As shown in the previous sections, the climate is a major determinant for the phenology, distribution and productivity of plants, as well as for the distribution of the released pollen. With climate change, the margins of species ranges, their relative abundances and boundaries may start to shift (Walther 2003), which will result in changes in local allergenic pollen spectra. For example Finland will probably experience an annual mean temperature increase of 3-5°C during the next
100 years (Carter et al. 2005) and the CO2 concentration increase of 0.5-2% per decade (Vingarzan 2004). Forest resources are expected to increase throughout the country, and at the same time, the proportion of coniferous trees will be reduced with an increased dominance of birches (Kellomäki et al. 2001). Likewise, eastern cottonwood (Populus deltoides) that is a fast-growing broad-leaved tree, able to respond rapidly to changing environment, is expected to benefit from rising CO2 concentration (Gielen and Ceulemans 2001).
A meta-analysis of reports on 79 crop and wild plant species indicated that CO2 enrichment resulted in 19% more flowers, 18% more fruits and 16% more seeds (Jablonski et al. 2002). Studies with several species have indicated that increase in ambient growth temperature may accelerate onset of flowering (Blázquez et al. 2003). Stinson and Bazzaz (2006) observed that elevated CO2 stimulated significantly the stand-level reproduction of common ragweed and minimized the differences in the reproductive output of large and small plants.
A number of recent changes in pollen patterns have been reported regarding onset (earlier) and duration (longer) of the pollen season (Clot 2001, 2003; Emberlin et al. 1997, 2002; Frenguelli et al. 2002; Galán et al. 2005; Inoue et al. 2002; Newnham 1999; Tedeschini et al. 2006), and also the airborne pollen load (Bortenschlager and Bortenschlager 2005; Corden and Millington 1999; Frei 1998; Frei and Leuschner 2000; Jäger et al. 1996; Rasmussen 2002; Spieksma et al. 2003). Most of these changes have been attributed to the increased levels of air pollutants or the higher temperatures associated with global warming, while some others have been attributed to urbanisation and land use change (Voltolini et al. 2000). Nevertheless, the ever increasing eutrophication of the planet due to increased CO2 or nitrogen concentrations and its effect on pollen production (Wayne et al. 2002; Townsend et al. 2003) should also be taken into consideration.
Several reports suggest that the synchronous increase in pollen abundance or pollen season length and in respiratory allergy severity or prevalence are to some extent interdependent, the latter being potentially the result of the first (Ault 2004; Clot 2003; Frei 1998; Frei and Leuschner 2000; Voltolini et al. 2000; Wayne et al. 2002; Ziska et al. 2003; Menzel et al. 2006). The number of taxa and covered area vary between the studies. Several reports have concentrated on a selected taxon (Clot 2001; Corden and Millington 1999; Emberlin et al. 1997, 2002; Frenguelli et al. 2002; Galán et al. 2005; Inoue et al. 2002; Orlandi et al. 2005; Osborne et al. 2000; Rasmussen 2002; Tedeschini et al. 2006). Others considered up to six (Bortenschlager and Bortenschlager 2005; Frei 1998; Frei and Leuschner 2000; Jäger et al. 1996; Voltolini et al. 2000; Menzel et al. 2006), for the territory covering up to a whole of Europe. However, Beggs (2004) has argued for the inclusion of more taxa in pollen studies of each study area so as to quantify how global the observed trends are, or how intense is the regional factor.
Shifts to an earlier onset of the pollen season have been more frequently observed than increases in airborne pollen abundance (Clot 2001, 2003; Emberlin et al. 1997, 2002; Frenguelli et al. 2002; Galán et al. 2005; Inoue et al. 2002; Orlandi et al. 2005; Osborne et al. 2000; Tedeschini et al. 2006). For other pollen variables, the picture is more complicated. Often no trends are observed (Frenguelli et al. 2002; Leuschner et al. 2000) and when they are, they tend not to be consistent across different taxa and regions (Spieksma et al. 2003; Tedeschini et al. 2006; Voltolini et al. 2000). Overall more attention has been paid to pollen distribution patterns than to pollen abundance.
Clot (2003) and Damialis et al. (2007) have examined a wider sector of the regional pollen flora (more than 15 plant taxa) concentrating on the onset and length of pollen season, together with the atmospheric pollen loads. Clot (2003) found mainly earlier onset of pollen season in Neuchatel, Switzerland, whereas Damialis et al. (2007) found mainly increasing trends in atmospheric pollen levels in Thessaloniki, Greece, with no systematic shift in the onset of the pollen season.
Differences in regional responses to climatic change are related to differing geographical location (viz. latitude) or to specific regional characteristics (i.e. local climate). The Mediterranean environment is characterised by an annual alternation of a hot and dry period with a cold and moist one. Species living in this environment are adapted to large changes of environmental factors, among which is air temperature. Under such strongly varying conditions, systems have developed high resilience and species are able to survive environmental fluctuations (Dell et al. 1986). The increasing trends of annual pollen levels in Thessaloniki (Greece) could be interpreted in terms of increased daily atmospheric pollen concentrations deriving from a higher reproductive effort of various plant species in response to a changing environment. This is supported by the fact that the peak daily count also showed an increasing trend for a large number of taxa and that the two (annual load and daily peak of pollen counts) were interrelated (Damialis et al. 2007).
