Carbon isotope composition of foliar bulk material and carbohydrates. Implications for assessing hydraulic limitation in Northwest conifers.
Schotzko, Alisa*,1, Pangle, Robert1, Kavanagh, Kathleen1, 1 Forest Resources Department, Moscow, ID, USA
ABSTRACT- Limited temporal integration of bulk foliar material and within-canopy variation of photosynthetic capacity may confound changes in foliar 13C with height making it difficult to assess hydraulic limitations. To explore this possibility, western white pine (WP) and western hemlock (WH) across three height classes, short (7-17m), medium (27-34m) and tall (47-58m), were felled in September 2004. Foliar samples were collected from the top of each canopy third for bulk, (13Cbulk), sugar (13Csugar), and starch (13Cstarch) isotopic analysis. Nitrogen content per unit leaf area (Narea) was also measured as an indicator of photosynthetic capacity. Narea increased significantly from the bottom to the top of canopies in the medium and tall height classes (p=.001, <.0001 for WP and WH respectively), and decreased significantly from the tops of short WP to tall WP (p=.0087). Therefore Narea was included in all model statements when assessing hydraulic limits. Hydraulic limitation to stomatal conductance, as indicated by enrichment in 13C, was observed both between canopy height classes and within canopies. Enrichment in 13C, was significant in the sunlit foliage of top canopy samples for WP (p=.0129) and WH (p=.0218). Additionally, there was significant within-canopy enrichment of 13Cbulk between bottom and top canopy samples in both WP (p=.0004) and WH (p=.0016). Similar shifts were observed in 13Csugar and 13Cstarch. We found that foliar samples taken from the top third of the canopy mirrored samples taken from the top of the canopy, making them an acceptable surrogate when the very top of canopies are inaccessible. We also found evidence that foliar carbohydrates may provide a signal with a different temporal resolution and thus be able to detect changes in 13C on a finer scale. In summary, it is important to account for changes in Narea when using 13C to quantify the effects of hydraulic limitation on stomatal conductance.
Key words: hydraulic limitation, carbon isotopes, carbohydrates, photosynthetic capacity
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