Changes in the Poaceae pollen season in Gothenburg

(1979-2012) and the synchronization between

pollen season andflowering phenology

Index

Sammanfattning på svenska

Keywords

Introduction

Plant phenology

Poaceae pollen season

Observed changes in pollen season and meteorological parameters influencing it

Flowering phenology and pollen season

Aim

Background

Anemophily

Phenology

Basic Poaceae morphology

Roots

Culm

Inflorescence

Anthesis

Anther dehiscence

Poaceae pollen production

Pollinosis

Materials

Alopecurus pratensis (meadow foxtail)

Dactylis glomerata (cock’s foot)

Deschampsia cespitosa (tufted hair-grass)

Deschampsia flexuosa (wavy hair-grass)

Festuca pratensis (meadow fescue)

Festuca rubra (red fescue)

Lolium perenne (perennial rye-grass)

Molinia caerulea (purple moor-grass)

Poa pratensis (smooth meadow grass)

Methods

Location

Part one – Changes in the pollen season and meteorological parameters influencing it

Pollen data

Meteorological data

Statistical analysis

Part two – Comparing local flowering phenology with the pollen season 2013

Phenological data

Pollen data

Analysis

Geographical position of locals

Results

Significant changes in temperature and precipitation 1979-2012

Temperature

Precipitation

Changes in pollen season 1979-2012

The effect of meteorological parameters on the pollen season in Gothenburg, Sweden, during 1979-2012

The beginning of the pollen season (BPS)

Day when the 1st pollen grain is found

Pollen peak (PP)

End of pollen season (EPS)

Duration

Days with pollen amounts exceeding 80 grains/m3

Total pollen amount

Flowering phenology and pollen season 2013

Discussion

Atmospheric Poaceae pollen trends

Changes in climate in Gothenburg

Temperature

Heat accumulation

Precipitation

Environmental factors in combination

Why is so much of the variation in pollen counts not explained?

Previous studies

Land use and cultivation

Definition of the main pollen season

Pollinosis

Phenology

Pollen season and weather 2013

Flowering phenology vs. pollen season

Conclusion

Acknowledgements

References

Sammanfattning på svenska

I denna studie har jag undersökt vilka förändringar som har skett i gräspollensäsongen under 1979-2012, och hur meteorologiska variabler påverkar utvecklingen av pollensäsongen. Syftet var att öka kunskapen om hur pollen-säsongen påverkas av pågående klimatförändringar.Även blomnings-fenologin hos nio vanliga Poaceae arter (Alopecurus pratensis, Dactylis glomerata, Deschampsia cespitosa, Deschampsia flexuosa, Lolium perenne, Festuca pratensis, Festuca rubra,Molinia caerulea, Poa pratensis) observerades och jämfördes med pollensäsongen 2013. Detta gjordes i syfte att undersöka hur väl den lokala blomningen synkroniserar med de uppmätta pollenhalterna, och hur detta det skiljer sig mellan arter.

Pollen-data från 1979-2012 användes för att undersöka förändringar i pollensäsongens start, slut, varaktighet och intensitet. Temperatur- och nederbörds data samt värmeackumulering i form av GDH (Growth Degree Hours) användes sedan i regressioner mellan meteorologiska parametrar och pollensäsongen.

Minst två lokaler i Göteborg valdes ut för fenologi-studien. Observationer av blomnings-fenologin utfördes varannan dag (bortsett från Festuca pratensis som observerades mer sällan) och start-, slut- samt full-blomning beräknades för att sedan jämföra med pollensäsongens utveckling.

Resultaten visar att pollensäsongen i Sverige har förändrats dramatiskt under 1979-2012. En förlängning av pollensäsongen på en månad, fyra gånger fler dagar med pollen-halter överstigande 80 pollenkorn/m3 och en fördubbling av den totala pollen-summan har skett på 30 år. Detta beror delvis på klimatförändringar, eftersom temperatur och nederbörd utan tvekan influerar på pollensäsongen, men förmodligen har även de förändringar i markanvändningen som skett de senaste 60 åren i Sverige samt ökade kvävenedfall till följd av trafikutsläpp påverkat förlängningen och intensifieringen av pollensäsongen.

