**7. Consequences and effects of climate change over the growing season period**

Plants, whether natural plants or crops and horticultural plants, to complete their growth and development, need to spend a period with optimum temperature and humidity conditions.

Various definitions and criteria have been proposed to determine the growing season, the first and last frost has been used in many studies [19–22].

The length of the growing season, the length of the days between the last hard frost and the first hard frost in spring are also defined. Hard frost refers to a minimum daily temperature that 50% of plants exposed to it are destroyed [23].

The length of the growing season, the length of time between the last killing frost in spring and the first killing frost in the fall are also defined (U.S. Army Agronomists, 1987). Killing frost is 28°F (−2.2°C) or colder [24].

According to the definition provided by the variation group of the Climatology Commission of the World Meteorological Organization, in the northern hemisphere, the interval between the first period after July 1st (10th of Tir), which is at least 6 consecutive days, the average daily temperature is more than 5°C and the first 6-day period with an average daily temperature of less than 5°C (in autumn) is considered as the growing period. In the southern hemisphere, this interval is considered from January 1st [25].

The length of plant growing season varies not only in terms of each plant species, but also there are differences in one species. Determining the growing season in each region has an effective role in selecting crops and cultivars and determining planting time and other crop decisions.

Many phenological, meteorological, and satellite researches report an increase in the length of the growing season due to rising temperatures in the northern regions during the twentieth century [26].

Chemielewki [27] investigated the relationship between annual and periodic changes of the growing season in Europe with air temperature changes, the results show that in Europe during 1989–1998 with an increase of 0.8°C the average temperature in the last decade, was occurred 8 days earlier at the beginning of the growing season. Research conducted in the United States [28] Australia, China [29], also shows a decrease in the number of freezing days at high and middle latitudes of the northern hemisphere and an increase in the length of the growing period compared to the twentieth century. Regarding the effects of climate change on growing period, researches such as temporal and spatial variation of phenological seasons in Germany from 1951 to 1996 [30], changes in the growing season in the last century [26] can be noted.

One of the important consequences of climate change is the change during the growing season, which follows the change in the beginning and termination of zero temperature and temperature of 5°C (thermal base of cold crops). Changes in the time and length of the growing season may not only have far-reaching consequences for plant and animal ecosystems, but a steady increase in the length of the growing season may lead to a long-term increase in carbon storage and changes in vegetation cover that may affect the climate system. The decrease in the length of the growing season leads to changes in the cultivation calendar and a decrease in yields of crops that have not yet fully reached their maturity and final growth, while increasing the length of the growing season may provide more opportunities for earlier cultivation, ensuring final growth and maturity, and even the possibility of further harvesting (if water is available) [30].
