Projets Tutorés

Estimation of recent climate variations in Neotropical opened forest based on node length fluctuations in Cecropia species

Claire Montigny, Clément Andrzejewski, Maxime Roumazeilles under the supervision of Patrick Heuret.

Dendroclimatology could help in monitoring climate variation in a context on global climate change. However, in tropical forest, this kind of approach is difficult because only a few trees show relevant seasonal markers. Cecropia sciadophylla showed the smallest internode length during the dry season. 29 Cecropia from 3 species (C obtusae, C. sciadophylla and C. distachya) were measured from stump to apex in Saul and Regina in French Guyana. We managed to compute a method in order to agglomerate daily rainfall data on a node level with significant correlation coefficient. Pair-wise alignment constraint with the dates extracted from the method revealed similar alignments with a semi global alignment with affine gap cost which indicates that node length fluctuated according to the same factor which is precipitation. A model was deduced from a power regression of cumulative rainfall and residual length. A prediction of available water was possible knowing node length.

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Growth responses of neotropical trees to resource availability and individual condition : an innovative method using LIDAR canopy height model

Elias Ganivet, Romain Gaspard under the supervision of Mélaine Aubry-Kientz, Elodie Allié, Bruno Herault et Stéphane Traissac

In complex ecosystems like neotropical forests, it is largely unknown how tree species differ in their response of growth to resource availability and individual conditions (e.g. size, density...). Previous studies having usually focused on the seedling or sapling life stage, an understanding of the drivers of tree growth is required to predict changes of community composition and biodiversity, as well as to apply detailed process-based simulation models to predict forest dynamics when environmental conditions change. We used a linear mixed effect approach to quantify the impact of light availability and individual conditions on growth of 134 woody species in a 1 ha plot of lowland tropical rainforest at the Paracou experimental site in French Guiana. To estimate the light avalability, we used an innovative technique by estimating the illumination of the crown from LIDAR data, especially from the LIDAR canopy height model (CHM). A program was created to estimate canopy openings at the crown height of each tree. Results confirm that the annual growth is higher when the tree is well illuminated (high crown illumination), facing few competition and with high maximum size. Moreover, the most important result is that after a certain age, the
variation of resources influences no longer the growth of trees. Together resource availability and individual conditions only explained on average 25% of the variation in growth rates, a large part (46%) is explained by mixed effects (species effect).
Thereby other non-investigated additional environmental variables (topography, soil, etc.) may contribute to shaping tree growth in tropical rainforests.

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Impact of habitat shift driven by humans on ants biodiversity and foraging strategies

Isabelle Kozon, Jean-Romain Roussel under the supervision of Alex Salas-Lopez

Ants are ecologically successful animals present in most ecosystems. Despite a strong competition for such dominant taxa, communities of ants can be extremely rich in terms of species number in a same habitat. Here we explore how foraging
strategies can promote such coexistence in two contrasted habitats: well preserved forests, and slash and burn crops in the coastal area of French Guiana. Four types of baits were used to emulate major foraging strategies described for ants in the literature. Our results indicate a richness three times lower in ant slash-and-burn communities than in forest communities. The equitability is also lower in crops. Human activities have therefore a negative impact on ant communities biodiversity.
Ants have a complex relationship with biotic and abiotic environment. So, we observed a difference of species composition in forest and between forest and crops. Ant communities present a niche partitioning, through stress tolerance, trophic levels, and time activity. Habitat conversion (from forest to crops) seems to increase ant activity. We observed a reduction of specialization in ant slash-and-burn communities. Foraging strategies have been studied according to different trade-offs. In this study trade-offs cannot explain the coexistence of many species according to niche theory.
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