Relationships of environmental factors and plant traits to fog frequency gradient in cloud forest of northern Taiwan

Master thesis at the Institute of Ecology and Evolutionary Biology, National Taiwan University (2023)

https://doi.org/10.6342/NTU202300700

Author: Lin, Shu-I (林書逸)

Advisor: David Zelený

Abstract:

Frequent fog occurrence shapes special environmental conditions and unique vegetation in the montane cloud forest (MCF). As MCF is one of the rarest and vulnerable forest types in the world, understanding how fog affects the plant community is an urgent task for MCF conservation under ongoing climate change. In this study, we aim to clarify the relationship between fog frequency and environmental factors and evaluate the effect of fog on plants’ functional trait variations. We set up a transect along the fog frequency gradient in northeastern Taiwan, by confining our sampling plots between 1850 and 2150 m asl in the winter monsoon-affected MCF. The transect includes nine localities along an increasing fog frequency gradient, with three 20 m × 20 m plots established in each locality. In each plot, we collected various climatic and soil properties. Also, all woody species with a diameter at breast height > 1 cm were identified, measured, and recorded. Five leaves’ and one branch’s functional traits were measured for up to three individuals of each species in each plot, namely leaf thickness, leaf chlorophyll content, specific leaf area (SLA), leaf dry-matter content (LDMC), equivalent water thickness (EWT), and wood density. First, we used Pearson’s correlation to clarify the fog-environment relationships (temperature, relative humidity, precipitation, soil nutrient, and freezing event). Second, we used simple linear regression to test the relationship between fog frequency and six community-level plant functional traits calculated by the community-weighted mean (CWM) method. In addition, we used three types of CWMs to separate the functional trait changes caused by the turnover of species composition (inter-specific) and the changes caused by species adaptation to the environment (intra-specific).

For the relationship between fog frequency and other environmental factors, we found that higher precipitation may contribute to the stronger leaching effect, which can lead to lower soil phosphorus content in foggier sites. Also, in this winter-monsoon-influenced cloud forest, foggier sites have a longer time under freezing temperatures (-2.22˚C). These environmental relationships could explain the fog-trait pattern in which intra-specific CWM of SLA is lower and intra-specific CWM of LDMC is higher in foggier sites. Also, owing to the unbalanced freezing stress on leaves between evergreen and deciduous species in winter, deciduous species (with high SLA) aggregate more in foggier sites. The phenomenon may lead to higher inter-specific SLA CWM observed in foggier sites.

中文摘要

霧林帶因頻繁起霧的現象而擁有獨特的環境條件,加上分佈侷限且面積狹窄,在面臨氣候變遷與開發行為下顯得脆弱易危,了解起霧影響森林之機制以幫助保育更顯其重要性。本實驗欲釐清起霧頻率與其他環境因子的關聯性,並且探討起霧對於木本植物功能形質(Functional trait)之可能影響,故於台灣東北部受季風影響的霧林帶中,在限制海拔於1850公尺至2150公尺內沿著雲霧頻度進行取樣,選擇九個區域內各自劃設三個邊長20公尺的方形樣區,收集氣候與土壤相關因子,同時調查胸徑一公分以上之木本植物物種與胸高直徑,並且在各樣區的每個物種中挑選三個個體,採集五片葉片與一枝條,測量其葉厚度(Lth)、葉綠素濃度(CHL)、比葉面積(SLA)、葉乾重含量(LDMC)、等效水厚度(EWT)與枝條密度(WD)。首先,我們使用皮爾森積動差相關係數討論霧與其他環境因子(溫度、濕度、降雨量、土壤養分與結凍條件)之關係,而後利用簡單線性回歸來探討霧與六種功能形質的群集加權平均(CWM)正負相關性。另外,我們還使用三種群集加權平均來分離物種組成(inter-specific)與種內適應(intra-specific)形塑植群功能形質差異的貢獻。

在起霧與其他環境因子的相關性分析中,我們推測高降雨量帶來的高土壤磷淋洗,可能是造成土壤磷含量於起霧頻率高的樣區較低的原因。並且我們也發現在起霧頻率高的樣區,在冬天時處於結凍溫度(攝氏-2.22度)的時間較長。上述兩個現象都可以解釋在種內適應的面向上,物種於起霧頻率高的樣區傾向於製造低SLA、高LDMC的葉子。與此同時,由於冬天時的結凍溫度施於常綠樹種的逆境壓力較落葉樹種大,可能讓落葉樹生長之優勢增加,使得在起霧頻率高的樣區在物種組成上有比例稍微高的落葉樹種,進而提高SLA在物種組成上群集加權平均。

關鍵字:群落加權平均、雲霧頻度、結凍、功能形質、山地霧林帶、比葉面積、葉乾物質含量