Response of mature Norway spruce (Picea abies) to elevated atmospheric CO

Abstract

The aim of this thesis was to identify pathways and rates of C allocation in tall forest trees, and to identify effects of elevated CO2 on respiratory processes and root growth. Correspondingly, this thesis is divided into three separate parts: Chapter 2) Long-term 13C labeling provides evidence for temporal and spatial carbon allocation patterns in mature Picea abies (published in Oecologia) Chapter 3) Respiratory fluxes and fine root responses in mature Picea abies trees exposed to elevated atmospheric CO2 concentrations (published in Biogeochemistry) Chapter 4) Photosynthetic enhancement and diurnal stem and soil carbon fluxes in a mature Norway spruce stand under elevated CO2 (published in Environmental and Experimental Botany) The work was conducted at the Swiss canopy crane (SCC) research site in Hofstetten near Basel, Switzerland, and explored signals produced by free air CO2 enrichment (FACE) in 110-year-old, ca. 37m tall P. abies trees. Chapter 2 capitalizes on the isotopic signal carried by the CO2 gas used for CO2 enrichment, yet does not address effects of elevated CO2 as such, but rather deals with basic questions of C transfer in tall trees. Chapter 3 explores the longer-term CO2 effects on mature P. abies (i.e. 2.5 years), whereas chapter 4 reports short-term (diurnal) responses to elevated CO2. In the following, I will provide a summary of the results of the three chapters of my thesis, extended by a conclusion that links these chapters. Chapter 2) Long-term 13C labeling of Picea abies As a side effect, the FACE technique provided the unique opportunity to study C translocation within the tree body using the stable isotope 13C signal the FACE gas carries. Since control trees are not (can not) be similarly labeled with 13C the tree responses to elevated CO2 were not the subject of this chapter. Yet, FACE resembles a continuous 13C labeling of new assimilates. Tracking the fate of these assimilates over a period of 2.5 years in tall trees offers new insights in tree C relations under steady state conditions. We tracked 13C signals in mature P. abies trees at a high spatial and temporal resolution, i.e. from the canopy (needles and branchlets), down to the tree trunk (year rings and stem CO2 efflux), and into the soil compartment (fine roots, fungi, soil CO2 efflux). The following key questions were answered: 1. How long does it take for new C to arrive at a certain tissue type or respiratory flux? 2. What is the proportional contribution of newly assimilated C to concurrent tree tissue production and maintenance? 3. How long does it take until old C is replaced by new C in various tissues? Generally, we observed a reduction of new assimilate investment with distance from the canopy, which can be explained by a progressive dilution of new C into the existing C storage pools in the tree. New sunlit needles (and adjacent branchlets) exhibited a nearly 100% share of new C, whereas shaded needles also used some older C. Stem wood isotope signals evidenced a complete exchange of…