拉拉山森林動態樣區木本植物組成與環境之關係

Abstract

亞熱帶山地雲霧森林是一種台灣獨特的植群類型，因受到規律且高頻的雲霧影響，使山地雲霧森林中的植物需適應空氣濕度高、光線少、氣溫低及土壤中長期缺乏養分的環境。為了詳細地瞭解決定亞熱帶山地雲霧森林植物群落組成的生態過程，本研究於台灣北部插天山自然保留區內，位於近塔曼山-拉拉山鞍部拉拉山側之檜木山地針闊葉混合雲霧森林中設立了拉拉山森林動態樣區(24°42ʹ N, 121°26ʹ E)。 該一公頃樣區設立於2019年7月，並於2020年8月完成第一次樣區木本植物普查，普查資料內容包含樣區中所有胸高直徑 ≥1公分木本植物之物種、胸高直徑、位置及編號。同時，也於樣區中蒐集和地形及土壤性質相關之環境因子以供研究分析(其中有些環境因子含樣區中所有樣方之資料，有些則只含樣區中的25個取樣樣方之資料)。 第一次拉拉山森林動態樣區木本植物普查共記錄隸屬於29科、42屬、65種之5220株個體，胸高斷面積共為69.1平方公尺/公頃。以種的優勢度而言，台灣扁柏(Chamaecyparis obtusa var. formosana)和台灣杜鵑(Rhododendron formosanum)之重要值指數(IVI)皆為14%、次為毽子櫟(Quercus sessilifolia)佔9%、昆欄樹(Trochodendron aralioides)佔7% 、假柃木(Eurya crenatifolia)佔5%，前五優勢樹種之累積重要值指數達49%。本研究利用雙向指標種分析(TWINSPAN)將樣區森林以10公尺×10公尺之樣方為單位分為三種植群型後，透過變異數分析(ANOVA)測試植群型間環境因子之差異。 薄葉虎皮楠-台灣扁柏型(Daphniphyllum himalayense subsp. macropodum-Chamaecyparis obtusa var. formosana type)共佔74個樣方，是樣區中主要的植群類型，主要分布於樣區西側及中央寬闊的稜線上。分類至該植群型之樣方擁有相對平緩的地形及酸鹼值較低之土壤，並擁有較高的平均胸高斷面積及較低的密度。小葉石楠-台灣杜鵑型(Pourthiaea villosa var. parvifolia-Rhododendron formosanum type)共佔20個樣方，主要分布於樣區東側的迎風坡上，受東北季風的影響最甚。分類至該植群型之樣方之迎風程度高且土壤酸鹼值高，並擁有較低的平均胸高斷面積和較高的密度及物種豐富度。狹瓣八仙花-假柃木型(Hydrangea angustipetala-Eurya crenatifolia type)相對較少、僅佔6個樣方，主要分布於樣區西側的溪谷中。分類至該植群型之樣方普遍坡度較陡、地勢較低窪、土壤含石率及土壤酸鹼值高，且擁有所有植群型中最低的密度。 為了瞭解樣區形成現今木本植物物種組成背後的主要成因，我們使用降趨對應分析(DCA)，並將所有通過測試、與樣區物種組成具有顯著相關之環境因子陳列於圖中。DCA的結果顯示拉拉山森林動態樣區之木本物種組成主要受DCA之第一及第二軸所影響。於該分析中可以見得第一軸和海拔及土壤化學性質包含碳氮比、磷、鎂、鋅呈正相關，而和坡度及土壤酸鹼值呈負相關。第二軸則和土壤含石率呈正相關，並和迎風程度、凹凸度及土壤分解中的穩定因子呈負相關。其中，樣區中的木本植物物種組成主要於擁有較高土壤酸鹼度之較陡的迎風坡和擁有較低土壤酸鹼度之平坦且較高之稜線間發生變化。而次要的物種組成變化則發生於擁有較高凹凸度之地形及擁有較低凹凸度且土壤含石率較高的溪谷兩者之間。 本研究所蒐集之拉拉山森林動態樣區木本植物分布及相關環境狀況，亦可作為未來複查樣區之基線資料，以供未來分析及監測台灣亞熱帶山地雲霧森林動態之所需。

Subtropical montane cloud forest (SMCF) is a peculiar vegetation type, affected by the regular occurrence of dense clouds, which influences plant species due to high air humidity, lower light availability and air temperature, and chronic soil nutrient limitation. To understand the ecological processes behind the SMCF community in Taiwan, we established the Lalashan Forest Dynamics Plot (LFDP) in Chamaecyparis montane mixed cloud forest near the saddle between Lalashan and Tamanshan, inside the Chatianshan Nature Reserve, northern Taiwan (24°42ʹ N, 121°26ʹ E). The 1-ha plot was established in July 2019, and in August 2020, we finished the first census of all woody species with a diameter at breast height ≥ 1 cm. Each individual was identified, tagged, mapped, and its diameter at breast height (DBH) was measured. We collected environmental factors related to the topography within each of the 100 10 m × 10 m subplots and soil properties within selected 25 10 m × 10 m subplots. In total, we recorded 5220 individuals belonging to 65 species, 42 genera and 29 families, with a basal area (BA) of 69.1 m2/ha. The forest is dominated by Chamaecyparis obtusa var. formosana (14% of importance value index, IVI), Rhododendron formosanum (14%), Quercus sessilifolia (9%), Trochodendron aralioides (7%) and Eurya crenatifolia (5%), and the cumulative IVI of these five most dominant species reached 49%. We applied modified two-way indicator species analysis (TWINSPAN) to classify the vegetation into three vegetation types at subplot-level, and tested differences in environmental conditions between these types by ANOVA. The ridge type (Daphniphyllum himalayense subsp. macropodum-Chamaecyparis obtusa var. formosana type, 74 subplots) is the main vegetation type of LFDP, mainly distributed on the wide ridge in the west and middle part of LFDP, with relatively flat topography and soils with lower pH, with higher mean DBH and lower density of individuals. The east-facing slope type (Pourthiaea villosa var. parvifolia-Rhododendron formosanum type, 20 subplots) is mainly distributed in the eastern part of the plot on the steeper windward slopes facing the northeast monsoon, with soils of higher soil pH, with low mean DBH, higher density of individuals and higher species richness. The valley type (Hydrangea angustipetala-Eurya crenatifolia type, 6 subplots) is rather rare, distributed in the ephemeral streams in the western part of LFDP, in steeper slopes and concave shapes with high soil rockiness and soil pH, and with the lowest density of individuals. To uncover the main gradients in species composition, we used detrended correspondence analysis (DCA) with passively projected topographical and soil environmental factors. The results of DCA showed that the vegetation of the plot is structured along two main compositional axes, the first related positively to elevation, soil chemical properties including C/N ratio, available P, Mg, and Zn, and negatively to slope and soil pH, while the second related positively to soil rockiness, and negatively to windwardness, convexity and stabilization factor of decomposition. The main changes in species composition are between steeper, windward slopes with less acid soils, and flatter higher ridges with more acid soils. The second main changes are between more convex types of topography and concave valley types with rockier soils. This study provides baseline data about the distribution of woody species and relevant environmental conditions in LFDP, which can be used as references for future resurveys, and analyses for monitoring the dynamics of SMCF in Taiwan.