Faculty of Bioscience Engineering 2011 – 2012 How are anatomical and hydraulic features of Avicennia marina and Rhizophora mucronata trees influenced by siltation? Hannes De Deurwaerder Promoters: Prof. dr. ir. Kathy Steppe1 Dr. Nele Schmitz2 Prof. dr. Nico Koedam2 Master’s dissertation submitted in partial fulfilment of the requirements for the degree of Master of Bioscience Engineering: Agricultural Sciences 1 Laboratory of Plant Ecology, Universiteit Gent 2 Laboratory of Plant Biology and Nature Management, Vrije Universiteit Brussel I, HANNES DE DEURWAERDER, declare that this is the result of my own work and that no previous submission for a degree has been made here or elsewhere. Works by others, which served as sources of information, have been duly acknowledged by references to the authors. The author and the promoters give the authorisation to consult and to copy parts of this work for personal use only. Any other use is under the limitation of copyrights laws; specifically it is obligatory to specify the source when using results from this thesis after having obtained the written permission. Ghent and Brussels, August 2012 Promotor, Promotor, Prof. dr. ir. Kathy Steppe1 Prof. dr. Nico Koedam2 Promotor, Author, Dr. Nele Schmitz2 Hannes De Deurwaerder 1 Laboratory of Plant Ecology, Universiteit Gent 2 Laboratory of Plant Biology and Nature Management, Vrije Universiteit Brussel Acknowledgement Acknowledgement ‘IF I have seen further it is by standing on ye sholders of Giants’ (Isaac Newton,15 February 1676) Firstly, I want to thank my tree promoters for giving me the thrust and the opportunity of working out this interesting master thesis about mangroves. Dr. Nele Schmitz, thank you for always making time for me even when your time was scarce. Thanks for all your input and for your exchange of view and encouragement. Prof. dr. ir. Kathy Steppe, thank you for your infectious enthusiasm which made me believe that this thesis was the most interesting and ground breaking subject of modern research. Prof. Dr. Nico Koedam, I explicitly want to thank you for your companionship during my first days in Kenya and for your voluntary involvement and your critical points of view. In addition, I would like to thank you for your snorkelling invitation which was one of the highlights of my stay in Kenya. Secondly, I want to express my gratitude to both Dr. Elisabeth Robert and Dr. ir. Veerle De Schepper for correcting my text. I know lots of time and patience were needed for this task which both of them voluntarily performed. I appreciate this enormously. Thirdly, I want to thank all my friends at KMFRI in Kenya. Especially, Judith Okello and Eric Okuku for arranging my stay in Mombasa. Thanks for taking care of me and dragging me around your country to let me see the culture and the beauty of Kenya and the Kenyan people (and the Kenyan nightlife and Kenyan beers). Your hospitability and helpfulness were and still are heart-warming. When speaking of giants, I certainly refer to my companion during the fieldwork: George Onduso, a great man in every sense of the word. When I reached my physical and/or mental limits during the long measurement days in the blistering heat and in the mud, he always kept me going until all work was done. I am most grateful for all the things you did for me and this research. I would like to apologise to both my taxi drivers, Samuel Njoroge and Naftali Mukua, for the times I deprived their sleep when the measurements had to start before sunrise. I would like to thank Sturcky Okumu and Oliver Ochola for performing the soil analysis. I would like to thank Oduor Nancy Awuor for her lab assistance, which included awful assignments like counting the leaves on Avicennia marina branches. Also in Belgium, many people have their share in the establishment of this master thesis. I would like to thank dr. ir. Wouter Maes and ir. Maurits Vandegehuchte for their help with the porometer; dr. ir. Jan Van Den Bulcke and Piet Dekeyser for their assistance with microscopic photography of vessels and stomata; Prof. dr. Klaartje De Buysser and Prof. dr. Isabelle Van Driesche for the donation of millimetre pipettes. iii Acknowledgement I would like to acknowledge the following persons for allowing me to use their laboratories and lab equipment: Jared Bosire (KMFRI), Prof. dr. Nico Koedam (Laboratory of Plant Biology and Nature Management, VUB), Hans Beeckman (Laboratory of Wood Biology and Xylarium, Royal Museum for Central Africa) and Prof. dr. ir. Joris Van Acker (Laboratory of Wood Technology – Woodlab, UGent). I want to thank the ‘Laboratory of Plant Biology and Nature Management’ at the VUB and the ‘Laboratory of Plant Ecology’ at the UGent for their generous donations and financial support. I still want to thank a few people for their help and theirs support despite the fact that their contribution to this research is less directly visible. Firstly, I’m grateful to both my parents for all the chances and opportunities they gave me, which cannot be taken for granted. Thanks for all the great and small things you gave me and especially for financing my studies, my food and my chamber in Ghent which all attributed to pleasant student days. Jennifer, Korneel and Lize, I am so glad that I have such nice friends spoiling me with love, friendship, amusement and sweets. Thanks for the many times that you invited me for dinner when I had no time (or no desire) to do it myself. Especially for Jennifer, your intelligence and opinions in many subjects