Ragweed Pollen Concentrations Research Articles

Semiparametric regression models under skew scale mixtures of normal distributions

Semiparametric models (SM) are an important tool in modeling environmental data where generally a covariate presents an unknown nonlinear behavior. Usually, the error component is assumed to follow a normal distribution. However, in some situations, the response variable is skewed and heavy-tailed. This paper aims to extend the SMs allowing the errors to follow a skew scale mixture of normal distributions, increasing the model’s flexibility. In particular, we develop the EM algorithm for the proposed model, diagnostic analysis via global, local influence, and generalized leverage. A simulation study is also conducted to evaluate the efficiency of the EM algorithm. Finally, a suitable transformation is applied in a data set on ragweed pollen concentration to illustrate the utility of the proposed model.

A temporally and spatially explicit, data-driven estimation of airborne ragweed pollen concentrations across Europe

Ongoing and future climate change driven expansion of aeroallergen-producing plant species comprise a major human health problem across Europe and elsewhere. There is an urgent need to produce accurate, temporally dynamic maps at the continental level, especially in the context of climate uncertainty. This study aimed to restore missing daily ragweed pollen data sets for Europe, to produce phenological maps of ragweed pollen, resulting in the most complete and detailed high-resolution ragweed pollen concentration maps to date. To achieve this, we have developed two statistical procedures, a Gaussian method (GM) and deep learning (DL) for restoring missing daily ragweed pollen data sets, based on the plant's reproductive and growth (phenological, pollen production and frost-related) characteristics. DL model performances were consistently better for estimating seasonal pollen integrals than those of the GM approach. These are the first published modelled maps using altitude correction and flowering phenology to recover missing pollen information. We created a web page (http://euragweedpollen.gmf.u-szeged.hu/), including daily ragweed pollen concentration data sets of the stations examined and their restored daily data, allowing one to upload newly measured or recovered daily data. Generation of these maps provides a means to track pollen impacts in the context of climatic shifts, identify geographical regions with high pollen exposure, determine areas of future vulnerability, apply spatially-explicit mitigation measures and prioritize management interventions.

Ragweed pollen concentration predicts seasonal rhino-conjunctivitis and asthma severity in patients allergic to ragweed

In this work, we investigate the correlation between ragweed pollen concentration and conjunctival, nasal, and asthma symptom severity in patients allergic to ragweed pollen using ambient pollen exposure in the Milan area during the 2014 ragweed season We calculate the pollen/symptom thresholds and we assess the effectiveness of ragweed allergen immunotherapy (AIT). A total of 66 participants allergic to ragweed (Amb a 1) were enrolled in the study and divided into two groups: AIT treated (24) and no AIT treated (42). Pollen counts and daily symptom/medication patient diaries were kept. Autoregressive distributed lag models were used to develop predictive models of daily symptoms and evaluate the short-term effects of temporal variations in pollen concentration on the onset of symptoms. We found significant correlations between ragweed pollen load and the intensity of symptoms for all three symptom categories, both in no AIT treated (τ = 0.341, 0.352, and 0.721; and ρ = 0.48, 0.432, and 0.881; p-value < 0.001) and in AIT treated patients (tau= 0.46, 0.610, and 0.66; and ρ = 0.692, 0.805, and 0.824; p-value < 0.001). In both groups, we observed a positive correlation between the number of symptoms reported and drug use. Mean symptom levels were significantly higher in no AIT treated than in AIT treated patients (p-value < 0.001) for all symptom categories. Pollen concentration thresholds for the four symptom severity levels (low, medium–low, medium–high and high) were calculated. Ragweed pollen concentration is predictive of symptom severity in patients with a ragweed (Amb a 1) allergy. Patients treated with AIT had significantly reduced mean symptom levels compared to those without AIT.

Unusual early peaks of airborne ragweed (Ambrosia L.) pollen in the Pannonian Biogeographical Region.

