Forêt tropicale | EPFL Graph Search

thumb|Forêt tropicale primaire de Bébour (Île de La Réunion). La forêt tropicale est la forêt caractéristique des régions tropicales et équatoriales. Sous ce terme se cachent des réalités très différentes, des forêts tropophiles, composées d'arbres assez épars poussant sous un climat tropical de savane, à la forêt dite tropicale humide dans des zones à climat équatorial, en passant par les forêts de nuages relativement froides. En 2015, les forêts tropicales couvrent d’hectares, dont environ en Amérique du Sud (dont la forêt amazonienne), en Afrique (dont la forêt du bassin du Congo) et en Asie (dont la forêt indonésienne). N'ayant pas ou peu été affectées par les dernières glaciations, ce sont les forêts les plus riches du monde au regard de la biodiversité. Les forêts tropicales humides abritent la moitié des espèces animales terrestres et les deux tiers des espèces végétales existantes. Cependant, elles sont souvent menacées par la conversion en zones agricoles ou sylvicoles,

Tropical soils degraded by slash-and-burn cultivation can be recultivated when amended with ashes and compost

Muhammad Saleem Arif, Alexandre Buttler, Leia Soraya Véronique Falquet, Justine Marie Gay des Combes, Vincent Eric Jules Jassey, Robert Thomas Edmund Mills, Bjorn Jozef Maria Robroek, Clara Sanz Carrillo

In many tropical regions, slash-and-burn agriculture is considered as a driver of deforestation; the forest is converted into agricultural land by cutting and burning the trees. However, the fields are abandoned after few years because of yield decrease and weed invasion. Consequently, new surfaces are regularly cleared from the primary forest. We propose a reclamation strategy for abandoned fields allowing and sustaining re-cultivation. In the dry region of south-western Madagascar, we tested, according to a split-plot design, an alternative selective slash-and-burn cultivation technique coupled with compost amendment on 30-year-old abandoned fields. Corn plants (Zea mays L.) were grown on four different types of soil amendments: no amendment (control), compost, ashes (as in traditional slash-and-burn cultivation), and compost + ashes additions. Furthermore, two tree cover treatments were applied: 0% tree cover (as in traditional slash-and-burn cultivation) and 50% tree cover (selective slash-and-burn). Both corn growth and soil fertility parameters were monitored during the growing season 2015 up to final harvest. The amendment compost + ashes strongly increased corn yield, which was multiplied by 4-5 in comparison with ashes or compost alone, reaching 1.5 t/ha compared to 0.25 and 0.35 t/ha for ashes and compost, respectively. On control plots, yield was negligible as expected on these degraded soils. Structural equation modeling evidenced that compost and ashes were complementary fertilizing pathways promoting soil fertility through positive effects on soil moisture, pH, organic matter, and microbial activity. Concerning the tree cover treatment, yield was reduced on shaded plots (50% tree cover) compared to sunny plots (0% tree cover) for all soil amendments, except ashes. To conclude, our results provide empirical evidence on the potential of recultivating tropical degraded soils with compost and ashes. This would help mitigating deforestation of the primary forest by increasing lifespan of agricultural lands.

Wiley2017

The response of stomatal conductance to seasonal drought in tropical forests

Charlotte Grossiord

Stomata regulate CO2 uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (g s) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water‐use‐efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψleaf), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf‐mass‐per‐area (LMA), leaf carboxylation capacity (V c,max), leaf water content, the degree of isohydry, and predawn Ψleaf. We first investigated how the choice of four commonly used leaf‐level g s models with and without the inclusion of Ψleaf as an additional predictor variable influence the ability to predict g s, and then explored the abiotic (i.e. month, site‐month interaction) and biotic (i.e. tree‐species‐specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψleaf did not improve model performance and that the models that represent the response of g s to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each g s model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R 2 = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of g s dynamics over seasonal drought, and identifies a practical, trait‐based approach to improve modeling of carbon and water exchange in tropical forests.

2020

The response of stomatal conductance to seasonal drought in tropical forests

Charlotte Grossiord

2020

Tropical soils degraded by slash-and-burn cultivation can be recultivated when amended with ashes and compost

Wiley2017