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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Energy, water, and carbon fluxes in a loblolly pine stand:Results from uniform and gappy canopy modelswith comparisons to eddy flux data
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Energy, water, and carbon fluxes in a loblolly pine stand:Results from uniform and gappy canopy modelswith comparisons to eddy flux data

机译：火炬松林分中的能量，水和碳通量：均匀且蓬松的冠层模型的结果，并与涡流数据进行比较

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This study investigates the impacts of canopy structure specification on modeling netradiation (R), latent heat flux (LE) and net photosynthesis (A) by coupling twocontrasting radiation transfer models with a two-leaf photosynthesis model for a maturingloblolly pine stand near Durham, North Carolina, USA. The first radiation transfer model isbased on a uniform canopy representation (UCR) that assumes leaves are randomlydistributed within the canopy, and the second radiation transfer model is based on a gappycanopy representation (GCR) in which leaves are clumped into individual crowns, therebyforming gaps between the crowns. To isolate the effects of canopy structure on modelresults, we used identical model parameters taken from the literature for both models.Canopy structure has great impact on energy distribution between the canopy and the forestfloor. Comparing the model results, UCR produced lower R_n,higher LE and higherA_nthanGCR. UCR intercepted more shortwave radiation inside the canopy, thus producing lessradiation absorption on the forest floor and in turn lower R. There is a higher degree ofnonlinearity between A estimated by UCR and by GCR than for LE. Most of the differencefor LE and A_nbetween UCR and GCR occurred around noon, when gaps between crownscan be seen from the direction of the incident sunbeam. Comparing with eddy-covariancemeasurements in the same loblolly pine stand from May to September 2001, based onseveral measures GCR provided more accurate estimates for R_n, LE andA_nthan UCR. Theimprovements when using GCR were much clearer when comparing the daytime trend ofLE and A_nfor the growing season. Sensitivity analysis showed that UCR produceshigher LE and A_nestimates than GCR for canopy cover ranging from 0.2 to 0.8. There is ahigh degree of nonlinearity in the relationship between UCR estimates for A_nand those ofGCR, particularly when canopy cover is low, and suggests that simple scaling of UCRparameters cannot compensate for differences between the two models. LE from UCR andGCR is also nonlinearly related when canopy cover is low, but the nonlinearity quicklydisappears as canopy cover increases, such that LE from UCR and GCR are linearly relatedand the relationship becomes stronger as canopy cover increases. These results suggestthe uniform canopy assumption can lead to underestimation of R_n, and overestimation ofLE and A. Given the potential in mapping regional scale forest canopy structure withhigh spatial resolution optical and Lidar remote sensing plotforms, it is possible to useGCR for up-scaling ecosystem processes from flux tower measurements to heterogeneouslandscapes, provided the heterogeneity is not too extreme to modify the flow dynamics.
机译：这项研究通过耦合两个相对的辐射传输模型与北叶达勒姆附近成熟的多叶松林的两叶光合作用模型耦合，研究了冠层结构规范对建模净辐射（R），潜热通量（LE）和净光合作用（A）的影响。美国卡罗来纳州。第一个辐射传输模型基于统一的树冠表示（UCR），假定叶片在树冠内随机分布，第二个辐射传输模型基于空子冠层表示（GCR），其中叶子聚集成单个冠，从而在两个树冠之间形成空隙。冠。为了隔离冠层结构对模型结果的影响，我们对两个模型使用了与文献相同的模型参数。冠层结构对冠层和林地之间的能量分配有很大影响。比较模型结果，UCR产生的R_n较低，LE较高，A_n高于GCR。 UCR在冠层内部拦截了更多的短波辐射，因此在林地上产生的辐射吸收较少，从而降低了R。UCR和GCR估算的A之间的非线性程度比LE高。当从入射光束的方向可以看到冠之间的缝隙时，UCR和GCR之间LE和A_n之间的大部分差异发生在中午左右。与2001年5月至2001年9月在同一块松木林中进行的涡度-协方差测量相比，GCR根据多项测量提供了比UCR更准确的R_n，LE和A_n估计值。比较生长季节的LE和A_n的白天趋势，使用GCR时的改进要明显得多。敏感性分析表明，对于树冠覆盖范围为0.2到0.8的情况，UCR比GCR产生更高的LE和A_nestimates。 A_n和GCR的UCR估计之间的关系存在高度非线性，尤其是在树冠覆盖率较低时，这表明UCR参数的简单缩放无法补偿两个模型之间的差异。当冠层覆盖率较低时，来自UCR和GCR的LE也是非线性相关的，但随着冠层覆盖率的增加，非线性会迅速消失，因此，来自UCR和GCR的LE呈线性相关，并且随着冠层覆盖率的增加，这种关系变得更强。这些结果表明，统一的冠层假设可能导致R_n的低估，以及LE和A的高估。鉴于在绘制具有高空间分辨率光学和Lidar遥感图形式的区域尺度森林冠层结构的潜力时，有可能使用GCR来扩大生态系统过程从通量塔测量到异质景观，前提是异质性不能太极端地改变流动动力学。

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