Assessing the proportion of “extra-local” pollen by means of modern aerobiological and phenological records — An example from Scots pine (Pinus sylvestris L.) in northern Finland

Abstract

Detecting “extra-local” pollen, and especially long-distance transport events, in fossil pollen data is a problematical task, particularly when one species dominates both the vegetation and the pollen counts, as is the case with Scots pine (Pinus sylvestris) in Fennoscandia. As a step towards elucidating this, modern airborne pollen data were analysed in northern Finland along a north–south transect (Kevo north of the pine forest limit, and Rovaniemi and Oulu within the pine forest) for the years 1977–2008 (Kevo), 2002–2008 (Rovaniemi) and 1997–2008 (Oulu). By using phenological observations, the “extra-local” (“preflowering”) pine pollen, occurring before the onset of local male flowering, could be detected. Preflowering pine pollen represents only that “extra-local” pollen deposited before the onset of local pollen shedding and, therefore, underestimates the annual “extra-local” pine pollen amount. For Kevo a maximum estimation of total “extra-local” pine pollen could be calculated, which was 34%. The preflowering values nevertheless provide a minimum estimation of total “extra-local” pine pollen, because, within the period studied, the pollen shedding onset dates at the three sites were delayed in a northwards direction by, on average, 8–9days from one study site to the next, which was mostly longer than the duration of local flowering. Kevo had the lowest total pine pollen values due to its position north of the pine forest limit, of which 11% was preflowering. The highest average total pine pollen values and 15% preflowering were found at Rovaniemi, while Oulu had the smallest amount of preflowering pollen (6%) but a high total pine pollen sum. Rovaniemi and Kevo had several years with more than 10% preflowering pollen. The variation in preflowering data was highest at Kevo. In five years, its percentage was remarkably high (>20%) and occurred at least two weeks before the local male flowering onset, suggesting a pollen source further away. Clear long-distance transport events could be detected at Kevo in 2001 and 2008. In the 2001 event, the wind came from southerly directions, so the possible pollen source was in southern Finland several hundred km to the south. The amount of “extra-local” preflowering pine pollen is on average only 6 to 15% of total pine pollen at the northern Finnish sites, and much of it would seem to originate from sources closer to the site than expected. These average preflowering pine pollen percentages provide a minimum estimation of the proportion of pollen originating from “extra-local” sources, which can be applied to fossil pollen data, where one sample comprises several years.