MSc thesis (Completed)

Functional traits reveal how pioneer tree species cope with drought
Coen van den Brand

The rationale. Pioneers are the first plant species to establish during forest succession. By forming a first canopy layer they improve microclimatic conditions, thus paving the way for the establishment of other species. It is not clear how pioneers are able to cope with the extremely hot and dry environmental conditions. A clue may be find in their hydraulic traits; how do plants take up, transport, and use water? Insights into hydraulic traits may allow to better understand under what conditions pioneers are able to establish, and to improve species selection for restoration activities. 

Aims. This study aimed to understand how pioneer species from dry and wet tropical forests differ in their hydraulic traits.  

Approach. Seventeen stem and leaf traits were collected for 10 pioneer species from dry forest and 8 pioneer species from wet forest in Ghana.  

Findings 

  • Dry forest species were more drought tolerant than wet forest species; they had a more negative leaf water potential, which allows them to continue leaf functioning during drought stress.  
  • Dry forest species had a higher wood density which ensures hydraulic safety and continued water transport during drought. They also had higher leaf phosphorus concentration which may increase photosynthesis.  
  • Wet forest species had a larger leaf stomatal area which allows them to transpire more water, that is in abundant supply. 
  • Across species, drought tolerance (i.e., a high absolute leaf water potential) was positively correlated with wood density and negatively with leaf area and stomatal leaf area, indicating a continuum in drought strategies. 

Implications. Tropical pioneer species and secondary succession may help to restore landscapes naturally and combat climate change. To enhance the success of restoration programs in dry environments, drought tolerant species can be selected that have a high wood density and a high absolute leaf water potential. 

Citation. Coen van den Brand (2020) Tropical pioneer species show a drought tolerance gradient and differ in hydraulic traits. MSc thesis, Wageningen University and Research, Wageningen, the Netherlands 

Coen van den Brand graduated in Forest and Nature Conservation. He is interested in restoration of tropical forest landscapes, co-organized the FEM restoration sessions, and currently works as a forestry consultant. 

How do functional traits differ amongst secondary forest species?
Edoardo Nevola

Why traits? Plant species differ tremendously in their characteristics (i.e., ‘traits’). To design efficient forest restoration strategies, insight is needed how species vary in their traits, and how that allows them to become successful in a specific environment.

The aims. This study aimed to evaluate how functional traits of secondary tree species differ between ecological groups related to successional stage (early vs. late), forest type (dry vs. wet), adult stature (small vs. large), life form (shrubs vs. trees) and origin (exotic vs. native).

The approach. 11 leaf-, stem- root- and reproductive traits were compiled for 392 tropical secondary forest species from Queensland, Australia.

Findings

  • Late-successional species and exotic species had larger animal-dispersed seeds and therefore a better establishment success than early-successional species and native species.
  • Trees and wet forest species had larger leaves, fruits and seeds and lower wood density than shrubs and dry forest species. 
  • A principal component analysis revealed two plant strategy axes; 1) a reproductive axis reflecting a trade-off between small-seeded species with high colonization potential and large-seeded species with high establishment success, and 2) an economic axis reflecting a trade-off between dense-wooded evergreen species and soft-wooded deciduous species that can avoid drought.

Implications. To increase the diversity and resilience of restored forests, a mix of species with different trait values could be selected for mixed species plantings. Native and pioneer species have widely shown to be successful in forest restoration. If carefully selected, additionally species with different traits values can be included, such as late-successional species or exotic species. Late-successional and exotic species have larger seeds, and therefore a larger establishment success than native and early-successional species. At the same time they can bring several benefits to the ecosystem. Because of their fleshy seeds they can attract frugivore seed dispersers. They can shade out herbaceous weeds, may act as nurse trees for the establishment of other species, and provide forest products and resilience to environmental change.

Citation. Eduardo Nevola (2020) How do functional traits differ amongst tropical secondary forest species? MSc thesis, Wageningen University and Research, Wageningen, the Netherlands.

Edoardo Nevola graduated in  Forest and Nature Conservation from WUR. He is interested in forest ecology, and is currently working at the University of Milan on forest carbon storage.

