Palaeoclimatic interpretation of a topographic profile across middle Holocene regressive shorelines of Longmu Co (Western Tibet)

Most lakes and closed basins in Tibet are surrounded by abandoned shorelines attesting for high-water levels in the past. Two such lakes, Longmu Co and Sumxi Co (Western Tibet), were the focus of detailed palaeoclimatic work during the 1989 Sino-French expedition based on the study of lake sediments and analysis of a piston core. Here it is shown that additional information may be deduced from the abandoned shorelines around Longmu Co. A 1800 m long topographic profile was levelled from 75 m up to 230 m above the present lake elevation (5008 m), on which the shorelines form a flight of horizontal terraces separated by more or less degraded scarps. The elevations of the terraces were spaced regularly about 1.35 m, except for 3–5 m high lacunae reccurring every 15–16 m. The potential annual evapo-transpiration was computed based on meteorological data collected at Tien Shui Hai from 13/08/89 to 11/09/90. Using a modified Penman formula a value of 1.60 ± 0.3 m/yr was obtained under close system conditions (no outlet) and a synthetic curve of lake regression was derived which appears to be consistent with the measured shoreline heights. It is concluded that the 1.35 m terraces represent annual regression of the lake level. By contrast, the 3–5 m high lacunae might reflect the periodic return, about every 9–12 yr, of particular climatic conditions responsible for a perturbation of the annual process of shoreline formation. All the shorelines of this profile have formed probably within one century, during a continuous and abrupt regression of the lake. Such evolution has also been identified in the sediments of a piston core collected at Sumxi Co, and dated at about 6-5.5 kyr B.P. The lake level must have been at its highest stand, shortly before that major regression. The mechanism responsible for this middle Holocene high stand and for the major sudden drop of the lake level remains uncertain

Interannual variations of forest floor vegetation communities in the French RENECOFOR: level 2 network

International audienc

Interannual variations of forest floor vegetation communities in the French RENECOFOR: level 2 network

International audienc

Can we reliably estimate species richness with large plots ? an assessment through calibration training.

The number of species (species richness) is certainly the most widely used descriptor of plant diversity. However, estimating richness is a difficult task because plant censuses are prone to overlooking and identification errors that may lead to spurious interpretations. We used calibration data from the French ICP-level II plots (RENECOFOR) to assess the magnitude of the two kinds of errors in large forest plots. Eleven teams of professional botanists recorded all plants on the same eight 100-m2 plots in 2004 (four plots, eights teams) and 2005 (four plots, nine teams including six from 2004), first independently and then consensually. On average, 15.5% of the shrubs and trees above 2 m were overlooked and 2.3% not identified at the species level or misidentified. On average, 19.2% of the plant species below 2 m in height were overlooked and 5.3% were misidentified and 1.3% were misidentified at the genus level (especially bryophytes). The overlooking rate also varied with plant species, morphological type, plot and team. It was higher when only one botanist made the census. It rapidly decreased with species cover and increased with plot species richness, the recording time of the census in the tree layer and the number of the censuses carried out during the day in the ground layer. Familiarity of the team with the local flora reduced the risk of overlooking and identification errors, whereas training had little impact. Differences in species richness (over space or time) in large plots should be cautiously interpreted, especially when several botanists participate in the survey. In particular, the quality of the data needs to be evaluated using calibration training and, if necessary, may be improved by involving more experienced botanists working in teams and by fixing a minimum recording time