Physiological And Ecological Mechanisms Of Natural Regeneration Saplings Reflecting To Light Factor Of Three Confer Species

Pinus thunbergii, Pinus tabuliformis and Pinus densiflora are main afforestation tree species with a long history and vast planting area and proportion in the north China. To date, the physioecological mechanisms underlying the effects of light on these three species remain elusive. In order to investigate the adaptability strategies of P. thunbergii, P. tabuliformis and P. densiflora to different light environments,and provide theoretical foundation for thinning which facilitate the close-to-natural management and natural regeneration of plantation in the north China, we simulated the light levels in open field(0 shading), gaps(40% shading) and understory(80% shading) through shading, and compared the photosynthetic characteristics and chlorophyll fluorescence parameters among these conditions. The results indicate that:(1) Different shading conditions exhibited different effects on the photosynthesis of P. tabuliformis, P. thunbergii and P. densiflora young trees. The photosynthesis of P. thunbergii decreased by increasing shading, while that of P. tabuliformis showed highest value under 40% shading and was inhibited by full exposure. The photosynthesis of P. densiflora did not show obvious different conditions, indicating that this species could adapt to different light conditions. The time of shading exhibited different effects on the photosynthesis of the three species. The net photosynthetic rate of P. thunbergii showed the most obvious reduction with the prolongation of shading time. The photosynthesis of P. tabuliformis and P. densiflora did not changed evidently at the beginning of shading, but reduced rapidly after a period of shading.(2) The initial fluorescence of the three species showed different varying patterns under different shading conditions. The initial fluorescence P. thunbergii decreased by increasing shading, that of P. tabuliformis reached a maximum under 40% shading condition, and that of P. densiflora did not elustrate exhibited obvious changes. The PSII max photochemistry quanta output of P. tabuliformis and P. densiflora was relatively higher under 80% shading condition, which might be resulted from a request for high energy utilization efficiency of the photochemical reaction center under high shading conditions. PSII max photochemistry quanta output of P. thunbergii showed relative higher value under 40% shading condition, indicating that moderate shading confers high efficiency of PSII reaction center. High light level might cause photoinhibition for P. thunbergii, and 80% shading condition might impair the photosystem and result in attenuated energy utilization efficiency of the PSII reaction center.(3) In the young trees of P. tabuliformis, P. thunbergii and P. densiflora, the absorption flux, trapped energy flux, electron transport flux and heat dissipation of per reaction center were remarkably reduced as the shading time prolonged. Extended period of shading impaired the photoactivity of PSII reaction center, which restrains the photosynthetic efficiency and organic material accumulation, and thus caused reduction of tree growth. The absorption flux, trapped energy flux and electron transport flux of P. densiflora were higher than those of P. thunbergii and P. tabuliformis, suggesting that P. densiflora possesses higher efficiency of light absorption and utilization as well as adaptation to lower light conditions, and thus could grow under open field, gaps and understory. The electron transport flux of P. thunbergii reached the highest level under 40% shading condition, suggesting that young trees of this species adapt well to low light level conditions and could grow well in gaps. The electron transport flux per reaction center of P. tabuliformis gradually reduced as the increase of shade, indicating that P. tabuliformis young trees prefer to conditions with abundant sunlight rather than low light levels, and thus could grow well in understory areas.