於台灣中低海拔森林檢驗葉面積指數間接量測法的測量表現

葉面積指數是重要的生態參數，葉面積多寡影響生態系的初級生產力、能量的流轉與水循環等重要生態過程。因此，測量與描述生態系的葉面積指數及其時空變動，有助於了解生態系的特性、結構、功能、抵抗力與回復力。量測葉面積指數的方法可分為直接量測法與間接量測法。直接量測法較為準確但操作費時而不易在大空間範圍施作；間接量測法乃透過偵測光的削減、林冠孔隙率等方式估算葉面積，操作較為快速簡便，但其運算過程中有許多假定與現實有一定落差，因此需以直接量測法校正或驗證。已有許多研究於溫帶檢驗葉面積指數間接量測法中的植冠分析儀與半球面影像的測量表現，多數前人研究指出兩者容易低估葉面積，但測量結果與實際值間具有強烈線性關係。然而少有研究於熱帶與亞熱帶進行。因此這兩種儀器於台灣的森林環境的誤差情形仍有待探討。本研究於奧萬大設置落葉陷阱檢驗植冠分析儀與半球面影像的準確性，並於蓮華池試驗中心之檳榔林、肖楠林、香杉林、天然闊葉林檢驗兩種儀器是否能反映葉面積於生長季增加之趨勢。研究結果顯示台灣中低海拔、枝幹較密集、冠層較密閉的森林中，兩種儀器皆會低估葉面積，但植冠分析儀的低估情形較不嚴重且較半球面影像更能反應葉面積於時間上的變化。但冠層較開闊時，半球面影像可能有更好的表現，但會高估葉面積。除上述測量表現外，兩種儀器實際使用、分析時可能面臨不同的問題或限制，如天氣、放置位置、分析者等，這些也應納入考量來選擇最適合的儀器。

關鍵字

葉面積指數；半球面影像；植冠分析儀；亞熱帶森林

並列摘要

Leaf area index (LAI) is an important ecological parameter. Leaf area affects various important ecological processes, such as primary productivity, energy flow, and water cycle. Therefore, measuring LAI and its dynamics help our understanding of the features, structure, function, recovery, and resistance of ecosystems. Direct and indirect methods are widely used to measure LAI. Direct methods are accurate but are time consuming and spatially limited. With some inherent assumptions, indirect methods estimate LAI through light transmittance or gap fractions and can be operated at broad scales. However, the deviation from the assumptions varies among tree species or ecosystems. Thus, it is important to test the validity of indirect methods. Numerous studies have tested the validity of plant canopy analyzer (PCA) and hemispherical photography in estimating LAI. However, most of the studies were conducted in temperate region, which has very different forest composition from those in the subtropics. Thus, it remains unclear if the results of these studies can be directly applied to subtropical Taiwan. This study use direct method - litter trap to estimate the LAI and compare it to the estimates made by PCA and hemispherical photography and thereby test their accuracy. How the two types of equipment perform at different leaf area levels area are also important in the applications. Thus we measure leaf area index in four different forests from March to July, the growing season, in Taiwan to examine the viability of the two types of equipment by checking the trend of increase in leaf area estimated by PCA and hemispherical photography. The result shows that in areas with high leaf area such as low and mid altitude forest in Taiwan, both types of equipment tend to underestimate LAI. However, compare to hemispherical photography, PCA is more accurate and can better capture the temporal changes in LAI. In contrast, in areas with low leaf area, the performance of hemispherical photography is better, although it overestimated LAI. In addition to the patterns described above, the performance of PCA and hemispherical photography is although affected by weather, the location of positioning of the equipment and the personal bias of analyzers. Researchers should be aware of the limitation of both types of equipment in their application.