Differences in carbon isotope leaf‐to‐phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

Abstract

Summary While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport‐related fractionation mechanisms that influence phloem and subsequently tree ring δ13C are less investigated and may vary among species. We studied the seasonal and diel courses of leaf‐to‐phloem δ13C differences of water‐soluble organic matter (WSOM) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii). δ13C of individual sugars and cyclitols from a subsample was determined by compound‐specific isotope analysis. In beech, leaf‐to‐phloem δ13C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. 13C‐Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf‐to‐phloem δ13C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM. Trunk phloem WSOM at breast height reflected canopy‐integrated δ13C in beech but not in Douglas fir. Our results demonstrate that leaf‐to‐phloem isotope fractionation and δ13C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ13C.

Estimating canopy gross primary production by combining phloem stable isotopes with canopy and mesophyll conductances

“…Isotopic changes in phloem contents could also arise from compound-specific isotopic signatures in the phloem. Such differences amongst compounds have been observed in phloem contents (Smith, Wild, Richter, Simonin, & Merchant, 2016;Offerman, Ferrio, Holst, Grote, Siegwolf, Kayler, & Gessler, 2011) and they were especially noteworthy in the polyols in Douglas-fir xylem sap (Bögelein, Lehmann, & Thomas, 2019), which represented 37% of the phloem solutes and were approximately 2‰ more depleted than sucrose. It is not clear where the heavy carbon would go at polyol synthesis, but one might expect that it is retained in the substrates.…”

Whole‐tree mesophyll conductance reconciles isotopic and gas‐exchange estimates of water‐use efficiency

“…Our low sampling intensity could have obscured potential variation in δ 13 C ph during the course of the day (Gessler et al ., 2007) or with height along the trunk (e.g. Bogelein et al ., 2019). Still, our sampling protocol likely captured the actual variability in δ 13 C ph that would have had an imprint on monthly whole‐tree iWUE Δ .…”

Seasonal and diurnal trends in progressive isotope enrichment along needles in two pine species

“…Leaf water Δ 18 O imparts its isotopic signature on many downstream processes including phloem sugars (Bögelein, Lehmann, & Thomas, 2019), needle cellulose (Wright & Leavitt, 2006) and tree‐ring cellulose (Roden et al, 2000). Given the magnitude of progressive isotope enrichment and the ample evidence for its prevalence, it is surprising that (to our knowledge) only one study has sought to track the implications of progressive enrichment of leaf water for leaf cellulose isotope ratios (Helliker & Ehleringer, 2002).…”