How resilient are Polylepis reticulata and Escallonia myrtilloides to drought? Klara Bouwen Student number: 01403834 Promoter: Prof. dr. ir. Kathy Steppe Tutor: ir. Fran Lauriks A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Science in Bioscience Engineering: Forest and Nature Management Academic year: 2018-2019 “For the world is changing: I feel it in the water, I feel it in the earth, And I smell it in the air.” ~ Treebeard, J.R.R Tolkien, Many Partings, The Return of the King Declaration of Authorship “De auteur en de promotor geven de toelating deze scriptie voor consultatie beschikbaar te stellen en delen ervan te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron te vermelden bij het aanhalen van resultaten uit deze scriptie” “The author and the promoter give the permission to use this thesis for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using results from this thesis”” Ghent, August 2019 The promoter, The tutor, The author, Prof.dr.ir. Kathy Steppe Ir. Fran Lauriks Klara Bouwen i Thank you Yes, it is cliché to say, but it is also very true: you do not write a thesis alone… and there are so many people that contributed to this work in one way or another. First of all, I want to thank Kathy for giving me the opportunity to collaborate in such an unique project. Polylepis forests are truly special and without this thesis, I would never have known they even existed. Thank you for believing in me and reviewing my work. Next of course, I would like to thank my tutor: Fran, without your help, this research project would maybe still be stored on my computer by the name of ‘thesis_draft.doc’, instead of being the fancy booklet it is today. Thank you so much for all your time and patience, but most of all, for supporting me along the entire way. You are the best! Also, thank you, Niels, Linus, Roberto, Jonas and Olivier for helping me with my experiment. Your advices helped me to come to these results This thesis would of course not be same without all the new people I met and learnt from during this last year. Thank you Heidi, for giving a Belgian girl the chance to work in such an unique environment. I wish you all the luck with the further development of the project and I really hope this thesis can contribute in some kind of way. Also, my special gratitude goes to Ximena. Your kindness and hospitality are beyond this world. Thank you for all your help during this amazing experience! Alberto, Aldemar, Franklin and Fausto, muchas gracias for making me feel at home on the other side of the world. I really hope we could see each other again someday/somewhere! Liselot and Wouter, it would not have been such an wonderful experience without you two. I really cannot believe it already has been a year ago. When do you want to leave again? To all my friends and family, thank you for believing in me and making me the person I am today. ii Summary Fuelled by climate change, drought induced tree mortality increases in forest ecosystems around the world. Especially montane ecosystems, in which plant species are already surviving at the edge of their physical boundaries, are considered very vulnerable to changes in temperature and precipitation regimes. Degradation of these unique ecosystems can have disastrous consequences for local communities, depending directly on the high- altitudinal regions in their water supply. Despite the importance of montane ecosystems and the predicted increase of climate change induced drought stress, research assessing their vulnerability to drought remains very limited. Predicting drought responses at ecosystem level starts with a detailed and mechanistic understanding of soil-plant-water interactions at plant level. In this research project, we quantified drought vulnerability and hydraulic capacitance of two abundant páramo trees, Polylepis reticulata and Escallonia myrtilloides. Water potentials were measured in the field (Zhurucay Ecohydrological Observatory (Ecuador)) for twelve different days in September 2018. At the end of the measurement campaign, branches were collected and transported to Ghent University (Belgium) to establish vulnerability and desorption curves using acoustic emission sensors and continuously weighing of branches during dehydration. Plant anatomy of two branches was analysed and structural xylem features including vessel diameter, vessel area, and the degree of vessel connectivity were determined. Our results show that both páramo species are highly resilient against drought, with P. reticulata (P50 = -4.97 ± 1.39 MPa) being less vulnerable than E. myrtilloides (P50 = -9.11 ± 0.25 MPa). The low hydraulic