Nahuelbuta sites, respectively, but did not vary by more than 10 % over the 21 000 year simulation. A sensitivity study suggests that [CO2] is an important driver of vegetation changes and, thereby, potentially landscape evolution. Comparisons with other paleoclimate model drivers highlight the importance of model input on simulated vegetation. In the near future, we will directly couple LPJ-GUESS to a landscape evolution model (see companion paper) to build a fully coupled dynamic-vegetation/landscape evolution model that is forced with paleoclimate data from atmospheric general circulation models.

Description: Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding the risk of fires is quantifying the mediating effect of vegetation on fire regimes. Here, we explore the relative importance of Holocene land cover, land use, dominant functional forest type, and climate dynamics on biomass burning in temperate and boreo-nemoral regions of central and eastern Europe over the past 12 kyr. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and statistical modelling. Biomass burning was highest during the early Holocene and lowest during the mid-Holocene in all three ecoregions (Atlantic, continental and boreo-nemoral) but was more spatially variable over the past 3–4 kyr. Although climate explained a significant variance in biomass burning during the early Holocene, tree cover was consistently the highest predictor of past biomass burning over the past 8 kyr. In temperate forests, biomass burning was high at ∼45 % tree cover and decreased to a minimum at between 60 % and 70 % tree cover. In needleleaf-dominated forests, biomass burning was highest at ∼ 60 %–65 % tree cover and steeply declined at 〉65 % tree cover. Biomass burning also increased when arable lands and grasslands reached ∼ 15 %–20 %, although this relationship was variable depending on land use practice via ignition sources, fuel type and quantities. Higher tree cover reduced the amount of solar radiation reaching the forest floor and could provide moister, more wind-protected microclimates underneath canopies, thereby decreasing fuel flammability. Tree cover at which biomass burning increased appears to be driven by warmer and drier summer conditions during the early Holocene and by increasing human influence on land cover during the late Holocene. We suggest that long-term fire hazard may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene.