Study On The Regeneration Strategy And Ecological Adaptive Mechanism Of Celtis Sinensis Population

The costal windbreak of this study is located in Langqi Island, Fujian province. Celtis sinensis is the only dominant tree in this sand dune system. The forest stand structure of this windbreak was destroyed seriously by long term effects of southeastern monsoon, typhoon and heavy rainfall. Because of coordinately functioning of natural disturbances and topography, the gaps in different sizes and shapes are formed by stem breakage, uprooting and branch broken. Previous researches have shown seedlings were dominated in gaps, while root sprouts were dominated under the canopy. This research was conducted in three aspects:(1) Soil seed bank:if the number of seeds in gaps is larger than that of understory?(2) Environment characteristics: what are the differences of the micro-environment between gap and understory?(3) The growth of seedlings and root sprouts:how do they adaptive the microenvironment?The results are summarized as follows:(1) The air temperature and the10cm soil temperature in gaps were higher than in understory; the light intensity in gaps was larger than in understory and the light intensity has significant difference in these two habitats;(2) There were much higher quantities of nutrients in gap soil than of soil under canopy except for total phosphorus;(3) The number of seeds in soil seed banks got peak at the end of seed rain. The total number of viable seeds in gaps were larger than in understory under the natural disturbances.(4) in terms of height and basal diameter of the young trees, seedlings showed the better growth rate than root sprouts; relative humidity played the most important role in inhibiting the growth of root sprouts, so did soil total potassium for seedlings;(5) Both types’young trees contributed most to the increment of stem which took more than60%above-ground biomass;(6) The nutrient contents in leaves are highest than in other organs of both young tree types. The average contents of N, P and Mg of seedlings were higher than that of root sprouts except for K and Ca. Average contents of nutrient elements in various organs and with different regenerations were varied. Average nutrient contents of N and Mg of root sprouts were in the descending order as:leaves> branches> roots> stems; other elements:P and K in leaves> branches> stems> roots, Ca in leaves> roots> stems> branches. The contents of two nutrient elements (N and Mg) in different organs were in the order as:leaves> roots> branches>stems, while for other elements (P, K and Ca):leaves> branches> roots> stems. The abundance of different elements of root sprouts in roots, in stems, in branches and in leaves respectively ranged as:Ca>N>K>Mg>P. The abundance of different elements of seedlings in stems and leaves respectively ranged as:N>Ca>K>Mg>P, while for roots and branches respectively ranged as:Ca>N>K>Mg>P. The N/P ratios of two young tree types were all more than21, which implied that P might be the limiting nutrient of Celtis sinensis.This study suggests that the gap formation in this coastal windbreak has greatly improved the light condition which changes thermodynamic characteristics in the air and heat balance of soil. Furthermore, the gap formation increases the litter fall mass and nutrient contents which improve soil fertility by promoting the accumulation of organic matter, total nitrogen, total potassium, available phosphorus, available potassium, exchangeable calcium, and exchangeable magnesium. This results in the differences of gap and understory even thought the soil in these two habitat are still poor. The gap formation has also provided more opportunities to the seeds to fall into the gap region, so that the numbers of seeds in gaps are higher than in understory, which means forest gap is with better regeneration potential. Celtis sinensis adjusts the relationship between the nutritional physiology and reproductive physiology and changes its reproductive modes in order to adapt the different microenvironments. Most of the resources are allocated towards sexual reproduction in gaps with fluctuating environmental conditions and strong competition, while asexual growth is dominant in stable understory habitats.