Den lokala blomningsfenologin hos de studerade Poaceae arterna matchar pollensäsongen väl. Fullblomningen av Dactylis glomerata och Poa pratensisavspeglas tydligt i pollenkurvan i form av en pollenpeak. Även fullblomningen av Lolium perenneoch Festuca prantensisreflekteras i en senare pollenpeak. Detta antyder att dessa fyra arter producerar mycket pollen, och eftersom de alla är kända för att orsaka pollenallergi, så är de till stor del de ansvariga arterna att orsaka gräspollenallergi.

Studien visar att med fortsatt ökad temperatur och nederbörd i Sverige, så kommer pollensäsongen troligtvis att ytterligare förlängas och intensifieras. Detta skulle inte enbart innebära en förvärrad situation för gräspollenallergiker, utan dessutom medföra en förändring i Poaceae ekologi som kan leda till en förändring i skördar av viktiga grödor (majs, ris och vete tillhör alla familjen Poaceae).

Observationer av lokal blomnings-fenologi hosPoaceae kan troligtvis med mer kunskap och utvecklade metoder användas som komplement för dagens dyra pollenmätningar och leda till förbättrade pollenprognoser. Resultaten antyder att Dactylis glomerata, Poa pratensis och Lolium perenne tillhör de stora pollen-producenterna i Sverige och därmed är några av de vikigaste arterna ur allergi synpunkt.

Abstract

Phenology observations of Poaceae are valuable in many aspects. First, they work as indicators to detect temporal changes that can be due to climate change. Additionally, information on temporal differences between species is important to better understand their ecology. By knowing which species are the major contributors of pollinosis, which is a severe health problem, it is possible to improve the situation for allergy sufferers.

Several studies have reported changes in the Poaceae pollen season caused by climate change. Yet, depending on geographical location, results differ and therefore observations on smaller geographical scales are necessary. When present, temporal changes in the pollen season will most probably concern the human population, not only by pollinosis, but also by influencing the harvest of important crops such as maize and wheat.

This study is separated in two parts, where the first one aims to detect possible changes in the Swedish pollen season from 1979-2012, and which variables that are responsible for these potential changes. Regressions between the meteorological variables and the pollen season were made to discover how temperature and precipitation influence the pollens season. In the second part, the flowering phenology of nine common Swedish Poaceae species was observed and compared with the pollen season of 2013, to discover how the flowering phenology matches the pollen season curve.

The results confirm that the Swedish Poaceae pollen season has changed drastically to become prolonged and more intense. The duration of the pollen season is prolonged by one month, the number of days with pollen amounts exceeding 80 grains/m3 has become four times higher, and the pollen index has almost doubled in 30 years. The variables that mainly influence the pollen season are the mean temperature in April and accumulated spring temperature (Growth Degree Hours). The changes in the pollen season are, at least partly, due to climate change. The local flowering phenology matches the pollen season well. Dactylis glomerata, Poa pratensis,Lolium perenneand Festuca pratensisseem to be the major pollen producers, since full-flowering of these species was clearly reflected in the pollen season curve as pollen peaks. The flowering of Poaceae is clearly temporally separated between species.

The results show that the pollen season may be even more prolonged and intensified if temperatures and precipitation continue to increase in Sweden. This will result in more severe consequences for pollinosis sufferers and changes in the ecology of Poaceae. The study also found that phenological studies can probably be used as a component in the basis for pollen forecasts, and that four common Poaceae species probably are mainly responsible for the airborne Poaceae pollen in Sweden.

Keywords

Flowering phenology, Poaceae, atmospheric pollen season, trends in the grass pollen season, meteorological variables, climate change

1

Introduction

Climate change and global warming are nowadays frequently mentioned in media, politics and not least, in science.

Overall the global surface temperature has increased with 0.74 ᵒ C the past 100 years, and the rate of warming is increasing (IPCC 2007). The severity of climate change has been even more acknowledged after alarming reports of extreme weather events causing humanitarian disasters around the world, which many scientists suggest are partly due to climate change (IPCC 2012). Ecosystems and species have been observed to respond to the changing climate and new conditions, the question is how far species adaptability reach when the rate of climate change increases, and what the consequences will be. Plants are sessile and are therefore sensitive to climate change in particular.

