Fabiënne de Jager: The effect of stressors on the onset of pneumatophore development of black mangroves

Wageningen University & Research

For my Master Thesis I went to Semarang to study the effects of flooding events on the Avicennia mangrove species in the Demak Regency (Timbulsloko). Seawater with a high salinity (30 ppt) flows further into the estuaries and rivers, which causes salinity concentrations in aquifers, aquaculture ponds and in ground water. Pneumatophores act as mechanical barrier and reduce the speed of the waves coming from storm surges during rain season (November till March) or during spring tides (mostly occurring in May-June). Therefore, the need of maintaining and rebuilding these mangroves are crucial for coastal protection. Mangroves are the most important plant species in this area to protect the coastal villages around Timbulsloko. I mainly found two Avicennia species in the area; Avicennia marina and Avicennia alba. These species are recognizable by their aerial roots which grow above the sediment to catch extra oxygen from the air. This happens when the soil conditions are anoxic. My hypothesis is that salinity and sediment type is also a stimulating factor for pneumatophore growth. Results showed that salinity of the surface water did have a significant effect on the pneumatophore growth of the mangroves. The other variables didn’t seem to have an effect on the pneumatophores. This could be due to the big differences in sites and trees during the sampling and the wide variance of the data.

In short during this field research I gained a lot of knowledge about mangroves and the species differences, which I can take with me in the future.

Xenia Davide: Effects of Temperature and Rainfall on Stem Growth in the Tropical Tree Species Toona ciliata

Maastricht University and Wageningen University

Tropical forests are essential to the regulation of climate and the global carbon cycle. They account for a third of global primary productivity, drive fluctuations in the terrestrial land sink, and thus have the potential to influence the pace of climate change. Multiple investigations revealed declining tree growth rates and forest productivity, making it increasingly important to comprehend how climate variability, particularly temperature and rainfall patterns, affects tropical trees. Both rainfall and maximum temperature have been found to be crucial factors in influencing stem growth. The predicted increase in climate change-related variability also increases the necessity of understanding tree growth responses. Despite their critical function, there is currently an absence of comprehensive understanding of tropical tree physiological and growth responses to climate variability.
With this project, I aim to bridge this gap by analysing samples of Toona ciliata, a tropical tree that was shown to make tree rings, with standard dendrochronological methodology. The samples were collected in multiple locations in the wet tropics world heritage, Queensland, Australia. By doing so, I want to provide new insights into the effects of climate on the growth and physiology of this species in the region.

Alba Gimeno: Secondary forest regeneration in Mexico

Wageningen University and Research

Tropical forests are disappearing rapidly due to deforestation and forest degradation. However, secondary forests can regrow once the land is left fallow through a process called secondary succession. This study aims to (1) understand the effect of the surrounding landscape in terms of distance to the nearest forest and percentage of surrounding forest cover on seed dispersal and (2) the effect of site factors, such as soil characteristics and plant resources (i.e., light and water) on seedling establishment. Fieldwork was conducted in 3 years old abandoned pasture fields in twenty 25x25m plots in the wet forest region of Loma Bonita, Chiapas, Mexico. Results showed that a higher pioneer forest cover within a 1km radius around the plot increased seed species richness. Higher tree presence in the plots increased seedling abundance and richness, as well as the number of pioneer seedlings. Moreover, seedling abundance was positively influenced by soil organic matter and the percentage of pioneer seedlings was positively correlated to soil nitrogen. Pioneer trees in the surrounding forest up to 1km are important seed sources, and remnant trees and soil organic matter of the regenerating forest are important in creating favourable conditions for seedling establishment.