Early peaks of airborne ragweed (Ambrosia L.) pollen concentrations were observed at several monitoring stations in Hungary in June 2017 and 2018, onemonth before the usual start of the pollen season at the end of July. Backward trajectories were calculated to simulate potential sources of pollen collected at different locations in the Pannonian Biogeographical Region. In a collaboration between aerobiological and phenological networks, a nationwide campaign was conducted to collect field data of ragweed blooming. During field surveys, ragweed plants having extremely early blooming were found most abundantly in a rural site near Vaja (North-East Hungary) and other locations in Hungary. Field observations matched with source areas identified by trajectory analyses; i.e., early-flowering ragweed plants were found at some of these locations. Although similar peaks of airborne pollen concentrations were not detected in other years (e.g., 2016, 2019-2021), alarming results suggest the possibility of expanding seasons of ragweed allergy.

Dynamics of Ragweed Pollen Concentration in the Air of Ivano-Frankivsk

The article updates the conduct of aeropalynological monitoring in Ukraine because today, there are only a few studies in the country that cannot be automatically extrapolated to other regions, as the diversity of climatic zones affects the duration of pollination and pollen grains in the air. Furthermore, this type of research is needed due to the increasing number of people suffering from pollen allergies (hay fever).The research was conducted in the city of Ivano-Frankivsk (western Ukraine) from early February to mid-October 2019, using a gravimetric Durham pollen collector installed on the roof of the Faculty of Natural Sciences of Vasyl Stefanyk Precarpathian National University (24 m from the ground). The pollen grains were counted using an Olympus CX-300 light microscope (magnification x 400). The main goal was to study the aeropalynological spectrum of the city and the dynamics of concentrations of ragweed pollen grains (the most important plant allergen).The study determined the aeropalynological spectrum of Ivano-Frankivsk and provided data on pollination of the following taxa: Betula, Quercus, Acer, Fraxinus, Pinus, Juglans, Carpinus, Alnus, Populus, Urticaceae, Poaceae, Artemisia, and Ambrosia; conducted a comparative characterization with the spectra of the cities of Vinnytsia (central Ukraine) and Zaporizhia (southern Ukraine); determined the beginning and end of pollination of ragweed and the peak of its flowering, as well as conducted a comparative description with last year's data.Seeing an alarming trend, there is a need to continue to conduct aeropalynological monitoring to create calendars of sawing and alleviate the course of allergies in people prone to it.

Heteroscedastic partially linear model under skew-normal distribution with application in ragweed pollen concentration

We introduce a new class of heteroscedastic partially linear model (PLM) with skew-normal distribution. Maximum likelihood estimation of the model parameters by the ECM algorithm (Expectation/Conditional Maximization) as well as influence diagnostics for the new model are investigated. In addition, a Likelihood Ratio test for assessing the homogeneity of the scale parameter is presented. Simulation studies for assessing the performance of the ECM algorithm and the Likelihood Ratio test statistics for homogeneity of variance are developed. Also, a study for misspecification of the structure function is considered. Finally, an application of the new heteroscedastic PLM to a real data set on ragweed pollen concentration is presented to show that it provides a better fit than the classic homocedastic PLM. We hope that the proposed model may attract applications in different areas of knowledge.

The influence of source maps on SILAM performance in modeling ragweed pollen concentrations in the area of a major European source.

The Pannonian Plain is one of the centers of ragweed distribution in Europe. The province of Vojvodina (Serbia) is located on the southern part of the Pannonian Plain, representing a highly infested region. In this study, we have used the SILAM atmospheric dispersion model to simulate ragweed pollen concentrations during the season 2016 in the Vojvodina region. SILAM was tested with three different source maps of ragweed distribution in Vojvodina only: (1) map used in operational SILAM, which was calibrated with the SILAM model and observations, (2) map derived using "top-down" approach with land cover data inventory, and (3) map obtained with "top-down" approach using crop classification from the satellite data. Additionally, the sensitivity studies were done using two modified maps to study the effect of the source strength and long-range transport. Results of simulations were validated with the bi-hourly, daily, and seasonal pollen concentrations measured at five stations in Vojvodina. Overall Pearson correlation coefficients were 0.51 (Map 1), 0.50 (Map 2), and 0.42 (Map 3), while debiased scores were 232.95 pollen m-3 (Map 1), 245.59 pollen m-3 (Map 2), and 258.24 pollen m-3 (Map 3). Even though Vojvodina is in the area of a major European source, regional transport of ragweed pollen from a few hundred kilometers of the surrounding area was important in explaining the presence of pollen in the afternoon hours, although it could not completely explain total pollen quantity. The results confirmed that it is vital to calibrate source maps using atmospheric dispersion model with the observed pollen data.

