Old-growth tropical forests harbor an immense diversity of tree species but are rapidly being cleared, while secondary forests that regrow on abandoned agricultural lands increase in extent. We assess how tree species richness and composition recover during secondary succession across gradients in environmental conditions and anthropogenic disturbance in an unprecedented multisite analysis for the Neotropics. Secondary forests recover remarkably fast in species richness but slowly in species composition. Secondary forests take a median time of five decades to recover the species richness of old-growth forest (80% recovery after 20 years) based on rarefaction analysis. Full recovery of species composition takes centuries (only 34% recovery after 20 years). A dual strategy that maintains both old-growth forests and species-rich secondary forests is therefore crucial for biodiversity conservation in human-modified tropical landscapes.

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International audienceThe census of vascular plants across a 10-year interval (1995-2005) at the fringe of a neotropical rainforest (Nouragues inselberg, French Guiana, South America) revealed that species richness decreased, both at quadrat scale (2 m2) and at the scale of the inselberg (three transects, embracing the whole variation in community composition). Juvenile stages of all tree and shrub species were most severely affected, without any discrimination between life and growth forms, fruit and dispersion types, or seed sizes. Species turnover in time resulted in a net loss of biodiversity, which was inversely related to species occurrence. The most probable cause of the observed species disappearance is global warming, which severely affected northern South America during the last 50 years (+2° C), with a concomitant increase in the occurrence of aridity

THfiRY, M. 1990. Ecologie et comportement des oiseaux Pipridae en Guyane: leks, frugivorie et dissemination des graines. These de Doctoral de PUniversite Paris 6.

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International audienceWe questioned whether and how plant communities vary in space and time along an inselberg-rainforest ecotone in relation to present-day warming and whether biotic and non-biotic factors could explain the observed patterns. The study took place on a granitic inselberg in the French Guianan (South America) rainforest (Nouragues Natural Reserve: 4°5'N, 52°41'W). In a diachronic study (1995-2005) embracing a severe El-Niño event in 1997, we analysed vegetation structure and composition along three transects subsuming whole environmental and topographical variations in the transition zone from shrub vegetation at the fringe of open-rock vegetation to tall-tree rainforest. Data were analysed by PCA. Major variations in species and trait distribution were described in the low forest, with two floristic types evidenced by first PCA component and verified by cluster analysis: one with floristic composition reminiscent of open-rock vegetation but with higher and continuous canopy, the other typical of the low forest. There is no clear-cut boundary between typical open-rock and low forest vegetation. Variation in species composition of typical low forest was evidenced by second PCA component, which displayed differences according to slope and altitude. Small (~1.5 m), although significant, shifts in the spatial distribution of plant species pointed to possible slow encroachment of typical low forest vegetation in the absence of disturbance. However, the stability of species and trait distribution was remarkable within the 10-yr interval considered, despite an otherwise recorded decrease in species richness and recruitment. The boundary between typical low forest and open-rock-like vegetation coincided with the spatial limit of the mineral soil above granite. Despite demographic accidents due to severe El Niño events, plant communities at the fringe of a tropical inselberg are stable at short-time both in composition and spatial distribution. In the absence of strong disturbances such as wildfire and further erosion, soil availability for roots could be interpreted as an environmental constraint to the successional development of forest vegetation. Soil development might thus act as an ecological barrier to forest encroachment, which could only be alleviated by erosion recovery, as otherwise demonstrated

Secondary forests are a prominent component of tropical landscapes, and they constitute a major atmospheric carbon sink. Rates of carbon accumulation are usually inferred from chronosequence studies, but direct estimates of carbon accumulation based on long‐term monitoring of stands are rarely reported. Recent compilations on secondary forest carbon accumulation in the Neotropics are heavily biased geographically as they do not include estimates from the Guiana Shield. We analysed the temporal trajectory of aboveground carbon accumulation and floristic composition at one 25‐ha secondary forest site in French Guiana. The site was clear‐cut in 1976, abandoned thereafter, and one large plot (6.25 ha) has been monitored continuously since. We used Bayesian modeling to assimilate inventory data and simulate the long‐term carbon accumulation trajectory. Canopy change was monitored using two aerial lidar surveys conducted in 2009 and 2017. We compared the dynamics of this site with that of a surrounding old‐growth forest. Finally, we compared our results with that from secondary forests in Costa Rica, which is one of the rare long‐term monitoring programs reaching a duration comparable to our study. Twenty years after abandonment, aboveground carbon stock was 64.2 (95% credibility interval 46.4, 89.0) Mg C/ha, and this stock increased to 101.3 (78.7, 128.5) Mg C/ha 20 yr later. The time to accumulate one‐half of the mean aboveground carbon stored in the nearby old‐growth forest (185.6 [155.9, 200.2] Mg C/ha) was estimated at 35.0 [20.9, 55.9] yr. During the first 40 yr, the contribution of the long‐lived pioneer species Xylopia nitida, Goupia glabra, and Laetia procera to the aboveground carbon stock increased continuously. Secondary forest mean‐canopy height measured by lidar increased by 1.14 m in 8 yr, a canopy‐height increase consistent with an aboveground carbon accumulation of 7.1 Mg C/ha (or 0.89 Mg C·ha−1·yr−1) during this period. Long‐term AGC accumulation rate in Costa Rica was almost twice as fast as at our site in French Guiana. This may reflect higher fertility of Central American forest communities or a better adaptation of the forest tree community to intense and frequent disturbances. This finding may have important consequences for scaling‐up carbon uptake estimates to continental scales.

The Muséum national d'histoire naturelle (MNHN) in Paris holds ca. 70 million specimens. The collections were in need of a strategy to ensure their long-term conservation. We discuss how the Department of Botanical and Zoological Gardens (DJBZ; tropical living collections), and the Department of Systematics and Evolution (DSE; herbarium) contribute to achieving GSPC's Target 1 ('a widely accessible working list of known plant species as a step towards a complete world flora'). The DJBZ started encouraging better management of the collections, evolving towards focused reference collections, where all specimens have well-documented collection data. The objective is to link all collections to a scientific referee. This has already been achieved for a number of taxa. The herbarium of the DSE (acronym P) is among the world's largest (11 million specimens, including 400,000 types). The collection's heterogeneity impedes access to its data, since P is a mix of recent well-documented collections and historical collections at various curational levels. P is currently under renovation, which started by mounting all ca. 2 million unmounted specimens. The project also includes databasing and imaging of every specimen. The database now holds around 1,000,000 records. For taxonomic studies, living collections are crucial, especially for plants that are not easily preserved as herbarium specimens. Living collections also enable studies impossible to forecast at the time of collecting. Herbaria and living collections should therefore be conceived as interoperable entities requiring common scientific curation. Through a combination of its assets and the expertise of its researchers, the MNHN is well prepared to tackle the new objectives of the GSPC beyond 2010.