Assessing the carbon sequestration potential of Populus euphratica and Haloxylon spec. in Central Asian Desert ecosystems

In terms of climate change and climate change mitigation, the quantitative knowledge of global carbon pools is important information. On the one hand, knowledge on the amount of carbon cycling among – and stored in – global pools (i.e. Atmosphere, Biosphere, Cryosphere, Hydrosphere, and Lithosphere) may improve the reliability of models predicting atmospheric CO2 concentrations in terms of fossil fuel combustion. On the other hand, the carbon sequestration potential of specific ecosystems allows for estimating their feasibility regarding carbon trade mechanisms such as the Clean Development Mechanism or the Reducing Emissions from Deforestation and Degradation Program (REDD+). However, up to date, the majority of terrestrial carbon assessments have focused on forests and peatlands, leaving a data gap open regarding the remaining ecosystems. This data gap is likely to be explained by the relatively high carbon densities and/or productivities of forests and peatlands. Nevertheless, to get a precise as possible global picture, information on carbon pools and sequestration of other ecosystems is needed. Although desert ecosystems generally express low carbon densities, they may absolutely store a remarkable amount of carbon due to their large areal extent. In this context, Central Asian Deserts (in particular within the Turanian Deserts, i.e. Karakum, Kysylkum, Muyunkum) likely inhibit comparably high carbon pools as they express a sparse vegetation cover due to an exceptionally high annual precipitation if compared to the World’s deserts. In this dissertation, three important woody plant species – Populus euphratica and Haloxylon aphyllum and Haloxylon persicum – of Central Asian Deserts were investigated for their carbon pools and carbon sequestration potential. These species were chosen as they I) locally express high carbon densities, II) are dominant species, III) have a rather large spatial distribution, and IV) have experienced a strong degradation throughout the 20th century. Thus, they likely show a remarkable potential for carbon re-sequestration through restoration and thus for an application of carbon trade mechanisms (CHAPTER I). P. euphratica was investigated in the nature reserve Kabakly at the Amu Darya, Turkmenistan and in Iminqak at the Tarim He, Xinjiang, China. The assessment of Haloxylon species was restricted to the Turanian deserts west of the Tain Shan. To achieve a first scientific basis for large scale estimates, different methodologies, ranging from allometric formulas, over dendrochronology to remote sensing were combined (CHAPTERS II-V). In CHAPTER II allometric formulas were successfully developed for Haloxylon aphyllum and Haloxylon persicum and applied to six study sites distributed over the Turanian Deserts to represent the allometric variability of Haloxylon species in Central Asia. CHAPTER III derives another allometric formula (only based on canopy area) for H. aphyllum and combines it with a remote sensing analysis from the nature reserve Repetek. Thereby, a first large scale estimate covering the Northeastern Karakum Desert of carbon pools related to mono specific H. aphyllum stands is achieved. CHAPTER IV describes the wood structure of Populus euphratica forests in the nature reserve Kabakly (Turkmenistan) and in Iminqak (Xinjiang, China). In CHAPTER V a dendrochronological approach derives models for predicting the Net Primary Productivity (NPP) and the age of P. euphratica in the nature reserve Kabakly. Thereby, a first feasibility assessment regarding remote sensing analyses and the upscaling of the obtained NPP results is carried out. First estimates based on these local studies (CHAPTER VI), reveal carbon densities ranging from 0.1 – 26.3 t C ha 1 for the three investigated species. Highest maximum and median carbon densities were found for P. euphratica, but Haloxylon aphyllum expressed remarkable maximum carbon densities (13.1 t C ha-1), too. The total carbon pools were estimated at 6480 kt C for P. euphratica, 520 kt C for H. aphyllum stands and 6900 kt C for Haloxylon persicum shrubland. Accounting for the extent of degraded areas, the total re-sequestration potentials of the respective species were estimated at 4320 kt C, 1620 kt C and 21900 kt C, this highlighting the remarkable absolute re-sequestration potential of H. persicum shrubland despite its low average carbon densities. In the end, the main results were put into a broader context (CHAPTER VI), discussing the general feasibility of reforestations both in ecological terms as well as in terms of carbon trade mechanisms. A short example highlights the strong connection between the feasibility of reforestations and the global carbon market. Finally, open research questions are brought forth revealing the yet large research potential of Central Asian Desert ecosystems in general and in terms of carbon sequestration.Im Rahmen des erwarteten Klimawandels und der Verhinderung dessen, ist das Wissen über die globalen Kohlenstoffvorräte von besonderem Interesse. Kohlenstoffvorräte können durch Verschiedene Prozesse (u. A. Landnutzung) an die Atmosphäre abgegeben werden, was zu einer Erhöhung der atmosphärischen CO2 Konzentration – einem klimawirksamen Prozess - führen würde. Auf der anderen Seite kann Kohlenstoff aktiv gebunden werden (z. B. durch Anreicherung in Mooren oder Böden aufgrund von Netto-Anreicherung von Pflanzenmaterial). Bisherige Untersuchungen zum pflanzlichen, globalen Kohlenstoffvorrat konzentrieren sich vor allem auf Moore und Wälder, da diese wahrscheinlich den größten Anteil hierzu liefern. Nichts desto trotz, ist das respektive Wissen bezüglich der anderen Ökosystem von Interesse um das komplette Bild der globalen Kohlenstoffvorräte zu erlangen. Obwohl Wüstenökosysteme durch geringe Kohlenstoffdichten (Kohlenstoffmasse pro Fläche) charakterisiert sind, können sie aufgrund ihres großen terrestrischen Flächenanteils unter Umständen einen signifikanten Beitrag zum globalen Kohlenstoffvorrat leisten. Vor allem zentralasiatische Wüstenökosysteme stechen hier aufgrund ihrer spärlichen Vegetationsbedeckung hervor. Im Rahmen dieser Dissertation wurden drei wichtige Gehölzpflanzen – Populus euphratica, Haloxylon aphyllum und Haloxylon persicum – in Zentralasien bzgl. Ihres rezenten Kohlenstoffvorrates und der potentiellen Kohlenstoffsequestrierung untersucht. Der Fokus wurde auf diese drei Arten gelegt, da sie I) lokal hohe Kohlenstoffdichten aufweisen, II) dominant sind, III) eine große räumliche Ausdehnung aufweisen, und IV) ein großes Wiederaufforstungspotential aufweisen, da sie im 20. Jahrhundert stark anthropogen degradiert wurden. Die Untersuchungen fanden hauptsächlich in den Turanischen Wüsten (westlich des Tian Shan) statt und haben allometrische Formeln, fernerkundliche Analysen und dendrochronologische Methoden interdisziplinär verknüpft. Erste Schätzungen basierend auf den Resultaten dieser Dissertation belaufen sich auf 0.1 – 26.3 t C pro ha bzgl. der Kohlenstoffdichten und einem gesamt-Wiederaufforstungspotential von 21900 kt Kohlenstoff. In der Diskussion werden zudem offene Forschungsfragen und Anwendungsmöglichkeiten behandelt