Pollen Season Trends (1973-2013) in Stockholm Area, Sweden

In the present study, the phenological and quantitative changes in the pollen seasons between 1973 and 2013 in the Stockholm region of Sweden were studied for nine types of pollen (hazel, alder, elm, birch, oak, grass, mugwort, willow and pine). Linear regression models were used to estimate the long term trends in duration, start- and end-dates, peak-values and the yearly accumulated pollen sums of the pollen seasons. The pollen seasons of several arboreal plant species (e.g. birch, oak and pine) were found to start significantly earlier today compared to 41 years earlier, and have an earlier peak-date, while the season of other species seemed largely unaffected. However, the long term trends in the end-dates of pollen seasons differed between arboreal and herbaceous species. For herbaceous species (grass and mugwort), a significant change towards later end-dates was observed and the duration of season was found to have increased. A significant trend towards an earlier end-date was found in the majority of the arboreal plant species (i.e. elm, oak, pine and birch), but the length of the season seemed unaffected. A trend towards an increase in yearly concentrations of pollen was observed for several species; however the reasons for this phenomenon cannot be explained unambiguously by the present study design. The trend of increasing yearly mean air temperatures in the Stockholm area may be the reason to changed phenological patterns of pollen seasons

Pollen trends 1973–2013.

<p>The significant trends (CI>95%) are marked with <b>bold</b> style. The negative sign in front of a value indicates a trend towards an earlier start-date, peak-date or end-date, a shortening trend in the duration of the pollen season or quantities of pollen produced. Positive numbers indicate a trend towards a later start-date, peak-date or end-date, a lengthening trend in the duration of the pollen season and increased quantities of pollen produced. DOY indicates the no of day of year. The 25 and 75 percentile values for all pollen types were determined by first calculating the average number of pollen grains/m³/day for each season and thereafter obtaining the average percentile values across the whole 41 year period.</p

Pollen Season Trends (1973-2013) in Stockholm Area, Sweden - Fig 1

<p>The change in pollen season characteristics per year for hazel (Co), alder (Al), elm (Ul), Pine (Pi), oak (Qu), willow (Sx), birch (Be), grass (Po), and mugwort (Ar) a) Duration b) Start-date c) End-date d)Peak-date. Error bars depict 95% confidence interval.</p

The proportional change/year in total yearly pollen counts.

<p>Error bars depict 95% confidence interval.</p

Average yearly temperatures in the Bromma measuring station, Stockholm area 1973–2013 (Swedish Meteorology and Hydrology Institute).

<p>Average yearly temperatures in the Bromma measuring station, Stockholm area 1973–2013 (Swedish Meteorology and Hydrology Institute).</p

European phenological response to climate change matches the warming pattern

Global climate change impacts can already be tracked in many physical and biological systems; in particular, terrestrial ecosystems provide a consistent picture of observed changes. One of the preferred indicators is phenology, the science of natural recurring events, as their recorded dates provide a high-temporal resolution of ongoing changes. Thus, numerous analyses have demonstrated an earlier onset of spring events for mid and higher latitudes and a lengthening of the growing season. However, published single-site or single-species studies are particularly open to suspicion of being biased towards predominantly reporting climate change-induced impacts. No comprehensive study or meta-analysis has so far examined the possible lack of evidence for changes or shifts at sites where no temperature change is observed. We used an enormous systematic phenological network data set of more than 125 000 observational series of 542 plant and 19 animal species in 21 European countries (1971–2000). Our results showed that 78% of all leafing, flowering and fruiting records advanced (30% significantly) and only 3% were significantly delayed, whereas the signal of leaf colouring/fall is ambiguous. We conclude that previously published results of phenological changes were not biased by reporting or publication predisposition: the average advance of spring/summer was 2.5 days decade1 in Europe. Our analysis of 254 mean national time series undoubtedly demonstrates that species' phenology is responsive to temperature of the preceding months (mean advance of spring/summer by 2.5 days°C1, delay of leaf colouring and fall by 1.0 day°C1). The pattern of observed change in spring efficiently matches measured national warming across 19 European countries (correlation coefficient r=−0.69, P<0.001)