An assessment of ragweed pollen and allergen loads in an uninvaded area in the Western Black Sea region of Turkey

Ambrosia artemisiifolia L., known as common ragweed, is an invasive and allergenic plant that has begun to expand its presence in Turkey. Although ragweed pollen has been monitored for a long time—especially in the atmosphere of cities in Northern Anatolia—the invasion of the plant is more limited compared to that of other European countries. Here, we present the highest ragweed pollen concentration recorded in Turkey, as well as changes in the Amb a 1 allergen level in the air during 2015 and 2016 in Zonguldak, measured using a BGI 900 cascade impactor. The Seasonal Pollen Integral (SPIn) was found to be 709 pollen day/m3 and 1542 pollen day/m3 in 2015–2016, respectively. The sum of Amb a 1 in both stages was 8.42 × 10−3 U/m3 and 13.1 × 10−3 U/m3 over the same period. The number of days in which the Ambrosia pollen concentration was higher than 50 pollen/m3 doubled in 2016 compared to 2015, despite the negative effect of rainfall on pollen and allergen concentration in 2016. The pollen and allergen sources were evaluated by using simulated air mass movement with a Hybrid Single-Particle Lagrangian Integrated Trajectory model. Our findings suggest that individuals in Zonguldak are exposed as much to pollen—and possibly allergens—transported from the Ukraine, Russia and to a limited extent from Moldova. Furthermore, the allergen content of pollen in Zonguldak was much higher than other cities where Amb a 1 is monitored. We also demonstrated that the date of the peak pollen day may be an indicator of the source of long distance transportation.

Comparative study of ragweed pollen seasons (2008–2017) in the air of Croatian tourist cities of Zagreb (continental area) and Zadar (Mediterranean area)

The majority of tourists visit Croatia during the summer months, which overlaps with the ragweed pollination season, and this might be the main health issue, which could affect Croatian tourism. Therefore, the aim of this research was to perform the preliminary analyses of ragweed pollen seasons in two different Croatian tourist cities—continental Zagreb and Mediterranean Zadar, and to define researched areas according to the allergenic risk assessment. The research was performed during a 10-year period (2008–2017), and aerobiological samplings, as well as qualitative and quantitative analyses, were carried out by standard methodology recommended by EAN/REA. The results showed differences between cities in the seasonal pollen integral (SPIn) and intradiurnal variations index (IDI) of ragweed pollen, and similarities in the features of the main pollen seasons (MPS). The SPIn was higher in the air of Zagreb during the whole studied period; it was on average 2899 pg/m3 in Zagreb and 579 pg/m3 in Zadar. The MPS in both cities were similar (from the first half of August until the mid-September) and relatively short, around 40 days. The IDI indicates that Ambrosia pollen in Zagreb derives from local sources, whilst in Zadar is probably a result of long-range transport. According to the allergenic risk assessment, Zagreb is placed in the intermediate-risk area and Zadar in the low-risk area. Relatively short exposure times during the summer periods enable both cities to be favourable tourist destinations even during the ragweed seasons, especially Zadar, with its low ragweed pollen concentrations.

Ragweed pollen and allergic symptoms in children: Results from a three-year longitudinal study

