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.

The biodiversity value of carbon sequestration in secondary forests
Misaki Takahashi

Why carbon and diversity? Tropical forests can sequester large amounts of carbon by removing CO2 from the atmosphere, thus contributing to climate change mitigation. Carbon credit mechanisms such as REDD+ (Reduced Emissions from Deforestation and forest Degradation) have been developed to conserve forests while paying for their climate change mitigation. These carbon credit mechanisms tend to focus on mature forests and ignore young secondary forests, that have large carbon sequestration rates and species accumulation rates. Additionally, policymakers tend to regard tree species diversity as a co-benefit, rather than as a requirement to increase carbon stocks.

The aim. This study examines the relationship between climate change mitigation and biodiversity conservation by analyzing 1) how individual tree species contribute to aboveground biomass stocks, and 2) when both carbon sequestration rate and tree species diversity are relatively high during succession, thus maximizing conservation benefits.

The approach. Chronosequence data were used from wet and dry secondary tropical forests in Mexico. Plots were established in regrowing forests differing in age (1-60 years) since agricultural abandonment, and compared to neighbouring mature forests.

Findings.

  • AGB stock and tree species diversity increased over time in both wet and dry forests, indicating that their carbon storage and conservation potential accrue over time.
  • Early successional forests are dominated by a few species, which contribute most to stand biomass
  • In 9-year old secondary forests eight species make up 90% of the forest biomass in wet forests, compared to less than two species in dry forests. Hence, for the same amount of carbon a larger diversity is conserved in wet than in dry forests.
  • More species contributed to forest AGB stock at later successional stages: in mature forests 25 species contributed to 90% of AGB in wet forests compared to 8 species in dry forests. Hence, for the same amount of carbon a larger diversity is conserved in old than in young forests.
  • During succession, tree species diversity increased whereas carbon sequestration rate shows an optimum with time. Carbon sequestration rate and diversity were relatively equally high after 4 decades in wet forest and 7 decades in dry forest.

Implications. Tree species diversity and AGB are positively correlated, indicating that high biodiversity may increase AGB stock through species complementarity in resource use. We propose a portfolio of carbon and tree species diversity conservation strategies, that  can be used alone or in combination: 1) young forests can be conserved for the highest carbon sequestration rate and rapid species accumulation, 2) mid-successional wet forests and old-growth dry forests can be conserved for relatively high carbon sequestration rate and biodiversity conservation, and 3) old-growth forests can be conserved for carbon storage and a large number of (old-growth) species.

Citation. Misaki Takahashi (2020) The biodiversity value of carbon sequestration during secondary succession in dry and wet tropical forests: a bridge between ecology and policy. MSc thesis, Wageningen University & Research, Wageningen, The Netherlands.

Misaki Takahashi graduated in Forest and Nature Conservation. She is interested in the policy-ecology interface, and currently works in Japan as a forestry consultant in tropical environment and development. 

MSc thesis (Undergoing)

The effect of previous land use on tropical secondary dry forest recovery in Mexico 

Jansma Robyn

The rationale. Abandonment of agricultural and pastoral lands after losing their productivity favors the partial or total recovery of vegetation through secondary succession, leading to the emergence of young secondary forests. The previous land use, however, imposes ecological disturbances affecting the forest recovery potential and inseparably the forests’ value for ecosystem services such as carbon sequestration or biodiversity conservation. As the vegetation regenerating during the first years of succession appears to be the foundation for the subsequent successional developments, it is essential to uncover how land use prior to abandonment affects the regenerating forest structure and growth form composition in its early stages.   

Aims. To identify how human land use prior to abandonment affects the forest recovery potential in a tropical dry forest in Nizanda, Mexico, and how different components and specific management practices of the land use history are responsible for diverging successional trajectories.  

Approach. I will obtain the level of previous land use disturbance in several abandoned agricultural and pastoral plots through semi-structured interviews with farmers and landowners and quantify this data by using an ecological disturbance index. By simultaneously evaluating the regenerating forests’ structural attributes and vegetation growth form composition in the first years after abandonment, I hope to identify management practices that do not hamper forest recovery and thereby provide a base for more sustainable future land use.   

Landscape drivers of regrowth

Jules Koppen

The rationale. Agricultural expansion is a main cause of tropical deforestation, but 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 affected by the land-use history, but it is also driven by the landscape context: the structure and composition of the surrounding landscape. Different successional pathways may establish early in succession, and assessing the effect of the surrounding the forest cover gradient may identify tipping points that can determine alternative pathways. 
 
Aims. To examine the effects of surrounding landscape on secondary succession of tropical wet and dry forests in Mexico.  
 
Approach. Spatial data of the surrounding landscape will be obtained using satellite imagery. The surrounding landscape will be classified into six different categories, and their landscape cover will be determined for increasing spatial scales. Forest regeneration will be measured by assessing vegetation structure and diversity. This landscape-scale analysis may identify potential thresholds in the cover of surrounding landscapes to aid in the recovery of secondary tropical forests, which is pivotal in the light of tropical deforestation. 

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.