in life has taught me to see things from different angels, which definitely made me a better scientist/person. Last but not least, I want to thank my fellow ‘Blekersdijk 51’ occupants which became like a family to me during the years. So Jorunn, Katelijne, Niels, Robbe, Sacha and Titus, thank you for all the good times, all the pranks, all the good chats near the water and for your friendship during my study period. Hannes De Deurwaerder, August 2012 iv Abstract Abstract Mangrove ecosystems are known as very unique ecosystems with a large biodiversity, dominating the coastal zones in tropical and subtropical regions. The mangrove trees have typical characteristics to survive in a salty environment which is subjected to tides of the oceans, seas or rivers. Pneumatophores, salt glands, salt exclusion and vivipary are some adaptations that can be found among the mangrove species. Mangroves are important and essential ecosystems. For example, they function as a sediment trap preventing the destruction of coral reefs and sea grass beds, they house many different and endangered species and they are excellent fish nurseries. The local people depend on the mangrove forests for timber, charcoal and food. Despite, large scale destruction and fragmentation among mangrove forests are observed. Mangroves are even destroyed at a higher rate compared to tropical rainforests and coral reefs. This destruction is caused by intensive cutting, land use changes by humans, pollution, shrimp ponds and oil spills. Accumulating evidence points out that the siltation process particularly causing high tree mortality mangroves. Due to siltation, high amounts of suspended particles cover and thus smother the roots causing oxygen deficiency and possible death of the trees. The aim of this study was to investigate whether siltation causes stress in mangrove trees and in the worst tree mortality. It was investigated how siltation influenced the physiological and anatomical properties of the trees. Therefore, a measuring campaign was performed in the mangrove forest of Mikindani, near Mombasa (Kenya) from 10 of July till 15 August 2011. This region is known to be silted due to a flush flood in 1997 and the recent land use changes which induce anthropogenic land erosion and run off. A difference in siltation degree was observed in the region and correspondingly the region was divided in high siltation sites and low siltation sites. Measurements were performed on two local and abundant species, Avicennia marina and Rhizophora mucronata. The measurements can roughly been subdivided in two parts. First, the diurnal patterns of stomatal conductance and o hydraulic branch conductance were measured using a porometer (AP4) and by applying the field method, respectively. A decline of one or both of these physiological variables indicates more stressful conditions for the tree due to siltation. Secondly, it was studied how trees adapt their anatomical and morphological characteristics in an attempt to prevent fluctuations in their metabolic and ecophysiological pathways due to siltation. Area and density of both vessels and stomata were measured together with other characteristics such as leaf number and sizes. The amount of available oxygen for the roots was estimated by the amount of crab burrows and pneumatophores. Lower values of stomatal conductance were found in the high siltation sites indicating that siltation causes higher stress levels in trees. The stomatal conductance of Avicennia marina was abnormally high in the morning. No significant differences in hydraulic branch conductance were found between v Abstract sites and species during this study. However, during the measurements of hydraulic conductance water flowing in the opposite direction was more observed in branches collected in the highly silted sites, indicating also more stressed levels. Consistent or significant differences of anatomical features were observed between high and low siltation sites. Some adaptations to siltation were found for both species, whereas others were species-specific. Both the number of crab burrows and the number of pneumatophores were lower in the high siltation sites. It is hypothesized that the high morning values of stomatal conductance for Avicennia result from fresh water uptake by the leaves from dew which probably condensed on the leaves in the early morning. Due to the high salinity in the xylem sap, an osmotic gradient probably pulls the external fresh water through the stomata into the internal tissues. In addition, the water flows observed in the opposite direction during the hydraulic conductance measurements are presumably caused by the high amount of dehydrated cells in the branches. Dehydration of cells is higher in silted trees, since water supply is lower or even absent. The dehydrated cell attract water by creating an osmotic gradient. When the xylem sap flow is not sufficient and fast enough to neutralise this osmotic gradient, other water sources, such as water from the leaves or in the case of the experiment, water from the loading dye are drawn towards the dehydrated cells. Moreover, trees subjected to siltation have higher water us efficiencies. This is concluded from the combined changes in anatomical and morphological characteristics such as smaller and more leaves, smaller stomatal area, lower leaf water content and a better vessel-leaf correlation. To cope with siltation stress, Avicennia marina trees adapt their stomatal anatomy by increasing the stomatal