Key message

Evidence supporting hydraulic limitation hypothesis was found using foliar δ¹³C in combination with nitrogen content per unit leaf area and statistical partitioning for three conifer species.

Abstract

One theory behind the productivity decline of mature forests is the hydraulic limitation hypothesis (HLH); leaf-level gas exchange is reduced with increasing forest canopy height via increased hydraulic resistance in the xylem pathway, which in turn limits photosynthesis via stomatal regulation. Foliar \(\delta\)¹³C can be used to assess the HLH as it reflects the history of leaf-level gas exchange. However, this method should be used with caution as co-varying factors, including light levels and foliar nutrient status, can also influence foliar \(\delta\)¹³C. We explore the potential use of foliar \(\delta\)¹³C to assess leaf-level hydraulic limitation using three coniferous species across three height classes (short, intermediate and tall) in northern Idaho, USA. Foliar samples were collected from multiple canopy locations varying in height from each height class to measure \(\delta\)¹³C of bulk foliar materials as well as sugar and starch extracted from the samples. We also quantified nitrogen content per unit leaf area (N_area) as an integrated measure of nutrient status and light environment of a given foliar sample, which can partly account for various non-stomatal limitations for photosynthesis, and thus affect foliar \(\delta\)¹³C. Using sequential ANOVAs, we tested the hypothesis that foliar \(\delta\)¹³C variation was attributable to foliar sample height changes after accounting for N_area. The hypothesis was supported by analyses using foliage samples from the top canopy location across the three height classes for each conifer species, especially for bulk foliage and extracted sugar. In conclusion, we found evidence supporting the HLH using foliar \(\delta\)¹³C from three conifer species.

中文翻译：

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树冠$$ \ delta $$δ13 C的变化是否可以归因于树高的变化？三种针叶树种的调查

关键信息

证据支持液压限制假说是使用叶面发现δ ¹³与单位叶面积和统计分区氮含量为3针叶树种组合下进行。

抽象

成熟森林生产力下降的一个理论是水力极限假说（HLH）。通过增加木质部途径中的水力阻力，随着森林冠层高度的增加，叶片水平的气体交换会减少，这反过来又通过气孔调节限制了光合作用。叶面\（\增量\）¹³ C可用于评估HLH，因为它反映叶级气体交换的历史。然而，这种方法应该谨慎作为共可变因素，包括光水平和叶面营养状态使用，也可以影响叶面\（\增量\）¹³ C.我们探索的潜在用途叶面的\（\增量\）¹³C以评估美国爱达荷州北部三个高度级别（短，中和高）的三个针叶树种的叶水平水力限制。叶面样品从复合群体的位置从每个高度类的高度变化的测量收集\（\增量\）¹³的散装物料叶面C作为以及糖和从样品中提取的淀粉。我们还定量了每单位叶面积（N_面积）的氮含量，作为给定叶面样品营养状况和光照环境的综合度量，这可以部分解释光合作用的各种非气孔限制，从而影响叶面\（\ delta \）¹³ C.使用顺序方差分析，我们检验了叶面\（\ delta \）的假设考虑到N_面积后，叶面样品高度的变化是¹³ C变化的原因。对于每个针叶树物种，尤其是散装叶子和提取的糖，使用三种高度类别的顶部树冠位置的叶子样本进行分析，从而支持该假设。总之，我们发现的证据使用叶面支持HLH \（\三角洲\）¹³从三个针叶树种℃。