**3. General phenological behaviour of all studied tree taxa**

The revised species were selected because of their flowering time in early spring or late winter and because of their strong presence and distribution in Europe. They are anemophilous, deciduous, and perennial trees growing in different climatic areas of Europe: hazel (*Corylus avellana* L.) in Central and South Europe, oak (holm oak (*Quercus ilex* subsp. *ballota,* (Desf.) Samp.)) in Southern Europe, common oak (*Quercus robur* L.) in Central Europe, birch (*Betula* spp.) in Central and North Europe, willow (*Salix alba* L.) in Central and South Europe, ash (*Fraxinus angustifolia* Vahl.) in South Europe, and white mulberry (*Morus alba* L.) in South Europe [14]. All of them are endemic European species expect for white mulberry [7].

Most of the revised species belong to the Fagales order, divided into the *Betulaceae*, *Salicaceae*, and *Fagaceae* families [14]. On the other hand, *Fraxinus* genus is in the *Oleaceae* family of the Lamiales order, and *Morus* in the *Moraceae* family of the Rosales order.

Fagales order comprises three families: *Betulaceae*, including the genera *Betula* (birch), *Alnus* (alder), and *Corylus* (hazel); *Fagaceae*, including the genera *Quercus* (oak) and *Fagus* (beech); and *Salicaceace* including the genus *Salix* (willow) [14]. These wind-pollinated trees have catkins, which dangle from the branch so that pollen is easily shaken loose in the wind. Interestingly, catkins in deciduous species emerge before the leaves, allowing the pollen to travel further away from the parent without the obstruction of foliage [15].

Birch is the major pollen allergen-producing tree in Northern Europe, although there are high levels of allergenic cross-reactivity between the representative plants of the genera of the order Fagales [16].

As it has been already mentioned, all the revised species are foliating or flowering in early spring in Europe and North America; nevertheless, there are some specific characteristics for each one.

Hazel and alder are the first (December–April) to blossom and to shed pollen in the outdoor air in Europe, followed by birch. This fact joint to an allergenic crossreactivity between hazel and alder provokes that pollen from these species can act as a primer of allergic sensitization to *Betulaceae* pollen allergens. Consequently, clinical symptoms become more marked during the birch pollen season [17, 18]. In the central Alpine regions, the highest concentrations of *Alnus* pollen are found at the end of May and in early June [17].

In the case of *Betula*, the budburst occurs at March–April depending on the latitude and altitude. In South and Western Europe, the main flowering period usually starts at the end of March, whereas in Central and Eastern Europe, it occurs at early April. In northern areas the flowering season starts from late April to late May depending on the latitude [8]. Pollen values peak 1–3 weeks after the start of the season, so they are recorded in April in South Europe and in May in Northern Europe. Far shorter or longer periods, with yearly alternating low and high pollen production, have been observed in various European regions [17].

On the other hand, the onset of the oak season in spring, shortly before the beech pollen season, which is usually quite mild, can prolong the season in western, central, and eastern Europe [8]. One important characteristic of the oak pollen is the fact that it includes many species. In South Europe perennial species such as holm oak, kermes oak, and cork oak flower through all the spring from March to June [19]. In Central Europe, the pedunculate oak and the sessile oak usually flower in April–May [8].

Mulberry plants are normally dioecious, but they can also be monoecious on different branches of the same plant. The pendulous pistillate (female) and staminate (male) catkins are arranged on spikes and appear in April and May [20].

All the studied species have their main flowering season on early spring; nevertheless the different phenological phases vary among species, sites, and years depending on the bioclimatic characteristics and fluctuations [8].