Plant phenology

Plant phenology observations have been important ever since man started farming land, since knowledge of when flowering, pollination and seed-set took place was, and still is, crucial to predict and improve the crops yield. With the increased awareness of climate change, plant phenology studies have increased in both frequency and status. Phenology studies can provide excellent bio-indicators of climate change. By knowing when a plant species normally enter a phenophase, temporal changes can be detected. And by knowing which biotic or abiotic variables that influence the phenology, these potential changes in phenology could be explained. Furthermore, if changes in phenology can be understood, predictions can be made of how the phenology will develop and what consequences this would have in the future when climate change is continuing.

Poaceae pollen season

The pollen season is a crucial part of the phenology of plants and does not only determine the reproduction and continuity of plants but also have a large impact on human health. In Europe pollinosis is a major health problem, and in Sweden 25% of the population suffers from pollinosis (Sahlgrenska UniversityHospital 2013), mainly induced by the family of Poaceae and the order of Fagales. The symptoms of pollinosis differ in severity, but can gravely lower life quality both physically and psychologically (Laforest 2005).

Additionally, Poaceae is probably the most important angiosperm family from an economical and agricultural point of view since the world’s most important food sources, maize (Zea mays), rice (Oryza) and wheat (Triticum spp.), all belong to the Poaceae family.

There is no doubt about the importance of Poaceae in many aspects and therefore the pollen season of Poaceae is of great interest to study. Potential changes in the pollen season of Poaceae will not only have consequences on the reproduction of Poaceae but will also concern the human population.

Observed changes in pollen season and meteorological parameters influencing it

Several previous studies have observed changes in plant phenology in Europe (Sparks & Carey 1995, Chmielevski & Rötzer 2001, Fitter & Fitter 2002, Van Vliet et al. 2002, Menzel et al. 2006, Jato et al. 2009, Dahl et al. 2013, amongst many others). Observations from all over Europe overall show changes toward advanced flowering, pollen season and seed set.

An extensive phenology study including many species and countries (Chmielevski & Rötzer 2001) showed that in the Baltic Sea region (where Sweden is included) the start of plant growth is overall advancing with 4.3 days/decade and the growth season is extended by 5.9 days/decade. The trends were explained to be caused by increased temperatures during late winter and early spring. Another study, including 21 countries and 125 000 observations in Europe, observed an advance in leafing, flowering and fruiting due to an increase in monthly temperature in 78% ofthe 542 plantspecies included (Menzel et al. 2006). The advance of flowering of Poaceae due to climate change has also been confirmed by IPCC in the chapter “8.2.7 Aeroallergens and disease” in the fourth assessment report from 2007.

Many studies agree that temperature, precipitation and day length are the main responsible meteorological variables that influence the pollen season. Especially increased temperatures have a significant effect on pollen season and flowering phenology (Sparks et al. 2000, Chmielevski & Rötzer 2001, Van Vliet et al. 2002, Badeck et al. 2004,Green et al. 2004, García-Mozo et al. 2008, Jato et al. 2009, Recio et al. 2010), mainly by advancing the pollen season. When knowing which meteorological variables that influence the pollen season and how, it is possible to discuss and understand the future development of the pollen season with ongoing climate changes. More profound studies also contribute to the possibility of improving annual pollen season forecasts.

Flowering phenology and pollen season

There are several studies that aim to improve the pollen forecasts in different ways. Since phenology studies lately are understood to be useful, there is an interest in understanding how local flowering phenology synchronizes with measured airborne pollen. Studies of the match or mismatch between pollen season and local flowering phenology, in order to improve and better understand pollen forecasts have taken place all over Europe (Jato et al. 2001, Estrella et al. 2005,Tormo et al 2011). Complementing the airborne pollen counts with phenological observations would improve the understanding of which species that contribute to the highest amounts of pollen and the temporal distribution of pollen dispersal among species and thus improve the interpretation of the pollen curve. It will also make it possible to understand the influence of long-distance traveling pollen grains on the pollen season curve. Thus, by this information the taxonomical and geographical interpretation of the annual pollen curve could be improved. It will also serve to find out to what extent phenological records and local observations could be used when predicting and analyzing the pollen season.

Despite the fact that many exhaustive studies concluded a general European trend of an advanced Poaceae pollen season start, a deeper analyze of previous studies shows that there are clear differences between regions and countries in the trends of pollen season and the meteorological variables influencing it. The differences are too large to make any general conclusions over widespread areas. Therefore it is important to make observations in how phenology and pollen season have developed on smaller geographic scales.

