| Forest natural regeneration is an important part of forest ecosystem succession and also an important part of forest management research.At present,the research on natural regeneration of forests is mainly concentrated on common species,and the research on natural regeneration of rare and endangered species is less.In recent years,Dacrydium pierrei has become a rare and endangered tree species influencing by climate change and human disturbance,and it has serious obstacles to natural regeneration.The purpose of this study was to reveal the natural regeneration mechanism of Dacrydium pierrei by analyzing the temporal and spatial dynamic characteristics of seed rain and soil seed bank,the distribution and growth characteristics of seedlings(seedling height ? 1.3 m)and saplings(seedling height > 1.3 m,DBH < 10 cm)and their adaptation mechanism to composition structure of community,soil nutrients,climate and geographical environment,which provides a theoretical basis for the protection,restoration and development of Dacrydium pierrei.From September 2013 to August 2016,in this study,types,quantity and quality types of seed of soil seed bank,community composition and structure,density,plant number,height,diameter(ground diameter or diameter at breast high)of seedlings and saplings,soil nutrients,temperature and humidity,microclimate(temperature,humidity,illumination,wind speed and wind direction)of Dacrydium pierrei in Bawangling of Hainan Province in different habitat conditions were measured through a series of experiments.The temporal and spatial dynamic characteristics of seed rain and soil seed bank of Dacrydium pierrei were analyzed.Geostatistical methods were used to analyze the correlation between seed rain,soil seed bank and environmental factors.The effects of soil nutrient on the density,ground diameter/ diameter at breast high and height of seedlings and saplings were studied based on single factor analysis of variance and non-linear mixed model.The above-ground biomass of seedlings and saplings of Dacrydium pierrei was estimated by using generalized regression neural network,adaptive neuro-fuzzy inference system,artificial neural network and support vector machine model etc.The response characteristics of seedlings and saplings of Dacrydium pierrei to climate change were analyzed under the background of global climate change.Distribution of seedlings and saplings under different community,geographical environment,soil and climatic conditions was determined by establishing discriminant models.The main results are as follows:1.Spatial-temporal pattern characteristics and driving mechanism of seed rain of Dacrydium pierrei:(1)the number of intact seeds in the rain was less(5.07%),the damage rate of birds was higher(39.38%),and the empty shell rate was very high(47.83%).(2)Seed fall began at the end of September or the beginning of October every year and lasted until the end of June of the second year.It lasted for a long period,during which there were two peaks of seed fall(April,December and May of next year).In addition,the unstable yielding phenomenon in fruiting characteristics of Dacrydium pierrei are obvious.(3)Gravity is the main driving force for the seed transmission of Dacrydium pierrei.The seeds of Dacrydium pierrei mainly scattered within 5 m from the center of the mother tree by the action of gravity.(4)In the east,south,west and north four directions,the dispersal distance of seed rain is positively correlated with the size of mother tree crown in the same direction,and also has certain spatial heterogeneity at the same time.2.Spatial-temporal pattern characteristics and driving mechanism of soil seed bank of Dacrydium pierrei:(1)in the soil seed bank of Dacrydium pierrei,mildewed seeds account for 45.55% of the total seeds,hollow seeds(29.68%)and defective seeds(24.30%)take the second place,and few sound seeds account for only 0.48% of the total seeds.(2)The horizontal distribution pattern of seeds in soil seed bank is obvious,and the number of seeds decreases with the increase of distance from mother tree.(3)In vertical direction,seeds were mainly distributed in litter layer(87.78%),that of 0-5 cm layer(11.01%)was in the middle,and that of 6-10 cm layer was the least,accounting for only 1.20% of the total number of seeds.(4)The number of viable seeds in seed bank is small,the germination rate is low,and the life span of seeds is short and does not exceed one year.3.The adaptability mechanism of seedlings and saplings of Dacrydium pierrei to soil nutrition: The soil nutrient contents of different plots in the study area were significantly different.The contents of six main nutrient elements(available potassium,alkali-hydrolyzed nitrogen,organic matter,water content,total nitrogen and available phosphorus)in soil were almost exponential and logarithmic functionsrelated to seedling density,ground diameter and seedling height.4.Community habitat adaptation mechanism of seedlings and saplings of Dacrydium pierrei:(1)Air temperature,humidity,illumination,soil temperature and humidity are the main factors affecting the distribution and growth of seedlings and saplings of Dacrydium pierrei under forest microclimate conditions.(2)Among the geographic environmental factors,slope,slope aspect,longitude and latitude are the dominant factors affecting the distribution and growth of seedlings and saplings of Dacrydium pierrei.(3)Among stand factors,DBH > 5 cm tree number,canopy density and adult tree number were the dominant factors affecting seedling distribution and saplings growth.(4)A Fisher discriminant model for the distribution of seedlings and saplings of Dacrydium pierrei was established.The discriminant accuracy of models was higher than 80%.5.Forecast of natural regeneration of Dacrydium pierrei under the background of global climate change:(1)Under three typical climatic scenarios(RCP)2.6,RCP 4.5 and RCP 8.5,the aboveground biomass of seedlings of Dacrydium pierrei will increase by 13.0(+31.4%),16.6(+30.7%)and 18.5(+30.9%)during 2041-2060 years period,respectively.It increased by 15.6(+32.1%),21.2(+33.2%)and 24.8(+32.7%)during 2041-2060 years period,respectively.During 2041-2060 years period,the aboveground biomass of saplings will increase by 16.3(+32.3%),21.7(+32.5%)and 22.9(+32.6%)respectively.It increasedby 22.3+30.8%,27.2+31.8% and 30.1+34.4% during the 2061-2080 years period,respectively.(2)Higher air or soil temperatures are beneficial to the seedling establishment and saplings growth of Dacrydium pierrei under the future climate change conditions.The results showed that the natural regeneration of Dacrydium pierrei in Bawangling of Hainan was not good,and the main reasons for its formation includewas few effective seed resources,weak seed vitality,limited seed dispersal range,and inappropriate habitat conditions for seed germination and survival and development of seedlings and saplings.Therefore,in order to improve the quality of natural regeneration of Dacrydium pierrei,The following measures can be adopted: firstly,to increase the number of healthy seeds and improve the quality of seeds through the cultivation of superior mother trees of Dacrydium pierrei;secondly,to adopt the liberation tending method to clean up the plants around the seedlings and saplings of Dacrydium pierrei,so as to form a canopy gaps and create suitable habitat conditions for the survival of the growth of seedlings and saplings of Dacrydium pierrei. |
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