capacitances of both species suggest low reliance on internal water reserves and high adaptation of the xylem tissue to avoid embolism formation. This was also supported by the microscopic analysis indicating low vessel connectivity and small vessel diameters. Overall, these results show that P. reticulata and E. myrtilloides are highly resistant to drought and suspects that both páramo species are resisted against increasing drought stress. Key words: acoustic vulnerability curves – hydraulic capacitance – embolism formation– Polylepis reticulata – Escallonia myrtilloides – páramo iii Samenvatting Massale boomsterfte neemt onder invloed van klimaatsverandering steeds meer toe. Vooral bergecosystemen, waarin planten omwille van het barre klimaat sterk aangepast zijn aan specifieke standplaatsen, blijken erg kwetsbaar voor veranderingen van temperatuur- en neerslagregimes. Degradatie van deze fragiele ecosystemen kan rampzalige gevolgen hebben voor bevolkingsgroepen die rechtstreeks afhankelijk zijn van ecosysteemdiensten die worden geleverd door de captatie van water op grote hoogte. Hoewel het essentieel is om te kunnen inschatten hoe bergecosystemen zullen reageren op toenemende droogtestress, is er tot vandaag weinig gekend over hun vatbaarheid op droogte. Om te voorspellen welke invloed toenemende droogtestress zal hebben op ecosysteemniveau, is een goede kennis van bodem-plant-water relaties op plantniveau noodzakelijk. In dit onderzoek werd de droogteresistentie van twee veelvoorkomende páramo-bomen, Polylepis reticulata en Escallonia myrtilloides, bepaald. Waterpotentialen werden opgemeten in het veld (Zhurucay Ecohydrological Observatory (Ecuador)) gedurende 12 verschillende dagen in september 2018. Takken werden op het einde van de meetcampagne verzameld en overgebracht naar de Universiteit van Gent (België) voor de bepaling van vatbaarheids- en uitdrogingscurves met behulp van akoestische emissie sensoren en door continue weging van takken tijdens dehydratatie. Plantanatomie van twee takken werd in detail bekeken en structurele xyleemkenmerken waaronder vatdiameter, vatoppervlakte en onderlinge vatconnecties werden bepaald. De bekomen vatbaarheidscurves tonen de hoge droogteresistentie aan van beide páramo- boomsoorten. Bovendien blijkt P. reticulata (P50 = - 4.97 ± 1.39 MPa) gevoeliger aan droogte dan E. Myrtilloides (P50 = -9.11 ± 0.25 MPa). De lage hydraulische capaciteiten (< 100 kg m-3 MPa-1) voor beide soorten wijzen op de lage afhankelijkheid van interne waterreserves en de hoge adaptatie van het xyleem om embolisatie te vermijden. Dit werd ook bevestigd in de anatomische analyses door de beperkte connecties tussen vaten onderling (Solitary vessel index = 0.72) en de lage vatdiameter (~ 20 µm). Deze resultaten tonen de hoge droogteresistentie van P. Reticulata en E. Myrtilloides aan en doen vermoeden dat beide páramo soorten bestand zijn tegen toenemende droogtestress. Trefwoorden: Vatbaarheidscurves – Hydraulische capaciteit – Embolisatie – Polylepis reticulata – Escallonia myrtilloides – Páramo iv Table of contents Declaration of Authorship i Thank you ii Summary iii Samenvatting iv Table of contents v Abbreviations & Symbols vi 1 Introduction 1 2 Literature review 2 2.1 Movement of water in plants 2 2.2 Drought-induced forest mortality across the globe 14 2.3 Tropical montane ecosystems under threat 18 3 Materials and methods 24 3.1 Zhurucay Ecohydrological Observatory 24 3.2 In situ measurements 28 3.3 Branch sampling procedure 30 3.4 Measurements during dehydration 30 3.5 Microscopic analysis 34 4 Results 35 4.1 In situ measurements 35 4.2 Dehydration experiment 37 4.3 Desorption curves 44 4.4 Microscopic analysis 48 5 Discussion 50 5.1 Vulnerability of P. reticulata and E. myrtilloides to drought-induced embolism 50 5.2 Resilience of P. reticulata and E. myrtilloides to drought under a changing climate 55 6 Conclusion 57 7 Further research 58 8 Appendix 59 9 Bibliography 61 v Abbreviations & Symbols Abbreviations AE Acoustic Emission a.s.l Above sea level C-T Theory Cohesion-Tension Theory DC Desorption Curve ITCZ Inter Tropical Convergence Zone IPCC Intergovernmental Panel on Climate Change RH Relative Humidity (%) PLC Percentage Loss of hydraulic Conductivity (%) Px Water potential at x percentage loss of hydraulic conductivity (MPa) SPAC Soil-Plant-Atmosphere Continuum TMCF Tropical Montane Cloud Forest VC Vulnerability Curve VPD Vapour Pressure Deficit (kPa) VWC Volumetric Water Content (kg m-3) VSH Vulnerability Segmentation Hypothesis ZHU Zhurucay river Ecohydrology Observatory PLC Percentage Loss of hydraulic Conductivity (%) Symbols -1 Cel