Éditeur(s) : HAL CCSD Public Library of Science
Résumé : International audience
Forest successional processes following disturbance take decades to play out, even in tropical forests. Nonetheless, records of vegetation change in this ecosystem are scarce, increasing the importance of the chronosequence approach to study forest recovery. However, this approach requires accurate dating of secondary forests, which until now was a difficult and/or expensive task. Cecropia is a widespread and abundant pioneer tree genus of the Neotropics. Here we propose and validate a rapid and straightforward method to estimate the age of secondary forest patches based on morphological observations of Cecropia trees. We found that Cecropia-inferred ages were highly correlated with known ages of the forest. We also demonstrate that Cecropia can be used to accurately date disturbances and propose twenty-one species distributed all over the geographical range of the genus as potential secondary forest chronometer species. Our method is limited in applicability by the maximal longevity of Cecropia individuals. Although the oldest chronosequence used in this study was 20 years old, we argue that at least for the first four decades after disturbance, the method described in this study provides very accurate estimations of secondary forest ages. The age of pioneer trees provides not only information needed to calculate the recovery of carbon stocks that would help to improve forest management, but also provides information needed to characterize the initial floristic composition and the rates of species remigration into secondary forest. Our contribution shows how successional studies can be reliably and inexpensively extended without the need to obtain forest ages based on expensive or potentially inaccurate data across the Neotropics.
ISSN: 1932-6203

halsde-00757518
https://hal.archives-ouvertes.fr/halsde-00757518
DOI : 10.1371/journal.pone.0042643
PUBMED : 22916144

Éditeur(s) : HAL CCSD Société nationale de protection de la nature
Résumé : International audience
The island of Moorea in French Polynesia is representative of a relatively well preserved small tropical oceanic island but both its terrestrial and marine ecosystems are currently under strong anthropogenic pressures. The terrestrial ecosystems are facing an increasing urbanization, and the destruction and modification of their natural habitats by cultivation and forestry plantations, accidental or intentional fires, and particularly by the invasion of alien plant and animal species. Coastal areas and reef ecosystems are threatened by shoreline artificialization, sedimentation due to soil erosion, increasing uses of marine and lagoon resources and frequency of coral bleaching events. A significant erosion of terrestrial and marine biodiversity is observed on the island since the last decades. In the context of the Land Management Plan (PGA) and Marine Area Management Plan (PGEM) recently set up in the island by local authorities, scientific surveys and inventories were made in 2006 in order to assess the status of terrestrial biodiversity (vascular plants) and marine biodiversity (reef fishes) and compare areas with different anthropogenic pressures. Studies conducted on the flora and the vegetation at low and middle elevation along altitudinal transects show that the plant invasion process is related to climatic and topographic factors, and species autoecology. The marine study aims to compare the protected areas with control areas in order to test the effectiveness of protection measures and to propose a streamlined monitoring protocol. A methodological study in both terrestrial and marine ecosystems was conducted in order to propose biodiversity indicators which are crucial for decision making in conservation and sustainable development of island biodiversity. We used Geographic Information System (GIS) as a fast and intuitive tool for mapping indicators and their variability. This methodological approach developed on the island of Moorea could be used as a model for other tropical islands
Moorea est une petite île océanique tropicale de la Polynésie française relativement bien préservée, mais dont les écosystèmes terrestres et marins sont actuellement soumis à de fortes pressions anthropiques. Le milieu terrestre est confronté à une urbanisation croissante, à la destruction ou à la modification des habitats naturels pour l’extension des cultures ou des plantations forestières, aux incendies volontaires ou accidentels, et particulièrement à l’invasion d’espèces végétales et animales introduites. Les zones côtières et les récifs coralliens sont menacés par de multiples perturbations telles que l’artificialisation des lignes de rivage, l’apport de sédiments dus à l’érosion des sols, l’augmentation des usages des ressources marines et lagonaires et de la fréquence des phénomènes de blanchissement du corail. Une érosion sensible de la biodiversité terrestre et marine de l’île est observée depuis les dernières décennies. Dans le cadre du Plan Général d’Aménagement (PGA) et du Plan de Gestion de l’Espace Maritime (PGEM) mis en place par les autorités locales, des inventaires scientifiques ont été réalisés en 2006 dans le but d’évaluer l’état de la biodiversité terrestre (plantes vasculaires) et marine (poissons récifaux) et de comparer des zones soumises à différentes perturbations. Les études réalisées sur la flore et la végétation de basse et moyenne altitude le long de transects altitudinaux montrent des processus d’invasion végétale différents en relation avec les facteurs climatiques, topographiques et autoécologiques des espèces. Les études en milieu marin visent à comparer des aires protégées à des aires témoins afin de suivre l’efficacité des mesures de gestion. La réflexion méthodologique menée parallèlement sur les deux écosystèmes a pour but d’illustrer différents indices de biodiversité nécessaires à la prise de décision en matière de conservation ou de développement durable de la biodiversité insulaire et de réaliser un état de référence pour établir un diagnostic sur l’état de la biodiversité. La représentation cartographique des indices à l’aide d’un SIG, illustrée dans cette étude avec la richesse spécifique terrestre et marine, permet une lecture intuitive et rapide de la variabilité de cet indice dont la restitution est basée sur une standardisation du nombre d’espèces. Cette approche méthodologique développée sur l’île de Moorea pourrait servir de modèle pour d’autres îles tropicales
EISSN: 2429-6422

hal-01306285
https://hal-univ-perp.archives-ouvertes.fr/hal-01306285

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : Patterns of leaf production and leaf fall directly influence leaf area index and forest productivity. Here, we focused on Cecropia sciadophylla individuals inhabiting the extremes of the gradient in seasonality in rainfall at which C. sciadophylla occurs. In Colombia and French Guiana we compared the intra-annual variation in leaf production as well as the intra-annual fluctuation in internode length on a total of 69 saplings ranging in size from 1 to 2 m. The mean rate of leaf production was similar to 2 leaves mo(-1) in both populations, and the rate of leaf production was constant throughout the year. Our results showed monthly variation in internode length and the number of live leaves per sapling in the seasonal habitat and variation only in internode length in the everwet habitat. Because the rate of leaf production is constant at both localities, the difference in number of live leaves per sapling at the seasonal site must reflect seasonal variation in leaf life span. We show that in Cecropia, internode length can serve as an indicator of precipitation seasonality. Finally an open question is whether leaf production in other pioneer species is also independent of climatic seasonal cues. This information could allow us to link growth and climate of secondary forest species and better understand how past and future climate can affect plant growth trajectories.
ISSN: 0266-4674

hal-01032402
https://hal.archives-ouvertes.fr/hal-01032402
DOI : 10.1017/S0266467413000394