Angélique Meyer: Explaining Zostera noltii seed distribution in Arcachon Bay

Rijksuniversiteit Groningen

Seagrass meadows, found in shallow coastal waters, provide valuable ecosystem services such as nutrient filtering, carbon storage, and supporting biodiversity. However, human pressure has led to rapid losses of these ecosystems on a global scale. Traditional methods of restoring seagrass meadows through transplanting shoots and rhizomes have limitations, leading to an increased interest in seed-based restoration as a cost-effective and feasible method for larger-scale restoration. In France’s Arcachon Bay, which hosts the largest Zostera noltii beds in Europe, there has been a 33% loss in the total area of Z. noltii beds. The Marine Natural Park of the Bay is working to restore the seagrass meadows to their early 2000s level. As part of this restoration effort, I investigated the environmental factors that affect the presence of a Zostera noltii seed bank at both a large and small scale. Results showed that seagrass cover, bottom flow speed, shear stress, immersion time, salinity, and sediment grain size were the main variables influencing seed presence and density. However, seed density varied greatly on a local scale, and no predictive model of seed presence could be constructed. Despite this, understanding seed bank presence can still guide seed-based seagrass restoration efforts, with the potential to contribute to seagrass recovery and the colonization or recolonization of zones globally.

Julian Voet: Population dynamics of Persoonia arborea

Rijksuniversiteit Groningen

Tree Geebung (Persoonia arborea) is een boomsoort die alleen voorkomt in de Central Highlands van Victoria, Australië. Deze bossen zijn doorgaans onderhevig aan bosbranden en houtkap. Veldobservaties suggereren dat Tree Geebung bosbranden kan overleven (ondanks dat de soort beschouwd wordt al brandgevoelig) en dat het massaal kan kiemen na houtkapactiviteiten. Hoewel er weinig bewijs voor is, wordt er gedacht dat Tree Geebung zeer oud kan worden en daardoor erg kwetsbaar is voor verstoringen. De soort is momenteel opgenomen als (ernstig) bedreigd in relevante inventarisatielijsten.

Het doel van mijn onderzoek is om belangrijke eigenschappen van Tree Geebung op te helderen, bijv. tijd tot maturatie en respons tegen vuur en houtkap, ter bevordering van het beheer. Mijn bevindingen zijn dat Tree Geebung tenminste 74 jaar oud kan worden, en dat individuen tussen 28-40 jaar 50% kans hebben om fruit te produceren. Een individu beschermd onder het huidige protocol (≥10 cm DBH) heeft een kans van 40-70% om fruit te produceren. De kiemrespons van Tree Geebung is sterker na houtkapactiviteit dan na een bosbrand. Daarnaast kan Tree Geebung een bosbrand overleven mits deze van lage intensiteit is. Hoewel Tree Geebung niet zo oud wordt als gedacht, geven mijn resultaten wel weer dat Tree Geebung door de bijzondere respons tegen verstoringen nog steeds een kwetsbare soort blijft.
Meer weten? Ik heb tijdens mijn verblijf YouTube-lessen gemaakt die ingaan op de ecologie van (bos)branden en vertel er ook over mijn eigen onderzoek en de belevenis van het studeren in Australië.

Tim Claerhout: Deelname botanische expeditie Sint-Helena

PhD student Hortus Botanicus Leiden

Sint-Helena, een eiland gelegen op zo’n 2000 km van de Afrikaanse kust, is een van de meest afgelegen plaatsen op aarde. Ondanks het enorme potentieel voor endemische soorten kwam de eerste verregaande lichenologische expeditie pas in 2006. Onze expeditie bouwt hierop voort door het inschatten van de populatiegroottes en de verspreiding van de endemische lichenen met een extra focus op de Ramalina spp. (Ramalina geniculatella, R. ketner-oostrae, R. rigidella en R. sanctae-helenae). Tijdens deze expeditie reisde ik mee met een internationaal team van lichenologen. We bemonsterden +/- 50 punten, verspreid over 14 locaties. Op elke locatie werd een inschatting gemaakt van de populatiegrootte van elk van de endemische soorten. Wanneer lichenen werden gevonden die potentieel nieuw zijn voor het eiland en/of de wetenschap, werden collecties aangelegd. Deze zullen geïdentificeerd en/of beschreven worden in de maanden die volgen. Ook van de Ramalina spp. werden collecties aangelegd om de soort en/of de endemische status te bevestigen. Dit alles zorgt voor een bredere kennis over de lichenologische flora van Sint-Helena en de verspreiding van de endemische lichenen op het eiland. Dit laatste helpt natuurbeschermers om “areas of interest” aan te duiden om de restanten van hun endemische biodiversiteit te beschermen.