Abstract
The early culmination of maximum radial growth (RG) in late spring has been found in several coniferous species in a dry inner Alpine environment. We hypothesized that an early decrease in RG is an adaptation to cope with drought stress, which might require an early switch of carbon (C) allocation to belowground organs. To test this hypothesis, we experimentally subjected six-year-old Norway spruce saplings (tree height: 1.35m; n=80 trees) to two levels of soil water availability (watered versus drought conditions) and manipulated tree C status by physically blocking phloem transport at three girdling dates (GD). The influence of C availability and drought on tree growth (radial and shoot growth; root biomass) in response to girdling was analyzed in both treatments. Non-structural carbohydrates (NSCs, soluble sugars and starch) were measured in the stem, root and current leader to evaluate changes in tree C status due to girdling. The main finding was a significant increase in RG of the girdled trees compared to the controls above the girdling zone (UZ). At all girdling dates the RG increase was significantly more intense in the drought-stressed compared with watered trees (c. 3.3 and 1.9-fold higher compared with controls in the drought-stressed and watered trees, respectively), most likely indicating that an early switch of C allocation to belowground occurs as an adaptation to maintain tree water status under drought conditions. Reactivation of the cambium after the cessation of its regular activity was detected in UZ in drought-stressed trees, while below the girdling zone no xylem formation was found and the NSC content was strikingly reduced. Irrespective of water availability, girdling before growth onset significantly reduced the progression of bud break (P<0.05) and the length of the current leader shoot by −47% (P<0.01) indicating a reduction in xylem hydraulic conductance, which was corroborated by significantly reduced xylem sap flow (P<0.001). Based on our findings, we conclude that during the growing season drought stress prioritizes an early switch of C allocation to the root system as an adaptation to maintain adequate tree water status in drought-prone environments.
Highlights
Drought is a major factor affecting growth and wood formation in trees
It is well known that plants can adjust their carbon (C) allocation patterns to optimize resource uptake under prevailing environmental constraints and can increase C allocation to roots in response to drought, i.e., tree species adapted to dry conditions generally have higher root-to-shoot ratios and deeper root systems than species that occur under more mesic environmental conditions (Kozlowski and Pallardy 2002; Brunner et al, 2015)
The early decrease in stem growth of conifers exposed to low soil water availability can be regarded as an adaptation to maintain adequate tree water status, which might require an early switch of C allocation to the root system and associated mycorrhiza
Summary
Drought is a major factor affecting growth and wood formation in trees Both cell division and cell enlargement are affected under drought due to decreased energy supply, loss of cell turgor and impaired enzyme activities (Pallardy, 2008). Several dendroecological studies conducted within dry inner Alpine environments have shown that drought stress in terms of reduced soil water availability in April to June impairs the annual increment of coniferous species (e.g., Zweifel et al, 2006; Pichler and Oberhuber 2007; Schuster and Oberhuber 2013). The early decrease in stem growth of conifers exposed to low soil water availability can be regarded as an adaptation to maintain adequate tree water status, which might require an early switch of C allocation to the root system and associated mycorrhiza
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