density and pore area index. In addition, they protect themself against cavitation by producing smaller vessels and a higher phloem ratio. In response to siltation, Rhizophora mucronata trees adapted their vessel anatomy: an increase in vessel density and total lumen area were found in the high siltation sites. Furthermore, the decrease of crab burrows in the silted sites is probably due to the changed soil texture, hindering crabs for digging holes. Pneumatophores are most likely covered by siltation, decreasing the number of pen roots counted in the high siltation site. In conclusion, siltation imposed water and oxygen stress in both studied mangrove species. Both species responded in a similar way to siltation such as smaller stomata and more and smaller leaves for trees in high siltation sites. Although, some adaptations are species-specific, including the increased vessel density for Rhizophora and the higher stomatal density for Avicennia found in the high siltation sites. Mangroves are thus negatively influenced by siltation and the trees will adapt their anatomical and physiological characteristics. The interesting hypothesis of water uptake by the leaves within Avicennia marina trees needs further research. vi Samenvatting Samenvatting Mangrove ecosystemen staan gekend als zeer unieke ecosystemen met een hoge biodiversiteit. Ze domineren in de kustzones van tropische en subtropische gebieden. De mangrove bomen hebben typische karakteristieken om te overleven in een zout milieu dat onderhevig is aan de getijdewerking van oceanen, zeeën en rivieren. Pneumatoforen, zoutklieren, zout exclusie en viviparie zijn enkele adaptaties die worden aangetroffen onder de mangrove soorten. Mangroves zijn belangrijke en essentiële ecosystemen. ze functioneren bijvoorbeeld als een val voor sediment en beschermen zo de koraalriffen en zeegrasbedden van vernieling, ze zijn een huisvesting voor vele verschillende en bedreigde soorten en ze zijn ideaal als paaiplaats voor vissen. De lokale bevolking is afhankelijk van de mangrove wouden voor hun hout, houtskool en voedsel. Desondanks wordt grootschalige verwoesting en fragmentatie van de mangrove wouden waargenomen. Bovendien worden mangroves vernietigd tegen een hogere snelheid in vergelijking met tropische regenwouden en koraalriffen. Deze vernieling wordt veroorzaakt door intensieve houtkap, veranderingen in landgebruik door mensen, vervuiling, garnaal/scampi-kwekerijen en olielozingen. Accumulerende bewijzen wijzen erop dat het proces van aanslibbing hoge mortaliteit onder de mangrove bomen zou veroorzaken. Door aanslibbing bedekken grote hoeveelheden opgeloste partikels de wortels en verstikken deze door zuurstofgebrek, wat uiteindelijk kan leiden tot sterfte van de boom. Het doel van deze studie was het onderzoeken of aanslibbing stress en in het ergste geval de dood van mangrove bomen, veroorzaakt. Er werd bestudeerd hoe aanslibbing de fysiologische en anatomische eigenschappen van de bomen beïnvloedt. Hiervoor werd een meetcampagne uitgevoerd in het mangrove woud van Mikindani, dichtbij Mombasa (Kenia) van 10 juli tot 15 augustus 2012. Deze regio staat gekend als zijnde aangeslibd door de vloedgolf in 1997 en door de recente veranderingen in landgebruik die aanleiding geven tot antropogene erosie en run-off. Er werd een verschil in aanslibbing graad binnen het gebied geobserveerd waardoor het gebied werd opgedeeld in een hoge aanslibbing en een lage aanslibbing site. Metingen werden uitgevoerd op twee lokale en abundante soorten, Avicennia marina en Rhizophora mucronata. De metingen kunnen ruwweg worden opgedeeld in twee delen. Vooreerst werd de dagelijkse patronen van stomatale conductiviteit en hydraulische tak conductiviteit gemeten door respectievelijk gebruik te maken van een porometer (AP4) en de veldmethode. Een daling in beide fysiologische variabelen wijst op de meer stressvolle condities voor een boom door het proces van aanslibbing. Ten tweede werd er bestudeerd hoe bomen hun anatomische en morfologische karakteristieken aanpassen in een poging om fluctuaties in metabolische en ecofysiologische pathways, veroorzaakt door aanslibbing, te verhinderen. Oppervlakte en densiteit van zowel vaten als van stomata werden opgemeten evenals andere karakteristieken, zoals het aantal en de grootte van de bladeren. De hoeveelheid aan beschikbare zuurstof voor de wortels werd geschat aan de hand van het aantal pneumatoforen en het aantal holen van krabben. Lagere waarden voor stomatale conductiviteit werden aangetroffen in de hoge aanslibbing sites, wat erop wijst dat aanslibbing hogere stress niveaus creëert in bomen. ’s Ochtends werd een abnormaal hoge stomatale conductiviteit van Avidennia marina gevonden.. Gedurende de studie werden geen significante verschillen gevonden in hydraulische conductiviteit tussen de sites en de soorten. Echter gedurende de metingen van hydraulische conductiviteit werden meer waterstromingen in vii Samenvatting tegengestelde richting geobserveerd in takken verzameld uit de hoge aanslibbing sites, wat ook wijst op hogere stressniveaus. Consistente of significante verschillen in anatomische karakteristieken werden waargenomen tussen hoge en lage aanslibbing sites. Sommige aanpassingen voor aanslibbing werden gevonden voor beide soorten, terwijl andere soort-specifiek waren. Zowel het aantal holen van krabben als het aantal pneumatoforen was lager in de hoge aanslibbing sites. De voorgestelde hypothese stelt dat de hoge ochtend waarden van stomatale conductiviteit bij Avicennia