Projets tutorés

Towards a better understanding of trees’ futures in a natural tropical forest through the architectural tracking of their vitality

Timothée Audinot and Lefebvre Laurie under the supervision of Lilian Blanc and Eric Nicolini

Because of climate change, drought episodes and El Niño events are becoming more frequent and intense. The rainforest is particularly sensitive to such events. However, while the impact of these events on temperate forests is well studied, only a few studies focus on rainforest vitality. In the present study, we investigated crown fragmentation through primary branch mortality, secondary branch mortality, and trunk mortality in order to better understand the evolution of tree vitality and to predict future tree mortality. The main findings of this study are that trees of the past which have a highly-fragmented crown are more likely to die and that regeneration, even though it is quite rare, happens across several tree species. Most of the trees are in the present and death rate is quite low. The majority of trees die standing, meaning without any external factor such as impacts from tree falls. The ability to predict trees’ futures can be very useful for forest management.

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Increased cohorts beta-diversity in response to logging over 29 years

Job Knoester and Sylvain Schmitt under the supervision of Ariane Mirabel et Camille Piponiot-Laroche

Selective logging, the targeted harvesting of timber from commercial species in a single cutting cycle, is increasing in tropical forests, and its long term impact is still relatively unknown. We thus need to study the impact of selective logging on tropical forest dynamics. We designed communities from individual cohorts with similar histories discriminating new recruits from the original community. We used beta-diversity from Tsallis entropy of order 1 (Shannon) between those communities to study and compare turnover of logged and unlogged forests over time. We show that logged forests had higher turnover rates than unlogged, even 29 years after the log date. And we also find that selective logging mainly affect —diversity with species distribution evenness. Consequently, evenness is needed to assess selective logging’s impact on tropical forests. Additionally, our results suggest that models assuming mid-term recovery to selective logging are inadequate for forest management and conservation policy.

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Short-term impacts of selective logging on functional diversity in a tropical forest

Amandine Confais and Oksana Grente under the supervision of Ariane Mirabel et Camille Piponiot-Laroche

Tropical forests have been widely logged and it keeps expanding. Numerous studies only focused on effects of this logging on taxonomic diversity, or they analysed functional diversity several years after logging. Our objective was to determine the functional diversity and community structure changes in selectively logged plots immediately after disturbance, in a French Guiana tropical forest. We chose eight functional traits related to leaf and wood economic spectra, and life history. The analyses were performed with the three independent and complementary indices, FRic, FEve and FDiv, and the CWM index. We found significant decrease of functional richness and increase of functional evenness, presumably resulting from the recruitment of heliophilous species. Logging did not affect the community structure on short-term, which was based on the “limiting similarity” assembly rule.

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Speciation between two ecotypes of Symphonia globulifera in French Guiana

Solène D’Angelo, Pauline Guillaumeau and Mathieu Jegu under the supervision of Niklas Tysklind

Symphonia globulifera is widely distributed in South America and mostly one phenotype occurs throughout the species range. In French Guiana and Surinam, S. globulifera presents two ecotypes (Göte Wilhelm Turesson, 1922) with low genetic differentiation, despite the presence of two apparent phenotypes (Tinaut et al., 2015). This second ecotype usually called S. sp1 is also found in terra firme contrary to the main phenotype which is only found in bottom-lands. Local adaptation has been observed through a reciprocal transplantation in experimental gardens (Tinaut et al., 2015). This study allows us to think that this species is trying to adapt by genetic innovation strategies. S. sp1 is colonizing a new habitat (terra firme), and we can use it as a model of sympatric speciation process. We have compared these 2 ecotypes for adults followed Tiraut et al, who has already demonstrated difference in gene expression for juveniles. We found a lower differentiation in gene expression for adults but still observable for 261 genes, showing a relation with the ontogeny of Symphonia globulifera.

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