图 6不同施氮处理下气孔长度与气孔密度的线性回归关系

Fig. 6 Linear regressions between stomatal length and stomatal density across all species under different nitrogen treatments

ACTA Scientiarum Naturalium Universitatis Pekinensis - - Contents -

气孔导度与气孔长度和密度的这种关系是否在我国东部森林具有普适性, 仍需要在更多区域和更多物种内进行验证。

3.4 氮添加对气孔特征的影响

本研究发现, 南方木本植物对氮添加的响应比北方敏感。亚热带地区木本植物的气孔导度以及热带地区乔木的气孔密度和SPI指数与施氮处理显著正相关, 施氮会促进阔叶乔木气孔导度增加以及常绿乔木气孔长度和气孔导度增加。此外, 从受施氮影响的物种数角度分析, 南方8个树种均至少有一个气孔特征受施氮影响, 而北方有约40%的物种叶片气孔特征不受施氮影响。这些结果都表明, 施氮会促进南方(GNJ及以南地区)木本植物叶片气孔的生长, 这可能与南北方木本植物采取不同的生长和适应策略有关。Bowsher等[35]指出, 植物对养分添加的形态可塑性反应与植物所处环境的养分状况有关, 生长在养分含量较高环境中的植物一般采用积极的生长策略, 而生长在养分匮乏区域的植物则反应相对保守, 生长相对缓慢[36]。与北方相比, 我国南方氮资源较充足, 并且周转快[37], 导致生长在南方的植物形态可塑性较高。此外, 综合分析氮添加对全部木本植物的影响, 发现氮添加对气孔长度的伸长和SPI指数的增加有抑制作用, 但促进气孔导度的增加, 这意味着植物叶片在增加气孔开张程度的同时, 降低了对气孔结构的成本输入。气孔长度与气孔密度的相关关系反映气孔大小与数量之间的权衡关系, 而这种最优化协调的结果是植物适应复杂环境的有效策略之一[4]。本研究发现, 在不同施肥处理下, 气孔长度与密度之间存在显著且稳定的负相关关系(取对数), 并且二者关系的斜率在3个施肥梯度下存在差异, 随着施氮浓度 的增加, 斜率降低, 这与SPI指数随施氮浓度增加而下降的结果一致。关于导致这种现象的原因及机理, 尚需进一步研究。

本研究发现, 不同木本植物的气孔特征对氮添加的响应不同, 一些植物的气孔特征对氮添加不敏感, 可能存在以下原因。1) 物种所处环境氮素营养的供应情况不同。虽然不同站点的施肥处理浓度一致, 但由于各地的基础氮沉降不同, 使得施氮的效果存在差异。2) 物种叶片结构的差异。例如不同的气孔器类型可能导致物种对外界影响的生理反应有差异。3) 植物可通过改变气孔大小和密度来适应长期的环境变化。已有报道指出, 往年的环境印迹会影响下一年的气孔特征[38]。5年的施肥时间可能相对偏短, 气孔特征的变化规律尚未完全显现。4) 植物对环境的响应是通过各器官间的协同变化实现的, 植物往往通过性状组合的动态变化来适应环境的变化。应对不同的环境胁迫时, 因功能的不同, 各器官组织结构特征的敏感程度不同, 植物可能通过改变其他器官的形态结构来应对氮添加浓度的变化, 导致气孔特征对氮添加的响应不显著。

4 结论

1) 不同生活型木本植物的气孔特征有显著差异。针叶乔木的气孔长度和SPI指数显著高于阔叶乔木, 但气孔密度较低; 落叶乔木的气孔长度和SPI指数显著高于常绿乔木, 但气孔导度较低。

2) 我国东部森林优势树种的气孔特征具有明显的纬度格局, 随纬度的增加, 气孔长度显著增加,而气孔密度显著降低(蒙古栎除外)。MAT, MAP和PET都是重要的影响因子。

3) 我国东部优势木本植物各气孔特征之间存

在显著的相关性。气孔长度与气孔密度存在显著的负相关关系, 气孔导度则随着气孔长度的增加而增加, 随着气孔密度的增加而降低。

4) 氮添加对不同物种的影响不同, 其中, 南方木本植物对氮添加的响应比北方敏感。 致谢 感谢北京大学蔡琼、马素辉、田地和徐龙超同学, 姜来和李鹏博士, 孙振中老师和郑成洋老师在野外调查采样、室内实验和数据分析中给予的帮助, 感谢朱剑霄博士在实验设计和论文撰写等方面提供的宝贵意见。

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