resulteren uit de opname van zoet water via de bladeren, wat afkomstig zou zijn van dauw dat waarschijnlijk in de vroege ochtend condenseert op de bladeren. Door de hoge saliniteit in het xyleem sap zal een osmotische gradiënt het externe zoet water doorheen de stomata trekken tot in de interne weefsels. Daarnaast werden de tegengestelde waterstromingen, geobserveerd tijdens de hydraulische tak conductiviteit, wellicht veroorzaakt door de hoge hoeveelheid aan gedehydrateerde cellen in de takken. Dehydratatie van cellen is hoger voor aangeslibde bomen aangezien de water aanvoer lager, tot zelf afwezig, is. De gedehydrateerde cellen trekken water aan door het creëren van een osmotische gradiënt. Wanneer de xyleem sapstroom niet bevredigend en snel genoeg is voor de neutralisatie van deze osmotische gradiënt, dan zullen andere bronnen van water, zoals het water in de bladeren of zoals in het geval bij dit experiment, water van de kleurvloeistof, worden aangetrokken naar de gedehydrateerde cellen. Bovendien hebben bomen, die onderhevig zijn aan aanslibbing, een hogere efficiëntie van watergebruik. Dit wordt geconcludeerd uit de gecombineerde veranderingen in anatomische en morfologische karakteristieken, zoals kleinere en meer bladeren, kleinere stomatale oppervlakte, lagere bladwaterinhoud en een betere vat-blad correlatie. Om het hoofd te bieden aan aanslibbing stress, passen Avicennia marina bomen hun stomatale anatomie aan door het verhogen van de stomatale densiteit en pore-area index. Daarnaast beschermen zij zichzelf tegen cavitatie door de productie van kleinere vaten en een hogere floëem ratio. Als antwoord op aanslibbing zullen Rhizophora mucronata bomen daarentegen hun vat anatomie aanpassen: een toename in vat densiteit en totale lumen-area werd aangetroffen in hoge aanslibbing sites. Bovendien is de afname in krab holen in de aangeslibde sites waarschijnlijk te wijten aan de veranderde bodem textuur die de krabben hinderen bij het graven van holen. De pneumatoforen zijn hoogst waarschijnlijk bedekt door sediment, wat het aantal luchtwortels in de hoge aanslibbing site doet dalen. Concluderend dat aanslibbing water en zuurstof stress oplegt voor beide bestudeerde mangrove soorten. Beide soorten reageerden op gelijke manier op aanslibbing door bijvoorbeeld kleinere stomata en kleinere maar meerdere bladeren te vormen. Echter waren sommige adaptaties soort- specifiek zijn, zo werden hogere vatendensiteit bij Rhizophora en hogere stomatale densiteiten bij Avicennia aangetroffen in de hoog aangeslibde sites. De mangrovebomen worden dus negatief beïnvloed door aanslibbing waarop de bomen hun anatomie en fysiologie zullen op aanpassen. Meer onderzoek naar de interessante hypothese van wateropname via de bladeren bij Avicennia marina bomen is nodig. viii Contents Contents Acknowledgement ................................................................................................................................... iii Abstract .................................................................................................................................................... v Samenvatting .......................................................................................................................................... vii Introduction ............................................................................................................................................ xii 1. Literature Review ............................................................................................................ 1 1.1 Mangrove Ecosystems ......................................................................................................... 2 1.1.1 Mangrove definition ........................................................................................................ 2 1.1.2 Distribution ...................................................................................................................... 3 1.1.3 Cause of distribution ....................................................................................................... 6 1.1.4 Spatial pattern of mangrove zones ................................................................................. 8 1.1.5 Adaptations to extreme environmental conditions ........................................................ 8 1.1.6 The benefits of mangroves ............................................................................................ 12 1.1.7 Threats for mangrove ecosystems ................................................................................ 13 1.2 Siltation ................................................................................................................................ 14 1.2.1 Definition of siltation ..................................................................................................... 14 1.2.2 Origin of siltation events ............................................................................................... 16 1.2.3 Anatomical and physiological impact of sedimentation ............................................... 17 1.3 Avicennia marina and Rhizophora mucronata ............................................................. 17 1.3.1 Classification .................................................................................................................. 17 1.3.2 Adaptation to cope with high salinity ........................................................................... 