The Photosynthetic Adaptability Of Korean Pine To Forest Gaps And Its Seasonal Changes In The Birch Forest In Xiaoxing’anling Of China

The study aims to reveal that the photosynthetic adaptation rules of Korean Pine（Pinus koraiensis）sapling to forest gaps in the secondary forest is of great importance to the restoration of zonal climax vegetation of broad-leaved Korean pine forest in Northeast China.The CIRAS-2 photosynthetic apparatus was used to measure the photosynthetic parameters（the maximum net photosynthetic rate,light saturation point,light compensation point,transpiration rate,stomatal conductance,etc.）and its micro-environmental factors（light intensity,leaf temperature,etc.）of Korean pine saplings（35a）at the central area,transition area and edge area of three different size forest gaps（large gap-201～211m²,medium gap-169～174m²,small gap-100～102m²）and the understory in the secondary white birch forest in the Lesser Khingan Mountains of China,to reveal the photosynthetic adaptation rules and mechanisms of Korean Pine saplings to the gap size and position within forest gaps in the secondary forest.The results showed that:（1）The average maximum net photosynthetic rate(P_max)(2.4～8.4μmol·m^-2·s^-1)of Korean pine saplings in the white birch forest during the growing season at all positions（except the edge area of small forest gap）of three different size forest gaps was significantly higher than that in the forest understory by 54.2%～250.0%,and showed a regularity of increasing with the increase of gap size.Its adaptive mechanism is as follows:the light saturation point,transpiration rate,stomatal conductance and water use efficiency of Korean pine needles in forest gaps were significantly higher than those in the forest understory（28.7%～250.0%）,but the light compensation point is significantly lower than that in the forest understory（27.1%～81.9%）;the photosynthetic parameters in large gaps were generally significantly higher than those in small gaps（except that the light compensation point was lower than that in small gaps）,while that in middle gaps was between large gaps and small gaps.（2）The seasonal pattern of Pmax for Korean pine needles in different gaps was different.The maximum net photosynthetic rate of Korean pine in the forest understory showed a seasonal pattern,namely summer>autumn>spring,but changed its seasonal pattern in the transition area of middle gap and all positions of large gap.Its adaptation mechanism is as follows:transpiration rate and stomatal conductance in the forest understory,small gaps and middle gaps were the main physiological parameters that caused the seasonal variation of Pmax（summer>autumn>spring）,while apparent quantum efficiency in central area,transition area in large gaps and transition area in middle gaps is the main physiological parameter that affects the seasonal variation of P_max.（3）The photosynthetic adaptation to gap size of Korean pine was obvious in leaf morphology and chlorophyll content.The specific leaf area of Korean pine needles(7.15～9.06 m²·kg^-1)in the central area of large gaps was significantly lower than that of small gaps and small gaps by 14.9%and 21.1%（P<0.05）;the content of chlorophyll（a,b,a+b）in the gap was significantly lower than that in the forest understory（15.6%～47.7%）,and showed a decreasing trend with the increase of gap size.（4）The average Pmax of Korean pine in the growing season in large and medium gaps showed a decreasing regularity,namely central area（18.2%～52.7%）>transitional area（20.0%～25.0%）>edge area;its adaptive mechanism is as follows:the light energy utilization capacity in the growing season showed that the light saturation point decreased and the light compensation point increased from central area to edge area,and the apparent quantum efficiency,transpiration rate and stomatal conductance in the central area were also significantly higher than those in transition area and edge area;moreover,the specific leaf area（large and middle gaps）and chlorophyll content of Korean pine needles in the central area of gaps were significantly lower than those in the edge areas,so the adaptation of Korean pine photosynthesis to various positions in gaps also existed adaptability in physiology,morphology and ultrastructure.（5）The average Pmax of Korean pine needles in each season（spring,summer and autumn）in large and middle gaps showed a decreasing regularity,namely central area>transitional area>edge area,but it changed with seasons in small gap;the adaptive mechanism is as follows:the adaptability of light compensation point transpiration rate and stomatal conductance of Korean pine needles to the position in gaps in spring summer and autumn is not affected by seasonal changes;However,the adaptation of light saturation point and chlorophyll content to the position in the gap changed with the change of gap size and season.（6）The main controlling factors of maximum net photosynthetic rate(P_max)of Korean pine needles changed with the change of gap size and position.Its Pmax is only promoted by light saturation point in the forest,promoted by light saturation point（58.2%）and stomatal conductance（41.8%）in the small gap,promoted by stomatal conductance（39.1%）and inhibited by light compensation point（60.9%）in the middle gap,but only promoted by transpiration rate in the large gap;the Pmax was only promoted by transpiration rate in the central area of the gap,only by stomatal conductance in transition area,and only inhibited by light compensation point in edge area.Therefore,the photosynthetic capacity of Korean Pine sapling（35a）in the white birch forest of the temperate Lesser Khingan Mountains of China showed an increasing law with the increase of gap size and a decreasing law along central area to edge area of the gap,Therefore,it is suggested that in the restoration practice of broad-leaved Korean pine forest in Northeast China,we should make full use of the resource advantages of the central area of large gaps（planting Korean pine in groups in the central area within the gap of≥200 m² or creating the corresponding central area of large gaps around the planted Korean pine for light transmission tending）,so as to accelerate the restoration process of climax vegetation of broad-leaved Korean pine forest.