Quantitative reconstruction of Holocene vegetation cover in Flanders, Belgium - a study based on pollen-records from alluvial floodplains

<p>Rivers and alluvial floodplains are dynamic environments facing natural and anthropogenic impacts. A thorough knowledge of the functioning of alluvial floodplains and their sensitivity to changes in internal and external driving forces is required for sustainable management of these ecosystems.</p><p>During the Early and Middle Holocene, most floodplains in northern and central Belgium were stable environments with limited floodplain aggradation resulting in the formation of peat. During these times, floodplains consisted mainly of large marshes where peat accumulated and river channels were absent or small. During the Late Holocene, these environments changed completely towards single channel meandering rivers with overbank deposits, impeding peat accumulation, largely as a result of increasing anthropogenic impact. However, this evolution in floodplain geoecology is diachronous as some river valleys transform a few thousand years before others.</p><p>Previous research already showed that river systems respond non-linearly to changes in land-use and land-cover in their catchments, as land-use intensity and slope-channel coupling need to cross a certain threshold to result in significant change. Hence, the differences in timing of floodplain response can to some extent be related to different land-use trajectories in the river catchments. Based on previous qualitative and semi-quantitative research the exact land-cover threshold, i.e. the land-use intensity required to result in transformation of the fluvial system, as well as the timing at which this threshold is crossed, could not be detected. Hence, a quantitative assessment of the resilience of floodplain environments to regional land-use changes is needed. A successful pilot REVEALS-based reconstruction of the Dijle catchment, showed a decrease in forest cover from the Bronze Age onwards, accompanied by an increase in the proportion of cereals.</p><p>In this study, we constructed a database of pollen-records collected in the eastern part of Flanders, mainly retrieved from river floodplains, as deposits from large lakes are not available in the area. We selected sites with varying soil properties, topographies, and histories of human impact in their catchments, to uncover regional differences in land-cover evolution through the application of the REVEALS model. To assess the applicability of this model to alluvial deposits, modern pollen data will be included and outcomes will be compared to modern vegetation maps. In addition, vegetation reconstructions will be compared with historical maps (available from 1778 AD onwards).</p><p>Results will help to answer questions regarding the sensitivity of Flanders to (future) environmental changes. Our study contributes to the understanding of Holocene land-cover change and its drivers, by providing quantitative vegetation cover reconstructions for Belgium that are currently lacking in the European REVEALS reconstructions. Moreover, it extends the application of the REVEALS model to pollen-records retrieved from alluvial deposits.</p>

Relative pollen productivity estimates and changes in Holocene vegetation cover in the deciduous forest of southeastern Quebec, Canada

The Regional Estimates of VEgetation Abundance from Large Sites (REVEALS) model was used to quantify Holocene changes in vegetation cover in the deciduous forest of southeastern Quebec, Canada. The Extended R-Value (ERV) model was used to obtain relative pollen productivity estimates (PPEs) for eight tree taxa and to determine the relevant source area of pollen (RSAP) for lakes in this ecosystem. Modern vegetation was estimated using pollen data from 16 small (<0.5 km2) lakes and a species-level vegetation survey of southern Quebec. The RSAP was estimated to be within 1600 m of the lakes. Tsuga, Fagus, and Quercus were the most productive taxa, and Populus and Acer were the lowest. Reconstructed changes in absolute vegetation cover show a high abundance of Picea followed by Populus in the early Holocene. The reconstructed values for Populus suggest that it was widely distributed across the landscape. Abies and Acer were dominant on the landscape during the late to mid-Holocene, and an increase in Picea during the Neoglacial is more significant than in percentage diagrams. The REVEALS results provide estimates of land-cover change that are more realistic and informative than the use of pollen percentages alone.

Holocene land-cover reconstructions for studies on land cover-climate feedbacks

The major objectives of this paper are: (1) to review the pros and cons of the scenarios of past anthropogenic land cover change (ALCC) developed during the last ten years, (2) to discuss issues related to pollen-based reconstruction of the past land-cover and introduce a new method, REVEALS (Regional Estimates of VEgetation Abundance from Large Sites), to infer long-term records of past land-cover from pollen data, (3) to present a new project (LANDCLIM: LAND cover – CLIMate interactions in NW Europe during the Holocene) currently underway, and show preliminary results of REVEALS reconstructions of the regional land-cover in the Czech Republic for five selected time windows of the Holocene, and (4) to discuss the implications and future directions in climate and vegetation/land-cover modeling, and in the assessment of the effects of human-induced changes in land-cover on the regional climate through altered feedbacks. The existing ALCC scenarios show large discrepancies between them, and few cover time periods older than AD 800. When these scenarios are used to assess the impact of human land-use on climate, contrasting results are obtained. It emphasizes the need for methods such as the REVEALS model-based land-cover reconstructions. They might help to fine-tune descriptions of past land-cover and lead to a better understanding of how long-term changes in ALCC might have influenced climate. The REVEALS model is demonstrated to provide better estimates of the regional vegetation/land-cover changes than the traditional use of pollen percentages. This will achieve a robust assessment of land cover at regional- to continental-spatial scale throughout the Holocene. We present maps of REVEALS estimates for the percentage cover of 10 plant functional types (PFTs) at 200 BP and 6000 BP, and of the two open-land PFTs "grassland" and "agricultural land" at five time-windows from 6000 BP to recent time. The LANDCLIM results are expected to provide crucial data to reassess ALCC estimates for a better understanding of the land suface-atmosphere interactions.