**2. Climate change**

Climate change due to human activities has been witnessed for at least the last 100 years and is projected to continue for centuries to come. Climate change involves the whole climate system, including not only our atmosphere but also our hydrosphere, cryosphere, land surface, and biosphere [10].

Greenhouse gases and atmospheric concentrations have exponentially increased since the start of the Industrial Era (1750). Moreover, from this time the CO2 concentrations have increased by 41% mostly due to the global use of oil fuel [3]. The latest measures of the year 2013 of the National Oceanic and Atmospheric Administration reveal that global annual mean atmospheric CO2 concentration was 395.22 parts per million (ppm) [11], an increase of over 100 ppm from the

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episodes [12].

**Figure 1.**

*Phenological Behaviour of Early Spring Flowering Trees DOI: http://dx.doi.org/10.5772/intechopen.88259*

pre-Industrial Era. **Figure 1** shows that the increase in atmospheric CO2 concentra-

This increase in the atmospheric concentration of greenhouse gases such as CO2 has led to warm the climate. Between 1880 and 2012, the Earth's average surface temperature warmed by 0.85°C [3]. Most of this warming occurred after 1951 [12]; the warming of the Earth's surface varies over space, the land surfaces tending to warm more than the oceans; and some parts of the Earth's surface temperatures have increased the double than other places. Changes in precipitation have also been observed. Since 1901, precipitation has increased over the mid-latitude land areas of the Northern Hemisphere, especially in intensity, with more frequent heavy rain

By the end of this century (2081–2100), global mean surface temperatures are projected to increase from 0.3°C to 4.8°C depending on the sites. The mean warming over land will be larger than over the ocean, and the Arctic region will warm more rapidly than the global mean [3]. With regard to precipitation, it is projected to increase by the end of this century [13]. However, there will be substantial spatial variation in precipitation changes, with some regions experiencing increases, some decreases, and some no change at all [13]. All these factors and the actual impact on

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].

tion since 1750 has not been linear, being higher in the last 60 years [11].

*Monthly mean atmospheric carbon dioxide concentration from March 1958 to July 2015. Source: [11].*

phenology of early spring trees will be reviewed in the present chapter.

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

*Phenological Behaviour of Early Spring Flowering Trees DOI: http://dx.doi.org/10.5772/intechopen.88259*

### **Figure 1.**

*Plant Communities and Their Environment*

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.

including those from the last quarter of the twentieth century.

hydrosphere, cryosphere, land surface, and biosphere [10].

Climate change due to human activities has been witnessed for at least the last 100 years and is projected to continue for centuries to come. Climate change involves the whole climate system, including not only our atmosphere but also our

since the start of the Industrial Era (1750). Moreover, from this time the CO2 concentrations have increased by 41% mostly due to the global use of oil fuel [3]. The latest measures of the year 2013 of the National Oceanic and Atmospheric Administration reveal that global annual mean atmospheric CO2 concentration was 395.22 parts per million (ppm) [11], an increase of over 100 ppm from the

Greenhouse gases and atmospheric concentrations have exponentially increased

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

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**2. Climate change**

*Monthly mean atmospheric carbon dioxide concentration from March 1958 to July 2015. Source: [11].*

pre-Industrial Era. **Figure 1** shows that the increase in atmospheric CO2 concentration since 1750 has not been linear, being higher in the last 60 years [11].

This increase in the atmospheric concentration of greenhouse gases such as CO2 has led to warm the climate. Between 1880 and 2012, the Earth's average surface temperature warmed by 0.85°C [3]. Most of this warming occurred after 1951 [12]; the warming of the Earth's surface varies over space, the land surfaces tending to warm more than the oceans; and some parts of the Earth's surface temperatures have increased the double than other places. Changes in precipitation have also been observed. Since 1901, precipitation has increased over the mid-latitude land areas of the Northern Hemisphere, especially in intensity, with more frequent heavy rain episodes [12].

By the end of this century (2081–2100), global mean surface temperatures are projected to increase from 0.3°C to 4.8°C depending on the sites. The mean warming over land will be larger than over the ocean, and the Arctic region will warm more rapidly than the global mean [3]. With regard to precipitation, it is projected to increase by the end of this century [13]. However, there will be substantial spatial variation in precipitation changes, with some regions experiencing increases, some decreases, and some no change at all [13]. All these factors and the actual impact on phenology of early spring trees will be reviewed in the present chapter.
