Horizontal Variations in Photon Flux Density under a Chamaecyparis formosensis Matsum Plantation

[[abstract]]本研究主要目的為探討紅檜人工林內光合光子流密度之空間變異，以及不同冠層密度下透光率之變化，作為生態學及育林學相關研究之基礎。在棲蘭山區紅檜不同密度人工林內，就不同栽植密度區內設置2個樣區，株數分別為74與115株。每個樣區內各設置2條互相垂直的軌道，進行水平光度的取樣，取樣頻率為l秒l次，各樣點的水平間距為8cm，每月至少測量l次，從1998年10月到1999年6月止。雖然試驗地的天候變動劇烈，但是冠層下光度的頻度分布大多集中在弱光度區，高於50μmol m^(-2)s^(-1)的光度出現頻率偏低，但卻是林下累積光量最主要的來源，約佔林下總光量的76至59%。地理統計之半變異圖分析顯示，陰天時，林內無直射光源進入林床，冠層下方光度較為穩定，樣點間的相關性較高，各軌道的半變異量皆小於晴天時所得的結果。晴天時，第一樣區的光環境有較均質的變異，不因測量方向的不同而有不同的變異，亦即林下光環境的變異具有空間上的等方性，此區的可預測範圍約30cm；而在第二樣區中，則呈現出不同的結果，光度的變化梯度主要是沿著東西走向改變，兩軌道所得的結果呈現出強烈的空間異方性，軌道三的可預測範圍為70cm，但是軌道四受冠層結構影響，具有很大的碎塊效應，可預測範圍僅有16cm。陰天時，各軌道的可預測範圍均增大，且碎塊效應均下降，可預測範圍在500至l,500cm之間。在陰天時進行取樣，除可減少樣點的密度外，亦較不受測量方向的影響，所測得之林分平均透光率可以有較理想的推估品質。[[abstract]]The study aimed to investigate horizontal variations in photosynthetic photon flux density (PPFD) and transmittances under a Taiwan red false cypress (Chamaecyparis formosensis Matsum) canopy as the basis for ecological and silvicultural researches. We sampled two 0.04-ha plots according to tree densities, at 75 and 115 trees plot, respectively. In each plot there was a pair of perpendicular trams. The PPFD was sampled horizontally with moving quantum sensors (LI-190SZ) laid on the tram cable in 1-s intervals, and the distance between sampled points was 8 cm. We sampled at least 1 day per month from October 1998 to June 1999. The diurnal change in local weather conditions was acute, and most PPFD values under the canopy were lower than 50 μmol m^(-2)s^(-1). However, the few occurrences of higher PPFD values comprised a high proportion of the accumulated PPFD of from 59 to 76%. Horizontal variations in PPFD were analyzed by geostatistical technique of semivariogram construction. The results showed that PPFD under the canopy was more stable of cloudy days. The light environment of plot 1 on a sunny day appeared as space isotropy as contrasted to the space anisotropy of plot 2. The predicted range of plot 1 was 30 cm, which different between tram 3 and tram 4 of plot 2, at 70 and 16 cm, respectively. The predicable range was greater on cloudy days than on sunny days, and varied from 500 to 1,500 cm. Therefore, we obtained a higher quality of PPFD measurements on cloudy days