啄花鳥活動對蓮華池桑寄生種子散播之影響

Abstract

植物的種子散播是決定其族群分布的重要因素。桑寄生是一類半寄生植物，在台灣，桑寄生主要透過啄花鳥直接散播其種子。其種子需要在特定的適合生長地才有機會成長，如光照、不同寄主樹種的相容性、枝條徑級等因素都會影響桑寄生種子是否能成功建立。因此，種子被散播的位置為決定成功寄生的第一步。本研究探討蓮華池桑寄生的種子命運，以及啄花鳥的活動如何影響桑寄生種子分布。我於2011年七月至九月在蓮華池研究中心，調查蓮華池桑寄生寄主樹木(油茶)的樹高、樹冠幅、與蓮華池桑寄生的數量及大小，並收集掉落在植物及地面的排遺種子、外果皮、與落果；持續觀察啄花鳥的出現位置，同時記錄排遺種子在寄主樹木上的垂直分布。結果顯示啄花鳥偏好有桑寄生植株的樹木，幾乎只把種子散播到有桑寄生植株的樹木上。若移除桑寄生花朵及果實，寄主樹木所接收到的排遺種子數會顯著減少，僅有控制組的15%，表示食物資源比起植株更能吸引啄花鳥前來覓食。寄主樹冠幅越大，會接收更多排遺種子。可能是因為寄主樹冠幅越大，所能承載桑寄生植株也較多，提供更多的食物資源吸引啄花鳥來覓食，因此得到越多的排遺種子，此正回饋機制造成桑寄生聚集分布現象。蓮華池桑寄生果實約有45.6%被取食；經過啄花鳥散播之後，四十棵樣樹及其地面所收集的排遺種子量僅有被取食量的一半，代表種子在樹木之間流動頻繁，而且有大量的種子可以被帶離原本的寄主樹木。排遺種子只有11.3%落在有效散播位置，大部分排遺種子落在無效散播位置。推測是因為排遺種子的黏性不強，排遺時種子串直接從鳥類的洩殖腔口排出落下，使得直接散播效率減低。啄花鳥是蓮華池桑寄生的唯一有效散播者，對於種子的散播有益處、也有限制，彼此之間的詳細關係值得進一步研究。

Seed dispersal is critical for the distribution of plant populations. Mistletoes is a groups of hemi-parasitic plants and their seeds are directly dispersed by flowerpeckers in Taiwan. Mistletoe seeds can establish only at particular safe sites of germination. Condition of seed deposit site, such as light, compatibility to host trees, and branch size, all affect the success of mistletoe seeds. Therefore, where mistletoe seeds are dispersed determines the establishment of the parasitic mistletoes. This study was aimed to determine the seed fate of Taxillus tsaii, and to examine how seed depositions are affected by the movement of flowerpecker. During July to September, 2011, I investigated the tree height, crown diameter and number of mistletoe plants of the major host (Camellia oleifera) of Taxillus tsaii in Lien-Hwa-Chi Research Center, and I counted the number of mistletoe seed deposition, exocarps of mistletoe fruits, and drop fruits attached to plants and on ground. I also recorded the movement of flowerpeckers among host trees and the horizontal distribution of mistletoe seeds on host plants. Results show that flowerpecker visit and seed deposition were higher in trees parasitized by mistletoes. The host trees that experimentally removed mistletoe flowers and fruits received significantly lower seed deposition, only about 15% of seed deposition of controls. I founded seed deposition were greater in larger crown trees. Trees with larger crown were parasitized by more mistletoes, offering more food resource for flowerpeckers and receiving more seed depositions in return. This positive feedback between mistletoes and flowerpeckers led to the clumpy distribution of mistletoes. About 45.6% of mistletoe fruits consumed and, after dispersed by flowerpeckers, seed deposition on the 40 sample hosts was only about half of fruit consumption, indicating frequent movement of flowerpeckers brought large percentage of seeds away from their previous hosts. Only 11.3% of the seed deposition was landed on safe sites, and most seeds were deposited on unsuitable sites (lower branch, mistletoe, and ground). This might because the defecated seeds of Taxillus tsaii are not as viscous as other mistletoe species and usually drop directly from cloaca, leading to decreased efficiency of seed dispersal. Flowerpeckers, as the only effective seed dispersers for mistletoes in Taiwan, provide benefit and limitation to the dispersal of mistletoes. Further studies are needed to examine their relationships in details.