Effects of changes in land management practices on pollen productivity of open vegetation during the last century derived from varved lake sediments

Abstract

Pollen productivity is a key parameter to quantify past plant abundances and vegetation openness. In this study we explore how changes in land management influence pollen productivity. We study pollen deposition in largely annually laminated sediments from Lake Tiefer See in the Northeastern German lowlands deposited between AD 1880 and 2010. During this period, land use intensity has increased predominantly through the widespread introduction of artificial fertilizers, herbicides and heavy machinery mainly since the 1950s. Although land use statistics show that overall vegetation openness remained largely constant, non-arboreal pollen deposition (from herbs and grasses) sharply declined over the study period. This decline can be partly explained by a shift towards crops that emit little pollen such as wheat and oilseed rape. Furthermore, intensified grassland management, including drainage, also contributed to lower pollen deposition because of the decline of Plantago lanceolata and Rumex from grassland communities. However, the most important effect is a decline in pollen productivity of grasses of about 60%, which most likely is a response to earlier and more frequent mowing, although changes in grass species composition may also have played a role. Our results show that the type and intensity of land use have a strong effect on pollen productivity of grasses (and smaller effects on further crops). Since grass pollen deposition is a main proxy for vegetation openness and grasses are the common reference taxon in most PPE studies, variations in the pollen productivity of grasses introduce so far neglected errors in the reconstruction of the past vegetation cover. Our study provides a first estimate about the magnitude of this effect.