**6. Conclusions**

*Plant Communities and Their Environment*

climate change on phenology [38, 39].

*\*p < 0.05, \*\*p < 0.01, and \*\*\*p < 0.001.*

and phenophases.

**Table 1.**

pollen [17, 50].

1960s phenological data indicated the advance of spring events, such as flowering (+6 days), whereas autumn phenophases have been delayed by 4.8 days [42]. Speaking about early spring species, in the case of *Corylus avellana*, an earlier flowering onset was observed at 80% of the studied localities of the Iberian Peninsula, earlier fruit ripening at all sampling sites, and earlier fruit harvesting at 75% of them [43]. *Salix alba* presented a trend towards earlier budburst and earlier leaf unfolding at 67% of the studied Iberian localities. In the case of autumn phases, delay in leaf fall at all sampling sites [40]. Holm oak is suffering a strong advance in the flowering start, as it was previously indicated in the Iberian Peninsula [41]. Northern species such as birch, poplar, or willow are also showing the impact of

*Statistically significant differences in start date between the start and the end of the time are indicated with* 

**Taxa Country Time period (a-b) Start (a) Start (b) Difference Reference** *Fraxinus* The Netherlands 1970–1990s 92 88 −3 [38] *Betula* Belgium 1982–2000 102 84 −18\* [39] *Betula* Finland 1975–2004 130 118 −12\* [40] *Betula* The Netherlands 1970–1990s 106 94 −10\* [38] *Betula* Switzerland 1982–2000 105 85 −20\* [39] *Quercus* The Netherlands 1970–1990s 135 117 −18\*\*\* [38] *Quercus* Spain 1970–1990s 89 78 −11 [41] *Corylus* The Netherlands 1970–1990s 84 66 −18\*\* [38] *Salix* The Netherlands 1970–1990s 82 70 −12\* [38]

As it was demonstrated by [41, 43] among others, the relationship between the phenological observations and weather is so clear for tree species and especially for early spring species. The statistical analyses show that in the 55% of the studied localities of the Iberian Peninsula, the temperature is influencing these trees' phenology. In 58% of the sites affected by temperature, the correlation between phenology and minimum temperatures was negative, which is provoking an

advance in phenology. The mean temperature results showed negative correlation in 54% of the sites, although different behaviour was observed depending on species

On average, the length of the growing season in Europe increased by 10–11 days during the last 30 years. Trends in pollen amount over the latter decades of the 1900s increased according to local rises in temperature [8, 44–46]. The increased CO2 concentration can be affecting pollen production as it has been demonstrated in experimental conditions [47, 48]. Regarding the pollen season length, it is also extending especially in late spring and summer flowering species [49]. Moreover, temperature is influencing towards stronger allergenicity in tree

An earlier pollen season starts, and peak is being more pronounced in early spring flowering species [43]**.** Due to this earlier onset, the seasons are more often

Finally, changes in climate appear to have altered the spatial distribution of pollens. New patterns of atmospheric circulation over Europe might increase the number of long-distance transport episodes of allergenic pollen, increasing the risk of new sensitizations among the allergic population [52]. On the other hand, the temperature increases, and the changes in rainfall regime are provoking the

interrupted by adverse weather conditions in late winter/early spring [51].

**32**

The review made about the recent response of the phenology of different species of anemophilous trees to climate change reveals that, apart from the field phenology data, aerobiological pollen data are a valuable tool to obtain reproductive phenological information of wind-pollinated species.

The response to climate of each studied taxon was different; most of the revised species and sites presented an advance of the early spring phenophases, especially budburst. The statistical analyses of the revised studies indicate that phenological advances are a consequence of the increasing temperature trend—minimum temperature being one of the most influential factors. The increase of temperature influenced that leaf unfolding and flowering dates showed a general advance expressed by negative correlations with temperature data, whereas the leaf colour change and leaf- fall presented positive correlations due to the delay of the colder temperatures.

On the contrary, some studies detected a delay in the autumn vegetative phases, especially on leaf-fall events. Both, leaf colour change and leaf-fall events showed positive correlations with temperature due to the delay of the colder temperatures.

The phenological revised results can be considered as reliable and valuable bioindicators of the impact of the recent climate change in the Northern Hemisphere and especially in the Central and Southern Europe.
