Zhou Zaixing, Ollinger Scott V, Lepine Lucie
Earth Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, 03824, USA.
Oecologia. 2018 Oct;188(2):595-606. doi: 10.1007/s00442-018-4223-2. Epub 2018 Jul 12.
Canopy nitrogen (N) is a key factor regulating carbon cycling in forest ecosystems through linkages among foliar N and photosynthesis, decomposition, and N cycling. This analysis examined landscape variation in canopy nitrogen and carbon assimilation in a temperate mixed forest surrounding Harvard Forest in central Massachusetts, USA by integration of canopy nitrogen mapping with ecosystem modeling, and spatial data from soils, stand characteristics and disturbance history. Canopy %N was mapped using high spectral resolution remote sensing from NASA's AVIRIS (Airborne Visible/Infrared Imaging Spectrometer) instrument and linked to an ecosystem model, PnET-II, to estimate gross primary productivity (GPP). Predicted GPP was validated with estimates derived from eddy covariance towers. Estimated canopy %N ranged from 0.5 to 2.9% with a mean of 1.75% across the study region. Predicted GPP ranged from 797 to 1622 g C m year with a mean of 1324 g C m year. The prediction that spatial patterns in forest growth are associated with spatial patterns in estimated canopy %N was supported by a strong, positive relationship between field-measured canopy %N and aboveground net primary production. Estimated canopy %N and GPP were related to forest composition, land-use history, and soil drainage. At the landscape scale, PnET-II GPP was compared with predicted GPP from the BigFoot project and from NASA's MODIS (Moderate Resolution Imaging Spectroradiometer) data products. Estimated canopy %N explained much of the difference between MODIS GPP and PnET-II GPP, suggesting that global MODIS GPP estimates may be improved if broad-scale estimates of foliar N were available.
冠层氮素是通过叶片氮素与光合作用、分解作用以及氮循环之间的联系来调节森林生态系统碳循环的关键因素。本分析通过将冠层氮素制图与生态系统建模以及来自土壤、林分特征和干扰历史的空间数据相结合，研究了美国马萨诸塞州中部哈佛森林周边温带混交林中冠层氮素和碳同化的景观变化。利用美国国家航空航天局（NASA）的机载可见/红外成像光谱仪（AVIRIS）的高光谱分辨率遥感数据绘制冠层氮含量百分比图，并将其与生态系统模型PnET-II相链接，以估算总初级生产力（GPP）。用涡度协方差塔的估算值对预测的GPP进行验证。研究区域内估算的冠层氮含量百分比范围为0.5%至2.9%，平均值为1.75%。预测的GPP范围为797至1622克碳/平方米·年，平均值为1324克碳/平方米·年。实测冠层氮含量百分比与地上净初级生产力之间的强正相关关系支持了森林生长的空间格局与估算的冠层氮含量百分比的空间格局相关的预测。估算的冠层氮含量和GPP与森林组成、土地利用历史和土壤排水有关。在景观尺度上，将PnET-II GPP与BigFoot项目以及NASA的中分辨率成像光谱仪（MODIS）数据产品预测的GPP进行了比较。估算的冠层氮含量解释了MODIS GPP和PnET-II GPP之间的大部分差异，这表明如果能获得叶氮的大范围估算值，全球MODIS GPP估算值可能会得到改善。
