热带云雾林林冠附生维管植物繁殖体扩散及物种共存机制研究

Vascular epiphytes, which occupy 25% of the plant species in tropic areas, are recognized as characteristic components in tropical forest communities, and contribute to tropical forest ecosystem functioning. How species diversity maintains in a complex community (e.g. tropical forests) is a fundamental ecological question which has been being explored by ecological scientists. Propagule dispersal and species coexistence, which are recognized as the two key ecological processes in mechanisms of maintenance of species diversity, however, are almost neglected and seldom known in vascular epiphytes. Tropical cloud forest is an unusual type of tropical forest in Hainan Island. Trees in these forests are typically deformed, elfin and covered with rich epiphytes. Environmental conditions in these forests are characterized by low air temperature, strong winds, frequent fog and high relative humidity compared with low altitudinal tropical forests. Epiphytic plants in tropical cloud forests are mainly ferns and orchids, and thus are rich in spores or seeds which are very small and light. Propagule dispersal of epiphytes in tropical cloud forests, thereafter is assumed to be affected by precipitation, winds and host trees in tropical cloud forests. Additionally, vascular epiphytes live on nutrients from atmosphere and canopy soils, instead of ground soils like trees. Epiphytic plants are expected to be prone to environmental conditions such as air temperature, strong winds and nutrient resources, and are assumed to not only compete with nutrient resources, but to be facilitated by neighboring plants due to the constraints of low air temperature and strong winds. In this study, we predict (1) that propagule dispersal of vascular epiphytes in tropical cloud forests would be affected by precipitation, winds and host tree; and (2) that vascular epiphytes in tropical cloud forests would be assembled with competition for nutrients, as well as facilitation due to environmental stresses such as low air temperature and strong winds. We will test our hypotheses according to the following steps. First, after investigating all the epiphytes which live upon host trees with diameter at breast height (dbh) over 5 cm in tropical cloud forest plots, we will assess the patterns of species diversity and variation in functional traits of epiphytes at different canopy height, and examine the patterns of environmental conditions. Second, we will design a device which manipulates precipitation volume and wind speed on epiphytic propagules, and will install the device at different canopy height which simulates propagule sources at different height of host trees. After establishing a fan-shaped area which takes the point of the device as a centre, and labeling all the trees with abh over 5 cm at different radius of the fan-shaped areas, we will survey the dispersed propagules at different height of host trees and assess the effects of precipitation volume, wind speed, host trees. Third, we will assess the species co-occurrence pattern and limiting dissimilarity of functional traits of epiphytes after selecting proper null models, to infer the biotic interactions and environmental filterings affecting epiphytic species assemblage. These biotic and abiotic processes, moreover, will be verified by epiphyte transplant experiments which measure the growth of different species combination of epiphytic plants. The results are expected to show the patterns of propagule dispersal and mechanism of species assemblage of canopy epiphytic plants in tropical cloud forests, and thus will contribute to mechanism of maintenance of epiphytic species diversity, and will enrich the knowledge and theory of tropical forest community ecology, as well as being helpful to the forest biodiversity conservation.

附生维管植物物种占热带地区物种数的25%，是热带森林中重要特征性组分，对维持生态系统功能具有重要作用。热带森林中物种多样性如何维持一直是生态学者努力探索的生态学基础问题，而繁殖体扩散及物种共存是植物物种多样性维持的两个关键过程。但目前有关附生维管植物这方面的知识知之甚少。本项目拟在空气湿度大、树木低矮、附生维管植物丰富的热带云雾林中，调查不同林冠高度附生维管植物的物种多样性、功能性状和环境特征；在各个林冠高度实施人工模拟试验，探究降雨量、风速、宿主树木特征等对附生维管植物繁殖体扩散的影响；结合零模型分析附生维管植物物种共有度格局及功能性状的限制相似性，分析共存物种间的生物作用和环境筛，并通过不同组合的物种移植生长观测试验验证。项目有望阐明热带云雾林林冠附生维管植物繁殖体扩散规律及物种共存机制，揭示附生维管植物物种多样性维持机制，有助于丰富群落生态学理论，为生物多样性保育提供借鉴。

附生维管植物是热带森林中重要特征性组分，对维持生态系统功能具有重要作用。复杂群落（如热带森林）中物种多样性如何维持一直是生态学者努力探索的生态学基础问题，但目前有关附生维管植物的这方面知识知之甚少。项目在海南霸王岭热带云雾林8400m2固定样地内发现附生维管植物2650株51种，附生兰科植物和附生蕨类植物为优势类群，生活在10.6%的宿主树上，70%以上的附生维管植物生活在苔藓基质上，且植物多度和丰富度与宿主树胸径显著正相关。2019-2021年间附生维管植物多度和丰富度均显著增加，群落物种周转较快，群落结构变化明显，温度是驱动群落物种多样性年际变化的重要因子。湿季附生维管植物的水资源近99%来自于叶片对雨水的吸收；干季附生维管植物的水资源主要来自于雾水(47%)和雨水(53%)；附生维管植物倾向于通过调节水分流失达到节水目的以抵御干季，为保守的水分利用策略。附生维管植物所有物种及不同多度物种在5 m、10 m样方尺度上的C-score观测值均比预期值显著低，在5 m、10 m样方尺度上V-ratio观测值比预期值显著高，表现为非随机聚集的共有度格局，且在较大样方水平上聚集度最高；在宿主树水平上，所有物种及多度大于50及100的物种的C-score观测值均比预期值显著低，V-ratio观测值比预期值显著高，也表现为非随机聚集的共有度格局。附生维管植物的生境分异水平显著高于木本植物，木本植物的周转型生境分异显著高于附生维管植物，占总变异的73.16-80.08%；附生维管植物的嵌套型生境分异显著高于木本植物，占总变异的46.82-67.5%；随机性过程是木本植物与附生维管植物群落构建的主要驱动力。项目阐明了热带云雾林林冠附生维管植物群落的物种多样性特征、功能性状特征及环境特征，揭示附生维管植物物种多样性维持机制，丰富了群落生态学理论，为生物多样性保育提供借鉴。