Vose James M., Swank Wayne T.
USDA Forest Service, Southeastern Forest Experiment Station, Coweeta Hydrologic Laboratory, Otto, NC 28763, USA.
Tree Physiol. 1990 Dec;7(1_2_3_4):125-134. doi: 10.1093/treephys/7.1-2-3-4.125.
We evaluated the ability of a portable light meter (Sunfleck Ceptometer, Decagon Devices, Pullman, WA, USA) to quantify seasonal photosynthetically active radiation (PAR, 400-700 nm) interception, projected stand leaf area index (LAI), and vertical LAI distribution in a 32-year-old eastern white pine (Pinus strobus L.) plantation. Canopy PAR transmittance measured with the ceptometer was converted to LAI with the Beer-Lambert Equation. The ceptometer was sensitive to changes in PAR transmittance resulting from foliage growth. Predicted stand LAI ranged from 3.5 in the dormant season to a maximum of 5.3 in late July. Predicted LAI values were within 9% of values determined from destructive sampling. Published canopy extinction coefficients (k) were inadequate for converting PAR transmittance data to stand LAI because a significant amount of PAR was intercepted by dead branches and stems below the forest canopy. Because of interception by dead branches and stems, we estimated k = 0.84, which is substantially higher than previously reported values. The ceptometer was also sensitive to seasonal changes in PAR transmittance within the canopy. However, in contrast to predictions based on the Beer-Lambert Law, the relationship between proportional PAR transmittance (Q(i)/Q(o)) and cumulative LAI within the canopy was linear. Thus, vertical LAI distribution was best estimated with a linear model, as opposed to the non-linear model assumed in the Beer-Lambert Equation. We hypothesize that the linear relationship was a result of a gap in the canopy which was not represented by the cumulative leaf area distribution estimation procedure.
我们评估了一台便携式光度计(Sunfleck Ceptometer,Decagon Devices公司,美国华盛顿州普尔曼)在一片32年生的东部白松(Pinus strobus L.)人工林中量化季节性光合有效辐射(PAR,400 - 700纳米)截获量、林分投影叶面积指数(LAI)以及垂直LAI分布的能力。用该光度计测量的冠层PAR透过率通过比尔 - 朗伯方程转换为LAI。该光度计对因叶片生长导致的PAR透过率变化敏感。预测的林分LAI范围从休眠季节的3.5到7月下旬的最大值5.3。预测的LAI值与通过破坏性采样确定的值相差在9%以内。已发表的冠层消光系数(k)不足以将PAR透过率数据转换为林分LAI,因为大量PAR被林冠层以下的枯枝和树干截获。由于枯枝和树干的截获,我们估计k = 0.84,这大大高于先前报道的值。该光度计对冠层内PAR透过率的季节性变化也很敏感。然而,与基于比尔 - 朗伯定律的预测相反,冠层内PAR透过率比例(Q(i)/Q(o))与累积LAI之间的关系是线性的。因此,垂直LAI分布最好用线性模型估计,而不是比尔 - 朗伯方程中假设的非线性模型。我们推测这种线性关系是由于冠层中存在一个间隙,而累积叶面积分布估计程序未体现这一点。