18 1.3.3 Wood anatomy .............................................................................................................. 18 1.3.4 Reproduction ................................................................................................................. 19 1.4 Study area description and research goals .................................................................... 19 1.4.1 Selection study site ........................................................................................................ 19 1.4.2 Climate ........................................................................................................................... 19 1.4.3 Study goals .................................................................................................................... 20 2 Material and Methods ............................................................................................... 23 2.1 Study site and measurement period .............................................................................. 23 2.2 General site characteristics .............................................................................................. 25 2.2.1 Micro-climate ................................................................................................................ 25 ix Contents 2.2.2 Soil characteristics ......................................................................................................... 25 2.2.3 Tree sampling ................................................................................................................ 25 2.3 Leaf characteristics ............................................................................................................ 26 2.3.1 Leaf sampling ................................................................................................................. 26 2.3.2 Leaf water content, leaf number and leaf size .............................................................. 27 2.3.3 Stomatal characteristics ................................................................................................ 28 2.3.4 Stomatal resistance ....................................................................................................... 28 2.4 Branch characteristics ....................................................................................................... 29 2.4.1 Branch sampling ............................................................................................................ 30 2.4.2 Branch anatomy ............................................................................................................ 31 2.4.3 Hydraulic conductivity ................................................................................................... 32 2.5 Root characteristics ........................................................................................................... 34 2.6 Statistical data analysis ..................................................................................................... 35 3 Results ..................................................................................................................................... 37 3.1 General site characteristics .............................................................................................. 37 3.2 Leaf characteristics ............................................................................................................ 38 3.2.1 Leaf water content, leaf number and leaf size .............................................................. 38 3.2.2 Stomatal characteristics ................................................................................................ 41 3.2.3 Stomatal resistance ....................................................................................................... 43 3.3 Branch characteristics ....................................................................................................... 47 3.3.1 Branch anatomy ............................................................................................................ 47 3.3.2 Hydraulic conductivity ................................................................................................... 50 3.4 Root characteristics .............................................................................................................. 51 4 Discussion ............................................................................................................................. 53 4.1 Division in low and high siltation site is justified ......................................................... 53 4.2 Siltation produces physiological stress in mangrove trees ......................................... 53 4.2.1 Stomatal conductance ................................................................................................... 53 4.2.2 Hydraulic conductivity ................................................................................................... 56 4.3 Anatomical adaptations to withstand the negative influence of siltation .............. 57 4.3.1 Similar adaptations to siltation for Avicennia and Rhizophora ..................................... 58 4.3.2 Species specific adaptations to siltation for Avicenia ................................................... 59 4.3.3 Species specific adaptations to siltation for Rhizophora ............................................... 60 4.4 Root characteristics ........................................................................................................... 61 x