Common ragweed is a highly allergenic invasive species in Europe, expected to become widespread under climate change. Allergy to ragweed manifests as eye, nasal and lung symptoms, and children may retain these throughout life. The dose-response relationship between symptoms and pollen concentrations is unclear. We undertook a longitudinal study, assessing the association between ragweed pollen concentration and allergic eye, nasal and lung symptoms in children living under a range of ragweed pollen concentrations in Croatia. Over three years, 85 children completed daily diaries, detailing allergic symptoms alongside daily location, activities and medication, resulting in 10,130 individual daily entries. The daily ragweed pollen concentration for the children's locations was obtained, alongside daily weather and air pollution. Parents completed a home/lifestyle/medical questionnaire. Generalised Additive Mixed Models established the relationship between pollen concentrations and symptoms, alongside other covariates. Eye symptoms were associated with mean daily pollen concentration over four days (day of symptoms plus 3 previous days); 61 grains/m3/day (95%CI: 45, 100) was the threshold at which 50% of children reported symptoms. Nasal symptoms were associated with mean daily pollen concentration over 12 days (day of symptoms plus 11 previous days); the threshold for 50% of children reporting symptoms was 40 grains/m3/day (95%CI: 24, 87). Lung symptoms showed a relationship with mean daily pollen concentration over 19 days (day of symptoms plus 18 previous days), with a threshold of 71 grains/m3/day (95%CI: 59, 88). Taking medication on the day of symptoms showed higher odds, suggesting responsive behaviour. Taking medication on the day prior to symptoms showed lower odds of reporting, indicating preventative behaviour. Different symptoms in children demonstrate varying dose-response relationships with ragweed pollen concentrations. Each symptom type responded to pollen exposure over different time periods. Using medication prior to symptoms can reduce symptom presence. These findings can be used to better manage paediatric ragweed allergy symptoms.

Detection of reactive allergens in long-distance transported pollen grains: Evidence from Ambrosia

The pollen of Ambrosia artemisiifolia (ragweed) is an important aeroallergen. The plant originated from North America and, has spread worldwide in recent decades. Air masses can transport pollen grains far from their source region and episodes of long distance transported (LDT) of ragweed pollen have been reported. The allergenic properties of LDT pollen grains can be altered by humidity, temperature and UV radiation. The aims of the study are to detect the major allergen of ragweed (Amb a 1) in the atmosphere of uninfested areas and to compare daily Ambrosia sp. pollen grains with Amb a 1 amount per m3 of air. Samples for allergen detection were collected by ChemVol® sampler. ChemVol® collects particles at 800 l/min and contains 2 impaction stages (PM > 10 μm and 10 > PM > 2.5 μm). Samples collected on to filters were obtained during a 63 day period in the year 2014. Amb a 1 was recorded on 54 days and pollen grains of ragweed on 43 days. Diurnal average Amb a 1 concentrations ranged between 0.29 and 263.3 pg per cubic meter of air and correlated positively with daily average ragweed pollen concentrations. The average seasonal Ambrosia pollen allergen potency was 2.57 pg Amb a 1/pollen. The air mass movement was simulated using the HYbrid Single-particle Lagrangian Integrated Trajectory (HYSPLIT) model and back-trajectory paths were computed for four episodes. The findings suggest that ragweed pollen allergens still survive after at least 48 h during transport over long distances from allochthonous regions and are therefore a potential trigger of allergic reactions even in areas where ragweed is not widely dispersed.

Estimating economic benefits of biological control of Ambrosia artemisiifolia by Ophraella communa in southeastern France

The North American common ragweed, Ambrosia artemisiifolia, which produces highly allergenic pollen, is invasive in different parts of the world, including Europe. In 2013, common ragweed in northern Italy was found attacked by another accidentally introduced species, the North American leaf beetle Ophraella communa, which is used for biological control of common ragweed in China. Since the establishment of O. communa, ragweed pollen concentrations in northern Italy have significantly dropped. Here we set out to estimate the potential economic benefits of establishment of O. communa in the Rhône-Alpes region in south-eastern France, where detailed data on the economic impact of common ragweed are available. Extrapolating from the change in airborne ragweed pollen concentrations in the Milan area, we estimated that establishment of O. communa in the Rhône-Alpes region will reduce the number of days with ragweed pollen concentrations at which sensitive people express symptoms by 50% and the medical costs due to common ragweed by 5.2–6.8M € annually. Our findings suggest that investments of public funds are justified to conduct a complete assessment of the potential risks and benefits associated with the accidental establishment of O. communa in Europe.