Landscape effects on early secondary forest succession in Nizanda, Mexico Jules Koppen

The rationale. Agricultural expansion is a main cause of tropical deforestation, with loss of biodiversity and ecosystem services and increased global carbon emissions as a result. With the abandonment of the agricultural fields, secondary succession is allowed to set in, forming secondary forests through natural regeneration. These tropical secondary forests are valuable as a carbon sink, but may also aid in biodiversity recovery and ecosystem provisioning. Secondary succession is thought to be driven by the land-use history, but it is also influenced by the landscape context: the structure and composition of the surrounding landscape that provide seed sources and habitat for dispersal agents. Discovering the effects of the surrounding landscape on forest recovery may grant a valuable insight for restoration purposes.

Aims. The aim of this research is to identify the landscape attributes that affect secondary tropical dry forest regeneration after land abandonment.

Approach. Fourteen 25 x 25 m plots were established on recently abandoned pastures and crop fields along a surrounding forest cover gradient (0-100%). Data on woody and herbaceous vegetation was gathered using a stratified plot design. Land cover types including old secondary forest (OSF, 25+ years old) and intermediate-aged secondary forest (ISF, 25 to 11 years old) in a circle of 100m and 1200m radius around the focal plots.

Findings.

  • Intermediate-aged forests in the direct proximity pre-eminently affected the recovery of secondary tropical forest during the first year of succession, enhancing forest structure, increasing LAI and constituting successional shifts towards pioneer tree dominated stages.

  • Contrastingly, old-aged forests negatively influenced the recovery of forest diversity.

  • Landscape attributes at larger spatial scales (up to 1200 m) are less important perhaps, because dry forests are dominated by wind-dispersed trees, and less on animals that need surrounding habitat.



Implications. This research indicates that natural regeneration can be a viable method of forest restoration, with secondary forest in the direct neighbourhood serving seed source and as adequate disperser habitat. Future restoration projects should not only focus on preserving mature forests, but should also consider mid-aged secondary forests for restoration efforts

Citation. Jules Koppen (2022) Landscape effects on early secondary forest succession in Nizanda, Mexico. MSc thesis, Wageningen University and Research, Wageningen, the Netherlands.

Jules Koppen graduated in Forest and Nature Conservation with the Management track. He now works as a project coordinator at IUCN NL.

Land use history drives early secondary succession in a Mexican tropical dry forest Robyn Jansma

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The rationale. Abandonment of agricultural and pastoral lands leads to the emergence of young, secondary forests in the tropics. The land use history generates variation in forest recovery and therefore to the ecosystem services these regrowing forests provide, such as carbon sequestration or biodiversity conservation. The first years of succession are the foundation for the subsequent successional developments and it is essential to uncover how land use prior to abandonment affects early forest succession.

The aim. This research evaluates how management practices during agricultural and pastoral use affect subsequent secondary forest recovery on abandoned fields in a tropical dry forest in Southern Mexico.

The approach. Semi-structured interviews with farmers and landowners were conducted to define each site’s land use history. In 14 abandoned agricultural plots trees and different growth forms were measured for their size and identified to species. 13 Forest recovery attributes were assessed related to growth form composition, forest structure and diversity, to find potential relationships between land use history and forest recovery.

Findings.

  • The use of heavy machinery increases herb cover but reduces tree cover, forest structure, and tree diversity, probably through destruction of regenerative legacies and soil compaction.
  • Past fire events reduce the relative abundance and density of trees only, likely through destruction of seeds, buds and resprouts and reduced soil nutrients.
  • Dual land use (i.e., the combination of crop cultivation and pasture), is surprisingly advantageous for the recovery of the vegetation cover and LAI as compared to single use (crop cultivation only) as it harbors more remnant vegetation at the time of abandonment.
  • Longer fallow periods stimulate the recovery of tree and seedling diversity, potentially because those sites are surrounded by relatively more mature forest.
  • Finally, variation in soil conditions has little to no effect on forest recovery.

Implications. The use of heavy machinery is the most influential management practice, affecting almost all forest attributes negatively. When natural forest regrowth is intended after land use, then farmers are advised to constrain from using machinery as far as possible and feasible. With the eye on natural regeneration, dual land use proves a tenable practice and burning is potentially a tolerable alternative for land clearing instead of using machinery. The incorporation of fallow periods is beneficial for recovery of forest diversity and should be encouraged. The previous application of these practices can serve as proxies in nature restoration efforts to identify those abandoned sites where assisted natural regeneration is needed.

