利用掃描器法調查臺灣中部孟宗竹林及柳杉林之細根動態

Abstract

細根是植物吸收水分、養分的重要構造，扮演著碳在植物與土壤間流動的重要路徑。細根生物量、生長量及死亡量具有時間與空間變異的特性並容易受到環境與生物因子的影響，在各種森林中的研究有助於瞭解地下碳循環對於周圍環境改變的反應。近年來孟宗竹林擴張至周圍森林(例如：柳杉林)的現象可能會影響區域碳循環，已受到亞洲國家關注。但過往研究多僅侷限於地上部，尚缺乏對於細根動態的研究，可歸因於野外調查困難度高。近年亦發展出掃描器法，能夠以較大的視野觀察細根，因此被認為具有潛力能夠應用在野外調查，但仍缺少研究以證實其適用性。 本研究目的為發展掃描器觀測細根動態的方法，並調查臺灣中部孟宗竹林及柳杉林的細根生物量、生長量、死亡分解量的時間與空間變異特性及其影響因子，以瞭解這兩種森林地下碳循環的差異。此研究分別討論：一、發展掃描器法於孟宗竹林及柳杉林的應用方法；二、瞭解孟宗竹林及柳杉林細根生物量、生長量、死亡分解量的時間變異與空間變異特性；三、檢測細根生物量、生長量、死亡分解量的時間變異與空間變異影響因子；四、推估細根生物量、年生長量、年死亡分解量及置換速率；五、瞭解細根動態對地下碳動態(例如：土壤呼吸速率) 於時間變異上的影響。 此研究於2015年12月在溪頭實驗林的孟宗竹林長期樣區埋設六個壓克力觀察箱，此樣區會於每年5月進行竹桿位置的量測。2016年1月和2月沿著兩棵柳杉的兩個不同方向分別在距離樹幹1公尺及2公尺處共埋設八個壓克力觀察箱。在壓克力觀察箱埋設後開始至2017年3月，每個月一至二次以掃描器法收集影像並同時測量相關的影響因子(土壤溫度、土壤濕度、葉面積指數)及土壤呼吸速率。在進行野外試驗之前，透過標準化影像分析過程，八位人員分析同組影像以評量人員的影像分析能力差異；結果顯示人為誤差僅約10 %。亦透過影像中的像素數量與細根乾重的關係建立迴歸式，可將掃描器獲得的影像資料由2-D資訊轉換成根生物量。 在測量期間，孟宗竹林、柳杉林細根生物量約在壓克力箱埋設後7個月趨於穩定。孟宗竹林細根生長量及死亡分解量高峰時間較柳杉林早發生，相對高的時期也持續較久；而柳杉林中地被植物細根動態與柳杉的細根動態相似。此外，兩種森林的細根生長量與死亡分解量的高峰時間及相對高的時期於各地點不太一致，且細根並非均勻地分布在各個測量地點。兩種森林的垂直空間分布，細根生物量、年生長及死亡分解量在上層土壤 (0-10公分深處) 及下層土壤 (10-20公分深處) 的分布皆無明顯差異。在柳杉林中，距離柳杉樹幹的遠近對細根生物量、年生長量及年死亡分解量的分布沒有影響。 時間變異上，土壤溫度與孟宗竹林細根生物量、年生長及死亡分解量呈現顯著正相關，但與柳杉林細根生物量則是顯著負相關。土壤濕度與葉面積指數並無發現明顯關係。林分結構在特定距離與細根生物量的空間分布負相關性雖然不高，但細根生物量與細根生長、死亡分解量有顯著正相關，因此仍可能對細根生物量的空間變異產生影響。 本研究中細根生物量與細根年生長、死亡分解量及置換速率分別為孟宗竹林 (290 g m-2, 429 g m-2 yr-1, 147 g m-2 yr-1, 1.48 times yr-1)；柳杉林 (126 g m-2, 151 g m-2 yr-1, 25 g m-2 yr-1, 1.20 times yr-1)，地被植物細根生物量、年生長及死亡分解量的貢獻約佔50%。孟宗竹林細根生物量與細根年生長、死亡分解量及置換速率都高於柳杉林，此種趨勢與前人研究之結果相符。 孟宗竹林細根生物量與土壤呼吸速率在時間變異上顯著正相關，因此細根生物量上升暗示著可能會對土壤呼吸速率有一定程度影響，但此趨勢在柳杉林中較不明顯。 總體來說，孟宗竹林相對於柳杉林輸入更多碳至細根中，此獨特的特性可能使其細根動態對土壤呼吸速率在時間變異上有顯著的影響。

