物種之間的交互作用是備受討論的議題 ，物種間除了負向的競爭作用外，還有正向的交互作用（positive interaction），然而探討物種間正向交互關係遠比探討負向的交互作用來的少。近來一些研究發現正向的交互作用有助於生物的多樣性的維持，全球變遷為許多生物帶來更多逆壓，而研究認為逆境中正向的促進作用更普遍。雖然已有研究探討附生植物與寄主樹種間的正向交互作用，但幾無研究檢驗附生植物之間的交互作用。在福山試驗林常可看到巢蕨（Asplenium antiquum） 與垂葉書帶蕨（Haplopteris zosterifolia）有共同出現的現象，本研究利用野外的調查及移除實驗，探討巢蕨與垂葉書帶蕨之間是否存在正向的促進作用，若有其機制為何? 田野調查顯示垂葉書帶蕨有偏與巢蕨一起出現的情形 (56%)，而巢蕨則無偏好與垂葉書帶蕨一起出現（18%）。單獨出現及與垂葉書帶蕨一起出現的巢蕨，其葉片展幅長度沒有顯著差異，而與巢蕨一同出現時的垂葉書帶蕨較單獨存在時葉片較長。此結果顯示，垂葉書帶對巢蕨可能是中性的作用而巢蕨對垂葉書帶蕨則可能有促進作用。移除實驗的結果顯示垂葉書帶蕨的移除對巢蕨葉片數量及葉片長度均無顯著影響，支持其對巢蕨的效應為中性。而同時移除巢蕨葉片基座會使其下方的垂葉書帶蕨其葉片長度下降，僅移除巢蕨葉則無此現象；蕨葉營養顯示，N、Ca、Mg在控制組及移除書帶蕨組無顯著差異；移除垂葉書帶組的巢蕨葉C含量較低，但差異甚小，P的含量則是較控制組高，但移除巢蕨基座並未使垂葉書帶蕨P含量下降，故書帶蕨對巢蕨營養的效應並不明顯；垂葉書帶蕨的營養含量不受是否移除巢蕨及其基座的影響。本研究結果顯示，巢蕨能促進垂葉書帶蕨的生長，因僅移除巢蕨葉並顯著影響故此促進作用並非來自遮光造成的輻射改變，而移除基座與否對垂葉書帶蕨的營養並無影響，故促進作用不是源自基座所提供的營養。因附生植物未能直接自土壤中取得水，而巢蕨基座具高的儲水能力，故推論巢蕨對垂葉書帶蕨產生的促進作用可能源自降低水的逆壓，惟此尚需進一步的研究加以檢驗。

Species interaction is a widely studied ecological issue. In addition to the most frequently studied negative interaction—competition, there is also positive interaction. However, positive interaction is much less explored compared with competition. Recent studies indicate that positive interaction helps to maintain biodiversity. Global change is expected to increase stress to organisms and studies suggest that positive interaction is more common in stressed environments. Although some studies have examined positive interaction between epiphytes and their host trees, barely any studies examined interaction between epiphytes. Asplenium antiquum and Haplopteris zosterifolia often co-occur at Fushan Experimental Forest. I conducted field survey and removal experiments to examine if there is positive interaction between the two epiphytic ferns and if so what are the causes? My field survey indicates that most H. zosterifolia (56%) co-occurred with A. antiquum whereas the latter showed no preference for the co-occurrence with the former. The size of H. zosterifolia plants that co-occurred with A. antiquum was larger than those that did not whereas the size of A. antiquum did not differ between those that co-occurred with H. zosterifolia and those that did not. The result suggests that H. zosterifolia probably has a neutral effect on the growth of A. antiquum whereas the latter has a facilitative effect on the growth of the former. Removing H. zosterifolia had no effect on the number and length of fronds of A. antiquum which supports its neutral effect on A. antiquum. Removing both the substrate and fronds of A. antiquum led to shorter fronds of H. zosterifolia and such effect was not detected when only the fronds of A. antiquum were removed. Chemical analysis of fronds indicates that the content of N, Ca, and Mg of A. antiquum did not differ between control and H. Zosterifolia removal treatment but the control group had higher C and lower P. I did not have explanation for such differences but the difference in P was not observed in the fronds of H. Zosterifolia among the three treatments (control, A. antiquum frond removal and A. antiquum frond and substrate removal). Overall the result shows only very weak evidence of the co-occurrence of the two epiphytic ferns on their nutrient content. Changes in solar radiation was not the major effect of A. antiquum on H. Zosterifolia because when only the fronds were removed although the light environment changed there was no significant effect on the growth of H. Zosterifolia. The removal of A. antiquum did not have significant effect nutrient content of the fronds of H. Zosterifolia so the facilitative effect should not come from nutrient enrichment. Because epiphytes have no direct asses to soil water and the substrate of A. antiquum has very high water holding capacity, I suspect that the facilitative effect comes from the alleviation of drought stress from the water in the substrate. However, more studies are needed to validate my inference.

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