Citation. Robyn Jansma (2022). Land use history drives early secondary succession in a Mexican tropical dry forest. MSc thesis, Wageningen University & Research, Wageningen, The Netherlands.

P.S. Due to covid-19, my fieldwork could not be conducted in Mexico. Luckily, I got the chance to perform similar research in Ghana, which resulted in the most amazing and educational experience.

Understory plant dynamics during tropical dry forest succession in Ghana Miho Tsujii

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The rationale. The area of tropical dry forests is decreasing rapidly because of land use conversion, but forests have also the potential to regrow naturally on abandoned lands. The secondary succession starts with the regeneration of tree seedlings and the pathway of succession is determined by three interlinked factors (stand age, stand structure, and environment).

The aim. This study aimed to get a mechanistic understanding of secondary succession by analysing 1) how does stand structure changes with time, 2) how changes in stand structure affect understory environment, and 3) how changes in environment drive plant regeneration.

The approach. In 19 plots aged from 0 to 6 years, stand structure (basal area, height variation, and crown area) and regeneration factors (growth form composition and seedling diversity) were measured. In each plot, a sensor was established to record temperature and soil humidity. Soil samples were taken to quantify bulk density and soil fertility. Light intensity was measured at 1 m above ground in each plot.

Findings

  • Forest structure increased linearly with time probably because the forests are in a young development stage and established trees grew continuously.
  • Increase in forest structure led to a darker and cooler understory microclimate and wetter soil probably due to more leaf mass and more light absorption by leaves.
  • Only soil phosphorus concentration among other nutrients decreased with the development of forest structure potentially due to the uptake by regrowing plants and immobilization.
  • The decrease in the proportion of herbaceous species was led by the decline in light and soil fertility as herbaceous species are usually short-lived and more light- and nutrient- demanding compared to woody species.
  • Changes in environmental conditions did not influence tree seedling diversity potentially because dispersal limitation was a stronger determinant of seedling diversity.

Implications

Because the establishment of forests in the early stage occurs rapidly and trees can outcompete herbaceous species along with the change in environment, nature-based forest restoration can be carried out. The change in soil factors and seedling diversity was not driven by the change in stand structure or microclimate and is supposed to proceed rather slowly. Therefore we should assist the natural regeneration by ensuring there is sufficient forests nearby that provide seed and seed dispersal agents.

Citation. Miho Tsujii (2022). Understory plant dynamics during tropical dry forest succession in Ghana. MSc thesis, Wageningen University & Research, Wageningen, The Netherlands.

MSc thesis (Ongoing)

Are hydraulic strategies of tropical forest trees globally determined by climate or soil properties?

Annabel Isarin 

The rationale. Human-induced climate change has increased the frequency and intensity of droughts worldwide. This may increase tree mortality and affect species distribution and diversity, both in dry and wet tropical forests. Therefore, there is an increased need to better understand the hydraulics and drought responses of tropical forests. Hydraulic strategies, characterized by different hydraulic functional traits, determine tree water use and drought resistance. Species may be either hydraulically efficient, or hydraulically safe, due to a tradeoff between a high xylem conductance and cavitation resistance. Furthermore, species may be drought tolerant, characterized by a low turgor loss point, or drought avoidant through shedding their leaves during the dry season. These hydraulic strategies have been found to be driven by differences in precipitation. However, recent research revealed high cavitation resistance to be associated with a low soil phosphorus content. This suggests that hydraulic strategies might have evolved to tolerate harsh growing conditions in general.  

Aims. My research is aimed to find out how hydraulic strategies are determined by climate and soil properties.  

Approach. My approach is to create an extensive database from several hydraulic traits of trees across the pantropics, focusing on the cavitation resistance (P50) and the turgor loss point (Ψtlp), by collecting available data from published studies. Climatic variables and soil properties are largely gathered from global databases.  

Implications. This study will contribute to understanding the complex interactions between the climate, soil properties and tropical tree hydraulics, which is essential for conservation and restoration of tropical forests worldwide.