How are anatomical and hydraulic features of Avicennia marina and Rhizophora mucronata trees influenced by siltation?
De Deurwaerder, H. (2012). How are anatomical and hydraulic features of Avicennia marina and Rhizophora mucronata trees influenced by siltation? MSc Thesis. Universiteit Gent: Gent. 80 pp.
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 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.
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