Can variation in canopy $$\delta$$ δ 13C be attributed to changes in tree height? An investigation of three conifer species

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. 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 (Narea) 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 Narea. 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.