Climate and atmospheric conditions

Climate is a combined effect of several environmental factors. The temperature and the amount and distribution of the rainfall are essential, but the prevailing winds, distance from the sea (continentality) and the topography of the land surface are also determining factors.

The Earth's climate zones can be characterized with the average annual temperature and the average annual rainfall. Based on the amount of rainfall and evaporation, climate types might be classified as:

- Humid climates: when an area has a positive water balance (more precipitation falls than evaporates)

- Arid climates: when there is a negative water balance, with extreme drought.

- Semi-arid climates: water shortage in certain periods of the year, including the growing season.

- Semi-humid climates: increased rainfall during certain periods of the year, mainly in the growing season.

Climatic conditions in an area can be expressed as climate diagrams. These diagrams show the monthly average temperature in relation to the distribution of monthly precipitation.

First represented by Gaussen-Walter, the climate diagrams are widely used in plant geography and ecology to describe and compare the climate of different vegetation zones. They are based on Szeljanyinov's hydrothermal coefficient (Qh) and Walter-Bagnouls xerothermic index (I). According to the Walter-Bagnouls xerothermic index, a month is considered dry if the total monthly rainfall is less than twice the temperature value. The graphic contains the temperature and the monthly distribution of precipitation shown on the two vertical axes of the graph, where 10µC correspondto 20mm of precipitation. The horizontal axis shows the months of the year, so it is possible to read the duration and intensity of the humid, arid and transitional periods (See textbox on how to construct climate diagrams).

The climate diagrams of various cities from the different climate zones of the Earth are shown in Figure 24.

Figure 24: Climate diagrams of major climate zones of the Earth (the vertical lines show the humid climate, and the dotted areas where the rainfall is below

the temperature curve shows arid climate) http://www.ucm.es/info/cif .

The climate of Hungary

The Carpathian Basin is influenced by several types of climate. This region is mainly characterized by a Central European continental climate with maximum precipitations during early summer and long lasting drought in the summer. The area receives a warm autumn- spring humid sub-Mediterranean climatic influence from South, and cool summers as Atlantic-Alpine climatic influence from West. The different climatic influences affect different parts of the country, which explains the uneven distribution and productivity of plant communities.

There is an important geographic line between the humid and semi-arid areas in Hungary, which separates the closed broadleaved forest zone (the vegetation of the Transdanubian region and of the Central Mountains) from the forest-steppe vegetation zone (the lowland area to East) (Figure 25).

Figure 25: Two different climate diagrams of Hungary in two distinct vegetation types: closed broadleaved forest (Bánkút) and forest-steppe

(Szeged).

Air composition and air movement

The lower part of the atmosphere in which due to the heating effect of the Earth's surface air currents and turbulence are common is called the troposphere. Its height ranges between 16km (at the tropics) to 10km (at latitudes of 60 degrees). Just above the troposphere lays the stratosphere, where the temperature is relatively constant, but in its upper third part strong gas movement can take place. At about a 32 km height in the stratosphere lays the ozone layer (O3), which filters out the harmful UV radiation.

Stratospheric gases also prevent the rapid cooling of the Earth's surface.

Above the stratosphere lays the ionosphere, in which large quantities of ions form under the effect of UV radiation and oxygen molecules disintegrate. This layer is situated at elevations above 100 - 110 km. Although all atmospheric parameters have contributed to the evolution of life, from an ecological point of view only the lower part of the atmosphere is considered an environmental factor. Therefore all air parameters refer to the troposphere.

The air contains constant (N2, O2, noble gases) and variable (CO2, water vapors) components. An increased air humidity is beneficial for plants because it balances the air temperature and decreases the evapo-transpiration (water loss). Dry air is much more unfavorable for plants.

CO2 is a variable component of the air. It is released into the air due to the respiration of organisms, microorganism activity in the soil and volcanic activity. Recently, the amount of CO2 released in the atmosphere increased critically as a result of the human activity (especially industrial exhausts).

