Mechanistic species distribution modeling reveals a niche shift during invasion.

Niche shifts of nonnative plants can occur when they colonize novel climatic conditions. However, the mechanistic basis for niche shifts during invasion is poorly understood and has rarely been captured within species distribution models. We quantified the consequence of between-population variation in phenology for invasion of common ragweed (Ambrosia artemisiifolia L.) across Europe. Ragweed is of serious concern because of its harmful effects as a crop weed and because of its impact on public health as a major aeroallergen. We developed a forward mechanistic species distribution model based on responses of ragweed development rates to temperature and photoperiod. The model was parameterized and validated from the literature and by reanalyzing data from a reciprocal common garden experiment in which native and invasive populations were grown within and beyond the current invaded range. It could therefore accommodate between-population variation in the physiological requirements for flowering, and predict the potentially invaded ranges of individual populations. Northern-origin populations that were established outside the generally accepted climate envelope of the species had lower thermal requirements for bud development, suggesting local adaptation of phenology had occurred during the invasion. The model predicts that this will extend the potentially invaded range northward and increase the average suitability across Europe by 90% in the current climate and 20% in the future climate. Therefore, trait variation observed at the population scale can trigger a climatic niche shift at the biogeographic scale. For ragweed, earlier flowering phenology in established northern populations could allow the species to spread beyond its current invasive range, substantially increasing its risk to agriculture and public health. Mechanistic species distribution models offer the possibility to represent niche shifts by varying the traits and niche responses of individual populations. Ignoring such effects could substantially underestimate the extent and impact of invasions.

Phenological Variation in Ambrosia artemisiifolia L. Facilitates Near Future Establishment at Northern Latitudes.

The invasive weed Ambrosia artemisiifolia (common ragweed) constitutes a great threat to public health and agriculture in large areas of the globe. Climate change, characterized by higher temperatures and prolonged vegetation periods, could increase the risk of establishment in northern Europe in the future. However, as the species is a short-day plant that requires long nights to induce bloom formation, it might still fail to produce mature seeds before the onset of winter in areas at northern latitudes characterized by short summer nights. To survey the genetic variation in flowering time and study the effect of latitudinal origin on this trait, a reciprocal common garden experiment, including eleven populations of A. artemisiifolia from Europe and North America, was conducted. The experiment was conducted both outside the range limit of the species, in Sweden and within its invaded range, in Croatia. Our main hypothesis was that the photoperiodic-thermal requirements of A. artemisiifolia constitute a barrier for reproduction at northern latitudes and, thus, halts the northern range shift despite expected climate change. Results revealed the presence of a north-south gradient in flowering time at both garden sites, indicating that certain European populations are pre-adapted to photoperiodic and thermal conditions at latitudes up to, at least, 60° N. This was confirmed by phenological recordings performed in a region close to the northern range limit, the north of Germany. Thus, we conclude that there exists a high risk for establishment and spread of A. artemisiifolia in FennoScandinavia in the near future. The range shift might occur independently of climate change, but would be accelerated by it.

Annual variation of airborne pollen in the city of Vinkovci, northeastern Croatia.

Pollen concentration in the atmosphere of Vinkovci (northeastern Croatia) has been analyzed using a Hirst-type volumetric spore trap. During the year 2005, 58 pollen types were recorded with the sum of annual totals of 14,011. The pollen spectrum reflected the floristic diversity of the region. Non-arboreal pollen predominantly contributed to the total pollen sum with a percentage of 81.80%. The main pollen producers characterized by allergenic pollen were: Betula, Quercus, Fraxinus, Populus, Pinus, Urticaceae, Ambrosia, Poaceae, Plantago and Artemisia. Urticaceae was the most frequent and most abundant pollen type accounting for 46.58% of the total annual pollen, followed by Ambrosia (19.66%) and Poaceae (11.01%).

Airborne ragweed pollen concentration in north-eastern Croatia and its relationship with meteorological parameters.

Airborne ragweed pollen concentration in the north-eastern part of Croatia was investigated in relation to some meteorological factors. Data was obtained for three consecutive years (2001, 2002 and 2003) using volumetric method (Burkard trap). The correlation between the concentration of pollen grains in the atmosphere and maximum, minimum and mean daily temperature, daily temperature range, sunshine hours, relative humidity and precipitation from daily monitoring was studied. The critical ragweed pollen season appears from 33rd to 38th week, with its peak at the beginning of September each year. Statistically significant correlation between the ragweed pollen count and selected meteorological variables were found in some study years, but only a correlation with the mean air temperature and ragweed pollen presence in the air was significant over time.