Responses And Adaptations Of Tilia Amurensis Seedlings And Saplings To Light Rgimes

Tilia amurensis, one of the mainly associated tree species in natural broad-leaved Korean pine mixtures and broad-leaved species mixtures of Changbai and small Xingan mountain area, is one of the important broad-leaved species for timber production. Knowing little about micro-site optimum suited to the linden growth, no successful management was offered to the linden plantations and natural regeneration. Therefor it is, in theory and in practice, essential to study the mechanism of the natural regeneration populations for determining reasonable management methodology for of natural linden stands and industrial plantations. Morphological plasticity of natural seedlings and saplings of the linden was analyzed in Changbai Mountain area by means of typical sample investigation, and physiological plasticity, the spatial orientation of branch and leaf and leaf movement, biomass allocation patterns and leaf assimilation efficiency of saplings were studied by light-control method. Some results have been gained as following: 1 Moderate shading and side competition in space were advantageous to sole bole development by strengthening apical dominance, but heavily overhead shading and high light intensity depressed bole elongation and promoted branch bifurcation. 2 The biomass allocation pattern is rather stable, that about 50% of biomass was assigned to leaves annually. The ratio of aboveground to underground biomass ranged from 1.75 to 1.83. Some 40% of total biomass of non-photosynthetic organs was allocated to bole, and less than 15% of product of assimilated carbon was utilized to develop lateral roots for absorbing soil water and mineral nutrients. Accumulation rate of biomass was intrinsically restrained by carbon assimilation efficiency of leaf for linden saplings exposed to different light regimes. High accumulation rate of biomass for saplings grown in forest gap attributed closely to high assimilation efficiency of leaf and low respirative consumption in fixed carbons. 3 Morphological and physiological plasticity in response to different light regimes for the linden seedlings and saplings were observed. Weekly shaded seedlings and saplings adopted shade-avoidance strategy of pioneer species for height growth. And they adopted light forage strategy of shade tolerants in lateral branch development. But heavily shaded individuals were observed to adopt typical shade tolerants in survival strategy. Gradual change of upper crown leaf aggregation and edge crown leaf scattering were observed for individuals grown in the light environments from full sun sites and gaps to under canopy. And the leaf area indices decreased sequentially due to leaves overlapping reduction. This morphological change was favor to the linden seedlings and saplings to intercept light more efficiently. 4 With decease in light intensity, leaf was thinner, palisade mesophyll thinner and looser, and the percentage of mesophyll cavity higher. Average leaf area and specific leaf area decreased with decrease in light intensity sequentially, specific leaf weight showed the opposite way. And with decrease in light intensity, the shape of palisade mesophyll changed from column into invert pyramid. The higher proportion of granum thylakoid, more granum in one ganum thylakoid and lower ratio of chlorophyll a to chlorophyll b were also the responses of leaves to shading. 5 Spatial orientation in branches and leaves was prevailing in the linden seedlings and saplings. Comparing with individuals exposed to full sun, seedlings and saplings shaded by 1 layer of shade-net distributed more branches in southern and northern part of crown and more leaves in eastern and western part of crown correspondingly. Young trees shaded by 3 layers of shade-net tended to distribute their branches and leaves randomly in crown. With branch and leaf orientation and leaf movement, young trees could modulate the interception of photosynthetic photon flux density (PPFD) in different solar time. That was, large quantity of PPFD was intercepted by leaves when net photosynthetic rate of leaves was high, and less PPFD was intercepted by leaves when leaf photosynthesis was restrained in noon. This modulating capacity was higher for young trees shaded by 1 layer of shade-net than young trees exposed to full sun. 6 CO2 dispersed through stomata and in mesophyll were two mainly limiting factors to leaf photosynthesis of the linden seedlings. Stomatal conductance of CO2 remarkably increased with decrease in PPFD, and was intensively restricted by high temperature and vapor pressure deficit (VPD). Rate of CO2 conductance in mesophyll which was closely related to CO2 supplication fluctuated by a large margin with change in carboxylation efficiency of CO2, stomatal conductance of CO2, VPD and respiration rate of leaves. The limitation of VPD to gas exchange was efficiently relieved by shading, and the resistant capacity of leaf against high temperature stress was obviously weakened. The impairment of high temperature on leaf photosynthesis of the linden seedlings and saplings was usually amplified when they were exposed to high light intensity. Reduction in net photosynthetic rate (NPR) of linden leaves was mainly caused by stomatal limitation when low stress of temperature and VPD occurred. But the reduction was mainly caused by mesophyll limitation when heavy stress of temperature and (or) VPD prevailed. The stomatal and mesophyll limitation to leaf photosynthesis were virtually relieved in a certain extent by moderate shading especially in initial growth stage of young tree. 7 With the reduction of PPFD, apparent quantum yield of leaf photosynthesis was obviously enhanced, and light compensation point and dark respiration rate leaf photosynthesis were declined in large scale. The carboxylation efficiency of CO2 and average diurnal NPR of leaves was the highest for seedlings shaded by 1 layer of shade-net. Based on the same leaf age, leaves of seedlings shaded by 1 layer of shade nethad higher capacity of photosynthesis and stronger vitality than leaves of seedlings exposed to full sunlight and shaded by 3 layers of shade net. High respiration rate of seedlings in peak growth stage was the main reason of which resulted in low growth of branches and boles and low biomass accumulation of young trees grown in full sunlight environments. 8 Comparing with young trees grown in full sunlight environment, leaves of seedlings shaded by 1 layer of shade net had the highest water use efficiency which was resulted from low transpiration rate, and had the highest carbon use efficiency which was obtained in low cost of respiration. Similar water use efficiency was received by seedlings exposed to full sunlight and shaded by 3 layers of shade net. Seedlings shaded by 3 layers of shade net, which had the lowest carbon use efficiency, received higher water use efficiency in the cost of lower NPR. 9 Light use efficiency of leaf photosynthesis of the linden seedlings increased with increase in shading. The highest of photosynthesis potential appeared in the linden saplings and seedlings shaded by 1 layer of shade-net. And the lowest befell in seedlings and saplings shaded by 3 layers of shade net. 10 Comparing with individuals exposed to full sunlight, higher average diurnal NPR and higher resources use efficiency for individuals shaded by 1 layer of shade net attributed to rapid responses of photosynthesis on light induction, to higher NPR based on constant PAR and to low carbon consumption in dissipation of excessive energy absorbing by leaves.