Oren R, Schulze E-D, Werk K S, Meyer J
School of Forestry and Environmental Studies, Duke University, 27706, Durham, NC, USA.
Lehfstuhl für Pflanzenökologie der Universität Bayreuth, Postfach 101251, 8580, Bayreuth, Federal Republic of Germany.
Oecologia. 1988 Nov;77(2):163-173. doi: 10.1007/BF00379182.
A declining, closed-canopy Picea abies (L.) Karst. stand produced as much crown biomass as a healthy stand, although some trees were chlorotic due to magnesium deficiency. The production of wood per unit of leaf area in both stands was related to the foliar magnesium concentration. Although leaf area index and climate were similar at both sites, stemwood production was 35% lower in the declining than in the healthy stand. Nutritional disharmony, rather than a deficiency in a single element, was identified as the mechanism for reduced tree vigor. The role of nutrient stress in forest decline was detected by partitioning the season into three periods reflecting different phenological stages: a canopy growth period in spring, a stem growth period in summer, and a recharge period during the non-growing season. Needle growth was associated with nitrogen supply. Most of the magnesium supply required to meet the demand for foliage growth was retranslocated from mature needles. Magnesium retranslocation was related to concentration of nitrogen and magnesium in those needles before bud break. Retranslocation from mature needles during the phase of canopy production resulted in chlorosis in initially green needles if the magnesium concentration before bud break was low. Nitrogen concentration in 0-year-old needles generally remained constant with increasing supply, indicating that foliage growth was restricted by the supply of nitrogen. In contrast, magnesium concentration generally increased with supply, indicating that magnesium supply for needle growth was sufficient. Much of the magnesium required for wood production was taken up from the soil because stored magnesium was largely used for canopy growth. Uptake at the declining site was probably limited because of restricted root expansion and lower soil magnesium compared to the healthy site. For this reason only wood growth was reduced at the declining site. Because the recharge of magnesium during the non-growing period is dependent on uptake from the soil, it was more limited at the declining that at the healthy stand. However, as nitrogen uptake from the atmosphere may account for an appreciable proportion of the total uptake, and as its supply in the soil at both sites was similar, an unbalanced recharge of nitrogen and magnesium may have occurred at the declining site. If mature needles are unable to recharge with magnesium in proportion to the uptake of nitrogen, chlorosis is likely to occur during the next canopy growth period.
一片树冠郁闭度下降的欧洲云杉（Picea abies (L.) Karst.）林分产生的树冠生物量与健康林分相当，尽管有些树木因镁缺乏而出现黄化现象。两个林分中单位叶面积的木材产量都与叶片镁浓度有关。尽管两个地点的叶面积指数和气候相似，但衰退林分的干材产量比健康林分低35%。营养失调而非单一元素缺乏被确定为树木活力下降的机制。通过将季节划分为反映不同物候阶段的三个时期来检测养分胁迫在森林衰退中的作用：春季的树冠生长期、夏季的干材生长期和非生长季节的养分补充期。针叶生长与氮供应有关。满足叶片生长需求所需的大部分镁供应是从成熟针叶中重新转运而来的。镁的重新转运与芽萌发前这些针叶中的氮和镁浓度有关。如果芽萌发前镁浓度较低，在树冠生产阶段从成熟针叶中重新转运镁会导致最初绿色的针叶出现黄化。随着氮供应增加，1年生针叶中的氮浓度通常保持不变，这表明叶片生长受到氮供应的限制。相比之下，镁浓度通常随供应增加而升高，这表明用于针叶生长的镁供应充足。木材生产所需的大部分镁是从土壤中吸收的，因为储存的镁主要用于树冠生长。与健康地点相比，衰退地点的镁吸收可能因根系扩展受限和土壤镁含量较低而受到限制。因此，衰退地点只有木材生长减少。由于非生长时期镁的补充依赖于从土壤中吸收，衰退林分比健康林分更受限制。然而，由于从大气中吸收的氮可能占总吸收量的相当大比例，且两个地点土壤中的氮供应相似，衰退地点可能出现了氮和镁补充不平衡的情况。如果成熟针叶不能按氮吸收比例补充镁，在下一个树冠生长时期可能会出现黄化现象。