It is also necessary to study if other parts of the pollen season than start date, such as peak and end dates and amounts of pollen have changed. In the majority of previous studies only the pollen start date is observed and other phases of the pollen season are ignored.

There is a tradition of predicting and analyzing the pollen season based on “generally accepted knowledge”, but deeper understanding of which, how and to what extent meteorological variables influence the pollen season is crucial. Even if it is well-known that temperature and precipitation influence pollen season, more detailed information is necessary to improve the precision of forecasts. What is also necessary to complement to previous studies is knowledge of how other meteorological parameters than just average spring temperature influence on the pollen season, such as days with no precipitation, rate of heat accumulation and minimum/maximum temperatures. In many studies mean spring temperature is the only parameter used to explain the changes in pollen season.

Since Sweden belongs to the region where the highest rate of change in plant phenology is estimated (Chmielevski & Rötzer 2001), effects on the pollen season are expected.

Aim

A Swedish study investigating temporal trends, the influence of meteorological parameters and phenology of the pollen season of Poaceae will provide valuable information for the fields of climate change, aerobiology and ecology.

The study will be answering following questions:

  1. How has the pollen season in Sweden changed from 1979 to 2012?
  2. How does meteorological parameters influence the pollen season?
  3. Does the local flowering phenology match the pollen season?
  4. How does matching between local flowering phenology and pollen season differ between Poaceae species?

Finding answers to these questions will increase the opportunities to manage and predict the effect of climate change on the pollen season of Poaceae, improve the methods of foreseeing the pollen season and therefore also improving the situation for pollinosis sufferers. It will additionally increase the understanding of the Poaceae family´s ecology.

1

Background

Anemophily

Anemophily is the most common type of abiotic pollen dispersal, and as all types of abiotic pollen dispersals it is one-sided (Fægri & van der Pijl 1979). This makes anemophilous plants independent from biotic vectors that sometimes can be scarce or absent. Abiotic dispersal of pollen is not directed as biotic dispersal (e.g. entomophily or ornithophily) and therefore a great quantity of pollen has to be produced for fertilization of an ovule to occur, which makes pollen dispersal a wasteful process with low fertilization per pollen grain. Every square meter of the plant’s habitat must receive around a million pollen grains to ensure pollination (Proctor & Yeo 1973). The pollen grains in anemophilous plants are small, smooth and dry, suggested to be adaptations that alleviate dispersal and decrease the air resistance.

Phenology

Phenology is the study of periodical events and repeated patterns in nature, such as bud formation, pollen dispersal and seed set in plants. Temporal variation in phenology among species can be explained by genetic variation as a result of selection pressure (Elzinga et al. 2007), and possibly, epigenetic changes. Phenology also depends on abiotic and biotic factors, where the most important factors are temperature, photoperiod and water ability (Dahl et al. 2013). The relative importance of these factors depends on geographical location, causing temporal variation in phenophases.

Basic Poaceae morphology

The morphology of Poaceae differs between species, but all grasses consist of a root-system, culm and inflorencence.

Roots

The root-system is an important storage of nutrients for the plant and is in Poaceae separated in adventitious and seminal roots (Bell & Bryan 1991). The adventitious roots grow just below ground while the seminal roots grow deeper in the soil. The seminary roots are highly branched and are important as they absorb high amounts of nutrients the first months of growth, after which they die in perennial grasses (Langer 1972).

Culm

The culm consists of nodes, internodes, leaves and meristematic tissue situated above the nodes on the axis of the leaves.

The first shoot from a seed is called the parent shoot. Secondary shoots (side shoot) are called tillers and develop from the axillary buds of the parent shoot. The production of tillers is called tillering. The tillers generally have the same morphology as the parent shoot and develop roots, leaves, flowers, an apical meristem and daughter tillers (Langer 1972, Bell & Bryan 1991). Multiple shoots can therefore develop from one seed. Tillers can develop in two ways. One is where the tiller does not break through the parent shoots leaf sheath (intravaginal development), and another one where the tiller breaks through the sheath of the parent shoot and grows horizontally (extravaginal development) (Bell & Bryan 1991). The amount of tillering is genetically controlled but also strongly influenced by environmental factors (Langer 1972). The tillers of perennial species can be annual and die the same season as they were produced but can also survive and flower the following year (Langer 1972). The tillers are essential for the persistence of the Poaceae population, since they represent a great part of the population.