Seasonal changes in plant pollen concentrations over recent years in Vinnytsya, Central Ukraine

&lt;p&gt;The control of plant pollen season patterns is especially important in the expectation of climate change, as the timing of potential varying pollen seasons affects the human population. An ever-increasing number of people suffer from hay fever symptoms with varying severity during the pollen season. This paper presents data on the seasonal variations of pollen concentration and the factors which are the likely causes of these variations in Vinnytsya, a city in Central Ukraine, in order to establish the apparent pattern of this variation and so improve the efficiency of hay fever control in Ukraine.&lt;/p&gt;&lt;p&gt;Pollen counts were obtained by gravimetric and volumetric methods employing a Hirst-type volumetric spore trap.&lt;/p&gt;&lt;p&gt;Alder (&lt;em&gt;Alnus&lt;/em&gt;) and birch (&lt;em&gt;Betula&lt;/em&gt;) peaks of pollen release occurred approximately 1 month earlier than was observed at the end of the twentieth century. This was due to the seasonal heat accumulation related to the appropriate temperature regimen registered in January and February prior to the growing season. Other trees – including poplar (&lt;em&gt;Populus&lt;/em&gt;), maple (&lt;em&gt;Acer&lt;/em&gt;), walnut (&lt;em&gt;Juglans&lt;/em&gt;), common hazel (&lt;em&gt;Corylus&lt;/em&gt;) – did not show distinct changes in pollen season pattern over the past decades.&lt;/p&gt;&lt;p&gt;Mean daily temperature seems to be the leading factor promoting early season onset and a seasonal pollen peak shift of the grass and herb flora such as ragweed (&lt;em&gt;Ambrosia&lt;/em&gt;). The shift of the ragweed seasonal pollen maximum towards later in the season correlated with higher temperatures during September. Our study has shown that droughts may also significantly decrease the ragweed pollen concentration.&lt;/p&gt;

Impacts of long-term ragweed pollen load and other potential risk factors on ragweed pollen allergy among schoolchildren in Hungary

Hungary is one of the areas in Europe most infected with ragweed (Ambrosia artemisiifoliaL.) and its pollen, and is the most important cause of seasonal allergic rhinoconjunctivitis in the country. The aim of the study was to investigate the association between ragweed pollen allergy and long-term ragweed pollen load, as well as analysis of the the impacts of additional potential risk factors on health outcomes. A modified version of standardized questionnaires, based on the International Study of Asthma and Allergy in Childhood, were completed by the parents of schoolchildren aged 8 - 9 attending 3rd grade classes throughout the country. Pollen load was calculated for each settlement from daily ragweed pollen concentrations monitored by 19 monitoring stations in the country. Descriptive and analytical statistical methods were applied. At national level there was a significant inverse association between prevalence of ragweed allergy and its pollen load, but significance was lost after excluding data from Budapest, the capital city, due to the impact of urbanization. In the adjusted model, parental atopic disease was the strongest risk factor (either parent had atopic disease aOR=2.60; 95% CI=2.31-2.93 or both parents had atopic disease aOR=4.56; 95% CI=3.71-5.60). Further significant risk factors were male gender (aOR=1.52; 95% CI=1.36-1.71), lower respiratory infection in the first two years of life (aOR=1.91; 95% CI=1.70-2.16), and unshared children's room (aOR=1.22; 95% CI=1.09-1.37). Allergy was significantly less common among children whose parents received social aid (aOR=0.83; 95% CI=0.72-0.97) and whose mother smoked during pregnancy (aOR=0.80; 95% CI=0.64-0.99). Higher ragweed pollen exposure was not found to be associated with higher risk of ragweed allergy.

