### Thermal requirements of plants - temperature sums

Heat is one of the most important environmental factors affecting the growth and development dynamics of plants during their vegetation period. Life processes in plants such as photosynthesis, dissimilation, transpiration, absorption of nutrients, etc. occur only within temperature limits set by four cardinal points: absolute minimum, absolute maximum, biological minimum, and the optimal temperature or optimum. (Penzar and Penzar, 2000).

Minimum and maximum air temperatures represent key points of plant development. Absolute minimum is a temperature at which the life process is halted due to the lack of heat. The processes of photosynthesis and dissimilation are fastest at optimum temperatures, and the absolute maximum is the upper limit above which the heat kills the plant. Values of these thresholds differ depending on the plant and their phenological phase.

The lowest mean daytime temperature at which the plant enters a certain development phase is called biological minimum. It varies depending on the plant species, e.g. sprouting of true grasses occurs at 5°C, corn sprouts at 10°C, etc. Temperatures above the biological minimum for given development stage are called active temperatures. Effective temperature is defined as the difference between the active temperature and the biological minimum.

In agriculture, instead of the quantity of heat necessary for plants to develop, sums of active temperatures in the vegetation period or degree days are used. They represent a cumulative sum of mean daytime air temperatures in the vegetation period for the given crop, from the beginning of growth until ripe.

Sums of active air temperatures above 10.0°C are used in agrometeorology as the basic thermal indicator for zoning of agricultural areas (Petrović, 1997), as well as for determining altitudinal thermal threshold for profitable production (Otorepec, 1991.).

By knowing mean values of temperature sums for certain temperature thresholds and certain plants, it is possible to predict the onset dates for individual phases of development of each plant, and the possibility for maturing of certain varieties.

Due to practical application of temperature sums in agriculture, for the needs of this Climate Atlas, we have processed and analysed spatial distribution of temperature sums for temperature thresholds of 5°C and 10°C (for the reference period 1961-1990), for the territory of the Federation of Bosnia and Herzegovina. To calculate temperature sums, we applied the following formula:

$$TS=\sum(S-P), \mbox{ za S>P }$$

$$S=\frac{t_{max}+t_{min}}{2}$$

• S = mean air temperature
• tmax= maximum daytime air temperature
• tmin = minimum daytime air temperature
• P= temperature threshold (5°C and 10 °C)
• ∑= sum from the first to the last day in the given month

Spatial distribution of cumulative temperature sums on the territory of the Federation of Bosnia and Herzegovina shows that average annual temperature sums above 5.0°C range between 2157°C and 3752°C; in the highlands, they range between 530°C and 1760°C. (Table attached)

In the lowlands, going from the west eastwards, average annual temperature sums above 5°C range from approx. 2560°C in Sanski Most to 2700 °C in Gradačac. During the vegetation period, sums of temperatures above 5°C range from 2140°C in Krajina to 2270°C in the northeast of the country.

In the central mountainous parts of the country, average annual temperature sums over 5°C range from 2160°C (Bugojno) to about 2550°C (Zenica); in vegetation period, from 1850°C (Bugojno) to 2160°C (Zenica ).

As the altitude increases, and depending on geomorphological characteristics of the terrain, annual value of temperature sums above 5°C decreases, ranging between 530°C (Bjelašnica) and 1760°C (Ivan-sedlo); in the vegetation season, it ranges between approx. 500°C on to approx. 1550°C on Ivan-sedlo.

Moving southwards, average temperature sums above 5 degrees increase reaching approx. 3752°C in Mostar, while in the vegetation season, the same station records approx. 2880°C.

As the temperature threshold moves up, the values of average temperature sums decrease. On the territory of the Federation of Bosnia and Herzegovina, average annual temperature sums above 10°C range between 970°C and approx. 2280°C.

In Krajina, they are at approx. 1380 °C, while the far northeast of the country (Gradačac) accumulates the average of 1520°C per year. In the vegetation period, these temperature sums range between 1260°C (Sanski Most) and approx. 1390°C in Gradačac.

In the central parts of the country, mean annual temperature sums range from approx. 1070°C in the Vrbas valley (Bugojno) to 1390°C in Zenica. At altitudes above 1000 m, the sum goes up to the maximum of 800°C.

During vegetation, values of these temperature sums range between approx. 1000°C in Bugojno and 1280°C in Zenica.

In the south of the country, sums of temperatures above 10°C range between 1920°C and 2290°C; in vegetation, 1670°-1980°C (Mostar).