台灣裸子植物之可能起源、傳播路線及分布

Hypothesizing Origin, Migration Routes and Distribution Patterns of Gymnosperms in Taiwan

本文旨在回顧整理台灣裸子植物分類群之化石紀錄，親緣關係及親緣地理研究，試圖回答下列的問題：（1）台灣裸子植物之高特有性是如何造成的?（2）台灣裸子植物之來源區域有哪些?（3）台灣特有裸子植物之分布樣式與他們的姐妹群之分布樣式有何關聯?（4）裸子植物如何由來源區域傳播到台灣?
台灣之裸子植物總共有28個分類群，包括19種及9變種。相較於台灣植物誌第二版，長葉竹柏（Nageia fleuryi）被排除於台灣之裸子植物相，而天龍二葉松（Pinus taiwanensis var. fragilissima）則加入。台灣肖楠（Calocedrus formosana）及鐵杉（Tsuga formosana）維持種的階級，香杉（Cunninghamia lanceolata var. konishi）則維持變種階級。香青之學名則以Juniperus morrisonicola代替J. squamata，清水圓柏之學名則以Juniperustsukusiensis var. taiwanensis代替J. chinensis var. taiwanensis.這28個分類群依其分布樣式可分成熱帶起源，南半球起源及北半球起源三類。台灣特有之裸子植物佔台灣裸子植物相之78.5%。如此高的特有性可能歸因於（1）木本的習性：若與草本習性相較，木本習性具有較寬的生態棲位，因此對環境的變異比較不敏感而更具適應性；（2）生存於溫帶的本質：台灣地形高低起伏，在溫度高低起伏變化大時，溫帶植物更容易在台灣找到避難所而存活下來。
屬於熱帶起源或南半球起源之分類群在台灣傾向於低海拔分布及零星分布，屬於北半球起源之分類群則傾向於中高海拔分布，尤其是中央山脈之中北部，而有些種類是成片分布，有些則是局限分布。
屬於北半球起源之台灣特有裸子植物，其分布樣式與他們的姐妹群的分布樣式具相關性。若姊妹群的分布在高緯度地區如日本，華西北，華中等，台灣之特有裸子植物屬高海拔分布，且其海拔分布中心高於1800公尺。若姊妹群的分布在低緯度地區如華南，華極南，華東南等，台灣之特有裸子植物分布屬低海拔分布，且其海拔分布中心低於2000公尺。北半球起源之台灣特有裸子植物之化石歷史，除香青（Juniperus morrisoniocola）可回溯至歐洲外，其餘種類可回溯至亞洲或北美洲。北美洲之始祖經白令海峽遷移至東北亞，再從東北亞往南傳播或到日本，或經華北而達華中或華東。定居在日本者，再從日本或經琉球而傳播到台灣，或當日本為亞洲大陸之一部分時，再傳播至華東，同時繼續往南傳播，經東海陸橋而到達台灣。定居華中或華東者，再往南傳播或經東海陸橋而到達台灣，或傳播至華東南經東山陸橋而到達台灣。歐洲之始祖則往南傳播到東喜瑪拉雅山，再經雲貴高原，南嶺到達廣東，福建等地，再經東山陸橋到達台灣。華西南及中南半島旣是避難所也是傳播中心。南半球起源之分類群可能由南半球經東南亞而到達中南半島及華西南。再從中南半島及華西南或經華極南，南海陸橋而到達南台灣，或經華南，再由東山陸橋到達中台灣。如果北半球起源之分類群與姐妹群之遺傳距離短，而且其姐妹群之分布在華西南，他們的傳播路徑與南半球起源之分類群由中南半島及華西南之傳播路徑一樣。如果北半球起源之分類群與姐妹群之遺傳距離長，而且其姐妹群之分布在華西南，則可能分布於華中之一支始祖往南傳播，經華東南，東山陸橋而到達台灣;另一支始祖則繼續由華中往華西南傳播，造成姐妹群間斷分布之現象。
雖然台灣裸子植物之垂直分布與姐妹群之分布樣式有關聯性，但是水平分布則缺少關聯性，可能是因其在台灣具有長久之演化及傳播歷史而混淆了分布樣式。因此欲解釋台灣裸子植物水平之分布樣式，需要更多有關台灣裸子植物的化石及其歷史生物地理學之資料。

