Exposés ATBC 2012, répétitions vendredi 15 juin 2012
14h-16h30 Salle silvolab
Mélaine AUBRY-KIENZ
Towards trait-based mortality models for tropical forests?
Mélaine Aubry-Kientz, Charles Ayotte-Trépanier, Bruno Hérault, Vivien Rossi
Global change and associated increases in the frequency, duration and/or severity of drought events
could fundamentally affect tropical forest dynamics. In this context, modeling the effects of climate
warming on tropical forest dynamics poses an urgent scientific challenge. Models of tropical forest
dynamics basically involve three ecological processes: recruitment, growth and mortality. In this
study, we focused on tree mortality, a key but understudied process.
We developed a model of tree mortality, working at the community scale, using 20-years of forest
dynamics data from a neotropical forest in French Guiana. The model computes individual
probabilities of tree death using functional traits as covariables in a generalized linear model. The
functional traits are proxies of tree ecological strategies and offer furthermore a way to solve
problems linked to over-parameterization in highly-diverse communities. Covariables were selected
and parameters were estimated in a Bayesian framework
We found that mortality is strongly linked to the ontogenetic status of individual trees, with a
typical U-shape curve of death probability with age. We describe a suite of functional traits (e.g.
wood density, maximum size etc...) that shape the individual curves.
Our modeling approach offers a promising way to predict mortality in species-rich communities
with little parameter inflation, bridging the gap between functional ecology and population
demography.
Fabien WAGNER
Which climate indices are relevant for predicting the response of tropical forests to future climate
scenarios ?
Fabien Wagner, Alana L. Moore, Vivien Rossi and Bruno Hérault
Climate models predict a range of changes in the amazonian region, including increased frequency
of extreme climatic events, increased average temperatures, increased atmospheric CO2 and reduced
rainfall intensity.
Understanding tree growth response to climate is important because wood production is the main
way carbon enters the forest ecosystem. The response of tropical tree growth to changing climate
could drive a change in the direction of the flux from terrestrial ecosystems to the atmosphere.
The last 20 years has seen a substantial increase in the number of publications with a focus on the
effects of climate on tropical tree growth. Citation reports extracted from the Web of Science with
the keywords ‘climate’, ‘tropical forest’, ‘growth’ and ‘trees’ found only 15 articles in 2000 and
more than 70 in 2011. These articles are based mostly on modelling approaches combining tree
growth and one or more climate indices.
We review the different approaches used in order to, (i) identify gaps in current research, (ii)
determine which climate index is the most relevant for a range of temporal and spatial scales and
(iii) establish which climate indices are the most appropriate to study tropical tree growth in regards
to the climate output of the IPCC scenarios.
Jérémie Lauth
A mutualistic community of ants, plant and fungi: How a third partner affects plant biotic
protection by ants
Jérémie Lauth, Pierre-Jean G. Malé2, Céline Leroy, Olivier Roux, Alain Dejean and Jérôme Orivel
In ant-plant mutualisms, the biotic protection of plants is ensured by the patrolling of ant workers on
the leaves. However, because such plant defense emerges from the predatory behavior of the
colonies, the biotic protection is considered as a by-product of selfish activities. To meet their need
for nitrogen in the restricted foraging environment provided by their host plants, some plant-ants
have developed alternative hunting techniques. This is the case for example in ants of the genus
Allomerus, which cultivate a fungus to build galleries on the stems of their host plants they use as
traps. If such predatory strategy enables the capture of prey larger than those the workers could
capture on the foliage, it generates, however, a spatial segregation of the workers, which could
reduce the conferred benefits to the plants. Here we explore this issue by quantifying several traits of
the predatory strategies of two Allomerus species coexisting on the same host plant species, Hirtella
physophora. These two ant species displayed significant differences, with the specialist ant A.
decemarticulatus preferentially preying in the galleries and the generalist A. octoarticulatus on the
leaves. Consequently, the plants inhabited by the ant A. octoacticulatus were better protected and,
thus, experienced a higher vegetative growth and fruit production. Our results suggest that although
A. octoarticulatus confers greater benefits to H. physophora, the higher specialization of A.
decemarticulatus towards its host plant and the maintenance of positive outcome of their association
through reciprocated investment seem to be sufficient to maintain an evolutionary stable interaction.
Moreover, providing that the galleries could be used as a proxy of the strength of the fungal
association, the integration of such protective mutualism in a larger community context highlights
how additional partners may modulate the basic outcomes and the evolutionary mechanisms
maintaining a bipartite mutualism.
Céline LEROY
Tank-bromeliads : under-studied freshwater micro-ecosystems in tropical rain forests
Christopher BARALOTO
Effects of road paving and land tenure on forest value in the tri-national frontier region of
southwestern Amazonia
Claire FORTUNEL
Environmental filtering and species distributions across lowland South American forests
Hélène MOREL
Studying phenology through a morphological and anatomical retrospective analysis under
tropical climate: An example on Parkia velutina Benoist (Mimosoïdeae).
Most studies on tropical plant phenology have focused on patterns of flowering, fruiting and leafshedding. Field surveys are used in most studies, some of them relying on remote-sensing data.
Phenological examinations of cambium are scarces, despite the importance of secondary growth to
dendrochronology and retrospective growth-ring analysis. We studied primary and secondary
growth at scales from within-individual to populations, using two complementary methods: a field
survey and a retrospective analysis based on morpho-anatomical growth markers.
We worked in French Guiana on Parkia velutina, an emergent tree of Neotropical rain forests. We
collected dominant branches from 14 adults and identified growth markers that delimited
longitudinal and radial increments. We coupled this retrospective analysis with a 2-year
phenological survey of (i) leaf-shedding and growth on twenty trees every month and (ii) the
repeated collection of microcores from five trees every fifteen days to determine intra-annual
cambial activity (i.e. formation of secondary xylem).
Successive growth units and growth rings were easily identifiable. These structural regularities
traduce the phenological cycle defined by leaf shedding, growth-unit elongation and growth-ring
formation. Within individual trees, we found this cycle to be synchronous across all growth axes.
The cycle was essentially annual, with some slight variability. At the population scale, trees were
unsynchronized. Primary and secondary growths were correlated: the peak of cambial activity
occurred when leaves were mature. Conversely, little cambial activity occurred when leaves were
senescent or young, or branches were leafless.
Our study shows that retrospective analysis and field survey are two complementary approaches to
understand plant phenology and to interpret morpho-anatomical structure. Dating a branch by
counting the number of growth units or growth rings is possible in many years with a reasonable
error. Nevertheless, estimating their precise month of formation in order to study climatic influences
remains difficult.