Kull Olevi, Koppel Andres, Noormets Asko
Institute of Ecology, Riia 181, EE2400 Tartu, Estonia.
Tree Physiol. 1998 Jan;18(1):45-51. doi: 10.1093/treephys/18.1.45.
Leaf nitrogen distribution pattern was studied four times during the growing season in a 2-year-old Salix viminalis L. and Salix dasyclados Wimm. plantation in Estonia. We measured the vertical distributions of leaf nitrogen concentration, dry mass, leaf area and light environment (as fractional transmission of diffuse irradiance, a(d)) in the canopy. The light-independent nitrogen pool was evaluated as the intercept of the leaf nitrogen concentration versus a(d) relationship, and the nondegradable nitrogen pool was evaluated as the nitrogen remaining in abscised leaves. A strong vertical gradient of mass-based leaf nitrogen concentration was detected at the beginning of the growing season, and decreased steadily during canopy development. This decline had at least three causes: (1) the amount of nitrogen in the foliage was larger at the beginning of the growing season than at the end of the growing season, probably because of pre-existing root systems; (2) with increasing leaf area index (LAI) during the growing season, the proportion of leaf nitrogen in total canopy nitrogen that could be redistributed (light-dependent nitrogen pool) decreased; and (3) the photosynthetic photon flux density gradient inside the canopy changed during the season, most probably because of changes in leaf area and leaf angle distributions. Total canopy nitrogen increased almost proportionally to LAI, whereas the light-dependent nitrogen pool had a maximum in August. Also, the proportion of the light-dependent nitrogen pool in the total canopy nitrogen decreased steadily from 65.2% in June to 17.2% in September in S. dasyclados and from 63.3 to 15.1% in S. viminalis. The degradable nitrogen pool was always bigger than the light-dependent nitrogen pool.
在爱沙尼亚一片两年生的柳条柳（Salix viminalis L.）和绵毛柳（Salix dasyclados Wimm.）人工林中，在生长季节对叶片氮分布模式进行了四次研究。我们测量了冠层中叶片氮浓度、干质量、叶面积和光环境（作为漫射辐照度的分数透过率，a(d)）的垂直分布。与光无关的氮库被评估为叶片氮浓度与a(d)关系的截距，不可降解的氮库被评估为脱落叶片中剩余的氮。在生长季节开始时检测到基于质量的叶片氮浓度有很强的垂直梯度，并且在冠层发育过程中稳步下降。这种下降至少有三个原因：（1）生长季节开始时叶片中的氮含量比生长季节结束时大，可能是因为预先存在的根系；（2）随着生长季节中叶面积指数（LAI）的增加，冠层总氮中可重新分配的叶片氮比例（与光有关的氮库）下降；（3）冠层内光合光子通量密度梯度在季节中发生变化，很可能是由于叶面积和叶角分布的变化。冠层总氮几乎与叶面积指数成比例增加，而与光有关的氮库在8月达到最大值。此外，在绵毛柳中，与光有关的氮库在冠层总氮中的比例从6月的65.2%稳步下降到9月的17.2%，在柳条柳中从63.3%下降到15.1%。可降解氮库总是大于与光有关的氮库。
