Ecology, syntaxonomy and recruitment of treeline ecotone vegetation in Rolwaling Himal, Nepal - Vegetation dynamics in response to climate change Cumulative Dissertation with the aim of achieving a doctoral degree at the Faculty of Mathematics, Informatics and Natural Sciences Department of Earth Sciences at Universität Hamburg Submitted by Birgit Bürzle from Illertissen, Germany Hamburg, 2020 Accepted as Dissertation at the Department of Earth Sciences Day of oral defense: 10.06.2020 Reviewers: Prof. Dr. Udo Schickhoff Prof. Dr. Jürgen Böhner Chair of the Subject Doctoral Committee: Prof. Dr. Dirk Gajewski Dean of Faculty of MIN: Prof. Dr. Heinrich Graener Für Paula Danksagung Die vorliegende Arbeit wäre ohne die vielfältige Hilfe und Unterstützung meiner Kollegen, Familie und Freunde nicht zustande gekommen. Euch allen gebührt mein herzlichster Dank. Besonders bedanken möchte ich mich bei Prof. Dr. Udo Schickhoff für das Vertrauen diese Arbeit zu schreiben, die inhaltlichen Freiheiten und die fachliche Hilfestellung, besonders aber für die Möglichkeit, die Schönheit und Faszination Nepals und des Himalayas kennen- lernen zu können. Prof. Dr. Jürgen Böhner danke ich für die Übernahme der Zweitbetreuung und die gute Zusammenarbeit innerhalb des TREELINE Projektes. Ich danke der Studienstif- tung des deutschen Volkes und der Stabsstelle Gleichstellung der Universität Hamburg für die umfängliche finanzielle Unterstützung. Für das angenehme und herzliche Arbeitsumfeld, die inspirierenden Diskussionen und die fachliche Unterstützung möchte ich mich bei der gesamten Physischen Geographie bedanken, insbesondere aber bei Niels Schwab, Alina Baranova, Maria Bobrowski, Marion Dohr, Franziska Hoppe, Elke Fischer, Johannes Weidinger und Julika Hellmold. Jens Oldeland danke ich für die statistische Beratung. Die langen Feldaufenthalte wären ohne die Unterstützung zahlreicher Helfer vor Ort nicht mögliche gewesen. Bedanken möchte ich mich insbesondere bei den Menschen aus Beding, die uns die Möglichkeit gegeben haben ‚ihren‘ Hang zu erforschen. Liebsten Dank an Lakpa und Tenzing Sherpa für die unbeschreibliche Gastfreundschaft, die Unterstützung im Gelände und die Hilfe in allen Lebenslagen. Danke an Yadu Sapkota für den zuverlässigen In- und Export der Untersuchungsgeräte und -proben. Ich möchte mich auch bei meinen Eltern Helene und Hans Bürzle bedanken. Danke für Euer Vertrauen und Euren bedingungslosen Rückhalt. Schließlich danke ich Stefan, für alles. Summary Mountains are characterized by a large diversity of plant species and plant communities over short horizontal and vertical distances, primarily corresponding to gradual changes of temper- ature. Specific zonations of vegetation types along elevational gradients are to be observed in mountains, with the most conspicuous change in mountain vegetation occurring at the alpine treeline ecotone, which represents the transition from closed forest to treeless alpine vegeta- tion. Rapid climate warming at high elevations is predicted to trigger extensive vegetation dynamics within treeline ecotones, primary related to recruitment processes. Vegetation dy- namics involve shifts in species distribution, community structure and treeline position. In the Himalayas, species richness as well as rates of climate change are among the highest on earth, yet studies addressing respective relationships are underrepresented in scientific research. The studies compiled in this thesis investigated the phytosociology and ecology of alpine treeline ecotone vegetation in the Nepalese Himalayas and analysed tree seedling establish- ment of dominating tree species as a critical component of the dynamics of spatial treeline position. All three parts of this work contribute to assessing vegetation dynamics in alpine treeline ecotones under climate warming conditions. During extensive field research in a near- natural treeline ecotone of Rolwaling Valley in east-central Nepal, we collected data on floris- tic composition and environmental conditions along three elevational transects. The micro- habitat characteristics of naturally germinated tree seedlings of Abies spectabilis, Betula utilis and Rhododendron campanulatum were recorded along the altitudinal gradient from subal- pine forests via treeline to alpine dwarf shrub heaths. By applying ordination methods and cluster analysis, we identified five communities belong- ing to two different classes: the Synotis alata – Abies spectabilis and Ribes glaciale – Abies spectabilis communities occupy the upper part of the subalpine forest zone, representing the uppermost forest stands below the treeline. Above the treeline, the mixed forest stands are replaced by the species-poor Boschniakia himalaica – Rhododendron campanulatum commu- nity, forming a dense