Carbon allocation and growth of Scots pine

Based on several dendroecological and dendroclimatological studies conducted within dry inner Alpine environments, drought stress in April through May limits radial growth of Scots pine (Pinus sylvestris L.). In a recent study we determined intra-annual dynamics of cambial activity and wood formation of P. sylvestris exposed to soil dryness and nutrient deficiency and found that maximum growth rate peaked early during the growing season in May and prior to occurrence of more favourable environmental conditions, i.e., repeated high rainfall events during summer. It is well known that plants can adjust carbon allocation patterns to optimize resource uptake under prevailing environmental constraints and increase carbon allocation to roots in response to drought and low nutrient supply. Hence, the early decrease in stem growth of P. sylvestris exposed to drought and nutrient deficiency can be regarded as an adaptation to cope with extreme environmental conditions, which might require an early switch of carbon allocation to belowground organs. However, an experimental approach confirming this hypothesis is still missing. Because radial growth depends on a continuous supply of carbohydrates, manipulation of the carbon status of the stem can reveal source limitation of radial growth. Girdling, i.e., physical blockage of phloem transport around a tree`s outer circumference, is frequently applied to investigate carbon relationships and causes accumulation and depletion of carbohydrates above and below the girdle, respectively. In a common garden experiment we will therefore aim at determining to what extent above- and belowground growth in P. sylvestris is influenced by carbon availability under different environmental regimes. To accomplish this, we will manipulate environmental conditions (i.e., water availability and soil nutrient content) and modify tree carbon status through physical phloem blockage to test the hypotheses that (i) phloem blockage at distinct phenological stages during the growing season differently affects above- and belowground growth, (ii) altered carbon availability changes temporal dynamic of growth processes and wood formation and (iii) soil related stress factors, i.e., soil dryness and/or nutrient deficiency, induce early cessation of aboveground growth in favour of belowground root growth. Hence, results of this project will largely contribute to the understanding of the physiological growth response of P. sylvestris exposed to extreme environmental conditions.

Carbon (C) availability plays an essential role in tree growth and wood formation and it is well known that plants can adjust C allocation patterns to optimize resource uptake under stressful environmental conditions. We evaluated the hypothesis that the early decrease of radial stem growth in late spring found in several coniferous species in a dry inner Alpine environment is an adaptation to cope with drought stress, which might require an early switch of C allocation to the root system. To accomplish this, we experimentally subjected six-year- old Norway spruce saplings to two levels of soil water availability (watered versus drought conditions) and manipulated tree C status by physically blocking phloem transport at different phenological stages during the growing season, i.e. several weeks before bud swelling and onset of cambial activity, during vigorous earlywood and shoot growth, and after cessation of shoot growth. The influence of manipulated C availability and drought stress on stem and root growth and wood structure was analysed. Results revealed C limitation of wood formation, which was deduced from significant increase in radial stem growth above the girdling zone and a significant increase of cell wall thickness in earlywood in both soil humidity treatments compared to the non-girdled controls. At all girdling dates the radial growth increase in the stem and coarse root was significantly more intense in the drought- stressed compared with watered trees indicating higher belowground C demand under constrained soil water availability. Furthermore, in response to phloem blockage reactivation of cambial activity in drought-stressed trees was detected above the girdling zone. While directly below girdling xylem formation in the stem stopped in both soil humidity treatments, root growth was not affected by girdling, indicating differences in hormonal control. Analyses of C status revealed significant decrease of non-structural carbohydrates (soluble sugars and starch) above girdling and in coarse roots, which is related to the considerable growth response. Additionally, ecophysiological field studies in a dry coniferous forest revealed that differences in intra-annual dynamics of radial stem growth and tree water status depend on species, tree size and forest canopy structure. Based on these findings we conclude that as an adaptation to maintain adequate tree water status drought-stressed Norway spruce trees prioritize an early shift of C allocation to belowground at the expense of radial stem growth, which explains early culmination of wood formation reported in coniferous species at drought-prone sites.