Biogeographical drivers of ragweed pollen concentrations in Europe

The drivers of spatial variation in ragweed pollen concentrations, contributing to severe allergic rhinitis and asthma, are poorly quantified. We analysed the spatiotemporal variability in 16-year (1995–2010) annual total (66 stations) and annual total (2010) (162 stations) ragweed pollen counts and 8 independent variables (start, end and duration of the ragweed pollen season, maximum daily and calendar day of the maximum daily ragweed pollen counts, last frost day in spring, first frost day in fall and duration of the frost-free period) for Europe (16 years, 1995–2010) as a function of geographical coordinates. Then annual total pollen counts, annual daily peak pollen counts and date of this peak were regressed against frost-related variables, daily mean temperatures and daily precipitation amounts. To achieve this, we assembled the largest ragweed pollen data set to date for Europe. The dependence of the annual total ragweed pollen counts and the eight independent variables against geographical coordinates clearly distinguishes the three highly infected areas: the Pannonian Plain, Western Lombardy and the Rhone-Alpes region. All the eight variables are sensitive to longitude through its temperature dependence. They are also sensitive to altitude, due to the progressively colder climate with increasing altitude. Both annual total pollen counts and the maximum daily pollen counts depend on the start and the duration of the ragweed pollen season. However, no significant changes were detected in either the eight independent variables as a function of increasing latitude. This is probably due to a mixed climate induced by strong geomorphological inhomogeneities in Europe.

Source regions of ragweed pollen arriving in south-western Poland and the influence of meteorological data on the HYSPLIT model results

We have investigated the relationship between the inflow of air masses and the ragweed pollen concentration in SW Poland (Wrocław) for a 10-year period of 2005–2014. The HYSPLIT trajectory model was used to verify whether episodes of high concentrations can be related to regions outside of the main known ragweed centres in Europe, like Pannonian Plain, northern Italy and Ukraine. Furthermore, we used two different meteorological data sets (the global GDAS data set and from the WRF mesoscale model; the meteorological parameters were: U and V wind components, temperature and relative humidity) into HYSPLIT to evaluate the influence of meteorological input on calculated trajectories for high concentration ragweed episodes. The results show that the episodes of high pollen concentration (above 20 pm−3) represent a great part of total recorded ragweed pollen in Wrocław, but occur rarely and not in all years. High pollen episodes are connected with air masses coming from south and south-west Europe, which confirms the existence of expected ragweed centres but showed that other centres near Wrocław are not present. The HYSPLIT simulations with two different meteorological inputs indicated that footprint studies on ragweed benefit from a higher resolution meteorological data sets.

Estimation and diagnostic for skew-normal partially linear models

ABSTRACTPartially linear models (PLMs) are an important tool in modelling economic and biometric data and are considered as a flexible generalization of the linear model by including a nonparametric component of some covariate into the linear predictor. Usually, the error component is assumed to follow a normal distribution. However, the theory and application (through simulation or experimentation) often generate a great amount of data sets that are skewed. The objective of this paper is to extend the PLMs allowing the errors to follow a skew-normal distribution [A. Azzalini, A class of distributions which includes the normal ones, Scand. J. Statist. 12 (1985), pp. 171–178], increasing the flexibility of the model. In particular, we develop the expectation-maximization (EM) algorithm for linear regression models and diagnostic analysis via local influence as well as generalized leverage, following [H. Zhu and S. Lee, Local influence for incomplete-data models, J. R. Stat. Soc. Ser. B 63 (2001), pp. 111–126]. A simulation study is also conducted to evaluate the efficiency of the EM algorithm. Finally, a suitable transformation is applied in a data set on ragweed pollen concentration in order to fit PLMs under asymmetric distributions. An illustrative comparison is performed between normal and skew-normal errors.

Technical and clinical validation of an environmental exposure unit for ragweed.

BackgroundAllergic rhinitis is a common condition, with ragweed pollen one of the more prevalent aeroallergens. Environmental exposure units such as the Allergen BioCube® are valuable models for clinical allergy studies. A study was conducted to validate the Allergen BioCube for uniform ragweed pollen concentrations and clinically relevant sign and symptom responses to ragweed exposure.MethodsRagweed pollen concentrations were measured on 3 consecutive days in the Allergen BioCube and verified by Rotorod collection and continuous laser particle count measurements. Subjects (N=10) were exposed to ragweed pollen in the BioCube for 3 hours per day for 3 consecutive days. Subjects assessed their nasal itching, sneezing, rhinorrhea, and nasal congestion during each BioCube exposure; total nasal symptom score was computed. Peak nasal inspiratory flow was also assessed during BioCube exposure.ResultsUniform ragweed pollen concentrations were obtained throughout each of the 3-hour testing periods in the Allergen BioCube, both spatially and temporally, at all subject positions, with a low mean standard deviation of 10%. Pronounced increases in mean total nasal symptom scores (6.7±0.94 to 7.6±0.86, last 90 minutes of exposure) occurred for all three BioCube ragweed pollen exposure visits. Mean peak nasal inspiratory flow decreased 24% at 3 hours of BioCube exposure on Day 3. No safety issues of concern occurred in this study.ConclusionThe Allergen BioCube achieved technical and clinical validation for ragweed allergen. Ragweed pollen concentration was uniform both temporally and spatially. Allergic rhinitis signs and symptoms were induced in subjects during exposure to ragweed in the BioCube at clinically meaningful levels for allergy studies.

