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During the attack of a forest fire, the vegetative organs of plants are affected by high temperatures, which lead to their stressful state. At the time of burning, it is quite difficult to record temperature changes in the tree crown and the associated reactions in the photosynthetic needle apparatus. This article presents the results of modelling a high-temperature effect simulating a convective flow from a ground fire. Experimental heating at 55° C lasted for 5 and 10 minutes. Evaluation of the response was carried out by the parameters of rapid fluorescence (Fv / Fm, ETR), the state of the pigment complex, the relative water content in the needles. To characterize the degree of heat endurance and short-term effects concerning thermal damage, saplings of Scots pine (Pinus sylvestris L.) in different periods of the vegetation phase were used. The researchers have discovered different levels of heat resistance of the needle assimilation apparatus. Usually heat resistance is rising by the end of the vegetation season. The data obtained in June show that heating of the saplings led to a significant suppression of the photosynthesis rate. In subsequent periods (July, August, September), the photochemical quantum yield (Fv / Fm) was restored to 75% and 60% from the initial level on average, after 5and 10-minute heating respectively. The values of the electron transport rate (ETR) for saplings selected in September restored to the initial level within 3 days after a short heat exposure. For the study of long-term effects after high-temperature exposure during the vegetation season, the undergrowth of Scots pine was used. Restoration of the photosynthetic activity in needles from model trees was observed only after a short-term (5-minute) impact, but by the end of the studied period the restoration had not reached the control values. A longer heating (during 10 minutes) resulted in an irreversible suppression of photosynthesis and destruction of the photosynthetic apparatus, as evidenced by the decrease in the number of photosynthetic pigments.

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