Anten N P R, Werger M J A
Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 800.84, 3508 TB, Utrecht, The Netherlands.
Oecologia. 1996 Jan;105(1):30-37. doi: 10.1007/BF00328788.
The objective was to investigate how nitrogen allocation patterns in plants are affected by their vertical position in the vegetation (i.e. being either dominant or subordinate). A garden experiment was carried out with Amaranthus dubius L., grown from seed, in dense stands in which a size hierarchy of nearly equally aged individuals had developed. A small number of dominant plants had most of their leaf area in the highest layers of the canopy while a larger number of subordinate plants grew in the shade of their dominant neighbours. Canopy structure, vertical patterns of leaf nitrogen distribution and leaf photosynthetic characteristics were determined in both dominant and subordinate plants. The light distribution in the stands was also measured. Average N contents per unit leaf area (total canopy nitrogen divided by the total leaf area) were higher in the dominant than in the subordinate plants and this was explained by the higher average MPA (leaf dry mass per unit area) of the dominant plants. However, when expressed on a weight basis, average N contents (LNC; total canopy N divided by the total dry weight of leaves) were higher in the subordinate plants. It is possible that these higher LNC values reflect an imbalance between carbon and nitrogen assimilation with N uptake exceeding its metabolic requirement. Leaf N content per unit area decreased more strongly with decreasing relative photon flux density in the dominant than in the subordinate plants showing that this distribution pattern can be different for plants which occupy different positions in the light gradient in the canopy. The amount of N which is reallocated from the oldest to the younger, more illuminated leaves higher up in the vegetation may depend on the sink strength of the younger leaves for nitrogen. In the subordinate plants, constrained photosynthetic activity caused by shading might have reduced the sink intensity of these leaves.
目的是研究植物中的氮分配模式如何受其在植被中的垂直位置（即占主导地位或从属地位）的影响。用从种子生长的糙果苋在密集林分中进行了一项园圃试验，在该林分中已形成了年龄几乎相同的个体的大小等级体系。少数占主导地位的植物其大部分叶面积位于树冠的最高层，而大量从属植物则生长在其占主导地位的邻居的荫蔽之下。测定了占主导地位和从属植物的树冠结构、叶片氮分布的垂直模式以及叶片光合特性。还测量了林分中的光分布。占主导地位的植物单位叶面积的平均氮含量（总树冠氮除以总叶面积）高于从属植物，这可以用占主导地位的植物较高的平均MPA（单位面积叶干质量）来解释。然而，以重量为基础表示时，从属植物的平均氮含量（LNC；总树冠氮除以叶片总干重）较高。这些较高的LNC值可能反映了碳和氮同化之间的不平衡，氮吸收超过了其代谢需求。占主导地位的植物单位面积叶氮含量随相对光子通量密度降低的下降幅度比从属植物更大，这表明对于在树冠光梯度中占据不同位置的植物，这种分布模式可能不同。从植被中最老的叶子向更高处更受光照的较年轻叶子重新分配的氮量可能取决于较年轻叶子对氮的库强。在从属植物中，遮荫导致的光合活性受限可能降低了这些叶子的库强。