Fine roots are responsible for resource acquisition (water, nutrients) in plants, which play important role in belowground carbon (C) cycle. As fine roots baiomass, production, and decompositon vary with season and locations, studying fine root dynamics in various vegetation types is crucial to know how belowground C cycle responsed to local environment. In recent years, invasion of Moso bamboo to surrounding forests (e.x., Japanese cedar) had been noticed in Asian countries, which might change belowground C cycle. However, few studies investigated fine root dynbamics in Moso bamboos resulting from difficulty of measurements. Recently, a new method, optical scanner method had been developed, which might have high applicability with larg viewing area. On the other hand, still few studies tested the method in forests. Hence, the aims of this study were 1) to develop methodology of scanner method in Moso bamboo (MBF) and Japanese cedar (JCF) forests, 2) to clarify the temporal and spatial variation of fine root dynamics, 3) to examine factors determining temporal and spatial variation of fine root dynamics, 4) to estimate fine root biomass (FRB), annual fine root production (AFRP), annual fine root decomposition (AFRD), and turnover (FTR), and 5) to understand the effect of fine root dynamics on belowground C cycle such as soil respiration (Rs). The 6 scan boxes were installed at December 2015 in a long-term study plot of MBF, in which each culm position was recorded every year. The 8 scan boxes were installed at January and February 2016 around two Japanese cedar trees with the distance of 1 and 2 m from each target tree along the two directions. After the box installation, fine root measurements with the scanner method were conducted immediatedly in both stands until March 2017. The controlling factors including soil temperature, soil water content, and leaf area indices were measured concurrently with scan image acquisition; soil respiration rate was also measured concurrently. Before the field measurements, the degree of human error in manual root extraction from scan images were tested by conducting the manual root tracking by 8 observers based on a standardized image processing procedure. This study confirmed human error of root extraction could be around 10 %. Also, to transform scanned pixel data to root biomass, the relationships were derived from projected root area and the corresponding dry biomass. During this study period, FRB had been stabilized approximately 7 months later at both stands since the box installation. FRP and FRD of MBF peaked earlier than those of JCF. Besides, the durations of high FRP and FRD values in MBF was longer than those of JCF. In JCF, the temporal variations in fine root dynamics of understory plant was almost same as that of tree roots. Moreover, we found the significant spatial variation of fine root dynamics at both stands, indicating the peak timing and duration with high values were not similar among locations. The vertical distribution of FRB, AFRP and AFRD were not distinctive in the comparisons between the upper (0-10 cm) and lower (10-20 cm) soil layer in MBF and JCF. In JCF, distances from the target trees did not affect FRB, AFRP, and AFRD. Ts was the main controlling factor of fine root dynamics which was positively correlated with temporal variations in FRB, FRP and FRD of MBF, although Ts was negatively correlated with FRB of JCF. Mostly, we found no relationships between SWC, LAI, and fine root dynamics. In the spatial variations, stand structure within specific distance and FRB in MBF showed weak negative correlation. Further, FRB in MBF was spatially correlated with FRP and FRD, suggesting the stand structure might affect the spatial variation of FRB, FRP and FRD in MBF. FRB, AFRP, AFRD and FTR were 290 g m-2, 429 g m-2yr-1, 147 g m-2 yr-1, 1.48 times yr-1 in MBF, respectively. In contrast, FRB, AFRP, AFRD and FTR were 126 g m-2, 151 g m-2 yr-1, 25 g m-2 yr-1, 1.20 times yr-1 in JCF. Understory plant roots accounted for about 50% of FRB, AFRP, AFRD in JCF. MBF had larger FRB, AFRP, AFRD and FTR than those of JCF in this study. The tendency was also confirmed in previous studies. This study also indicated Rs was positively correlated with the temporal variation of FRB in MBF but not found obvious relationship in JCF. Therefore, it seems that fine root dynamics may affect the temporal pattern of Rs in MBF with large FRB, AFRP, and AFRD than JCF. MBF invested much C to root biomass than that of JCF. The unique characteristics might lead to strong impact of fine root dynamics on the temporal variation of Rs.