krummholz belt. We assigned the forest and krummholz communities to the class Betula utilis – Abies spectabilis forests. In the alpine zone, two communities were classified: the species-poor Pedicularis cf. microcalyx – Rhododendron anthopogon commu- nity and the Anaphalis royleana – Rhododendron anthopogon community, which is character- ised by higher species richness of herbs and grasses. Both communities were assigned to the class Dasiphora arbuscula – Rhododendron anthopogon dwarf shrub heaths. The species composition of identified communities is related to an elevational gradient of deteriorating growth conditions, reflecting an increasingly unfavourable constellation of soil temperature, soil moisture and nitrogen supply and availability. i Our research on seedling establishment emphasises the importance of safe sites for successful tree regeneration in mountain environments. We found species-specific microhabitat prefer- ences for seedling establishment. While Abies spectabilis seedlings establish on ground that is covered with litter, Betula utilis and Rhododendron campanulatum seedlings primarily emerge on bryophyte mats. The microhabitat structure did not vary significantly in relation to different vegetation types encompassing the treeline ecotone. The majority of seedlings of all species grew close to at least one potentially protective element. Geomorphic elements and deadwood were the most important shelters for seedling establishment. The identified critical drivers for community differentiation and for tree seedling establish- ment - particularly soil temperature, water and nitrogen supply - are directly or indirectly re- lated to climatic conditions. Thus, we suggest distinct alterations to species distribution and treeline position in the Rolwaling treeline ecotone with climate warming. As growth condi- tions in high mountain ecosystems may substantially alter under contemporary climate change, the need for species-specific safe sites for tree recruitment to buffer potential ex- tremes will likely increase. This thesis provides novel findings on climate change-induced vegetation dynamics in Hima- layan treeline ecotones. The results on seedling establishment serve to enhance the existing knowledge on the prerequisites for successful seedling establishment and spatial alterations of treeline position. The classification of Rolwaling treeline ecotone vegetation also provides a suitable baseline for further phytosociological investigations and for the initiation of a long- term research approach to monitor the effects of climate warming on Himalayan mountain vegetation. This cumulative thesis is based on two peer-reviewed studies that have been published in two journals (see Appendix I for the abstracts and author contributions, and Appendix II for full articles): Study 1: ‘Phytosociology and ecology of treeline ecotone vegetation in Rolwaling Himal, Nepal’ Authors: Birgit Bürzle, Udo Schickhoff, Niels Schwab, Jens Oldeland, Michael Müller, Jür- gen Böhner, Ram Prasad Chaudhary, Thomas Scholten and Wolf Bernhard Dickoré Published in 2017 in Phytocoenologia 47:197–220 doi: 10.1127/phyto/2017/0130 ii Study 2: ‘Seedling recruitment and facilitation dependence on safe site characteristics in a Himalayan treeline ecotone’ Authors: Birgit Bürzle, Udo Schickhoff, Niels Schwab, Lina Marie Wernicke, Yanina Katharina Müller, Jürgen Böhner, Ram Prasad Chaudhary, Thomas Scholten and Jens Oldeland Published in 2017 in Plant Ecology 219:115–132 doi: 10.1007/s11258-017-0782-2 I contributed as a co-author to six additional studies that are closely linked to the objective of this thesis and thereby enhance our understanding on climate change-induced vegetation dy- namics in the Rolwaling treeline ecotone. Appendix III includes abstracts of these studies. iii Zusammenfassung Gebirge zeichnen sich durch eine hohe Vielfalt an Pflanzenarten und Pflanzengesellschaften auf kleinstem Raum aus, welche insbesondere auf den höhenabhängigen Temperaturgradien- ten zurückzuführen ist. Die Gebirgsvegetation ist gekennzeichnet durch eine charakteristische Abfolge von Vegetationstypen entlang des Höhengradienten. Am deutlichsten wird dieser Vegetationswechsel im alpinen Waldgrenzökoton. Dieses stellt den Übergang vom geschlos- senen Wald hin zur baumfreien, alpinen Vegetation dar. Aufgrund einer rasanten Klimaer- wärmung in den Höhenlagen von Gebirgen sind weitreichende Vegetationsveränderungen innerhalb des Ökotons, welche primär auf Verjüngungsprozessen basieren, zu erwarten. Die ablaufende Vegetationsdynamik zeigt sich in der Veränderung des Verbreitungsareals von Arten und des strukturellen Aufbaus von Pflanzengesellschaften, sowie in der Verschiebung der Waldgrenze. Der Himalaya gehört zu den Hotspots der globalen Artenvielfalt