Phytogeographical study of gymnosperms in Taiwan is carried out based on reviewing data gathered from published papers on fossils, phylogeny and phylogeography. Following questions are asked. (1) How is the high degree of endemism of gymnosperm flora of Taiwan derived? (2) How many source areas of gymnosperms in Taiwan are there? (3) Is there relation between distribution pattern of endemic gymnosperms in Taiwan and those of their sister species? (4) How do gymnosperms migrate to Taiwan?
In total, 28 taxa including 19 species and 9 varieties of gymnosperms are in Taiwan. Compared to the Flora of Taiwan 2nd edition, Nageia fleuryi is excluded and Pinus taiwanensis var. fragilissima is added in this paper. Species status of Calocedrus formosana and Tsuga formosana and variety status of Cunninghamia lanceolata var. konishii are retained. Scientific names are adopted for Juniperus morrisonicola instead of J. squamata and for Juniperus tsukusiensis var. taiwanensis instead of J. chinensis var. taiwanensis. According to distribution patterns, these 28 taxa may be categorized into tropical origin (TO), Southern Hemisphere origin (SMO) and Northern Hemisphere origin (NMO).
Gymnosperms in Taiwan with high degree of endemism, 78.5%, may owe to woody habit, which is wider in ecological niche compared to herbaceous one and would be less sensitive to the environmental changes, and owe to temperate essence that is more easily to find shelters during temperature fluctuations.
Taxa of TO and SMO are inclined to inhabit low altitudes and sporadically distributed, whereas taxa of NMO are inclined to inhabit middle to high altitudes, especially in northern and central Central Mountain Range and may be widely or restrictedly distributed.
Distribution patterns of endemic taxa of NMO in Taiwan are related with those of their sister species. Taxa with sister species in higher latitudes such as Japan, northwestern China and central China are distributed in higher altitudes with midpoint of altitudinal distribution over 1800 m, while those with sister species in lower latitudes such as South China, southern South China, southeastern China are distributed in lower altitudes with midpoint of altitudinal distribution under 2000 m.
Most fossil histories of endemic taxa of NMO may trace back to Asia or North America (NAM) except Juniperus morrisonicola that may trace back to Europe. For those traced back to NAM, ancestors in NAM migrated to northeastern Asia via Biringia, from where dispersed southward either to Japan, or to northern China and then to central and eastern China. From Japan, ancestors either migrated southward through the Ryukyus to Taiwan if sister species were restricted to Japan, or they might have dispersed to continental Asia and evolved when Japan was a part of continental Asia and further migrated southward via East China Sea’s land bridge to Taiwan. From central or eastern China, ancestors migrated southward either via East China Sea’s land bridge or through southeastern China via Tungshan land bridge to Taiwan. Ancestors in Europe migrated southward to the Himalayas, from where through the Yun-Kue Plateau, Nanling via Tungshan land bridge to Taiwan.
Southwestern China (SWC) plus IndoChina is both refuge and dispersal center. Taxa of SMO might have dispersed from the South Hemisphere through southeastern Asia to IndoChina, from where migrated either through southern South China via South China Sea’s land bridge to southern Taiwan, or through South China via Tungshan land bridge to central Taiwan. If taxa of NMO share short genetic distance with their sister species in SWC, their migration routes would be like those of SMO. However, if taxa of NMO share longer genetic distance with their sister species in SWC, one lineage of their ancestors, possibly distributed in central China then, migrated through southeastern China via Tungshan land bridge to Taiwan while another lineage in central China further dispersed to SWC and produced disjunct distribution patterns.
Taxa of gymnosperms in Taiwan distributed in higher altitudes are inclined to have sister species distributed in higher latitudes. However, horizontal distribution patterns of gymnosperms in Taiwan may be blurred by long history of colonization. Thus horizontal distribution patterns can only be explained by obtaining more data on fossils and paleogeography of such taxa in Taiwan.