Effect of pollen from regional vegetation on stand-scale forest reconstruction

To investigate the influence of regional pollen inputs on reconstructing local vegetation, we compared modern pollen assemblages deposited in forest hollow sediments from two study areas, Michigan and Wisconsin. Local forest-stand composition (within 50 m) at all sites is dominated by hemlock and northern hardwood trees, but the regional abundance of tree taxa in the two study areas is not the same. Modern pollen assemblages differ between the two study areas, corresponding with differences in regional vegetation. Oak and pine pollen are more abundant in Wisconsin samples, whereas sugar maple, birch and hemlock pollen are more abundant in Michigan samples. Pollen assemblages differed most between study areas for hardwood stands, reflecting lower pollen production of sugar maple and basswood, which exaggerates regional pollen inputs. However, within each study area, surface pollen assemblages are sufficiently different to permit differentiation of hemlock and hardwood stand types, suggesting that regional pollen inputs are similar on the scale of tens of kilometres. Therefore, stand-scale forest histories can be derived from forest-hollow sediments using modern analogues, but our results emphasize the importance of understanding the regional vegetation context and inferring how regional vegetation has changed in the past.