Defoliation constrains xylem and phloem functionality

Abstract

Insect defoliation contributes to tree mortality under drought conditions. Defoliation-induced alterations to the vascular transport structure may increase tree vulnerability to drought; however, this has been rarely studied. To evaluate the response of tree vascular function following defoliation, 2-year-old balsam poplar were manually defoliated, and both physiological and anatomical measurements were made after allowing for re-foliation. Hydraulic conductivity measurements showed that defoliated trees had both increased vulnerability to embolism and decreased water transport efficiency, likely due to misshapen xylem vessels. Anatomical measurements revealed novel insights into defoliation-induced alterations to the phloem. Phloem sieve tube diameter was reduced in the stems of defoliated trees, suggesting reduced transport capability. In addition, phloem fibers were absent, or reduced in number, in stems, shoot tips and petioles of new leaves, potentially reducing the stability of the vascular tissue. Results from this study suggest that the defoliation leads to trees with increased risk for vascular dysfunction and drought-induced mortality through alterations in the vascular structure, and highlights a route through which carbon limitation can influence hydraulic dysfunction.

Wood allocation trade‐offs between fiber wall, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees

“…This implies that across neotropical tree taxa, xylem embolism resistance is related to the ability of the xylem vessels to resist hoop and bending stresses experienced during water transport. Recent evidence suggests that carbon starvation, induced by defoliation, can cause the development of fibers with thin walls which in turn results in irregular shaped vessels that are less resistant against embolism (Hillabrand et al, ). These results suggest that fibers with thick fiber walls can assist the bordering vessels to resist partial implosion, prevent stretching or rupture of the pit membranes and microfractures in the vessel walls, lowering the risk of embolism (Hillabrand et al, ; Jacobsen et al, ).…”

“…Recent evidence suggests that carbon starvation, induced by defoliation, can cause the development of fibers with thin walls which in turn results in irregular shaped vessels that are less resistant against embolism (Hillabrand et al, ). These results suggest that fibers with thick fiber walls can assist the bordering vessels to resist partial implosion, prevent stretching or rupture of the pit membranes and microfractures in the vessel walls, lowering the risk of embolism (Hillabrand et al, ; Jacobsen et al, ). The exact causal relationships between wood fibers, vessel bending resistance and xylem embolism resistance are not entirely clear but seem crucial in understanding embolism sensitivity in neotropical plant communities.…”

“…Furthermore, the xylem axial parenchyma fraction also increases with increasing vessel diameter, suggesting a possible role of axial parenchyma in axial water transport (Morris et al, 2018;Zheng & Martínez-Cabrera, 2013). Fiber walls contribute to mechanical strength and support xylem vessels to withstand implosion under high tension while fiber lumen contribute to water storage and sapwood capacitance (Hillabrand, Hacke, & Lieffers, 2019;Jacobsen, Ewers, Pratt, Paddock, & Davis, 2005;Pratt & Jacobsen, 2017;Pratt, Jacobsen, Ewers, & Davis, 2007). Some angiosperms species also have tracheids, or transitional cell forms between tracheids and vessels (Carlquist & Schneider, 2002).…”

Physiological responses to fire that drive tree mortality

“…The results of Partelli‐Feltrin, Smith, et al (2020) hint that some meristems were killed, and this may have contributed to mortality, but crown scorch and bud kill were not explicitly assessed. Carbon acquisition and hydraulic function are linked in ways still not fully understood, as evidenced by a study of experimental manual defoliation that caused an increased vulnerability to embolism (Hillabrand, Hacke, & Lieffers, 2019). Fire may act in a similar way as defoliation to kill foliage, also causing changes to xylem anatomy.…”

Loading regulation prevents phloem failure during drought and widens the range of phloem and stomatal traits

“…Modeled phloem resistance could increase by˜1 order of magnitude due to the occurrence of callose blockage at sieve plates in comparison to plates which are free of callose (Stanfield et al, 2019), or up to 20% due to p-protein agglomerations (Froelich et al 2011). Insect attack (Hao et al 2008) leads to callose blockages, and defoliation due to insects may reduce sieve tube diameters by up to 20% (Hillabrand et al 2019) which would incur a large resistance to sap transport. The sieve tube has been hypothesized to compensate for these blockages by increasing its source-sink pressure differential (Knoblauch et al 2014) or increasing phloem area and/or decreasing height by not growing as tall (Hölttä et al 2006(Hölttä et al , 2009.…”