Community Ecology Of Tropical Monsoon Forest In Hainan Island, China

Climate change and biodiversity loss have been increasingly concerned by the whole world.In recent years humans have witnessed increases in the frequency of extreme climatic events and rapid extinction of biodiversiy.The tropical forest regions are experiencing decreased precipitation, increased duration of the drier season and greater variability in precipitation.Tropical forest biodiversity have been greatly impacted by these climatic scenarios. Drought in habitats is one of the most threatening factors to the tropical rain forests. The tropical monsoon rainforest has strong abilities and obvious advantages to adapt to drought stress since it has many deciduous species with special functional traits. Therefore, the ecological studies on tropical monsoon rainforest communities has both theoretical and practical implications for biodiversity conservation and ecosystem management of tropical forests under global climate change. The tropical monsoon rainforest on Hainan Island is a azonal vegetation type, mainly distributed in the similar elevational ranges with the tropical lowland rainforest but in locations where environmental conditions are more stressful, especially in the dry season. The tropical monsoon rainforest adapts to the seasonal droughts by defoliation of all or most of its trees ,which has significant effects on the community environments. In this study, the tropical monsoon rainforest in Bawangling forest region of Hainan Island was selected as the main study object, while two other forest types with similar elevational ranges were selected as comparisons.The two forest types for comparisons are the tropical lowland rainforest and the transformed monsoon rainforest,which is a disclimax recovered from serious disturbed tropical lowland rainforest. Based on the field investigations conducted in totally 8.75 hm2 sample plots ,the environmental feature, composition, structure, diversity, functional traits of plants, species-abundance relationships and biomass for the tropical monsoon rainforest community were studied. Through field seedling experiments, the factors influencing the survival and growth for seedlings of one dominant species(Terminalia hainanensis) in the tropical monsoon rainforest and seedlings of three typical species(Schima superba, , Vatica mangachapoi, and Peltophorum tonkinense) distributed in the similar elevational ranges with the tropical monsoon rainforest were also studied. These results can help us to understand the main features and maintenance mechanism of species diversity of the tropical monsoon rainforest, and explore the ecological adaptations of the tropical monsoon rainforest community to seasonal drought, which could provide scientific basis for the conservation, restoration and sustainable managements of tropical forests. The main results of the study are as follows:1. To analyze the characteristics of the tropical monsoon rainforest and similar elevational forest communities, quantitative classification and ordination were conducted by TWINSPAN and DCA based on field investigation plots. The investigated plots were classified into 3 communities by TWINSPAN and DCA, they are the tropical monsoon rainforest(TMR), the tropical lowland rainforest(TLR) and the transformed monsoon rainforest(TFMR).The results of ordination by the DCA also reflected the basic relationships between plant communities and environmental factors, the distribution of tropical monsoon rain forest was positively related to the slop and the coverage of exposed rock surface, and the distribution of tropical lowland rainforest was positively related to elevation and litter depth , while the distribution of the transformed monsoon rainforest was positively related to elevation and was negatively related to slop. The transformed monsoon rainforest was recognized as a community type between the tropical lowland rainforest and the tropical monsoon rain forest.2.The composition, structure and diversity of the tropical monsoon rain forest, community were analyzed by comparison with the TLR and the TFMR. The results indicated that family, genera and species richness were lower in the TMR than in the TLR and the TFMR. Many low density species existed in the three tropical forests,and it was the lowest in the TMR and the highest in the TLR. The mean DBH and the mean height in the three tropical forests from low to high were the TMR, the TFMR and the TLR, whereas, the stand density in three tropical forests from high to low were the TMR, the TFMR and the TLR. Species richness and abundance decreased with increasing diameter classes and height classes in the three forest types. Both the richness and abundance of deciduous species were the highest in the TMR than in the TFMR and the TLR. Most of the deciduous species in the TFMR were dispersed from the TMR. The percentages of deciduous species richness and abundance were the highest in the TMR, and the lowest in the TLR. In the tropical monsoon rain forest, the percentages of deciduous species richness and abundance in the trees were higher than in the shrubs and in the lianas.3. To explore the relationships between species richness, abundance and environments in the tropical monsoon rain forest and in the similar elevational ranged tropical forests in the study forest region,as, the vegetation investigations and environmental recording were carried out in field sample plots. Single factor correlation and stepwise regression analysis between species richness, abundance and environments were conducted. Single factor correlation analysis showed that the species richness of woody plants was positively correlated with canopy density, elevation and soil water content,repectively and was negatively correlated with the coverage of exposed rock surface. The richness of deciduous species was negatively correlated with canopy density, elevation and soil water content respectively, but positively correlated with slop and the coverage of exposed rock surface respectively.Stepwise regression analysis showed that the species richness of woody plants was only related to elevation( y = 0 .154(elevation)+10.153,R=0.925,P=0.000).The stepwise regression analysis also indicated that the species richness of decious woody plants was positively related to slop and the coverage of exposed rock surface,and negatively related to elevation and soil water ontent( y = 30. 