Functional Traits And Recruitment Of Woody Plant Seedlings During Natural Recovery Of A Tropical Lowland Rain Forest In Hainan Island, China

Understanding mechanism of species diversity maintenance has been one of the centralissues in community ecology. The seedling bank is a basic component in tropical forestecosystems, which play important roles in community dynamics, diversity maintenance andecosytem functioning.However, the seedlings with a diameter of less than1cm are usuallyneglected. However, seedling functional traits are often strongly related to the performance andfunction of adult plants in an ecosystem. Measurements of seedling traits are easy and accuratecompared to measurements of adult traits.In this thesis,we investigated the diversity andfucntional traits of woody plant(including trees,shrubs and lianas) seedlings in a tropicallowland rain forest of different recovery stages(30years and60years after shifting cultivationand old growth forest) on Hainan island, South China. The variation of seedling functionaltraits of woody plants during the recovery process and its influencing factors were explored.Seedling recruitment dynamics were analyzed by comparing the composition,abundance anddiversity of seedling and tree communities in each recovery stages of the tropical lowlandrainforest. The factors influencing the relative growth rate of8representative speceis of treeseedlings in the tropical lowland rain forest were examined through field experiments. Themain results are as follows.1. Seedlings in30-year and60-year secondary forests had higher specific leaf area,specific stem length and specific root length. Leaf mass fraction increased significantly, whileroot mass fraction reduced gradually during the recovery process. The seedlings can beaggregated into3functional groups by ordination of Principal compoent analysis The speciesin old growth forest clusted more closely than those in the secondary forests.2. Environmental factors changed significantly during the recovery process. Soil watercontent were significantly low in30-year forest, while there was no significant difference in60-year forest and old growth forest. Canopy openness and soil nutrient contents decreased withrecovery time. The correlations between seedling functional traits and environmental factors changed correspondingly with the natural recovery process. Most of the seedlings functionaltraits were significantly correlated with soil water content in the30-year forest but nocorrelations existed in the60-year and old growth forests. Seedling functional traits were notcorrelated with canopy openness in the30-year and60-year secondary forests while hadsignificant correaltions in the old growth forest. Seedling functional traits was correlated withsoil nutrient contents in each recovery stage.3. The functional traits varied significantly with life form and growth stage (height class).Seedlings adapte to the community environment by making the trade-offs between differentfunctional traits. Tree and shrub seedlings adapt to the stressful rainforest understoryenvironment (such as low light) by decreasing specific leaf area and increasing stem massfraction. However, lianas adapt to the environment through the opposite strategy. With thegrowth of seedlings, trees and shrubs reduced the investment to assimilation, and increased theinvestment to support organ. Lianas increased investment to assimilation organs, and reducedto support organs. This is consistent with characteristics of liana attanching to other plants togrowth.4. Functional traits of seedlings were significantly different from those of the adulttrees(DBH>7.5cm) in each recovery stage. Leaf dry mass content and specific stem density ofseedlings and adult trees were positively correlated. Specific leaf area of both seedlings andadult trees decreased, while leaf dry mass content and specific stem density increased duringthe recovery process. The environmental factors that affected functional traits of both seedlingsand trees were same to a certain extent in secondary forests, but different in old growthforest.These above results suggest that the patterns of change in functional traits and theirrelations with the environmental conditions for adult trees can be indicated in certain degree bythose of the the seedlings in this lowland rain forest.5. The species-area and species-individual cumulative rates of seedlings increased withthe recovery time. Species accumulation rates in old growth forest were faster than those in thesecondary forests. The seedling abundance and richness first increased and then decreased withrecovery time, which was in line with the intermediate disturbance hypothesis. Species richness of seedlings was correlated with soil water content, total nitrogen and total phosphoruscontent in the30-year forest,while it was correlated with canopy openness, soil water contentand potassium content in the60-year forest; and it was correlated with canopy openness,available phosphorus, and available potassium in the old growth forest. Seedling abundanceand richness showed a gradually decreasing trend with increasing growth stage (height class).6. The relationships between seedling species composition heterogeneity andenvironmental factors in three successional stages were assessed using redundancy analysis.The variance of environmental factors to interpret heterogeneity of seedlings were55.1%and53.5%respectively in30-year and60-year secondary forests, while lower (43.4%) in oldgrowth forest. The relationship between distribution of seedlings and environmental factors inthree recovery stages were different. Distribution of seedligns in30-year forest was mainlyaffected by soil water content and canopy openness. Distribution of seedligns in60-year forestwas mainly affected by soil nutrient content. Distribution of seedligns in old growth forest wasmainly affected by soil water content and total phosphorus.7. The species richness of seedlings in30-year forest,60-year forest and old growthforest.accounted for63.5%,60.6%and54.9%of those for the whole coummunity respectively.This result showed that seedling bank is an important component, which should not beneglected in the study of biodiversity in this lowland rain forest. The recruitment rates fromseedlings (DBH<1cm) to saplings (1cm≤DBH<7.5cm) and from seedlings to adult trees(DBH≥7.5cm) were71.9%and39.7%respectively in30-year forest, while those were65.0%and38.0%respectively in60-year forest. In old growth forest,74.9%and50.9%of seedlingspecies grew into saplings and adult trees.8. Eight representative species (Sapium discolor, Lannea coromandelica, Toxicodendronsuccedaneum, Peltophorum tonkinense, Castanopsis carlesi, Machilus suaveolens, Syzygiumcumini, Vatica mangachapoi) in the tropical lowland rain forest were selected to do thefollowing treatments: removal of the above vegetation (R), trenching (T), removal of the abovevegetation and trenching (R+T), and controlled (C). We measured the relative growth rate ofthese seelings to assess the factors affecting regeneration and growth of the seedlings. We found that the treatments of R and R+T significantly increased the growth rate of seedlings.But the effect of T was not significant. The growth rate of decidous species was higher thanevergreen species. Seedling growth rate increased with the increase of photosyntheticallyactive radiation. The relationship between them could be described by a linear equation. Allthese results showed that light was the most important factor for seedling growth in the tropicallowland rain forest. The below-ground competition had no significant effects on the growth ofseedlings.