臺灣南部南仁山下部山地常綠闊葉林林下層植群之研究

Abstract

林下植群為森林組成最多樣的一層，也是森林更新的重要一環。然而這一層卻僅少受到注目，對於亞熱帶季風地區來說也不例外。在這個研究中我將探討下列的問題：1. 在季風的影響下，森林各層之間是否會相互影響? (森林的上層如何影響下層的組成?) 2. 林下層對外在環境因子有何反應? 3. 控制木本小苗建立的主要因子是什麼? 本研究於台灣南部南仁山欖仁溪樣區進行取樣，在7條5 m寬300 m長的樣帶上進行林下常駐植物(無法長高一直待在林下的物種，包含高度小於1.5 m的草本、藤本植物、小型灌木及樹蕨)的調查，並選取其中的3條進行林下過渡植物(目前長在林下層但有潛力長超出此層的物種，即胸高直徑小於1 cm的冠層、次冠層及大型灌木的小苗)的調查。樣帶上調查到的林下層植物均被鑑定並分別5 m × 5 m的小樣方來記錄他們的覆蓋比例或是株數。調查資料1.先以空間自相關格局，初步探討影響林下植物數量可能的生物內部及外部環境因子；2. 再以支序分析結果比較林下與上層物種組成在空間上分布的情況；3. 應用直接梯度和空間模式檢查林下組成與外在環境因子的關係；4. 設定因果關係以路徑分析來定量林下覆蓋度受到季風直接與間接影響的程度；5. 利用相關分析來評估木本小苗與常駐植物的交互關係及6. 以回歸模式來探討母樹分布及環境因子對小苗數量的影響。 調查共記錄44科83屬94種的林下常駐植物及37科76屬102種共16,925株林下過渡植物。在50 m尺度下，98%林下物種有不同的空間自相關格局，18%物種180 m的尺度下出現顯著的空間自相關。根據各物種所表現的空間尺度，林下物種的空間格局受到自身特性(如：植株大小或具延伸性的組織)及外在地形特徵的影響。 林下常駐植物的組成沿著地形由迎風山頂到背風溪谷變化，且隨上層社會類型而改變；而林下過渡層的組成分布也與上層植物組成有一致的格局。分別進行林下及林上組成的直接梯度分析，顯示各環境因子對林下及林上物種組成有相似的影響。梯度分析結果顯示季風暴露度是林下常駐及過渡組成最重要的影響因子之一。然而林下光線指標-樹冠開闊度並未顯示出對林下層植物組成的重要性。因果關係模式中顯示季風暴露度是林下植群變異的決定因子。相較於樹冠開闊度，季風暴露度與蕨類覆蓋度有較高的相關係數。這顯示季風直接且強烈影響著蕨類覆蓋度，但季風透過改變樹冠結構而影響覆蓋度的路徑相對較弱。 在物種小苗對母樹分布及季風的反應上，許多物種在小苗數量迴歸模式中呈現對區域中母樹數量及季風暴露度有顯著相關。這顯示季風確實是亞熱帶林下木本小苗建立最重要的環境因子，而母樹也對小苗數量扮演了重要的角色。由於許多物種出現小苗與母樹數量的正相關，我認為欖仁溪多數的物種在傳播上有侷限的範圍，但沒有強烈的密度依賴效應。在與林下常駐植物的競爭上，由於缺乏物種與物種，以及結構與物種間的強烈負相關，且小苗數量與草本覆蓋度呈現負相關的物種中大多仍保持與母樹數量正向的關連，顯示草本植物的競爭效應並不強烈仍無法破壞母樹與小苗的分布關連。林下過渡植物與常駐植物在組成上的空間相關，來自於物種對外在環境因子的相似反應，而不是彼此相互影響。

Understory layer is the most diverse component of forest communities and understory woody seedlings play an important role in forest regeneration. However, this layer is least understood, not except for in subtropical region under monsoon climate. In this study, the following questions were asked: 1) under the influence of monsoon, do forest strata interact with each other? (how overstory vegetation affects the understory composition?) 2) how the understory vegetation reflect on exogenous factors? and 3) what are the factors that control tree seedling establishment? In order to better understand the ecological role of understory vegetation, resident species (including herbaceous species, climbers, small shrubs and tree ferns < ca.1.5 m high) were investigated on seven transects, each 5 m wide and 300 m long in a wind-stressed forest of Lanjenchi in Nanjenshan, southern Taiwan. Seedlings of Transient species (canopy, subcanopy and shrub species) with diameter at breast height < 1 cm were censused in three of the seven transects. Species of understory plants in transects were identified, and the percent coverage of residents or species abundance of transients was estimated for every 5 m × 5 m contiguous quadrat along transects. I detected possible intrinsic and extrinsic factors of understory abundance by detecting spatial autocorrelation patterns with Moran’s I. The composition patterns of understory plants and their corresponding overstories were compared with clustering analyses. The relationships between understory composition and environmental factors were examined with direct ordination and spatial models. The direct and indirect effects of monsoon on herb coverages were tested with path analysis based on a causal model. The interaction (competition or facilitation) between woody seedlings and resident cover was evaluated with correlation analysis. The contributions of parent tree distribution and environmental factors on seedling abundance were evaluated with regression models. A total of 94 resident species, belonging to 44 families and 83 genera were recorded. There were 16,925 transient individuals from 102 tree and shrub species, representing 37 families and 76 genera in seedling census. Most (98%) of understory species had significant autocorrelation in fine scale within 50 m. Eighteen % of species showed a significant autocorrelation at a 180m-class which correspond to the local replicates of topographical features in hilltops. According to the presence of spatial scales of understory species, it is suggested that the understory distribution patterns are influenced by species intrinsic traits (e.g. individual size and prolonged leaves and stems) and topographical characteristics in the plot. The resident compositions shifted with overstory community types along a topographical change, from windward ridges (northeast-facing slopes with strong effect of winter monsoon) to leeward creeks (southwest-facing slope with weak wind effect in winter). The spatial distribution of transient communities also presented a consistent pattern with the overstory composition. The separated direct ordination analyses showed that the understory resident and overstory compositions had very similar responses to every environmental vector, indicating similar habitat requirements of overstory and resident communities. It also showed that monsoon exposure (angle from zenith to skyline in northeast monsoon direction) was one of the most influential factors explaining spatial patterns of both resident and transient vegetations. However, canopy openness, a light availability index in understory, did not show significant correlation with the compositions/abundances of neither residents nor transients. A causal model indicated that monsoon exposure was the major determinants of understory vegetation variation. Path coefficient between monsoon exposure and fern coverage was higher than those between canopy openness/tree density and coverage of ferns which were considered as the indicators of wind effect. This suggested that monsoon might have a strong direct effect on fern coverage, but relatively weak indirect effect via altering canopy structure. Based on the abundance models of species seedlings, many species showed significant responses to parent abundance and monsoon exposure. This revealed that monsoon was the most important environmental factor for seedling establishment, and parent abundance also play an important role in shaping spatial patterns of seeding abundance. Owing to the positive correlations between local numbers of seedlings and parents for most species, it indicated that there was a short dispersal range but no strong density dependences for species in Lanjenchi. In the term of competitive effect of understory resident, strong negative correlations between resident structure/species and seedling abundances were absent. Among seedling species which abundances were negatively correlated with resident coverage, most of them exhibited positive correlation of abundance between seedling and parents. I concluded that the competitive effect of residents was not strong enough to attenuate seedling-parent co-occurrence. In summary, high similarity of spatial patterns between understory and overstory compositions should be attributed to that they both respond similarly to exogenous factors rather than interacted with each other.