Manipulating phloem transport affects wood formation but not local nonstructural carbon reserves in an evergreen conifer

Abstract

How variations in carbon supply affect wood formation remains poorly understood in particular in mature forest trees. To elucidate how carbon supply affects carbon allocation and wood formation, we attempted to manipulate carbon supply to the cambial region by phloem girdling and compression during the mid-and late-growing season and measured effects on structural development, CO 2 efflux and nonstructural carbon reserves in stems of mature white pines. Wood formation and stem CO 2 efflux varied with a location relative to treatment (i.e., above or below the restriction). We observed up to twice as many tracheids formed above versus below the treatment after the phloem transport manipulation, whereas the cell-wall area decreased only slightly below the treatments, and cell size did not change relative to the control. Nonstructural carbon reserves in the xylem, needles and roots were largely unaffected by the treatments. Our results suggest that low and high carbon supply affects wood formation, primarily through a strong effect on cell proliferation, and respiration, but local nonstructural carbon concentrations appear to be maintained homeostatically. This contrasts with reports of decoupling of source activity and wood formation at the whole-tree or ecosystem level, highlighting the need to better understand organ-specific responses, within-tree feedbacks, as well as phenological and ontogenetic effects on sink-source dynamics.

Drought impacts on tree carbon sequestration and water use – evidence from intra‐annual tree‐ring characteristics

“…Despite a reduction in leaf physiological processes during drought conditions, carbon assimilation still continues with a higher isotopic signal but incorporation into sinks is minimal (Tomasella et al ., 2017), owing to the different climatic sensitivities of photosynthesis and growth (Fatichi et al ., 2013; Körner, 2015; Hagedorn et al ., 2016). Previous findings of an accumulation of NSCs under low water availability (Dietze et al ., 2014; Piper et al ., 2017) and reduced transfer to storage tissues (Rademacher et al ., 2021) support our results. After the resumption of an improved tree hydric status, the assimilates that accumulated during drought are readily available and rapidly incorporated into wood formation processes, which are no longer constrained by water limitation (Gessler et al ., 2020).…”

Insights into source/sink controls on wood formation and photosynthesis from a stem chilling experiment in mature red maple

“…Carbon supply can either be sourced directly from recent assimilates or from NSC reserves, that can build up and be remobilized over multiple years (Carbone et al ., 2013; Muhr et al ., 2016). Like in other phloem transport manipulations (Maier et al ., 2010; Regier et al ., 2010; Rademacher et al ., 2021a), bulk NSC concentrations in stems and roots (here measured in the first centimeter of the stem and roots) reacted more sluggishly and show little to no treatment effect (Fig. 4).…”

“…However, below both restrictions NSC remobilization and/or reduced sink activity compensated for the reduced phloem carbon transport to maintain phloem sugar concentrations. This supports the idea that at low carbon supply, sugar concentrations near the cambium are homeostatically maintained within relatively tight limits that vary seasonally (Huang et al ., 2021; Rademacher et al ., 2021a). Such homeostasis is typical in biological systems and likely evolved to ensure metabolic stability for inter‐annual wood formation given the role of soluble sugar concentrations as a signal, resource and possible driver of wood formation (Riou‐Khamlichi et al ., 2000; Lastdrager et al ., 2014; Cartenì et al ., 2018).…”

Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest

“…Interestingly, even though the magnitude of MDS was similar in all the study years, dehydration effects on tree growth and its timing and on xylogenesis increased with recurrent drought. It is unlikely that the increased sensitivity of radial growth to TWD was due to progressive carbon limitation, given that leaves and branches had relatively constant levels of NSC under drought and that stem and roots may store substantial amounts of carbohydrates to sustain metabolism (Hoch et al, 2003; Rademacher et al, 2021; Rosell et al, 2021). Therefore, our findings suggest that the sensitivity of radial growth to TWD can increase under recurrent drought, which may be due to progressively decreasing stem hydraulic capacitance.…”