半球面影像在森林生態研究的應用

Applications of Hemispherical Photographs in Studies of Forest Ecology

林登秋(Teng-Chiu Lin) ； 江智民(Jyh-Min Chiang)

《台灣林業科學》 17卷3期 (2002/09) Pp. 387-400

https://doi.org/10.7075/TJFS.200209.0387

摘要

林下半球面影像可為林冠結構留下永久記錄，因此在森林生態的研究上有很大的潛力。早期以人工分析影像相當耗時，近年來由於電腦影像分析技術的快速發展，使得利用半球面影像來估算林下光照及葉面積指數的應用研究日益普遍，更已推展至林冠受干擾後的回復、苗木生長、棲地品質、森林生產力及植物生理生態等的研究。由於半球影像所攝取的方位與影像範圍對所分析之光立地係數有重大影響，拍攝時鏡頭宜用自動水平架維持水平，並在架上加裝微弱光源（如發光二極體）以界定影像範圍。影像對比程度是影響分析精確度最關鍵的因素，利用天空狀態均質的陰天或日出前的清晨及日落後的黃昏拍攝影像，有助於獲得林冠枝葉與孔隙呈現高對比而較易精確分析的影像，此比事後利用軟體進行影像編修更為實際有效。此外分析者的穩定性，對結果亦有重大影響，只要透過對少量影像進行多次分析即可有效增加穩定性。林下光照多偏離常態分布並易出現空間自動相關，故不符合許多統計分析的基本假說，因此在資料處理時應對此有明確的說明處理。此外利用半球面影像所估測之光立地係數未考量實際天空狀態，因此在比較不同地點的資料時必需非常小心。

關鍵字

半球面影像 ； 林冠結構 ； 林下光照環境 ； 葉面積指數

並列摘要

Undercanopy hemispherical photographs provide permanent records of canopy structure, and as such have great potential for studies of forest canopy dynamics. Rapid computerized image analysis has facilitated studies of undercanopy light environments and the canopy leaf area index (LAI). Temporal and spatial variations in the undercanopy light environment and canopy LAI derived from hemispherical photographs have been used to study recovery rates of forest canopies after disturbance, seedling growth, habitat quality, forest productivity, and plant ecophysiology. The orientation and range of hemispherical photographs greatly affect estimates of light indices. A self-leveling mount equipped with an LED (light-emitting diode) should be used to level the hemispherical lens and to identify the range of the image. To obtain the best contrast possible, which is critical for image classification and the most important process in image analysis, photographs should be acquired either early in the morning just before sunrise, just after sunset but prior to dusk, or on overcast days. Obtaining images with high contrast can save time and increase the efficiency and accuracy of the image analysis. Consistency of individual analyzers is critical especially when photographs from different sites are to be compared; it could be improved by re-analyzing small numbers of photographs. Undercanopy light levels often show spatial autocorrelation and lack of independence; and therefore violate assumptions required for many statistical procedures; they must be treated explicitly. In addition, the light environment estimated from hemispherical photographs does not take into account sky conditions, so such data from different sites must be compared with care.

並列關鍵字

hemispherical photographs ； canopy structure ； undercanopy light environment ； leaf area index