506?0.023(elevation)+0.151(rocky)?0.292(water)+0.152(slop),R=0.989,P=0.000). Single factor correlation analysis showed that the total stem abundance of woody plants was positively correlated with the coverage of exposed rock surface ,and negatively correlated with crown density, elevation and soil water content respectively. The stem abundance of deciduous woody plants was positively correlated with slop,but negatively correlated with crown density,elevation and soil water content respectively. The stepwise regression analysis revealed that both the total stem abundance of woody plants and the stem abundance of the deciduous woody plants were negatively related to the soil water content ( y = 3907. 472?100.46(9water),R=0.846,P=0.000; y = 1676. 237?59.60(6water),R=0.870,P=0.000).4. Based on the field investigation data,major functional traits of trees in the tropical monsoon rain forest and similar elevational forests in the study region were explored.The 7 functional traits included defoliation or evergreen, with or without thorns, potential maximum height, specific leaf area(SLA), seed-mass(SM), dispersal mode and wood density(WD). The results showed that species richness and abundance of deciduous species and species with thorns were higher in the TMR than in the TLR and the TFMR. The value of potential maximum height, species richness/abundance in different range of potential maximum height were the lowest in the TMR. There were no significant difference in species richness and abundance when SLA was higher than 20m2/kg, but the TMR had the highest abundance when SLA was lower than 10m2/kg. SM was lower in the TMR than in the TLR and the TFMR, however, no significant difference was found in the three tropical forests in species richness and abundance when SM was 50～200mg, and no species existed in the TMR when SM value was higher than 200mg. Species richness and abundance of animal dispersal were lower in the TMR than in the TLR and the TFMR. No significant difference were found in species richness and abundance in different WD ranges in the three tropical forests except when the WD was 400～600kg/m3.5. To explore the tree assembly rules in the tropical monsoon rain forest and similar elevational forest communities in the study region, the speices-abundance relationships were studied and simulated with population models ,including the Zipf- Mandelbrot(ZMM), Zipf(ZM), the broken stick model(BSM), the geometric series model(GOM), the lognormal distribution model(LN) and the neutral theory model(NT). We found that the best model to fit the TMR was the Zipf- Mandelbrot(ZMM), and the next was the Zipf(ZM). However, the best model to fit the TLR was the NT, and the next was the LN. Only the ZMM could predict well species-abundance relations in the TFMR. We hypothesized that in the study region,the neutral models fit species-abundance distribution patterns in the closed-canopy old growth forests(such as the TLR),but didn't fit those patterns both in the open-canopy old-growth forests(such as the TMR) and in the closed-canopy second-growth forests recovered from anthropogenic disturbance(such as the TFMR) , on the contrary, the niche models fit species-abundance distribution patterns in the open-canopied old-growth forests and the closed-canopy second-growth forests recovered from anthropogenic disturbance, but didn't fit the patterns in the closed-canopy old-growth forests.6. To find the factors affecting the survival and growth of the seedlings, a field seedling treatment experiment was conducted.We measured survival and height growth of 1200 seedlings of of dominant tree species in the tropical monsoon rain forest(Terminalia hainanensis) and three representative tree species distributed in the similar elavational range(Schima superba, , Vatica mangachapoi, and Peltophorum tonkinense)in the study region. The seedlings were planted for 17 months under one of four treatments: removal of aboveground vegetation(R), trenching(T), trenching plus removal of aboveground vegetation(T+R), and a control(C). We found that the treatments of R, T, T+R significantly increased the survival of Schima superba in the dry season, wet season and the whole experiment priod(17 months), increased the survival of Terminalia hainanensis in the dry season and increased the survival of Vatica mangachapoi in the whole experiment priod. Whereas, only the R treatment increased the survival of Peltophorum tonkinense in dry season and the whole experiment priod. Furthermore, the treatments of R and T+R increased the relative growth rates of Schima superba, Terminalia hainanensis, and Peltophorum tonkinense. The soil water content could increase the seedlings survivals of three species() and the seedlings relative growth rates of all the four species. The above- and below-ground competition and soil water content had significant controlling effects on the survival and growth of tree seedlings in the tropical monsoon rain forest.7. Biomass and its allocation in the TMR was studied using the data of field investigations and the biomass estimation models. We found that the total(evergreen+deciduous species) biomass was lower in the TMR than in the TLR, and no significant difference was found between the TMR and the TFMR, whereas, the TMR had the highest biomass of the deciduous species and the lowest biomass of the evergreen species. The TLR had the highest total biomass in the big diameter classes(> 5th diameter class), the TMR had the highest total biomass in the first diameter classes, however, no significant difference were found in the second, the third and the fourth diameter classes in total biomass among the three similar elevational tropical forests. Total biomass in the TLR mainly distributed in the big diameter classes, but total biomass in the TMR and the TFMR mainly distributed in the middle diameter classes. Biomass of deciduous species in the TMR was the highest in all diameter classes, however, its biomass of evergreen species was the lowest in most of the diameter classes among the three forest types. In the vertical direction, no significant difference was found between the low(H<5m) and middle(5≤H<15m) layers in total biomass among the three similar elevational tropical forests, however, the TLR had the highest total biomass in the high(H≥15m) layer. The TMR had the highest deciduous species biomass in most of the layers except in the lowest layer. Total biomass in the TLR mainly distributed in the high layer. Compared with the TLR and the TFMR, the TMR had the highest deciduous species biomass and the lowest evergreen species biomass in the high and the middle layers.