臺灣南部南仁山受季風影響森林中小苗的組成及草本層的促進作用

Seedling Composition and Facilitative Effects of the Herbaceous Layer in a Monsoon-Affected Forest in Nanjenshan, Southern Taiwan

木本小苗對於森林的更新扮演著重要的角色。為了瞭解影響木本小苗建立的主要因子，我們於臺灣南部南仁山一處受風影響的森林中進行調查，（1）比較其林下木本小苗與上層成樹的組成分布，（2）檢視影響木本小苗組成的環境因子以及（3）評估木本小苗與草本層間是否有競爭或促進的關係。我們於試驗地中建立了三條樣帶共計180個5×5公尺的樣方；並進行樣帶中所有冠層、次冠層及灌木種類的小苗（具有真葉且胸徑小於1公分）數量及草本層（表含草本植物、爬藤及小於1米的樹蕨）覆蓋度的調查。支序分析及序列分析被使用來顯示林下小苗組成的分群情況及小苗組成與環境因子的相關。各物種和類別組合的草本覆蓋度及小苗數量可以計算其相關係數，以用來評估草本層及木本小苗間的相互作用。木本小苗組成的空間分布與過去研究中所呈現的上層成樹組成分布十分吻合。木本小苗組成與草本層組成及地形特徵有強烈關聯，尤其是對季風暴露程度。由於缺乏木本小苗及草本層結構/物種的競爭與促進關係。我們認為小苗組成與草本層組成的關聯，來自於彼此對季風平行的反應。總而言之，研究推論小於1公分胸徑的木本小苗已受到強烈的季風影響，呈現與上層相似的組成格局，而草本層各物種及結構對木本小苗直接的影響較弱。

Tree seedlings play an important role in forest regeneration. To understand the factors that control seedling establishment, we (1) compared the composition patterns of tree seedlings and their corresponding overstories, (2) examined the relationships between seedling composition and environmental factors and (3) evaluated the interaction (competition or facilitation) between seedlings and herbaceous layer in a wind-stressed forest in Nanjenshan, southern Taiwan. In the study plot, seedling abundance of canopy, subcanopy and shrub species (with true leaves and < 1 cm diameter at breast height) and coverage of herbaceous species (including herbaceous species, climbers and tree ferns ≤ ca. 1 m in height) were investigated on three transects with a total of 180 contiguous 5 × 5 m quadrats. Clustering classification and ordination methods were used to reveal the tree seedling composition patterns and the relationships between seedling composition and environmental factors. Correlation coefficients were computed between herbaceous coverage and seedling abundance among herb-seedling species pairs and between tall (≥ 1 m high)/short (< 0.5 m high) herbs and seedlings pairs to test the herb-seedling interaction. The spatial distribution of tree seedlings presented a perfect match to the overstory vegetation pattern. There was a strong relationship among seedling composition, herbaceous composition and topographic features, especially exposure to monsoon winds. Because of the absence of strong correlations between herbaceous structure/species and seedling abundances, the strong linkage in spatial patterns between seedling and herbaceous compositions suggests that certain plant species in the study plot have similar responses to the monsoon exposure. Our results also indicated that seedlings < 1 cm in diameter were strongly influenced by wind stress, similar to the response of the overstory composition, and that the facilitative/competitive effects of the herbaceous layer on tree seedlings were relatively weak.