Modelling the introduction and spread of non‐native species: international trade and climate change drive ragweed invasion

Biological invasions are a major driver of global change, for which models can attribute causes, assess impacts and guide management. However, invasion models typically focus on spread from known introduction points or non-native distributions and ignore the transport processes by which species arrive. Here, we developed a simulation model to understand and describe plant invasion at a continental scale, integrating repeated transport through trade pathways, unintentional release events and the population dynamics and local anthropogenic dispersal that drive subsequent spread. We used the model to simulate the invasion of Europe by common ragweed (Ambrosia artemisiifolia), a globally invasive plant that causes serious harm as an aeroallergen and crop weed. Simulations starting in 1950 accurately reproduced ragweed's current distribution, including the presence of records in climatically unsuitable areas as a result of repeated introduction. Furthermore, the model outputs were strongly correlated with spatial and temporal patterns of ragweed pollen concentrations, which are fully independent of the calibration data. The model suggests that recent trends for warmer summers and increased volumes of international trade have accelerated the ragweed invasion. For the latter, long distance dispersal because of trade within the invaded continent is highlighted as a key invasion process, in addition to import from the native range. Biosecurity simulations, whereby transport through trade pathways is halted, showed that effective control is only achieved by early action targeting all relevant pathways. We conclude that invasion models would benefit from integrating introduction processes (transport and release) with spread dynamics, to better represent propagule pressure from native sources as well as mechanisms for long-distance dispersal within invaded continents. Ultimately, such integration may facilitate better prediction of spatial and temporal variation in invasion risk and provide useful guidance for management strategies to reduce the impacts of invasion.

Seasonal variation of airborne ragweed (Ambrosia artemisiifolia L.) pollen in Zagreb, Croatia

Previous research indicated that airborne ragweed pollen concentrations may be influenced by weather-related factors. Therefore, the object of this work was to examine the variation in daily pollen concentrations during four ragweed pollen seasons (2006–2009) in the highly urban area of Zagreb. Ragweed pollen grains were collected using a Burkard volumetric sampler (N45°49′55″, E15°58′54″). Meteorological data (maximum, minimum and mean temperatures, relative humidity, wind speed, precipitation, atmospheric pressure and irradiance) were related to daily pollen counts during the ragweed pollen season. The ragweed pollen season started around late July in 2007 and 2009, while it started on 15 August in 2006, the year characterized by a cold spring. However, the start dates of the pollen seasons were not related to the accumulation of thermal units. Maximum daily concentration of 363 grains m−3 was detected on 27 August 2008. Total airborne pollen concentrations ranged from 1188 grains m−3 in 2007 to 4384 grains m−3 in the following year, whereas the duration of ragweed pollen season varied from 50 days in 2008 to 72 days in 2007. The peak of the ragweed pollen season varied from 21 days in 2007 to 36 days in 2009 for airborne pollen concentrations ≥20 grains m−3 and from 1 day in 2007 to 20 days in 2008 for airborne pollen concentrations >80 grains m−3. Airborne pollen levels were affected by weather parameters such as temperature, sunshine, relative humidity, precipitation and wind speed in some ragweed pollen seasons in Zagreb, but these responses were inconsistent over the entire investigated period. Our study showed that large year-to-year variations in atmospheric pollen concentrations in Zagreb could not be consistently related to any of the analysed weather parameters.