Because of the increasing CO2 concentration, the intensity of photosynthesis can also increase to a certain extent and thus the primary production decreases the CO2 volume in the air by carbon fixation. Massive deforestations and conversion of grasslands to agricultural land has decreased considerably the carbon storage capacity of ecosystems, which then causes the CO2 accumulation in the air. The elevated CO2 levels contribute to the greenhouse effect. The greenhouse effect happens because the infrared radiation that warms the Earth surface cannot be reflected to the upper layers of the atmosphere, and is instead trapped in the troposphere because of the increased concentration of CO2 and other pollutant gases.

In the dry, hot seasons of the year CO2 is an important limiting factor for plants because CO2 uptake cannot happen through closed stomata. As a result, C4 plants are advantaged under decreased precipitation conditions due to their

more effective CO2 uptake. Carbon fixation is a key factor in Earth’s CO2

cycle and its effects on ecosystem functioning are manifold.

SO2, which results from industrial exhaust is also accumulating in the atmosphere and has a harmful effect on plants. The high SO2 content of the air and the acid rains produced as a result threatens many types of ecosystem.

The amount of solid components in the air is variable. These contribute to the air pollution, producing e.g., soot and smog. The smog which accumulates around the cities and industrial sites is harmful for all living beings. It is particularly problematic in calm and hot summer days or in foggy winter time.

Plants react differently to smog, but chlorosis and silvery, bronzing tissues are usually caused by smog. Some horticultural plants such as the spinach, Swiss chard, the tomatoes and the beets are very sensitive, whereas others such as the beans, the cabbage and the corn are resistant to smog damage.

Air currents

Winds can have both positive and negative effect on plants.

The positive effect is when winds mediate the dispersal of pollen and fruits.

Plants whose pollen is dispersed by wind are called anemogamous (anemophilous) species.

The most visible features of wind-pollinated species are:

- they usually produce a lot of pollen;

- the perianth is insignificant, reduced or absent;

- the flowers are often imperfect (diclinous), the male flowers are dense, form catkins and bloom early in the spring before the leaves appear;

- usually do not produce nectar;

Anemophilous species are e.g. the hazel (Corylus avellana), the walnut (Juglans regia), the Hungarian ash (Fraxinus angustifolia subsp. danubialis), and from among the herbaceous plants the grasses and the sedges (more anemophilous species in Figure 26).

A B

Figure 26: Anemochorous species

A. Betula pendula (Silver birch), B. Quercus robur (English oak)(Tuzson J.1926)

Anemochorous species spread their fruits and seeds using the wind.

These fruits/seeds are often:

- produced in large quantities - are lightweight

- are winged or hairy

Anemochorous propagules

The light fruits or seeds such as those of willows, poplars or orchids do not contain, or have just a small amount of nutrients. Such seeds have to germinate immediately (without dormancy) and very often only germinate when a fungal partner is present. In orchids, when the embryo starts to germinate it develops a symbiotic relationship with a fungal partner that helps it to absorb vital nutrients. (See more details about anemochorous fruits and seeds in Chapter 4).

Winds can also exert indirect effects on plants. The prevailing winds may be climate-driving factors and can be responsible for generating humid or arid

climate types. Winds may increase the aridity and as a result, the physiological water stress in plants.

Sudden, raging winds may cause forest blowdowns and may cause soils to dry up. Soil erosion caused by winds is called deflation. In tropical areas which once were covered with forests, deflation can have severe consequences (Figure 27).

Figure 27: Deflation and erosion on Hispagnola Island

An aerial view of Haiti and of the border with the Dominican Republic (Caribbean). In Haiti forests were clearcut, which triggered strong soil erosion

and landslides. (NASA/Goddard Space Flight Center (image from Seed)

Bibliography:

- MÁTYÁS CS. 1997: Erdészeti ökológia. Mezőgazda kiadó.

-PÁSZTOR E.,OBORNY B. (szerk.) 2007: Ökológia. Nemzeti Tankönyvkiadó.

- SIMON T. (szerk.) (2001): Növényföldrajz, Társulástan és Ökológia.

Tankönyvkiadó. Budapest.

-TÖRÖK P.,TÓTHMÉRÉSZ B.(2006):Növényökológiai alapismeretek. Kossuth Egyetemi kiadó. Debrecen.