Survival Strategy Of Understory Korean Pine Driven By Seasonal Light Availability

The mixed-broadleaved Korean pine(Pinus koraiensis Sieb.et Zucc)forest is a zonal climax vegetation type in Northeast China.Among them,the regeneration of Korean pine is a core issue for the restoration from secondary to primary forest.As we all know,the natural regeneration of Korean pine is extremely poor in the primary forest which is summarized as 'Seedlings only but no saplings',while it is very good in the secondary forest.Nonetheless,the mechanisms inhibiting the regeneration of Korean pine in the primary forest and causing such differences between two forests have not been satisfactorily explored.This study was conducted in a primary mixed-broadleaved Korean pine forest(Primary forest)and a secondary poplar-birch forest(Secondary forest)in the northern slope of Changbai Mountain.Photosynthetically active radiation sensors and hemispherical photographs were used for long-term monitoring understory light availability under two forests.Seasonal dynamics of photosynthesis of understory Korean pine were regularly observed with a Li-6400 portable photosynthesis system.Seasonal dynamics of growth,nonstructural carbohydrate content,and nonstructural carbohydrate pool of Korean pine seedlings under two forests were regularly observed by using harvesting method.Girdling and defoliation treatments were used to explore the carbon autonomy of Korean pine saplings at single branch scale.Seedling transplantation and shading experiments were conducted in nursery and forests to explore the effects of seasonal differences of light availability on the growth and survival of understory Korean pine.The main results are as follows:(1)Understory light conditions changed seasonally in both primary and secondary forests.Understory light availabilities in early spring and late autumn were significantly higher than that in summer.In the non-growing season,the side shading of the crown and trunk of the primary forest was significantly higher than that of the secondary forest,therefore,understory light availability of the later was significantly higher than that of the former in the non-growing season.However,in the growing season,because the canopy was completely closed,light availabilities under the primary and secondary forests were very low,and there were no significant differences.(2)The light-photosynthetic curves were well fitted with the four-parameter non-rectangular hyperbolic function,and the temperature-photosynthesis curves were well fitted with the empirical quadratic equation.Then,photosynthetic characteristics of Korean pine under different temperatures and light intensities were obtained based on the models.The model parameters suggested that Korean pine still has a certain photosynthetic capacity at low temperature.When light availability is sufficient,the minimum photosynthetic temperature of one-year,two-year,and three-year leaves could reach 3.3?,5.4?,and 7.8?.(3)The main C gain periods for understory Korean pine were the early spring and late autumn stages.The C gain in the growing season was negative for both primary forest and secondary forest,which indicated that understory Korean pine was in the condition of nutrient consumption in this period.More important,in early spring and late autumn,the C gains of one-year,two-year and three-year leaves of Korean pine in the secondary forest were all significantly higher than that in the primary forest.(4)In spring and autumn,organ biomass,nonstructural carbohydrate content and nonstructural carbohydrate pool of understory Korean pine seedlings all increased significantly,while they showed a significant downward trend in summer.These results might indicate that the understory Korean pine is in the stage of rapid nutrient accumulation in spring and autumn,while in the stage of rapid nutrient consumption in summer.(5)In spring and autumn,organ biomass,nonstructural carbohydrate content and nonstructural carbohydrate pool of understory Korean pine seedlings in the secondary forest were significantly higher than in the primary forest,while in summer there were no significant differences between two forests(6)Understory Korean pine have an intrinsic seasonal pattern of nonstructural carbohydrate.In spring,starch content increased rapidly,while the soluble sugar content decreased.In summer,soluble sugar and starch in all organs gradually decreased and touched bottom in August.In autumn,starch content did not increase and maintained a low level,while soluble sugar content presented a sharp increase to the highest level by the end of October.(7)For removing one-year,two-year leaves,and retaining all leaves treatments,there were no significant differences between girdling and non-girdling treatments in the growth of new branches and leaves.This result indicated that Korean pine saplings had carbon autonomy on a single branch scale.The growth of new branches and leaves may depend on the nutrient storage in current branches,and mainly on the nutrients accumulated in perennial needles.For understory Korean pine seedlings,the nutrient accumulation in the last year directly affected the growth and survival of Korean pine in the next year.(8)The growth and survival of Korean pine seedlings were closely related to light availability in non-growing season.There were significant differences in organ biomass,NSC concentrations,NSC pool size,and survival rates of Korean pine seedlings between different treatments in spring and summer.(9)The biomass of Korean pine seedlings was related to the light availability in spring.The photosynthate accumulated profoundly in spring with starch as the predominant compounds,which was used for the development of new organs.The NSC concentrations and NSC pool size of Korean pine seedlings was related to the light availability in autumn.The photosynthate accumulated profoundly in autumn with soluble sugar as the predominant compounds,which was used for winter maintenance and avoidance of cellular freezing.(10)The survival rates of Korean pine seedlings were significantly positively correlated with component NSC concentration and NSC pool size in autumn,while it was not significantly correlated with NSC concentration and NSC pool size in spring and summer.In addition,there were no significant correlations between survival rates and seedling biomass across the whole growing season.In summary,high light availability in the early spring and late autumn made pine seedlings acquire sufficient carbon to tolerate long-term dark summer period when the canopy is heavily closed,which is the key factor that made regeneration of Korean pine successful in the secondary forest.In addition,we suggest the differences in light availability and nonstructural carbohydrate accumulation may be the fundamental reason causing the difference in pine regeneration between the primary and secondary forest.