臺灣溪頭孟宗竹林之異戊二烯放出特性

Abstract

異戊二烯(2-甲基-1,3-丁二烯)為一種揮發性有機化合物(Volatile organic compound, VOC，該物質因會間接地造成空氣汙染和強化溫室效應而被關注。在過去研究中指出植物會為減緩熱傷害和暴露於臭氧造成的傷害等目的而製造並放出異戊二烯。因植物放出之異戊二烯總量極大，對於放出量及速度的估計十分重要。過去研究也指出，植物的異戊二烯放出速率受到許多外在環境因子(例如，溫度和光強度)和生理因子(例如，一樹冠層中的位置)。在過去，已有相當多對於植物異戊二烯放出的模型，卻多無考慮竹類之特性，一概以樹木或草地論之；但竹類為東亞地區最重要的森林組成之一，尤其孟宗竹因近年有快速擴張與入侵其他森林而受關注，基於其重要性，應要有針對之研究。因此，本研究以：一、確認孟宗竹是否有異戊二烯放出的能力；二、瞭解孟宗竹林的異戊二烯放出的空間變異；三、瞭解孟宗竹林的異戊二烯放出的時間變異；四、基於實驗資料製作適用於孟宗竹的放出速率模型。本研究利用葉箱法進行異戊二烯採樣。實驗在台灣中部，南投縣溪頭實驗林的竹類標本園及鄰近竹蘆的孟宗竹林進行。在2014年11月及12月，對14種竹類進行採樣；其中，綠竹(B. oldhami)、孟宗竹(P. Edulis)和石竹(P. lithophila Hayata) 有可觀的異戊二烯放出速率(32.02, 23.20 and 38.30 nmol m-2 s-1)；麻竹屬(Dendrocalamus)和剛竹屬(Phyllostachys)在實驗中的全部物種皆有放出能力；在實驗中，斑葉紅寒竹(C. marmorea cv. Variegata)、業平竹(S. fastuosa)、暹羅竹(T. siamensis)和玉山箭竹(Yushania niitakayamensis)沒有偵測到放出。在2015年11月、12月及隔年3月，本研究對不同垂直位置上的放出速率進行測試；然而，在7個實驗個體中，僅有一個個體在樹冠層底部與頂部的放出速率有統計顯著差異(P= 0.041)；但若在修除個體差異後將7個個體的資料混和測試的話，不同高度是有顯著的放出速率差異的(P= 0.0052)，且由低處到高處放出速率逐漸上升。在2015年9月到2016年3月間測試異戊二烯放出速率隨光度的變化和月間變異，本研究發現每個月放出速率都有類似的趨勢隨著光強度增加而上升並在一定的光強度後達到放出速率的飽和狀態，但2015年7月到11月的放出速率明顯高於12月到隔年3月；在每個月光強度一致(PPFD = 1000 mol m-2 s-1)的測量中，平均放出速率大致與平均葉溫的趨勢相同，但在2016年2月和3月出現不一致，故可能有其他影響因子造成的季節變異存在。另外，本研究利用月間測量的資料，製作考慮葉溫、光強度影響的孟宗竹異戊二烯放出速率模型；其中，以Gaussian分布來模擬葉溫影響並以Guenther等人(1993)描述的光強度影響分布來模擬光強度影響有較好的表現；此模型在從2015年9月到2016年3月每個月的RMSE分別為38.79、31.26、86.24、46.24、44.16、60.89和62.53。整體來說，本研究建立了孟宗竹林樹冠規模的異戊二烯放出估計方法的基礎，並建議為準確估計全年樹冠規模的異戊二烯放出量，在葉溫和光強度以外需考慮樹冠高層間變異和物候和生理因子的影響。

Isoprene (2-methyl-1, 3-butadiene), which is known as a volatile organic compound (VOC), has strong impacts on air pollution and global warming. Former studies indicated that plant can emit isoprene for multiple purposes including enhancing thermotolerance and preventing ozone-exposing damages. According to former estimations, the isoprene emission amount from plants was enormous, suggesting the importance of estimating fluxes of isoprene emission from plants. As well, former studies indicated that environmental factors (e.g., leaf temperature, light intensity) and physiological factors (e.g., position in the canopy) can affect isoprene emissions. Previously, several studies proposed models for estimating isoprene emission from plants, yet they did not consider bamboos. Nevertheless, moso bamboo is one of the dominant species in eastern Asia, currently showing rapid expansion and invasion into other forests. Hence, the objectives of this study were 1) to identify the ability of isoprene emission in moso bamboos and then to clarify 2) spatial and 3) temporal variations in isoprene emission in a moso maboo forest. Also, 4) this study developed a model reproducing the temporal changes in isoprene emission from the bamboos based on the measurements. This study conducted isoprene measurements based on a leaf chamber method in a bamboo specimen garden and a bamboo forest in Xitou Experimental Forest, central Taiwan. First, by checking 14 species of bamboo in November and December 2014, this study revealed that B. oldhami, P. Edulis and P. lithophila Hayata had significant isoprene emission which were about 32.02, 23.20 and 38.30 nmol m-2 s-1, respectively. All species of Dendrocalamus and Phyllostachys showed isoprene emission detected, but the isoprene emissions were not detected in C. marmorea cv. Variegata, S. fastuosa, T. siamensis and Yushania niitakayamensis. As the result, this study confirmed significance of isoprene emission in moso bamboos. Second, this study examined the pattern of vertical variations in isoprene emission within canopy under the standardized environmental conditions, and only one individual showed significant difference in isoprene emission rates between canopy top and bottom (P= 0.041) if we test the significance in each individual; however, if we consider total seven individuals measured, that is, canopy top and bottom tended to show higher and lower isoprene emission rates, respectively (P= 0.0052). Third, by measuring isoprene emission rate under fixed light intensity levels, the seasonal variation in isoprene emission during September 2015 to March 2016 increased with light intensity differently between months; where December 2015, January, February and March 2016 have lower emission at the given light level than those of September, October and November 2015. The seasonal variation in isoprene emission rates at the same light intensity (PPFD = 1000 mol m-2 s-1) generally corresponded to that of leaf temperature, although some discrepancy was found in February and March 2016, suggesting that there were other factors affecting the seasonal variation. Fourth, to develop a model, this study considered the effect of leaf temperature and light intensity, and better performances found in using the function based on Gaussian distribution for leaf temperature and the function for light-intensity proposed by Guenther et al. (1993) in the fitness to the measurements. The RMSE of each month in the model were 38.79, 31.26, 86.24, 46.24, 44.16, 60.89 and 62.53 in September 2015 to March 2016, respectively. Overall, this study established a foundation of estimating total amount of canopy-scale isoprene emission in the moso bamboo forest. For precise estimation, an isoprene emission model for annual canopy-scale should consider not only the effect of leaf temperature and light intensity but also variations in potential emission rates within canopy and phenological and physiological effects in spring.