Abstract
A bimodal radial growth (RG) pattern, i.e., growth peaks in spring and autumn, was repeatedly found in trees in the Mediterranean regions, where summer drought causes reduction or cessation of cambial activity. In a dry inner Alpine valley of the Eastern Alps (Tyrol, Austria, 750 m asl), Pinus sylvestris shows unimodal RG with onset and cessation of cambial activity in early April and late June, respectively. A resumption of cambial activity after intense summer rainfall was not observed in this region. In a field experiment, we tested the hypothesis that early cessation of cambial activity at this drought-prone site is an adaptation to limited water availability leading to an early and irreversible switch of carbon (C) allocation to belowground. To accomplish this, the C status of young P. sylvestris trees was manipulated by physical blockage of phloem transport (girdling) 6 weeks after cessation of cambial cell division. Influence of manipulated C availability on RG was recorded by stem dendrometers, which were mounted above the girdling zone. In response to blockage of phloem flow, resumption of cambial activity was detected above girdling after about 2 weeks. Although the experimentally induced second growth surge lasted for the same period as in spring (c. 2 months), the increment was more than twice as large due to doubling of daily maximum RG rate. After girdling, wood anatomical traits above girdling no longer showed any significant differences between earlywood and latewood tracheids indicating pronounced effects of C availability on cell differentiation. Below girdling, no reactivation of cambial activity occurred, but cell wall thickness of last formed latewood cell was reduced due to lack of C supply after girdling. Intense RG resumption after girdling indicates that cessation of cambial activity can be reversed by manipulating C status of the stem. Hence, our girdling study yielded strong support for the hypothesis that belowground organs exert high C sink strengths on the drought-prone study site. Furthermore, this work highlights the need of in-depth experimental studies in order to understand the interactions between endogenous and exogenous factors on cambial activity and xylem cell differentiation more clearly.
Highlights
Sporadic rainfall during June and July caused soil water content (SWC) to drop to c. 10 vol%
It has been found that the less-destructive phloem chilling method leads to similar growth responses as girdling growth pattern characterized by early peak of radial stem growth (RG) in mid-May to early June in coniferous species (P. sylvestris, P. abies, and Larix decidua) within the study area, extended dry periods frequently occur in spring and higher precipitation during summer would provide more favorable environmental conditions for tree growth
This hypothesis was confirmed by missing significant differences (p > 0.05) of wood anatomical parameters (i.e., cell lumen diameter (CLD), cell area (CA), CWT, and CLD:CWT) between earlywood and latewood tracheids after girdling, i.e., the characteristic wood anatomical pattern in conifers was not sustained as a result of increased C supply
Summary
Drought stress is a common trigger of growth reduction or premature cessation of cambial activity and cell differentiation in trees (e.g., van der Werf et al, 2007; Camarero et al, 2010; De Luis et al, 2011; Balducci et al, 2013), because cambial activity and cell differentiation are highly responsive to water availability (Sterck et al, 2008; Muller et al, 2011; Deslauriers et al, 2016). In several Mediterranean tree species (Pinus spp., Juniperus thurifera, Quercus ilex, and Arbutus unedo), cambial activity resumes, i.e., is reactivated in autumn if soil water availability increases again after prolonged summer drought (De Luis et al, 2007; Battipaglia et al, 2010; Camarero et al, 2010; Gutiérrez et al, 2011; Pacheco et al, 2016; Campelo et al, 2018) This bimodal pattern of cambial activity leads to the formation of intra-annual density fluctuations (IADFs; e.g., Campelo et al, 2007; Novak et al, 2013; Battipaglia et al, 2016; De Micco et al, 2016; Pacheco et al, 2018). This view is corroborated by several authors who reported that (i) plant growth is limited by competition between sinks rather than directly by C resources (e.g., Körner, 2003; Simard et al, 2013; Guillemot et al, 2017) and (ii) that the mycorrhiza-associated root system is a strong sink for C in plants experiencing water shortage during the growing period (Shipley and Meziane, 2002; Brunner et al, 2015; Hagedorn et al, 2016; Rainer-Lethaus and Oberhuber, 2018; Hartmann et al, 2020)
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