The glade effect: Vegetation openness and structure and their influences on arboreal pollen production and the reconstruction of anthropogenic forest opening

This study highlights the importance of landscape openness and structure as an environmental factor influencing the total arboreal pollen production of a landscape. This influence poses an unconsidered bias that potentially affects the results of landscape reconstructions. The so-called “glade effect” is an increase in arboreal pollen production as a result of landscape opening. This increase is based on the assumption that an opening up of the woodland canopy results in increased availability of light, and hence flowering activity in the crown areas of trees. A simple simulation of the glade effect suggests that: (a) the relationship of landscape openness and the arboreal/non-arboreal pollen ratio is non-linear and (b) the spatial pattern of the landscape opening process (e.g. uniform or patchy) is important for the degree of such underrepresentation. The simulation results reveal similar patterns found in empirical pollen data from two laminated lakes in Northern Germany. An increase in arboreal and non-arboreal pollen influx at the transition from hunter–gatherer to farming communities is regarded to result from increasing openness of the woodland canopy, reflecting the proposed glade effect. A subsequent decrease of arboreal pollen influx likely corresponds with an anthropogenic change in landscape structure, i.e. the creation of larger permanent open field systems. Therefore the interpretation of pollen influx data under consideration of the glade effect helps for interpretation of pollen diagrams and facilitates conclusions regarding the spatial pattern of early forest opening.