Such results, showing strong upward trends in annual pollen abundance that reflect increased levels of pollen production, but much fewer significant changes in phenological characters, have not been reported by other researchers. However, Menzel (2000) and Menzel and Fabian (1999), who studied flowering phenology of plant species across Europe, concluded that due to regional characteristics and in contrast to what is observed in western and northern Europe, no shifts to an earlier onset of flowering are observed in the Balkan peninsula, where Thessaloniki is located. The fact that Damialis et al. (2007) did observe trends in pollen production patterns suggests existence of a major factor leading to responses that can mask between-species variability in the temporal features of their reproductive output.
Increased reproductive effort, in terms of flower and/or pollen production, in response to higher temperatures is widely documented; this holds true for both animal and wind pollinated species (see, for example, Aerts et al. 2004; M0lgaard and Christensen 1997; Stenstrom and Jonsdottir 1997; Wan et al. 2002; Ziska et al. 2003). In the example of Greece, minimum air temperature presented an increase of the order of 1.0°C in 2005, compared to that in 1987. In this timescale, airborne pollen levels have displayed remarkable changes that suggest exponential increase. This is particularly true for Cupressaceae, Carpinus, Quercus. Nevertheless, a few taxa did not follow this pattern and either displayed decreasing trends (Populus) or no trends at all (i.e. Ambrosia). Regarding Populus, which is the only taxon with a negative trend in airborne pollen levels over the last 2 decades, we could remark that Populus species have high demands in water (Kailidis 1991). Increasing air temperatures influence water availability, as they influence evapotranspiration rates. For a most sensitive to water-shortage species, rising temperatures might result into a stress situation that could lead to reduced reproductive effort. In the case of Ambrosia, its low participation in the regional vegetation may be responsible for the inability to detect trends (Damialis et al. 2007). In the absence of commensurate changes in the abundance of the respective flora, Damialis et al. (2007) argued that changes of airborne-pollen load over the last 20 years in the area of Thessaloniki could be interpreted in terms of the concurrent temperature increase, without ignoring the eutrophication of the planet as an alternative or additional cause. The study has not revealed the influence of the other strong meteorological driver in the Mediterranean region - precipitation amount - onto the pollen abundance. The main reason for that is believed to be a weak and irregular trend of this parameter during the considered period. A significant decreasing trend, however, if projected for this region in the recent IPCC (2007) report (Intergovernmental Panel on Climate Change, http://www.ipcc.ch).
Climate change may also facilitate the spread and naturalization of some allergy plants in new areas. Common ragweed has its northern limits in the south of Sweden, but because the growing season is usually too short for the seeds to mature, the establishment of flowering plants require recurrent import of contaminated birdseed, for instance (Dahl et al. 1999). The establishment of flowering populations of ragweed in new areas increases also the risk of long-distance transport of its allergenic pollen to new areas (Dahl et al. 2000; Lorenzo et al. 2006). Some processes seem to be gaining the stream: in summer 2005 considerable amounts ragweed pollen originated from Eastern Europe was observed during 4 days in sub-arctic Scandinavia. There is also a possibility that the species not too sensitive to light limitations, such as pellitory-of-the-wall and other Urticaceae-plants, as well as some grass species, may benefit from longer warm season and extend their distribution towards the north resulting in longer pollen period.
As a result, a combination of the environmental changes and atmospheric pollution may in the near future considerably affect loads of airborne allergens via changes in vegetation structure, magnitude of flowering and allergen content of pollen (Singer et al. 2005; WHO 2003; Williams 2005). If such changes will be realised, the only way to cope would be an adaptation to the changing environment and, in particular, to longer and possibly more diverse flowering seasons. Adaptation The adaptation of people is possible via a set of pre-emptive measures of behavioural and medical kinds, which both require timely and detailed forecasts of forthcoming pollinating seasons. As shown by the existing systems, such forecasting is possible and useful. However, it heavily relies on many empirical models and statistical relationships, which may appear to be wrong in the future climate. Therefore, the new systems should be based on more universal parameterizations and include possibility for adjustment of the model setup and state using real-time observations.
Phenological and aerobiological data, reflecting the behaviour of various ecosystems, from regions differing in latitude or altitude and belonging into different climatic zones, have far-reaching importance. They can serve in predicting future patterns, in interpreting fossil pollen patterns, but they can also enable us to anticipate associated human health impacts and take appropriate measures, given that the prevalence and severity of respiratory allergies have been significantly increasing worldwide and several reports link them to increases in the pollen abundance or pollen season length. Taxa like Carpinus, Quercus, Cupressaceae or Populus that show trends might serve as bio-indicators of the expected climatic change in the Mediterranean region and possibly in other climatic zones as well.
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