**Abstract**

This chapter reports the phenological trends (reproductive and vegetative events) of some early spring and late winter flowering trees all around the world and especially Europe: *Corylus avellana* L. (hazel); *Quercus robur* L. (common oak); *Quercus ilex* subsp. *ballota,* (Desf.) Samp. (holm oak); *Betula* spp. (birch); *Salix alba* L. (willow); *Fraxinus angustifolia* Vahl. (ash); and *Morus alba* L. (white mulberry). They are deciduous and perennial trees growing in different climatic areas of Europe. They have anemophilous pollination liberating huge pollen concentrations to the atmosphere. Aerobiological surveys give us reproductive phenological information of these wind-pollinated species. The phenological response to climate during the last years was analysed, including budburst, leaf unfolding, flowering, fruit ripening, fruit harvesting, leaf colour change, and leaf fall. The response of each taxon to climate was different; most of the revised species and sites presented an advance of the early spring phenophases, especially budburst. On the contrary, some studies detected a delay in autumn vegetative phases, especially leaf fall events. The statistical analyses indicated 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. The phenological revised results can be considered as reliable and valuable bio-indicators of the impact of the recent climate change in the Northern Hemisphere, and especially Central and Southern Europe.

**Keywords:** phenology, anemophilous trees, climate change, *Corylus avellana* L. (hazel), *Quercus robur* L. (common oak), *Quercus ilex* subsp. *ballota,* (Desf.) Samp. (holm oak), *Betula* spp. (birch), *Salix alba* L. (willow), *Fraxinus angustifolia* Vahl. (ash), *Morus alba* L. (white mulberry)

## **1. Phenology**

Phenology is derived from the Greek word phaino, meaning 'to show' or 'to appear'. This science studies the recurring biological events as part of the animal and plant life cycles. These events are the phenological stages or phenological phases. Phenology not only studies the timing but also their relationships with weather and climate [1].

Sprouting and flowering of plants in spring and leaves' colour change in the fall are examples of plant phenological events [2].

Phenology has been used as a proxy for climate and weather through all the human history, particularly in relation with agriculture, but only from the last century has emerged as a science in its own right [1]. In last years it is being recognized as an integrative measure of plant responses to the environment changes that can be scaled from a local to a global scale, including climate change. During the last 100 years, the Earth's climate has warmed by approximately 0.6°C. In this last century, two main periods of warming have been detected. The first one was between 1910 and 1945, and the second one from 1976 onward [3]. In this second period, the rate of warming is being doubled than in the first and greater than at any other time during the last 1000 years [3]. The response of the different ecosystems and species is not a global response to a global climate average [4]. To know the regional responses can be more relevant in the context of ecological response to climatic change. In this sense, phenological behaviour data are the more reliable actual bio-indicator of the climate change response. Moreover, sessile life-style characteristic of plants has led them to develop high plasticity phenotypes in order to reach better phenological adaptations to deal with environmental changes [5]. These changes include climate changes that are of critical ecological importance as they affect species competitive ability and net primary productivity. These changes can even prompt ecosystem structure transformations [6]. Therefore, the analysis of trends of spring phenological phases for the past decades could provide important information about changes in climate and the impact on sessile organisms' phenology such us plants and specially trees, with longer lifetimes and shorter capacity of area distribution change.

This study presents a review of recent studies on both vegetative and reproductive field phenological development of different tree species characterized by their foliation or flowering during early spring. The phenological response of different tree species in the North Hemisphere was reviewed: hazel (*Corylus avellana* L.), alder (*Alnus glutinosa* (L.) Gaertn), willow (*Salix alba* L.*)*, birch (*Betula pendula* L.), holm oak (*Quercus ilex* subsp. *ballota,* (Desf.) Samp.) in South Europe and common oak *(Quercus robur* L.) in Central Europe, ash (*Fraxinus angustifolia* Vahl.), and white mulberry (*Morus alba* L.) [7]. All of them are anemophilous species producing high quantities of pollen grains spread to the atmosphere provoking allergy to the sensitized population [8]. Their huge quantities of pollen grains are also a phenological bio-indicator detected through aerobiological studies [9], also revised for the present review. Their phenological behaviour during last 40 years and the impact of the climate change on it were analysed. Particularly remarkable is the fact that the revised species are important for aerobiology and allergy studies, and therefore the changes experimented on their phenology have a special interest. This review offers valuable information due to the scarce number of researches studying field phenological data including those from the last quarter of the twentieth century.
